ok so heres the situation....
i bought my 91 z28 back in feb. of this year... it only has 43k original miles. its a top of the line z-28 with the 5.7l tpi, fully loaded etc.etc., ever since ive had the car it has had a weird, loping, surging, erratic, uneven idle. rpm's continually rise and drop anywhere from 300rpm - 800rpm's occasionally it will even itself out but then just "surges" again... the car drives like a dream with no power loss... in april i put a BBK performance throttle body on (58mm). after this mod the "problem" was magnified... please keep in mind there are no engine codes setting off.... after coming to a complete stop... sometimes when you go to give it gas.... it seems as if it stalls but then throws you into the seat as the rpm's suddenly rise... anyways back to topic... i finally decided to take the car to Bill Heard GM dealership for a diagnostics.... there conclusion was that the throttle body is too big and that the factory needed to be put back on.... so i did as they said, and now not only did i lease 15hp... but the problem still exists.... so after that.... i came to thirdgen to do some research and found that it is possible that it is the IAC.... I just replaced it two days ago, and yet the car runs even smoother then before... the loping is still there..... i managed to talk to a service manager at a different GM location and told him the whole situation.... in conclusion he advised that he thinks its a map sensor.... so i picked one up today and plan on replacing it tomorrow.... If anyone has any other educated guesses i could really appreciate the suggestions.... im making 350 car notes every month and still have yet to enjoy the car of my dreams....

 

Sorry to bust your bubble but you didn't loose 15 HP because you didn't gain 15 HP from that throttle body. You gained nothing.

 

alright master486 dyno tests dont lie... but anyways thanks for your comment

 

ok so.. i changed the map sensor today... and still no change in the way the car runs only now... it will lope then stay steady and then lope again.... any opinions are welcomed on what to do next....

 

I'd like anyone else who has seen 15 HP gain from a 58mm throttle body alone chime in please and tell me.

 

alright man....15hp or not...the guy needs help in solving the problem. Wish I could help, but im sure someone here can.

 

You are indeed correct.

Have you checked your TPS to make sure it's still set correctly?

 

im not sure about the tps.... ive been told that it is not adjustable so i really dont know... this is my first tpi ive owned... i could tell you about tbi... but anyways the answer is no....i dont know

 

oh and by the way... not that it matters or anything but the dyno test results were 213hp to the wheels with 315ftlbs. torque so i got carried away with the 15hp my apologies but it was a gain of 10.6hp... over factory.... (245hp from the crank) just thought id clear that up...

oh and for those of you who go to dealerships and are tired of not getting the help you need like me here is the customer assistance number to corporate GM 1800-222-1020

 

More than likely if a sensor is bad it's going to throw a code, if not then what's the point of having trouble codes. Sounds to me like a vaccum leak of somesort. A car with that few miles sitting around over the years in a garage has a tendency to have gaskets and rubber parts dry out and crack based on age. The best way to find out if you have a leak is spray carb cleaner around gaskets and vac hose lines with the engine running, you have a leak if the engine stumbles, and/or dies. Try the easy stuff first before changing parts for no reason.

 

No worries. I've just never heard of that kind of gain before just from a throttle body. In fact, I've heard everyone on here say it's pointless to get the 58mm for the stock engine because it's overkill. They all say the stock 48mm is more than sufficient unless the engine is making over 400 HP. If you say you got that gain, you probably did and I'm not going to ask for before and after dyno slips. It is just amazing to me that you yielded that much from just a TB. Did you get the plenum port matched?

