1989IROCTPI

Hello everyone,
I have a strange issue...I

So here it is...1989 IROC Z 305 TPI Auto. Bone Stock. Bought from original owner. Many parts replaced over the years but always with AC DELCO parts and serviced by the Chevy dealer, so no OEM parts used or local mechanics..not sure if that is good or bad but worth mentioning.

Ok here it goes. So whether it is cold or hot outside. When you first start the car (can be 1st thing in morning when cold or after driving, turn it off, turn it back on), it will idle at 1500 RPM. Turn the key off, turn it back on, Goes right to the proper Idle of 700-800. Every single time. We thought it was the IAC. So we replaced it with a new AC delco. Also cleaned the passages so they are clean as can be...Does the exact same thing. Vacuum gauge shows no fluctation and right at 18lbs. There is no adverse effects at all. Meaning at any time of driving (cold or hot) it does not run rough, stall, unresponsive, etc. Nothing. Drives like a brand new car with tons of power (well for a 305)

It is just driving me nuts...does anyone have a clue where to look why it would do this and does anyone else have this issue? It almost seems that the IAC is opening a bit to far initially, then when you turn it off and back on it goes to the correct spot but (2) IAC's do the exact same thing...I have disconnected the battery to reset computer, same thing. I do not want to mess with IDLE screws, etc. because it idles perfect after the 1st start.

Curious on what I should try next and if anyone else has this issue.

thanks for any advice!

Tuned Performance

iTrader: (92)

https://www.thirdgen.org/forums/tech...le-issues.html

TOM305T/A

im having a similar issue. if my 87 350 tpi, car is hot or cold. when i start it up. goes to 1200-1300 then eventually goes down to 750-800. been chasing that problem for a year. new iac and tps, no vacuum leaks. still no resolve.

TTOP350

iTrader: (8)

That's what its supposed to do on 1st startup till ecm sees the proper coolant temp. Very much how a choke works on a car with a carb.

1989IROCTPI

Thanks for all the replies and you are correct, when you first start it, and its cold, I would expect that however it should not do that after the car is hot.

To me it seems that the IAC is just not set right..and it figures it out after the second start whether its cold or hot.

thtanner

iTrader: (1)

It will do it for a second even when the car is warm due to the startup fueling I'm fairly certain. Mine settles down in under 3 seconds on warm startups.

1989IROCTPI

Thanks for that reply. That's what I would expect but it does not. So this is the scenario:

Start the car in the morning, cold. Idles at 1500-1600 RPM for as long as you let it run. Immediately turn it off, restart the car, Idles at 900-1000 cold.

Same thing when hot. Drive car 20 miles. Hot. Start car, Idles at 1500 RPM constantly. Turn it off. Turn it back on. Idles at 800-900 RPM.

Just seems like that IAC is not going to the correct spot the first start. After the first start, it finds the proper spot to position itself.

Thanks

Tuned Performance

iTrader: (92)

Sounds like it could be a bad ecm, sometimes the 1227165 circuit board flexes causing solder joints to come loose or odd running issues. You maybe able to gently tap with a screw driver handleon the ecm while it's doing this high idle . The purpose to see if it straightens out or idle surge happens.

thtanner

iTrader: (1)

That behavior is definitely different than my car, and other TPIs I've seen.

Edit: Just saw Tuned Performance responded; you're in good hands.

1989IROCTPI

Thanks for that! Yes, that is where we were going to look next....ECM...

I will try that gentle tap while its running in a few days and report back.

I have heard that buying remanufactured ECM's are hit or miss in quality. Is there one company that is better than others if I decide to swap out the ECM?

Thanks!

Tuned Performance

iTrader: (92)

Your right on hit or miss , for a reman maybe cardone 77-7165

TTOP350

iTrader: (8)

a vac leak and or a improperly set min air rate (to high) will act like that as well.

burnout88

I put a reman ac delco unit in mine and it had a totally new and redesigned board. Only thing reused was the case.

foxxtrot82

The minimum air rate like TTOP350 said. It helped me with my high and low idle. Its not just a plug it in and go thing like I thought it was. The Haynes manual will tell you the process of how to set it correctly. Its quite a ridiculous process, but it works. I suppose the computers cant make changes unless given the proper settings first.

