Morley

OUCH!

Have you tried replacing the TPS yet? The next thing I'd look into is the possibility that something has come loose in the distributor (reluctor ring maybe), you can try checking this by hooking a timing light to the car and while it is idling in park, run the throttle up and back smoothly, see if the timing mark moves smoothly. If it starys jumping around or moves jerky, then there is something up in the dist.

Deik

Ouch is an under statement! Did check voltages on TPS they were within specs @ idle and WOT. I will try replacing distributor because the timing mark does move eratically @ idle not alot but sort of like a twitch. What about the MAF is that typical for the car to die when you unplug it ?

Morley

Has been in my experiance. I can't get my car to run at all with it unplugged.

Mark_ZZ3

While trying to troubleshoot my surge, I have the following disconnected:

- IAC
- MAF
- Dist set timing connector
- O2

It idled at about 500-600 and still surged.

At this point, the only thing left that can cause a surge is the fuel or the dist. I am going to swap the injectors this weekend, but I expect the surge will still be there.

I am also thinking a bad coolant temp sensor could be the cause. If the ECM was getting erratic readings, it might be altering the fuel curve. I recall recently, when the engine was really warm, it idled quite well.

Mark.

Mark_ZZ3

Well a new set of 24 lb accel injectors and I still have my surge. I am convinced there is some play in the dist. I have a spare one that I will rebuild and swap when time permits.

Mark.

Kingtal0n

well now, let me share some experience.

I just got my TPI running after months of parts gathering..
this is my first fuel injected experience so i noted a rather large learning curve with this stuff.. but its been two weeks now and im beginning to figure this stuff out..

I am using the commander 950 instead of the stock ECM, so i probably have a bit more "control" over the motor while its running, considering im working real time with it. but its been helpfull to be able to make changes, and see the changes occur right after you make them.

my initial surging was caused by my fuel regulator diaphram leaking fuel into the vacuum side, which my plenum would then greedily suck up and basically cause the back 2 cylinder to get more fuel than the rest. this was a huge surging issue..

the next thing i noticed causing a surge was the knock sensor... it would pick up some knock, retard the timing, then set it back in... this caused a large surge. fixed it with teflon tape and not so tight.

the next thing causing a surge was a plugwire would fire perfectly fine for a bit... then suddenly stop firing. turned out it was a bit burned from the headers...

the next thing was the rate of change of MAP... since you have MAF there is probably a setting that watches the airflow into the motor. how big is you cam? I noticed my MAP sensor would pick up the cam's "lope" as a change in engine load, and add more fuel. (as if a power valve was opening in a carb). this caused it to surge... the fix for me was simple... i lowered that portion of the Rate of change so the fuel would be less incremential.

the next thing was simply the cam's powerband VS idle quality, or more simply, the cam was in "idle/lope" mode below 1000 rpms, and in "smooth" mode above 1000 rpms... so whenever the motor was trying to smooth the idle with the IAC moving about.. the cam's lope would make it hard to stabilize, and it would "overshoot" the desired idle speed to 1000RPMS or more... causing the engine to smooth out and consequently build RPMS really fast and cause the IAC to slam shut, which would cause the idle to stumble back down really fast and the IAC would catch it before it was about to die... and the process would repeat... 1300 rpms... 400 rpms... 1300 rpms... 400 rpms...
the fix was to slow the IAC movement down, and also to change the derivative so it would expect less of a change, sooner.

the other fix to the above was a smoother spark map... since right off idle (stock converter... 1000 rpms~~) i had a bunch of advance come in real fast to get the car going... when the idle shot above 1000 rpms the extra timing would "carry" the rpms really high... so smoothing out the advance map helped a lot.

another thing was the O2 mod.. my motor likes to idle slightly rich (cause of the cam i beleive) and therefore didnt want to idle closed loop... the O2 sensor would pull out fuel until 14.7:1 was acheived (as designed) but the motor would begin to die and idle sparotically causing an instant switch to open loop and it would go rich causing the idle to jump up and smooth out...
the process would repeat.

the fix was to go closed loop above 800 rpms... and return to open loop @ 700 rpms (desired idle is 650 RPMS).

also the coolant mod was screwing me up.. mine was adding extra fuel for temps above 195* causing a 5% enrichment of the fuel map.

and beleive it or not i had a huge vacuum leak in my plenum (below brake booster line) and it didnt do much at all except lower my need for air at the TB... fixing the vacuum leak didnt affect the idle hunting problem at all as i thought it would..

so it appears fuel plays the biggest role in idle hunting.. a slight change in Pulse width at idle changes the idle in a BIG way...

