Hello! I wanted to ask about my Fiero, a 2.5 1988. I was having issue with the car having a poor idle and dying when putting the car in gear. It has a new fuel pump along with filter. Also is new wires, plugs, and what I think is called the icm, the name is failing me right now. I also installed a throttle bottle kit and replaced all the gaskets and seals. I sprayed around and I cannot find any intake leaks. I also put in a new map sensor and a new IAC. Now the car wants to idle at like 500 rpm. The second you put it in gear it dies unless I rev it, and it won't start unless I give it gas. Before this happened the car seemed to be backfiring but it was coming out of the top of the engine, out of the air filter cover. Now it no longer does that, but I can't get the idle right. I know it is possible to adjust the idle but from what I read it is a bad idea. Anything I am missing?

Thank you!

Big one,


did you check your base timing. Poor timing can result a low and often unstable idle

I don't think that I can do much with the timing on an 88 duke. I don't think I even have a distributer. I have DIS Ignition, so basically my ecm handles all that. At least I think. Or am I thinking of something else?

500 idles not normal?if its an automatic it could be staying in lock up.

I don't think it's the lock up, it will die after around 5 seconds if I don't keep giving it gas regardless. It goes down to 500, then starts to sputter after a few seconds and dies. The car drives fine as long as I keep holding the throttle some. There is some hesitation to rev though if it is at around 500 and I stomp the gas though. Before I did all this work it always idled around 1000 now that I think about it, even when it warmed up. I may have fixed an intake leak and now it is idling where it is "suppose" to be.

[This message has been edited by Firezappy (edited 11-13-2024).]

quote Originally posted by Firezappy: It has a new fuel pump along with filter. Also is new wires, plugs, and what I think is called the icm, the name is failing me right now. I also put in a new map sensor and a new IAC.

There's no guarantee that a new anything is actually working properly.

If all the above have been checked and are functioning as they should, I'd test the Throttle Position Sensor (TPS) next.

I do need to check, I also need to make sure all the plugs are actually firing. I did put in a TPS, but while troubleshooting stuff I ended up putting the old one back in, which made no difference.

I just rememberd an important piece of information that may change things. So my throttle body is not the one my engine came with. I bought a junkyard engine because my original engine had warped, before I did all of this the car had a tendency to idle on the higher side. Now that I have replaced all this stuff it wants to idle to low. Once I verify the fuel and plugs are all fine, could I actually be one of the rare cases that I need to adjust my idle ofset screw thing? It is not tuned for my motor, it was tuned from the factory for my old engine.

The cam position sensor under the coil pack sets the timing on the 87-88 Duke engines. If it burns out it will set a code but erratic operation may not always be detected by the ECM. My guess is that you might have a fueling problem like a dirty/clogged injector.
Getting back to what I advise people to buy is a scanner that can help detect problems.
If you don't have one it becomes a guessing game.
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 11-15-2024).]

quote Originally posted by Firezappy: ...could I actually be one of the rare cases that I need to adjust my idle ofset screw thing?

Highly doubtful. As long as the butterfly valve is closed all the way without binding, it's probably fine. I suspect you've got something else going on with that engine that's preventing it from idling properly.

I adjusted the idler just to see what it would happen. With the idle set higher the car ran pretty much fine. But after a while this started happening...

https://youtube.com/shorts/...?si=xoJ0Iw8kqnoR6_yb

Loud popping noises when I rev, also lose all power. Maybe this is a timing issue?

It's obvious that something is wrong with this engine. Moving the idle stop screw has only added to what now needs to be fixed.

Thankfully it is easy to reset. I just got to follow the instructions here:

https://fierocave.shorturl.com/idlestop.htm

But this is not the first time it has done this popping, I just remembered when all this started it would do this once in a while, now it is even worse. And this of course was way before I even toughed the screw.

Edit

(The popping is coming from the top of the throttle body, that large plate that covers the air filter is flexing and making the popping noise.)
(I am going to clean the injector and replace it's filters, lean fuel mix can make this happen apparently. Also if the cam timing is off it can do it as well, so if nothing changes once I clean the injector and check the fuel pressure, that will be next. Cam sensor is only like $7 on Rock Auto anyway.)

Oh and I will reset the idle screw, I see now how that is not the way to go. I just had a mechanic friend pushing for me to do it and I probably should not have. But it did let me drive the car longer then I normally would to see how it behaves so not the worst thing to do. I will report back, thank you for all your input so far everyone!