1989IROCTPI

Thanks everyone! The minimum air rate was checked and was at proper spec..so were TPS values. there are no vacuum leaks found. vacuum gauge shows steady rate at proper value.

Important thing to remember, is that this only happens upon the 1st start (cold or hot). Idles high. Turn it off, Restart it idles fine. then while driving, absolutely no issues. Idles perfect, no hesitation, etc. Can put it in park, idles correctly, put back in drive, idles perfectly.

turn it off, go in and get coffee, come out. Idles high (1500 rpm). turn it off, turn it back on, idles perfect.

The only thing left to consider is ECM unless someone else has advice I have not tried. I will try the tap test this weekend and see if that does anything.

thanks for all the help and replies!

fervernt

Hey man, I've got an ECM for a 91 350 you auto, lemme know if that's the problem, I'll give you a good price on it if you need it. Remanned and purchased about 4 years ago. May be able to find receipt if necessary

Tuned Performance

iTrader: (92)

A 77-7730 reman , gm 1227730 is for 90-92 tpi speed density.

19875speed

iTrader: (2)

I had a similar problem and it turned out to be the throttle body was worn out and sticking. Replaced it with an aftermarket one and the problem was solved.

Fred SS

iTrader: (3)

One thing that strikes me odd about your problem is that on a cold start, your motor will idle high for as long as you let the motor run. Shouldn't the motor idle down as the coolant temperature increases? Could be a bad ecm as Tuned Performance suggested. I would suggest checking the wiring at the IAC solenoid for any damaged wires. Also I would suggest doing a pinout on the IAC connector to be sure it is wired correctly.

Fred SS

iTrader: (3)

I just looked at my shop manual. I would think that you have either a ecm problem or a wiring issue at the IAC solenoid. This is what my 87 shop manual says about the IAC valve (I know that you have an 89 TPI, but the IAC should still function the same): "Each time the engine is started and then the ignition is turned off the ecm will reset the IAC valve. This is done by sending enough counts to seat the valve. The fully seated valve is the ecm reference Zero. A given number of counts are then issued to open the valve, and normal ecm control of IAC will begin from that point. The number of counts are then calculated by the ecm. This is how the ecm knows what the motor position is for a given idle speed."


This is verbatim from my shop manual. It appears that your car is reseting the IAC when you turn ignition off then back on, and get normal idle. But for some reason your ecm is going back to some other value when you shut it down again.


That is weird.

1989IROCTPI

Thank You Fred SS. Yes at this moment it is looking like an ECM issue. Wiring checks out.

I am looking into best options on which remaned ecm I should purchase. Once I do and install, test I will update everyone on status.

Thanks for everyone's input. Great site!

Vader

I would connect a scanner and read the ECM target idle value. If the RPM is tracking that reasonably, there is nothing wrong with the TPS, the IAC, or the TB. That would be good to know.

86IROC112

My car does a similar thing. First startup it idles high, whether the engine is hot or cold.. However, turning it off then back on does not fix it. I have to drive it a while then I will be sitting at a red light and all of a sudden it drops down to where it should be. My issue seems to be more run time related than temperature. It's weird too.


Sometimes I only have to drive it for 5 minutes, sometimes 15 minutes.. But it always waits till I'm sitting at a red light before it adjusts itself, never does it while driving.

GTA1990

iTrader: (1)

as mentioned, check your throttle body to see if it is sticking; check that the throttle lever rests on the adjustment screw. Motion it slowly and see/ feel if it hangs or doesn't have as much tension nearer the stop.

Culprit for me was the cruise cable causing some interference with the articulation to the TB near the stop. I loosened it on the cruise module side by selecting a notch farther away from the module for the cable to connect. I haven't tested the cruise control since but the high idle issue has gone.