 

yeah im leaning towards vacuum leak too.... but like i said before this car is emaculate.... all the hoses gaskets etc are like "new" condition its just a pain trying to track every hose cause alot of them go underneath the plenum and its just the engine is so compact with all the emissions and a/c equipment.... and no i didnt port the plenum although i am thinking about it... im debating on leaving the car stock or doing a few mods... the factory throttle body is back on it right now but im holdin on to the bbk one...... but i called the 1800 no that i listed and they started a case for me so well see if gm can pull through ill keep you guys updated.... and no hard feeling master.... im not here to argue im a camaro lover just like everyone else on here... ive had an 86 3.8l mpfi, a 92 5.0tbi, and now the car of my dreams the 91 z28 with the preferred equipment group 3
you can view pics at www.cardomain.com user 91zzz28

 

yep... still having idle problems... and to make matters worse my case worker from GM called the local dealership and they told her that my car is too old and has "high mileage" for them to do any further diagnostics.... 43,000 is high mileage? so i think ive reached a dead end in fixing this problem. Im going to spend all day sunday under the hood trying to track down all the vacuum hoses... im hoping to find a diagram of some sort cause there are so many hoses... as always any opinions are always welcomed and i hope i can work this "bug" out. so please keep the suggestions coming thank you.

 

Could An Old Distributor Coil Be Causing My Problems? The Car Is Equipped With The Original One. The TPS Tested Fine, And There Are No Vacuum Leaks.

 

First:

A 58mm TB wont gain u 10 let alone 15hp(especially talking a stock car). Dyno sheet or not, I can fudge correction factors too :-)

By the way I put down 225 and 315 torque on a stock 91 GTA with nothing other than a catback and my own tuning.

Second:

Quit swapping parts. Actually take the time to investigate the issue.

Ensure you have no vaccum leaks.

Test and set the IAC and TPS to factory spec.

Surging is usually caused by things such as : vacuum leaks, lean conditions, extreme timing settings, even broken motor mounts or improperly set IAC's.

Get yourself a GM manual and learn how to use it.

U havent provided us with any real information other than u swapped out parts not bad in the first place.

later
JEremy

 

ok well heres the specs...
91 z-28 5.7l tpi
43,000 original miles all stock.
700r4 tranny
dual catalyst

only mod is a 3" catback exhaust
timing is set to factory, compression is ok, and 99..9 percent there are no vacuum leaks, no ses codes, and no broken motor mounts, the idle problem only persists in closed loop. idles between 500-850 rpm. repeatedly. sometimes it settles at around 600-650 rpm but then "searches for idle" again. also when the rpm's are doing this the car has a slight to intermediate vibration. sometimes it will stall. this condition occurs in park, neutral, drive and reverse.
computer and chip are factory. i run 93 octane fuel from either BP or Chevron and no where else. there are no leaks and occasionally, it will puff a little smoke if i kick the throttle. the car operates fine but i noticed if im cruising at, well lets say 10% throttle i can feel the car surge will im cruising and also when i let off the throttle to slow down to a stop the rpm's rise and drop as it is coming to a stop.

so far i have put the factory throttle body back on with new gaskets, new iac, new plugs and wires, new map sensor, and the tps has proper voltage.
hopefully this info will help you... i do not know anything about chip burning and stuff like that... if you need more info just ask and if i can give to you i will.

 

U still make no mention of properly setting the IAC and/or TPS(non adjustable on your year, but some cars do need to be adjusted).

Have u done that?

Is it missing or surging? Only at idle and coming to stops?

Have you verified fuel pressure? What are your BLM's looking like when this occurs?

Have you cleaned all grounds on the car, especially the ones on the back of the pass cylinder head?

Also ohm the injectors as 91 were one of the crap years for them. anything under 12 ohm is junk and around 16-17 should be good.