[This message has been edited by Firezappy (edited 11-15-2024).]

quote Originally posted by Firezappy: I don't think it's the lock up, it will die after around 5 seconds if I don't keep giving it gas regardless. It goes down to 500, then starts to sputter after a few seconds and dies. The car drives fine as long as I keep holding the throttle some. There is some hesitation to rev though if it is at around 500 and I stomp the gas though. Before I did all this work it always idled around 1000 now that I think about it, even when it warmed up. I may have fixed an intake leak and now it is idling where it is "suppose" to be.

I wouldn't knock out the lockup just yet. My fiero had the exact same issue you listed, bad idle, wouldn't start unless you gave it gas, wouldn't shift unless you gave it gas. Had to give it gas for a full on minute or two before it would idle by itself. (As in, car in neutral and make it idle yourself at around 1k, I wasn't just going hog wild)

I unplugged the tcc solenoid and the issue went away.

I also have a 3 speed iron duke, it's warm idle is 900-1,000 since I've unplugged the tcc. It's replaced now, but I haven't been able to test it.

[This message has been edited by Gizmo0816 (edited 11-17-2024).]

Thanks for the info, I did unplug the solenoid on the transmission and I had no change. It has got stuck once before but it has not since then. I got it running much better now, it will sit and idle between 500 - 600 no problem. I took the fuel filter off and it did not even look like gas, black sludge was coming out. Replaced it and it runs better. It still has hesitation to rev though. The injector was gross, I did clean it some and put new filters on it but I don't think I got it 100%. I may pay for a remain service just to be sure. Although I am thinking about dropping the tank and cleaning it, for the filter to look that bad after only being in a few years must mean that tank is nasty. I will update if cleaning the injector gets it to 100%, although my Fiero has had a random miss since they day I bought it at low rpm, maybe it has had a bad injector all these years.

quote Originally posted by Firezappy: It has a new fuel pump along with filter.

quote Originally posted by Firezappy: I took the fuel filter off and it did not even look like gas, black sludge was coming out. I am thinking about dropping the tank and cleaning it, for the filter to look that bad after only being in a few years must mean that tank is nasty.

How long is "a few years"? Why would the inside of the gas tank become "nasty" if it wasn't that way when the new fuel pump was installed?

The sock on the fuel pump inlet is probably plugged up. This is one of the reasons why basic tests such as fuel pressure need to be done when diagnosing a problem.

Well, to me it was a few years I guess but now that I think of it I put in a new fuel pump and filter back in 2018. I remember looking in the tank though and it looked fine. I guess it's possible I pulled some sludge from one of our questionable gas stations. Sadly it would not be the first time. Rural Ohio never change >.>

[This message has been edited by Firezappy (edited 11-17-2024).]

Aw, darn thought I could help for once. Good luck.

An update with some more questions. So I changed the fuel pump and put in another new fuel filter to be 100% sure. Car runs better but still not correct. But then I noticed something, my Fiero used to idle really high when it was cold, now it just tries to idle at 600 rpm regardless how cold the engine is. It used to sit at over 2000 rpm until it warmed up. I just changed the charge temperature sensor which seemed to help things even more but I still have trouble idling out of the 600 rpm territory unless I baby it. But if it is 1000+ rpm I can rev it all day. I take it on an 88 duke the ecm is looking at the engine coolant temperature to determine to rev in cold conditions correct? So I should verify that my coolant temp sensor is correct and reset the position of my IAC?

So the next steps I am going to take is:
1. New coolant sensor for the ecm
2. Put the computer in troubleshooting mode and reset the IAC and my idle screw.

Anything big I am missing here? Just want to thank everyone for their help so far.

quote Originally posted by Dennis LaGrua: The cam position sensor under the coil pack sets the timing on the 87-88 Duke engines. If it burns out it will set a code but erratic operation may not always be detected by the ECM. My guess is that you might have a fueling problem like a dirty/clogged injector. Getting back to what I advise people to buy is a scanner that can help detect problems. If you don't have one it becomes a guessing game. " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "

That would be the crank position sensor under the coil pack.

Thanks for the reminder. Sometime soon I will change that as well. I reset the iac along with the idle control and the car is still not running right. (It runs way better but still has a hesitation to rev and sometimes dies) I could not reach that sensor with my hands so I may end up taking off the intact manifold unless there is some secret method I am missing?

Update, so I replaced the crank position sensor and it has not fixed the issue. So far I have changed the fuel pump, filter, injector, coolant sensor, iac, wires, plugs, charge temperature sensor and of the course the crank position sensor. It runs better then it used to that is for sure, but it still does that popping when it warms up like that video I posted earlier. I am about ready to pull my hair out. Is the next step coils and an ignition control module? Or could it be the ecm? The fact that it not longer idles itself up when it is cold concerns me too.