I hope this gives you some direction. and BTW my combo is simple:
Stock TPI ported to hell (PTH). XE262 comp cam, Performer RPM heads. its a bit iffy off idle with the stock converter and gears, but it does ok. i get average 29-31 MPH highway too...

my fuel map has me @ about 3.2PW at idle (4-5%DC) and on the highway its about 8.5%DC(2.2PW) this with 45* of total timing anytime during cruise, and 36* total during any engine load. my AE enrichment "pump shot" is 1.77MS of fuel with slow throttle movement, and about 3.5MS of fuel with a "stab" from a dead stop. still tuning though...

2QUIK6

The only way I eventually solved my surge problem was selling the TPI motor and replaced it with an LT4! Sorry, but I gave up.

/\/3\/\/l8l3

THIS IS MY EXACT PROBLEM WORD FOR WORD

JPrevost

Not just to you but to everybody that's having the LEAN surging... guess what... YOU'RE ALL LEAN!!!!
All this playing around with replacing things that control spark and voltages, BAH, it's simplely lean.
I notice the people with these problems have large cams and have "custom" chips. After years of reading "custom chip" on these boards I've come to the conclusion that this is a huge problem. They aren't custom, they're crap, unless you DIY (DO IT YOURSELF).
I can't understand what people are thinking when they swap in a huge cam and expect the computer to compensate for it. That's wronger than wrong because if that was the case then GM would have had only 1 computer and it would never have been updated.
You all need to start doing yourselves a favor (and your engine) and tune your own chips. You would have been able to solve this lean surging in a matter of minutes compared $$$$ for parts and time wasted replacing perfectly good parts.
Mail order chips are crap, pure dung on a stick. The makers of these chips tell you they'll work with you but that's just a lie to get you to front up the money. Trust me, I was there and contemplating doing it . Nobody knows your car better than you and with the internet there is no reason why anybody couldn't tune their own vehicle without a dyno and without a wideband. I've taught more than 20 people how to tune EFI and I'm only 24 years old. I'm an espiring calibrations engineer so you know I'm not protecting my job security by telling you that YOU can tune. I'm just being totally frank with you because I'd rather have less money in my pocket than a bunch of crappy running chevy's getting beaten by Fords and Imports. Seriously though, why is it so many people actually listen to these mail order chip guys telling them to buy their solve all magic chip?
Get with the program people. Hot Rodders in the day had to learn how to tune carbs and they didn't even have the internet . EFI is just the new and improved "clean hands" way of running a performer.
If you need help tuning then find a person locally that'll show you hands on or it's nothing but a waste of your time and money. Case in point; DIY or don't even bother.

/\/3\/\/l8l3

i'm rich though

Mark_ZZ3

I had a 87 Z28 that did this. turned out the pickup coil wires were damaged ... or getting there. The only way it would idle would be if you disconnected the set timing plug. Try that ... if it makes a difference, then I would suggest you pull your cap and rotor off and have a look.

Mark.

Mark_ZZ3

I agree with you that "some" people might be experiencing the situation you describe, but you can't blanket everyone with that statement. First off, parts go bad, things wear out. You can tune your chips all you want, but if you have a vac leak, dead injector, low compression on one cylinder, dead wire or plug, and on and on ... you still have to check the basics.

No question ... once you start modifying, a custom chip is the best course of action. But you still need to know that your engine is in a good state of tune with working components.

I would be surprized to learn that I have a lean condition at idle. It is very rich. Rather I feel I am just out of the range where the ECM can compensate ... so I need to do the chip.

Mark.

JPrevost

Mark, if it's surging like the original poster then it's lean, plain and simple that's all that's wrong. Trust me, I've watched at least 10 TPI 305's and 350's all do the surge thing at idle when they're too lean.
You do know that when you're this lean it WILL smell rich. The only way to tell is to look at the spark plugs when the engine is reving up (turn off the ignition) and when it's coming back down (turn off the ignition). Look at the plugs and you'll find your answers if you know how to tell lean (white), normal (tan), and rich (black). It's more detailed than that but it's just a direction.
For those that don't tune their own chips, have you tried an adjusting your fuel pressure? Increase the fuel pressure and tell me where that lean surge has gone

Mark_ZZ3

I've spent a few hours in my day reading plugs, but I've never found my TPI to show anything out of the ordinary ... except for a weak injector.