later
Jeremy

 

ok i paid my mechanic to install the iac.... as far as setting the tps and iac... i dont know how to do that.... my tps isnt adjustable . and when i took my car to gm.... they "said" they did compression and fuel pressure checks.... and it turned up ok.... whether or not they did i dont know cause i didnt receive any print outs with the specs. all wires etc are covered in wire loom either from the previous owner or from the dealer. it feels more like a surge to me cause from what i can tell there is no loss in power or hesitation... but then again the problem existed ever since ive owned the car. i live in georgia, and my dad bought the car on ebay from a guy in pennsylvania, thats irrelevant to this i know but, this is how the car was when it was delivered to me. for all i know i could have loss in power and wouldnt even know it untill this problem is resolved. and like i said the rpms only "lope" at idle however when im cruising at lets say 55mph and start coming to a stop i can feel the rpm rising and dropping as im coming to a stop.... and when under a load like when im cruising it feels like the car goes into overdrive however the car is already in overdrive when i feel the "surge" or "miss" or whatever you want to call it. as far as the injectors ill have to buy a voltmeter and at a later date if you dont mind maybe you can tell me how to ohm them.... anyways i really do appreciate your feedback and i hope this information gives you a better understanding... this is my fist tpi so everything is new to me.... i had a 92 pos 305 tbi that i became really familiar with... but anyways hope to hear from you again. thanks

 

oh yeah i must apologize but what is BLM's?

 

oh and i must apologize but what is BLM's?

 

It's interesting becaise I have a '92 TransAm Convertible and I'm having the same surging problem...and yes....it is surging...not missing.

I just got through putting new 19# injectors in it with no change. I have a new cap, rotor, plugs, plug wires, and coil. The TPS is also new.

I checked for vacuum leaks and there's nothing. Everything's sealed up tight.

This is really starting to get annoying. Did anyone ever suspect that it could be the ECM or EEPROM going bad? I had that happen to my '91 TransAm TPI, but it threw a code. But I've heard that these cars aren't neccesarily known for having very reliable computers.

BTW, '92 TransAm Convertible 5.0 TPI Vin F, 5-speed, stock, 62K miles.

 

730 ecm is about as reliable as they come.

I would put money on 95% of failures of them are caused by water or bad injectors killing the drivers or shorted solenoids killing them.

By themselves they are very reliable. I have cut and hacked many and never had an issue with one.

Surges at lower speeds and idle can also be caused by bad valve stem seals. Get any blue clouds in the morning. Feels like a miss/surge almost.

These cars arent difficult. A stuck or sticking EGR valve can cause issues at lower speeds and idle as well.

I heavily suggest buying the Helms manual(dealer manual) and going by it.

later
Jeremy

 

i think i will take your advice and get that manual.... cause today. well long story short.... i drove 13 miles at speeds 70 - 90mph to a local theatre and when i pulled up i sat in the parking lot for about 20 mins waiting for others. during this time i let the car idle.... and you could definitely feel the "surge" however rpms were consistant at 650.... after the movie headed back home the problem started again... so this is why i dont think there is a vaccumm leak... but anyways... i appreciate all your feedback and will let you know if i get any leads... oh yeah bye the way... i also noticed as the rpms are "searching" the oil pressure gauge is in sync with the rpm rising and dropping so i dont know if that may mean something.... just thought id point that out.

 

you could also try the nohc sencer. if it is bad it will mess with tps, the are one in the seame.

 

i have an 89 T/A 305 TPI and i've had the same problems since i got the car a few months ago, ive checked for vaccum leaks, changed coil cap rotor plugs and wires, cleaned out my tb with intake medic, but the problem never goes away. after the ignition parts and the cleaning it got significantly better, but it still surges constantly in park neutral reverse or drive. the previous owner installed a B&M megashifter and mentioned something about a resistor or switch or something which he says never came with the shifter. ive never read anything about this and cant understand what he means.

anyone know anything about this?

 

Forgive my ignorance....but I don't recognize the name nohc? Could you elaborate?

 

Go to the tech pages and read up on how to adjust the TPS and IAC.
Do the mechanical adjustment farther open so the IAC has plenty of "steps" to work with.
You may be running out of control "room" because of the adjustments not being correct.
Using a scan tool or software logger to set the IAC is the best way to ensure the idle position is around 40-50 steps (possibly lower if desired).
Then go back and reset the TPS volts again.
If you don't understand what I've said, do some searching on the subject and read up on how the "system" works.
These are not just parts, they all work together.