1989IROCTPI

Thanks All for the replies. Work and the FLU got me over the past few weeks but I have some updates this weekend. Below are scanner results. Would be curious if anyone here feels they are good, bad, etc...and any suggestions to alter. they were all taken with a snap on MT2500 scanner.

BTW I did remove the wiring harness/connector to the ECM and cleaned both ends with spray electronics cleaner. Not sure if its me or if it did something, but it does seem to have settled down a bit. More to come as weather in NE PA has been real good lately so easier to mess with over winter...Definetly ran better but not perfect..see the below last 2 results. After it was hot, driven about 25 miles, put in park, idle went to 1100. Put back into drive, then back into park (still running) and it dropped down to 780 rpm which is just right I believe.

As for the folks who asked to check throttle body, that has all checked out with no leaks or free play so I feel confided that is not issue.

Scanner results. Any thoughts or recommendations let me know. i have alot of data from that scanner if anyone needs more (vader?)

(cold, overnight. Start, went to 1500 RPM, then dropped to 1100 after few minutes)


IAC 83
TPS .88

Driving at 55 MPH

IAC 42
TPS 2.70

Parked, Hot, Idling (IDLE went a bit high - 1,000 rpm).

IAC 23
TPS .52

Parked, Hot, Idling (Same as above but I put it in gear, then back to park and idle got much better)

IAC 11
TPS .52

GTA1990

iTrader: (1)

what's the TPS % showing on the scanner tool in the above mentioned scenarios?

Lucid

iTrader: (4)

My 86 TPI did the same exact problem. You would turn it on and it would idle crazy high, turn it off. Turn the key again and engine right back on and it would smooth out. It took me a long time to figure it out but I cannot quite remember what fixed it. I've been searching through the old threads to try to remember but you are on the right track. I'm thinking it's somewhere along the lines of TPS, MAF, ECM. I remember I had a quad driver go out on my ECM because I was grounding the fan a funky way to force it to come on earlier (which I later changed when I started burning my own chips).

Your TPS setting with the scanner connected, key ON engine off should be .54v. Another thing you can do with the TPS since you have a scanner: key on engine off, press the throttle really slowly, in a really smooth slow motion and watch the read out for the TPS on the scanner. If it goes crazy high or crazy low in a certain spot repeatedly, there could be a dead spot.

Do you have a buddy with a 86-89 TPI that you could borrow a '165 ECM from?

1989IROCTPI

Thanks Lucid. I hope you can find that thread because that sounds exactly like my problem. I would love to see what the rsolution was....

Yes, I have a buddy who has an 86 trans am TPI. It is in storage so trying to see if same ECM so we could swap, try.

Thanks for that info on the tps and the scanner. I will try that when I get a moment but thats a good thought regarding the dead spot.

1989IROCTPI

Thanks to everyone for their help on this one..unbelievably it turned out to the be the computer. Other than the High idle occasionally I never had any issues with the car at all, and never really thought the ECM could be the issue. Last week my Friend took his ECM out of his 1986 Trans Am (which was identical to my car), we swapped them. I drove my IROC for a week, never once had the high idle strange issue. Incredibly his Trans Am started having the occasional high idle after the swap!

So I have a re-manufactured one on order but that was the issue. Strange one for the books but thanks to everyone who offered advice!

86IROC112

I see it's been over a year.. Did you get a new ECM and did it work? Which one did you get and where? thanks!

1989IROCTPI

Funny. Every after market ECM I bought (autozone, napa, etc.) did the same thing. I (incredibly) found the same ECM number in a junkyard IROC. It was OEM. I put it in. Never had an issue since then. No clue how or why but something on the aftermarkets just was not right.....the way I solved the issue was taking a friends OEM computer from his 86 trans am which told me it was the computer.

haps

iTrader: (4)

Thanks very much for this thread. I'm having the same issues with my 91 and I've already put an aftermarket computer in it. May need to look for a used OEM.