I've varied my fuel pressure and changed to larger injectors and all that did is give me that rich fuel smell out the exhaust (actually the larger injectors slowed by 1/4 time by a bit - can you say too rich?).

As a test, are you saying that I should over richen my fuel curve at idle by upping the pressure and the surge should change or disappear? I understand that you don't do this and keep it this way, but just as an experiement? I'd guess I would have to really over richen it, not just a tad. But consider, I am running 24lb injectors now and I have my pressure at 40 or 43. I *should* be very rich at idle ... which my nose tells me. Although the BLM's still look like they are adding fuel (130-134).

I can't quite understand why you would need more fuel at idle given the same displacement. If the engine is still running at 650 rpm, it still is taking in a similar volume of air, but I understand the cam/heads/etc. will change that, however more air in, usually means a higher idle, and you would adjust the TB back down to 650. I can see how a better cam, heads, etc. can take a stock 5.7L and essence make it seem like a larger motor ... as in it will injest as much air as a 6.0L engine just because it is more efficient. I would expect a faster idle.

A PROM burner has always been on my list of things to buy, but apart my the surge, the darn thing runs very well ... surprizingly well. I've learked on the DIY PROM board for years. Lots of neat stuff happening there.

Mark.

JPrevost

I'm developing a website that shows all sorts of funky things with datalog examples. I plan on using a TBI 350 (mine), and a friends TPI 383 to show what happens when a car is lean idle, bad o2 sensor, rich idle, too low idle, vacuum leak, too much timing, not enough timing.
I think it'll help because I'll also have sound and video showing the tach. A lean surge idle is rather unique in that the exhaust is rather quiet. A rich idle stall into stall saver is much louder to the trained ear. You'd be suprised how much timing I do by ear and not smell. Smell can be so misleading it isn't even funny. Example, my car was lean and misfiring because the AFR in open loop cruising was over 16:1, sometimes the cylinder wouldn't even fire. The exhaust was rather quiet and I would smell raw fuel when I cam to a stop. I thought I was rich until I experimented and found out it was lean misfire (raw gas just gets pumped right out the exhaust).
MAF cars are different animals. At idle you usually have to increase the fuel, don't ask me why but it just does. While on a MAP car, an aggressive cam usually requires less VE because of the obvious self EGR level being so high (cylinders fill with a lot of exhaust gas because of the overlap and intake duration. Does this make sence?
Once you get into programming proms you'll never look back. Sure it takes time to learn but what doesn't? And the rewards of having a sweet running machine vs pulling your hair at trying to pick parts that do didlly squat for power but don't need a chip FAR outweighs the learning curve. Everybody, even stock motors, could benifit from chip tuning. The stock calibrations are great if you drive all around the world and need it to run but that's too generic. If you want best milage and don't drive through Alaska ever then tune your engine for your altitude and weather. Humidity like in Florida, cold winders like in the North, dry heat in Arizona all could use more specific tuning on a stock machine. All of this knowledge is being given to the members of this board for free so why not take advantage of it?

Mark_ZZ3

I'd like to understand why a MAF car needs more fuel at idle. I don't have any EGR (no EGR in the heads).

I understand the lean "miss" and raw fuel smell.

I suppose I'd have to crank the fuel pressure up to try and get the plugs to appear sooty and see what's up.

Perhaps the MAF cars can't compensate fast enough, hence the surge is the result? I wanted to run a 1989 MAF BIN and see how that affects it.

I can't agree more. However it is key to point out that you still need an understanding of the basics to do the programming. It's the same with any engine tuning ... you want to fix the cause of the problem, not the effect. No sense richening up the fuel mixture if all your injectors are just clogged and dirty.

Mark.


But as I said ... you still need to check the basics of your engine. It comes down to are you whether you are tuning for the cause or the effect. Too often, people focus on the result, in this case the surge.

irocdaddy

Mark, I have been following this thread. the last I heard was you where going to rebuild a distributor and pop it in.


My car and yours are pulling the same crap. I found one bad injector on mine thus far (5 ohms reistance).



