When my car idles in neutral my car does not know how to idle AT ALL, the RPM's drop to rite before where it would die, then rite back up, and rite back down...over and and over, what could it be?

 

easiest to fix first=1) needs t-body service 2)vacuum leak ck. pcv valve 3)swapped battery need to perform idle relearn 4)bad ps switch 4)bad p-n switch 5)general condition of tune-tune up time 6) pinhole in fuel pressure reg.diaghfram-spell'n stuff aint muh strong point, unghhh. 6)cannister purge 7)tps 8)cts skewed 9)alternator output low 10) thats enough for now

 

Check you throttle body mounting bolts. Mine where loose. EGR leak will cause bad idle also. Bad PCV valve also.

 

sounds like a vaccuum leak to me.. my car did the same thing after i got it all back together, and forgot the brake booster vaccuum Pluged it in, ran perfect. Can you hear a hissing sound when the car is warmed up?

 

My 2.8 does the same thing.... but only when it's cold. All summer, and after it warms up in the winter, it idles fine. It's done it for 3 years and I haven't found the problem yet. Granted, I'm not looking real hard. Lately my drivability has gone into the crapper though, so I may start looking soon. The car falls on its face at any point other than WOT. Now it's idling at 2k rpm all the time.

 

I know the symptoms. The engine is trying to idle the car, but it can't.

I run a 1986 TPI setup on a l98 and I use a stock 1987 PROM. I run a 180 thermostat. On summer days the car runs like a top. When it is cold out, the engine does not warm up enough so that it idles well. If I Start it in the driveway right now, it will do that serge thing. IF I drive it around and get it warmed up, it idles ok.

If you had a vacum leak, your engine would shake your fillings loose. You have a MAF car, so the vacum signal does not effect the car. I am assuming you have a MAF, if yours is SD, then that is another story.

A leak at the TB often causes a higher idle or a lean condition. More air getting in than is measured. Again, assuming a MAF car.

What thermostat are you running?
What chip?
What is the temp out when you notice the surge?
Is it only at start up, or does it happen after a warmup?

Mark.

 

Your problem is a vacuum leak somewhere. The most overlooked problem of all that resembles a vacuum leak is a bad IAC/clogged IAC passages. The IAC is basically "controlled vacuum". Inside the IAC a screw opens and closes electronically through a signal to and from the ECM. The IAC passage underneath the TB has a vacuum hose connected to it that comes from the vaccuum solenoid located on the back passenger side of the TPI setup. the vacuum goes through the IAC passage and into the IAC where it is then allowed into the plenum behind the throttle body. The amount of air that's allowed into the plenum is based on the position of the IAC screw that opens and closes the hole. If your IAC is not working properly it will affect how your car idles. It can idle erractically and even surge like you were referring too. If it stops surging after warming up then see below but if it surges like that even when fully warm then check the IAC


The problem you have Mark_ZZ3 is a little different. You have an '86 so it's got the cold start injector. When cold, the ECM sends a signal to the cold start injector switch and the cold start injector kicks on. The injector supplies extra fuel evenly to all the cylinder through passages in the intake. Once the engine warms up the and the coolant reaches a certain temp, the ECM knows this through the coolant temp switch and shuts off the signal to the cold start injector and then the cold start injector shuts off the extra fuel. What's happening with your car is that you're running on the brink of the "too rich" side right now for some reason (which could be a number of things) and when the cold start injector is on it's providing more fuel so you're running even richer and running "too rich" will cause idle surge. Then when the engine warms up, the injector shuts of and your idle returns to "normal".

 

The IAC gets it's air flow from the TB air intake. The vacum port you refer to is just that, a vacum port that provides ported vacum signal.

The cold start injector IS NOT controlled by the ECM. Check your diagrams. It is only controlled by the START circuit and is only used on start. It has nothing to do with the running of the engine. I'll send you a diagram if you disagree.

The ECM will add fuel to the engine when it is cold, but it does this with the regular injectors by varying the pulse width. There is an enrichment vs tempature config withing the prom.

Mark.
PS. My block learn is 132. I am not rich or lean.

 

Mark... have a problem with something you said... my car had the brake booster vaccuum disconnected when i put it all back together.. it didn't run rough enough to shake my filllings loose, all it did was rev up and down.. down to the point of almost stalling, and it almost did stall when it got warmer. Also, a vacuum leak on ANY engine will cause a bad running condition, SD or MAF.

As for the running better when it is at operating temperature, it could be a bad intake gasket that seals when the engine is warm....

Spray TB cleaner all aroung the intake base and runner base.

 

Perhaps my choice of words was not adequate. Simply offering one perspective. My thoughts are that it is not a vacumm leak, but I could be mistaken. For fun, I might create a leak just to see what happens. Hook the scanner up and watch.