86IROC112

Glad you got it fixed. I got my hands on a used OEM unit and there was no change at all. I did a datalog and Tuned Performance said I was running lean. He sent me a new chip and it runs great but the idle problem was not solved..

88IROCvertZ

iTrader: (1)

Yup.. This is how every thirdgen I’ve owned has been.. all were low mile and stock cars. First startup of the day they run about 1200-1500 RPM and after about 5 min drops to 750. I always let the car drop to 750 before I drive it.

After the first startup it’ll drop to 750 after about a minute.

I dont think you have to wait for the idle to drop, it I do.

Tuned Performance

iTrader: (92)

You might try cleaning your iac, setting the minimum air speed then double checking and adjust your tps in that order.

http://www.iroczone.com/articles/eng...rol-iac-valve/

http://www.iroczone.com/articles/eng...ir-idle-speed/

http://www.iroczone.com/articles/eng...on-sensor-tps/

SRKLEGIN

Morning,
Happy 4th of July to all third gen members. I am having the same throttle issues with a clean install of the IAC. This thing is stinking to high heaven of fuel...


Original harness set-up is set up for 1986 305 TPI


Have fresh 350 TPI Swap. Themed all sensors toward 1987 350 TPI. Have a modified prom with No VATS / No EGR/ TPS is set at .60


Mods: Comp Cam 8-502-08 cam .495 Int .503 Exh
Edelbrock High-Flo intake manifold set-up
TPIS 52mm Throttle Body
Edelbrock Heads #60975 64cc
Stock fuel rails and injectors

Hope someone can help....she wont idle well and backfiring is noted

Tuned Performance

iTrader: (92)

Have you tried advancing the timing more ?

Street Lethal

iTrader: (16)

Don't know if I agree with the ECM being the culprit in this case. Looking at his target RPM in Open Loop during a cold start leads me to suspect that someone before him tampered with his fast idle screw. Stock bin calls for 1100-RPM when cold, then settles down to desired RPM as coolant reaches operating temperature. His is going as high as 1500-RPM and stays there until turned off and restarted, that's a fast idle screw issue. His IAC steps show low 20's with RPM reaching 1000-RPM, and then down to 11 steps when desired his RPM is reached. This right there should tell you that the IAC is too closed at idle RPM and the air is being supplemented during idle through the throttle blades. IAC should be closer to 20/25 steps at 800-RPM when fully warmed up, not 11. Also need to confirm spark reference in the bin matches base timing, that too could have been touched...

- Rob

SRKLEGIN

That will be something I could take a look at. Right now cam specs are citing that it be set at 6 degrees BDTC. I live in Utah and the altitude here is up there. My buddy thinks that could be as well. Also, I have the high flo runner set sitting on my work bench in the middle of polishing it and ran the stock runners...he also thinks that we may have a vacuum leak common to the stock runners.


So Street Lethal, what would be your guess on timing? I plan on finishing the runners tomorrow and getting them up to his shop where the car is.


I will check the factory setting on the LH side of the throttle body, which is a TPIS 52mm Throttle body with a coolant bypass mod incorporated. what mm setting would that be if you know...


Thanks for your guys help. I don't want this car to sit around in the garage any longer. Its been two years down and I really want to drive it.

SRKLEGIN

oops, my timing question was to be aimed at tuned performance and the factory set screw on the throttle body would be for street lethal. Also this car is literally stinking of fuel when it does run. He's saying the pressure to the rails with stock injectors is reading 38.0 He also set the TPS at .54

SRKLEGIN

Thanks again...I will check tomorrow morning if anyone has ideas...

Tuned Performance

iTrader: (92)

What size injectors are you running ?
A small vacuum leak would cause a higher idle.
With that cam you should be able to idle fine on a stock tune.
Valves adjusted correctly ?
Have you checked for fuel pressure drop after prime maybe there is a leaking injector.

Tuned Performance

iTrader: (92)

Another test to see if the prom/memcal is working. Is the check enine light on solid?
Can you link a code 12 ?
Do the fans come on with the key on ?