Please let us know what has been done.

Mark_ZZ3

I did indeed replace my dist. I took an old one, put a new pickup coil on it, and a brand new vortec dist gear. The result was unchanged. :-(

So I have pretty much removed any possible physical cause.

I suspect I am outside the range of the PROM.

I do know, that if I set the idle up higher (use the set screw), the surge disappears (well more like the range is less). But stopping becomes an issue since the engine is idling higher.

I have not ponder this for a few months (parked my vehicle back in Oct). I will probably start working on it again.


Are you running a ZZ4 engine?

MArk.

Mark_ZZ3

I did indeed replace my dist. I took an old one, put a new pickup coil on it, and a brand new vortec dist gear. The result was unchanged. :-(

So I have pretty much removed any possible physical cause.

I suspect I am outside the range of the PROM.

I do know, that if I set the idle up higher (use the set screw), the surge disappears (well more like the range is less). But stopping becomes an issue since the engine is idling higher.

I have not ponder this for a few months (parked my vehicle back in Oct). I will probably start working on it again.


Are you running a ZZ4 engine?

MArk.

irocdaddy

Cool, your here.

Yes, I have the zz4 and t56 in my 89 iroc.

Back in 95'had the dealer install the motor. here are my mods.

52 mm.
slp runners
big mouth intake
ported plen
maf w/screens-sinks removed
1 5/8 slp headers/dual catco cats/slp catback
no adj. afpr.
22 lb. stock injectors
accel 300+ ign. and wires.
stock distributor innards
stock in tank fuel pump.
tpis burnt chip, but back when I had the automatic.
base timing @ 8 deg. btdc.
tps @ .58

the car had sat for about a year before resent start up.
drove the car to get my smog cert. running, car runs great. always had that rich gas smell out the pipes though. Anyway, once off the freeway, she starts surging at idle like a 500 rpm quick swing. from 500 rpm to 1000 rpm. no codes have set. let it just sit there and kick around forever and still no codes.

I have auto x-ray and after scan, nothing out of range. Unhooked the iac while running, no change. once I found a bad injector ( 5 ohms resist, i stopped trouble shooting. got 8 ford 22 lbs. coming in the mail via ebay. seller checked resistance/all checked @ 14.5 ohms (still awaiting them).

tried swapping out maf relays with others i had around/no change. when the car runs in open loop, there is no surge. go to closed loop scanner has o2 going rich to lean constantly. cleaned tb, no change. pulled plugs while waiting for injectors-they are very sooty/dark. never have the plugs looked this way. i think the tb cleaner jacked the plugs up. did a fuel pressure test at the rail, checked good and held pressure. no vacuum leaks were found using the spray around areas method. i have my egr hooked up/always have.

the only thing done between when it ran good was the tb cleaner. by the way, what is the sensor that is just to the left of the o2 sensor? knock sensor? that is on the passenger side. do i have two?

Mark_ZZ3

The sensor on the driver's side close to the O2 probaby is the engine tempature sensor. It is screwed into the cylinder head between Cyl 1 and 3 spark plugs.

Your surge sounds like mine. Nothing really affects it.

I noticed the close loop is where mine did it, however I attribute that to the fact that in open loop, the idle is kept higher.

So the big question ... what causes it. Mine smells rich at idle as well.

A posting above this suggest it is a "lean surge" ... but I have yet to understand how it is lean when all indications are that it is rich. I run larger injectors, and I adjusted the pressure to the point it HAD to be rich, so could it still be lean?

Like I said, if I could hold the fuel mixture constant, I could prove a point.

It's been a while since I thought about this ... need a cup of coffee to get the cob webs out.

MArk.

irocdaddy

Well, since my problem started AFTER years of service with the same chip and mods, I'm not sold on your problems being in the chip. I have my stock chip as well and have swapped it in/out before with little change from the tpis chip.

how bout' this conclusion....just until further testing.

1.my plugs are black after cleaning tb.
2.I found one bad injector (first found one on each side then after they were out, only one tested bad.

fouled plugs/bad injector cause the o2 to hunt?

if I unplug the o2 then run the car, will the car still go into closed loop? if so, then if still surging it is not that sensor because only surges in closed loop. I may have trashed the o2 with the tb cleaner, but scanner say different.

data logging may be our only real good clue to what's happening.

Have you tried a 89 maf 350 stock chip yet?