With all the new parts replaced have you tested the coils with a multimeter? You reported some popping noises coming from the exhaust. Its an easy test with a $10 multimeter or volt/ohmmeter. Fuel pressure should be around $15 psi, Good to check that as well.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Thanks for the tips. I actually went ahead and ordered a new coil ignition module, but I already have some spare coils to replace the old ones with. The popping is not out of the exhaust though, it's popping out of the intake. When I replace the coils and ignition I will report back. I don't have a fuel pressure tester but I did just replace the fuel pump along with a new filter so I won't say it's impossible that the pressure is low, but unlikely. I will rent a gauge and test the fuel pressure though before I go buying a new ecm or something. I have also done a compression test, the motor is not perfect but they are all close to each other. About what you would expect of a 200,000 mile duke.

Sort of fixed!

So after countless new parts the car is now fully drivable. The new coils and ignition control module got things working great. No more 'reverse' backfiring as far as I can tell. In fact, the car has regained some power. Problem is I have to leave the idle up at around 900. It will idle fine at 600, but the second you give it gas then let go of the throttle it will go down to 400 rpm cough and die. Sometimes it will recover but it takes a solid second or two for the computer to realize it needs to idle up. Leaving it slightly idled up to high makes this no longer happen. I think I may be dealing with an ecm issue at this point. Because not only does it seem to react slowly the car still no longer idles up at all in cold conditions on a first start. When you first start a Fiero they are suppose to idle up to around 2000ish rpm to quickly get the engine warmed up. Mine no longer does that at all. Checked the ECM temperature sensor and wiring and it is fine. If anyone has thoughts on this let me know! But I can finally drive my car around again.

Hello again, sorry to pull up an old thread. As you can see this has been an ongoing issue. Just to catch anyone up reading this so they don't have to re read this old post...

The main issue is the car does not want to idle at 500 rpm. It will sit there and run at 500 rpm forever but when you gas it and let go of the gas it drops below 500 rpm and dies. Otherwise the car runs perfectly fine. The computer refuses to idle up the car at first start regardless of temperature outside.

I have replaced the seals and gaskets in the throttle body, new fuel pump, filter, injector, coolant sensor, iac, ecm wires, ecm, spark plugs, wires, charge temp sensor, crank position sensor, among many other things. I have checked my grounds and wires, they appear to be fine now.

The question, how does the ecm control idle? I have replaced the iac, even pulled an old stock off another iron duke and nothing changed. I can feel it working when the car is on. And when the computer is in diagnosis mode I can clearly hear it running. Is there some plunger mechanism in there that can break? Is this something that has been known to plug up somehow? Any other ideas?

quote Originally posted by Firezappy: I have replaced the iac... Is this something that has been known to plug up somehow?

Did you make sure all the passageways for the IAC valve were free from obstruction? If not, and they're plugged, it's just like having the IAC valve permanently closed (even though the valve itself may be functional).

Thanks, I think my next step will be to take the throttle body apart and just carefully clean it, especially the iac area.

Actually fixed now. When I change the gaskets in the throttle body the replacement one I had was not correct. It had an additional path for the IAC that was not used on this year of Iron Duke. So it was always letting in some air regardless of what the computer wanted. Now the car runs perfect. I even used an ultrasonic cleaner so at least now I have a shiny throttle body sitting on a 200,000 plus mile duke. But I guess with all the new parts I basically did a full tune up, has not run this well in years. Idles nice and smooth,no more vacuum leaks, you would not know it was a high mileage 37 year old car.

quote Originally posted by Firezappy: I was having issue with the car having a poor idle and dying when putting the car in gear... Now the car wants to idle at like 500 rpm.

quote Originally posted by Firezappy: Actually fixed now. When I change the gaskets in the throttle body the replacement one I had was not correct. It had an additional path for the IAC that was not used on this year of Iron Duke. So it was always letting in some air regardless of what the computer wanted. Now the car runs perfect.

Glad to hear it runs so well now, but... If additional air was previously being let in, the idle speed would've been higher, not lower... so it's still unclear what the actual problem was.

There's no guarantee that a new anything is actually working properly.

I did clean up the throttle body as well, including an ultrasonic cleaner. So wrong gasket in combination with a filthy throttle body would cause a lot of problems. All I know is I have been driving it for the past few days and it runs great again.