My surge is only noticable when it is cold out, like near freezing. Otherwise it is fine. Hence my thoughts on the calibrations.

Mark.

 

the cold problem maybe as simple as a MAT sensor... have you checked the parameters pf the MAT yet? Or... how about the EGR valve?

You are right about the cold start valve though... the sensor in the front of the intake manifold is a temperature switch, and it simply tells the cold start injector to work, or not work. It may be hooked up to the ECM for pulse width purposes, I dunno... But I was looking at the schematics quickly, tryign to figure out why my car has a hard time starting when it is warm. May have fixed the problem.... dunno yet... still kind of cold out, and the garage never heats up over 70*F

 

Hard starting when it is warm ... could be leaking injectors. Try to crack the throttle 1/4-1/2 and see if it starts sooner than usual.

My cold start injector was leaking. Replaced it and volia ... back to quick starts.

Mark.

 

At idle the TB blades are almost 100% closed, the IAC provides "extra" air as needed to keep the idle proper. It uses the vacuum to do this and that's why it's referred to as "controlled vacuum". The vacuum line doesn't connect to the IAC passage for no reason.

The vaccuum solenoid has two vaccuum connectors. One connects directly to the EGR Valve and is ported vacuum. The other one, that goes to the IAC passage, is constant vaccuum if I recall correctly.

I beg to differ. The ECM reads the temp from the coolant temp sensor and tells the cold start injector switch (located right next to the coolant temp sensor on the front of the intake) when to kick the cold start injector on and off. When the coolant temp sensor is showing low temps during a cold start, the ECM tells the cold start injector switch to kick the cold start injector on. When the coolant temp sensor reaches a particular temp, set in the ECM, the ECM tells the cold start injector switch to turn the cold start injector off.

You are wrong again. With the '88 and up TPI, GM eliminated the cold start injector and added the cold start fuel enrichment table in the ECM programming to supply more fuel through the regular injectors. In pre-'88 TPI setups the cold start injector is the only way for the ECM to add more fuel during a cold start.

P.S. "Perfect" Block Learn is 128 .... lower than that is rich and higher than that is lean, 132 is lean (running lean can cause nasty idle on cold start as well)

 

Well... it may have been leaky injectors... I had all of them cleaned and flow matched by cruizin' performance. Before I put everything together, I had the fuel rail all on, and all injectors in (including CS injector) and saw no fuel leaking from the CS injector (I had clamped the return line to boost the pressur a bit.) I changed the MAT sensor before I put the plenum and runners on... figured maybe the computer was getting the wrong MAT reading, so it thought the engine was cold, when indeed it was hot :/ we'll see what happens on the hot days.

 

The ECM uses the coolant temp sensor located on the front of the intake, not the MAT sensor, to determine the difference between the engine being cold and hot. It (the ECM) uses the coolant temp sensor to provide fuel enrichment by telling the cold start injector switch to kick on and off and it uses the coolant temp sensor in conjunction with the O2 sensor to determine when to go from open to closed loop and vice versa. Coincidentally, the MAT sensor and coolant temp sensor have the same exact part number. One reads air temp, the other reads fluid temp but they're both the same sensor and interchangeable.

 

Actually... it was 89 and up, that had no CS injector

Ok.... just so we all don't kill eachother over the CS Injector.. here is a diagram out of the TPI Swappers Guide

It/s too big to put it in the message, so click on the link to view it

Cold Start Injector Wiring Diagram

 

Ok, I was wrong the ECM does not control the cold start injector but the function of the cold start injector is based on coolant temperature which is what I was getting at, I was just going the wrong route. I stand corrected on that part.

I still think it was '88 when they eliminated the cold start injector because I remember Ed Wright telling me I couldn't use an '88 chip in my '86 TA because of that.

 

That vacumm solenoid is ONLY for the EGR. It is not for idle. There are a bunch of us that have completly removed the EGR, including that solenoid.

The air that the IAC controls enters into the IAC port from an access hole between the throttle blades ... just look at the front of your TB, it is easy to see the passage.



It was 89 that GM removed the CS and used the main injectors for fuel enrichment on startup. In 1985-88 the cold start circuit was used for startup. Check the SLP catalog and you will see the runners are are for 1986-88 and 89 and up. nuff said. Then the main injectors are used for running. So back to your original statement ... the ECM does NOT conrol the cold start valve in anyway. That was my point.





Perfect may be 128, but that is why GM created a block learn and int learn ... so the it could compensate for changes in engine condition, air, humidity, etc. A typical car will never be 128 all the time. And 132 is not lean ... 132 means the computer is altering the fuel enrichment from base, so it is running within it's parameters. That is the whole point of the block learn.

... back to the original post ...