Mark_ZZ3

No I had not found a 89 chip with VATS to test with. I've always plan to burn my own chips, but the engine always ran so well I did not bother.

Here is something I discovered at the end of the season last year. I had a bad alternator. I had noticed my lights would flicker at idle. My gauge always read normal, and I verified it with a DVM. My engine actually died on the way to being parked. I unplugged the ALT and it fired back up.

I used a spare ALT, and the engine ran OK. Lights did not flicker either. So perhaps I had some noise or bad voltage in my system.

I don't recall if it surged when I parked it ... but I recall I would have been dancing in the streets if that had happened. I'll have to test my therory when the heck the snow goes away.

MArk.

VincentZ28

iTrader: (2)

This is a simple fix. First thing is to change the EGR to the 350 engine and the year of the car. Check the vacuum storage ball inside the fender for cracks. Adjust the fuel pressure regulator while the car is running and the surge should go away. My car has never surge again. This works every time on MAF cars.

Mark_ZZ3

I don't have EGR ... at all. My Cylinder heads don't have the cross over. I've disconnected the power to the EGR solenoid. No codes ever set. I've left the valve on the intake as I did not have a block off plate. Perhaps it is leaking causing a problem?

Perhaps it is related to EGR in some cases, but in mind it "should" be out of the equasion.

Mark.

VincentZ28

iTrader: (2)

Well what about the other things that I had mention?

Mark_ZZ3

>This is a simple fix. First thing is to change the EGR to the 350 engine and the year of the car.

No EGR on my car.

> Check the vacuum storage ball inside the fender for cracks.

No Vac storage ball on my car. Only one vac line on my TPI for fuel regulator. Rest are plugged.

>Adjust the fuel pressure regulator while the car is running and the surge should go away.

Varied pressure from 40-55. Upgraded to 24lb injectors with no change in surge.

Perhaps the surge is related to too much fuel rather than not enough?

>My car has never surge again. This works every time on MAF cars.

Mark.

MikeH

iTrader: (10)

I had a bad plug wire it burned on the inside looked fine on the outside. Found it with the meter, it broke in half after i removed it. The car idled choppy but not bad considering the dead cylinder. It would surge when in park and drive.

The surge went away after I put a good wire on.

Mark_ZZ3

I had checked all my wires (twice) and there were good. I installed new MSD wires a couple years ago and custom cut them for my setup. I measured them all, and I did the same last year just to check.

Agreed that a wire can cause the problem ... and a rich smell.

Mark.

VincentZ28

iTrader: (2)

What type of TPI system are you using?

Mark_ZZ3

It is based on 1987 5.7L Camaro TPI (IE. 1987 PROM, knock sensor).

Mark.

Mark_ZZ3

I should clarify that the TPI surge is on a different vehcile than what is listed. I have two Trans Am's, but they are fine. This is a modified TPI that I run in a "toy" of mine.

Mark.

86TA305Ga

tried playing with the Throttle Posisition Sensor?

Mark_ZZ3

Yes I had looked at the TPS. I re-adjusted my base idle and that required setting the TPS. I run it at 0.54 volts. I checked that it actually worked and there wasn't any perceived gaps or studder. It seemed smooth.

My TB is modified. I had larger butterflies installed (holley dominator parts equal to 52mm ... actually 51.x I recall). The throttle shaft has been milled. And smooth button head screws used. I had it flow benched, before, during and after, with and without air foil. For what it is worth, the air foil was worth a few cfm ... but that only matters if your engine can use it. I had this mod done at a professional race shop years back when "new" 52mm TB were not so common (or outrageous in price).

Mark.

Brazen

EGR functions don't kick in until the vehicle is moving at a good rate of speed, so I don't think that should be the problem. I have a 91 3.1L and have that same pain in the *** surging or lunging. My car will try to launch sitting at a stop light in gear. If I pull it down to neutral, it smooths out. Get's to be a pain though.... and I imagine the people next to me think I'm a 16 year old kid playing games at the light. ;]
Jim

Mangus

I did my best to read through this entire thread, but wasn't able to find if you've tried a difference ECM. The 1227165 ECMs are notorious for going south, and when they do they often have some of the strangest symptoms (speaking from experience! I've had one go bad that would idle at 1500 while the car moving in neutral or in between gears, had one where the CTS was off in a barely noticable manner, but enough to cause strange things to happen).