 

Well i had the same problem. it came out being my pick up coil in the distributor. fix the idle problem huge difference

 

I had vaccum leaks and they always seemed to just die. It would run , but let off the throttle and it would just barly hold idle then die. Check them how to articles on how to set minimum idea air. Or resetting IAC . If it's holding an idle at least but still surgeing . I would bet on something more electrical .

Best of luck man sounds like a sweet car there and , you like to take care of it.

 

well i ordered the helms manual and should be recieving it tomorrow so i will look through that... the car has been covered and dont plan on driving it untill its fixed.... some people tell me fix that while others say thats no the problem so i dont know. but i plan on reading all 3000 pages of this book when i get it so ill keep you guys posted .. appreciate all the feedback so talk to ya later.

 

I had the same surging problem when I had the 305.

Believe it or not, it was the ECM. I had it tested and it was half-in-the-bag.

After replacing the ecm the car idled perfect.

So yes, it could be an ecm problem.

Now, I have an idling problem myself just recently. Checked the ecm, replaced cap, wires, rotor, plugs, coil, and checked vacuum, shaft, and ignition. Turns out 6 of my 8 GM multecs have crapped out and are registering @ 7 ohms each.

So the new set of Ford Racing injectors are ready to go in (hehe). I belive this will take care of my problem.

 

Well, I dont have an update yet. However I did receive my service manaul today and wow... its like the holy grail of repair books. I definitly recommend everone going to www.helminc.com and order yours. This book has over 3000 pages and is strictly for my year model camaro.. im pretty confident that they carry every year... so go check it out... i plan on doing some thourogh reading before i start making repairs.. but anyways ill keep you posted.... the book and updated service bulletin is $150 plus i couldnt wait so i paid the next day shipping and total cost is 214.00 ok later all.

 

Any luck solving your problem im having the same surging problem keep posthing lets fix this

 

I had a surging idle too. The problem was a torn diaphragm in the AFPR. Check your fuel pressure.

 

i have the same problem but i'm pretty certain is due to an exhaust leak at the manifolds and i dont think i reconnected the ground cable to the back of my pass side cyl head when i changed the head gaskets so i'm gonna double check that.

 

what do you guys mean when you say surging or missing?

and im having idle problems too with a lean exhaust engine code. tps checks out but i cant keep the car running to do the IAC test with the brand new IAC disconnected to adjust minimum air.

also how do you check to see if you have a bad ECM?

 

Tell you what, mine just started surging, (new chip) when I had the stock chip in it, it would idle terrible. I went for the quick fix and picked up a Hyperchip($100.00, free shipping) for a quick fix for now, until I invest in some DIY. The car idled fine on MEMCAL only, not that it really means alot. With the Hyperchip, the idle straightened out dramatically, but I still have too high an idle after shifting out of reverse, symptoms of a lean mix/vaccuum leak, and it has begun surging in Drive (not Overdrive) at roughly 2250 to 2600 RPM rolling down the road between 45 and 55 MPH. The IAC and minimum idle air are set, but it still seems to be running a bit rich. I replaced all the injectors a year ago, and they are fine. The old ones had stock part #'s on them, just a bit old I would guess. I'm suspect of the fuel regulator, so next step is to modify the vaccuum applied version, to an adjustable one. New diaphragms are cheap, fuel isn't. I was planning on updating the ECM, it appears to be twenty two years old. I'll let you know how things go, and what I find out. P.S.

 

Just got in from the test drive, and this is a little odd sounding, but it seems that through the years, the plugs for the #1 and #3 injectors may have been reversed. The length of the wires just looked a little off, so I gave it a shot. I thought these opened in banks (I still have the "old" ECM), but it seems like it made a difference. The idle isn't perfect, but I don't have the problem with surging from Reverse anymore, and the idle in Park is a little better. Also, the off idle power SEEMS better. Anyone out there with more info on the injector timing?