 

That's a possibility to look into for sure but with the cold start injector being off when the engine is warm, how can it leak?? It could also be your starter solenoid if you have headers. Sometimes a weak solenoid getting hot from the headers will cause hard starting when warm.

 

injector leak because the pintle is dirty, so it does not close all the way. It does not matter if your cycling them on and off, they just aren't closed.

Simple proceedure. Pull out the injector rail and injectors, leave fuel system connected, place paper towels under injectors, pressure up the fuel system. The leaking injector will show a wet spot.

Cold start valves are a common injector to leak.

Mark.

 

BLM is long term and INT is short term. The INT will change regularly and the BLM follows. If your INT is 132 then yes that means the computer is altering the fuel and actually adding fuel due to a lean condition at that time. If your INT is below 128 then the computer is actually removing fuel due to a rich condition at that time. However, if your BLM is 132 and runs above 128 regularly then you are running universally lean. It's your rods, not mine.

 

Then why is that hole between the throttle blades not there on '89 and up I believe??

That hole you're referring too is an air inlet that's in some way related to the cold start injector. GM eliminated that hole when they eliminated the cold start injector.

 

Good point, makes sense to me.

 

I agree with your rich/lean, actually I never disagreed, rather we just said the same thing from different points.

Again another point of view, Running other that 128 means the ECM measured a rich lean and compensated for it. So the engine is not actually running rich or lean, it did, then the ECM corrected for it, so now it is not. Keeping it at 128 allows the ECM the most room for compensation.

Mark.

 

I guess that's a good way to put it. You can, however, get the engine to run 128/128 which is important for gas mileage in highway mode. Just ask Glenn about that, he talks about it all the time.

 

On the plenum, the cold start had a passage on the 86-88, yes that is correct.

The IAC does not use the vacumm port that goes to the EGR solenoid. That is ported vacumm.

You probable seen this before, but check out this diagram of a TB blowup: http://members.shaw.ca/s15/tpitb.gif
The IAC air flow is limited to the TB itself. That is all was trying to get across.

If you want to see the differences on the plenums, check this link out (second paragraph on second column):
http://members.shaw.ca/s15/tuneport_differences.jpg

Mark.

 

The 128/128 is probably the most optimal, as the steps used for adding/subtracting fuel are not optimal.

Another good point about 128/128 is for WOT. If your ECM is adding fuel on the base curve, then you will most likely run lean at WOT. I recall a discussion about this some time ago.

Mark.

 

Maybe I'm all wrong on the functioning of the IAC. However, it is still considered a "controlled vaccuum leak" and CAN very well mess up your idle and cause it to surge. I know this all too well from mine messing up.

I know the differences in the plenums. The weird part is that a good friend of mine has an '89 IROC and the passenger side runner still has that cold start air tube and the runners have never been changed. He took the runner off and the passages in the intake behind the cold start air tube are not there since GM eliminated them. The only thing we can figure out is that possibly GM had a surplus of these runners and decided to go ahead and use them on some of the '89 engines to save money and since the intake was modified to not have the air holes, these runners wouldn't cause any problems.

 

Controlled vacumm leak ... sounds like a problem when you use the term leak. I know what your getting at.

My idle *could* be a result of the IAC. It would not take much to cause that to be the culprit. Perhaps I will splurge this year and just replace it for a piece of mind. The IAC counts always seemed low to me, perhaps minimum air flow is still too high or else the dreaded vacum leak.


86FyrBrd - sorry about all the bandwidth. I hope some of it is helpful to your problem.

 

no, to classify airflow moving past the pintle of the iac as a vacuum leak is incorrect. the airflow is "accounted" for in a maf system because it is part of the total airflow into the motor. airflow that is quantified and sent to the ecm to make fueling decisions (along with several other decisions) the motor dosent care if the air squeezes past a butterfly or if it slides past a pintle or vacuum passage-as long as it went thru the hot wire its compensated for. sd cars are slightly different behaving beasts but if the prom is right, all the calcs. affecting map are plotted in the ecm including whatever reduction in pressure caused by idle airflow. check an lt1 "split blm's" and the lengths pursued to eradicate them. again fuel is added in the proper amount and its all good. this isnt covering a)egr flow b)cannister purge c)p-brake booster take-up or hvac control usage-all insignificant for this discussion :rockon:

 

Yeah, leak is a bad term but you get the point.

Your idle could very well be the IAC, I'm almost 100% my IAC is screwed up and I have similar symptoms to yours. Last night was the first night in nearly a month that we had temps in the lower 30s and this morning when I went out to start my car it was idling very low and surging. As it warmed up the idle increased and got smoother. Did fine all day long after that.

I'll be replacing my IAC sometime soon and if that fixes the problem I'll let you know.