Anyhow, it seems you've tried everything but that. It's worth a shot.

Mark_ZZ3

I have a spare ECM with a 1988 PROM. I recall it did the same thing. I've changed so many things, it is hard to recall.

Mark.

MikeH

iTrader: (10)

Yep...kinda scary when your two feet from the car in front of you.

Check every sensor connection i had a bad TPS connector that caused all kinds of little crazy things to happen. even though i didnt see anyhting out of the ordinary on the scanner.

Hope you get it figured out, i know it sucks, when you cant enjoy your car because of things like this.

trueburton

Did you ever find the problem my car is doing the same thing. sirry didn't have time to read the whole post

Mark_ZZ3

No I did not find my specific problem. Ruled out most things.

The answer is that there can be many causes of it, not any one thing. Best to check the defaults like the IAC motor and passage, timing, etc.

Mark.

trueburton

Yeah i've checked all that already too and still can't find the solution.

trueburton

What do you mean by TPS connector? What could be bad on it? Don't the wires just run straight through it?

1985WS6transam

iTrader: (4)

Just wondering, How do you adjust the fuel pressure regulator? is that just for the fuel pressure? For the vacuum storage ball to be bad does it have to have visible cracks or can it be internal and how do you know if it is?

PLANT PROTECTION

iTrader: (6)

My vote is for an internal engine problem. You seemed to have covered all the FI related troubleshooting.

Mark_ZZ3

I thought that as well. I did a compression test and there was very little difference between the cylinders. I can't recall the exact numbers, but the total variance from highest to lowest was under 5%.

For my case, it might be a combination of issues. I started it up after it's long winter rest. The surge was still there, but less. The difference this year is the alternator. I had an issue with my previous one which might have been giving a bad signal. In the ECM, it will add/subtract fuel based on the voltage setting. not a huge amount, but another variance.

If I could "fix" the injector pulse I could rule out a few more things.

If you have not noticed, I do enjoy the search. What better way to learn about it.

Mark.

VincentZ28

iTrader: (2)

You will need an adjustable fuel pressure regulator. The vacuum storage ball may have visible cracks. And also set the timing at 6 degrees.

stmotorsports

don't know how helpful this will be, but I've been having a similar problem with my truck ('92 S-10, '91 L05 swap), only mine is intermittent. It seems that my ecm connectors are either worn or a little corroded, or ... ?? I'll be cruising along and all of a sudden it will start acting up- rough, surging, no power, etc etc. Sometimes it will surge so strongly that the brakes almost won't hold the truck at a stop. Somehow, I figured out when I smack the ecm it will straighten right out, but once it straightens out I can beat the **** out of the ecm and it won't go backwards and start acting up again. Sooner or later it will start acting up again on its own, and I can reach over and smack the ecm again and it will be fine for awhile longer.
I've tried 3 known good ecms and different proms/memcals so thats not it. I think I just need some spare time to be able to pull all the pins and mess around with them

maroguy1

I have the same problem on my 85 but it is not constant. Only happens in closed loop. Sometimes it will go from 500 to 800 sometimes just from 500 to 600. I have replaced it all have a burned 85 corvette prom, no emissions new injectors everything replaced except for the throttle body. I have read in other sites that the bearing that controls the throttle doesn't seal and leaks air which causes the idle hunt. It can also happen to aftermarket throttle bodies. So that is my next step until I decide to live with it.

trueburton

What bearing are you talking about?

maroguy1

What is your throttle body bearing upgrade and why do I need it?

We perform our throttle body bearing upgrade on ALL throttle bodies we rework/bore. Do you currently have an unstable or high idle? Then you need throttle shaft repair / bearing upgrade. The GM throttle body is a great piece from the factory, however, with mileage the throttle shaft bore wears out which can lead to two problems. The worn and loose fit of the throttle shaft is a vacuum leak and second because the throttle shaft can wobble around, the blades can bind against the casting at idle thus sometimes closing completely, sometimes not. The end result can be an erratic idle, or high idle all the time. We clean up the throttle shaft and then use a high quality undersized throttle shaft bearing and seal in the Tbody casting (undersize to compensate for any wear of the throttle shaft). The potential for vacuum leaks is eliminated and the throttle blades return to the exact same spot every time - CLOSED. This means you get a consistent and stable idle.