I've been fighting stumbling and hesitation problems with my car. On startup it runs fine. Initially idles at about 1500 rpm cold and drops to 850 or so when warmed. If I start driving when the car is still cold all is well but as the car warms up it starts stumbling with a small backfire when giving it more throttle after cruising at a constant speed. Also, when warm, the idle hunts up and down starting with a small RPM fluctuation that gets worse as it idles. Kind of like it is overcorrecting. A tap on the throttle calms it down again.

I have replaced the O2 sensor. Searched for a vacuum leak and found none. Disconnected the EGR valve. Replaced the fuel filter. Dissassembled and cleaned the throttle body and IAC. Checked the TPS with a multimeter and it seems to be OK, no dead spots or sudden jumps in resistance as it is rotated.

While road testing The check engine light came on for a few seconds and then went out. The stored code was 24 ( three 24's actually ), VSS. I was moving when it set the code. My speedo works fine.

I assume the VSS is working since my speedo is. The computer is fed the VSS signal from the speedo board right? Is there a connector in the wiring from the speedo to the computer or is it just wire into the harness? Would a lack of VSS signal cause the problems I'm having?

I don't want to go easter egging with new parts anymore so any help is greatly appreciated.

what is the volt reading of the tps at idle?
some tps have some adjustment to them and if not you can enlarge the holes a bit so that it does have adjustment. It sounds like your tps voltage is too low at idle and that would carry thru the entire rpm range. Good luck!

Phil

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GTDude
OVER 25 years GM experience

Check your fuel pump. I went thru nearly the same list of goodies with my 88' coupe. My stumbling got REALLY bad. I thought it would die on the road somewhere. New TPS, IAC, and I even used my stumbling problem as an excuse to buy a new catalytic converter.

It straightened up for a while, and then she died on me going to work about 3 weeks ago.
Turns out my fuel pump was geting ready to let go. 40 bucks and two hours, I was rolling again.

I figure it was getting weak, and the longer I ran the engine, the hotter the fuel pump got. And it would work less efficiently and not support the fuel requirements.

I hope this helps.
88Ironduke

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Pilots without maintenance are pedestrians with a cool jacket and sunglasses.
I.F.Y.A.M.Y.A.S

I'll check the TPS voltage but I thought the ECM measured the voltage at startup and used that as base. Would the ECM get crabby if it read 0 volts?

Fuel pump. That one has already happened to me once before. We checked the fuel pressure and it's good. On my list of Things To Do is installing a remote fuel pressure guage so I can see whats going on while driving.

I think I found the problem. In the summer I run a 180 Deg. stat. I thought I might have forgotten to put the 195 back in for winter so I went to check. I forgot to put *any* stat back in after flushing the system. Duh. I'll roadtest after they finish scraping the snow off the roads.

While I'm at it, has anybody had any luck piggybacking a variable resistor onto the coolant temp sensor to fool the ECM into thinking the engine is colder than it is. It would probably just run rich but I thought I'd ask

Thanks folks!

The TPS on the 700 TBI is non adjustable. If the voltage is out of spec then the TPS is bad or there is something else wrong. Altering the TPS to be adjustable isn't likely to solve the problem.

EGR may be leaking around the pintle. that won't help anything.

low fuel pressure is always trouble.

could be a cooked MAT. Intake backfires will fry that sensor. (You've got to check this one no matter what else might be wrong.)

In youre case the speedo is working and the ECM still set 24. (It always repeats each code three times on the dash light.) This combined with the other issues....

While it could be the speedo board, There is a real posibility that the ECM itself is going south. You can get replacement ECMs from the cars listed in the DIS motor replacement article in my cave. (You need to reuse the Fiero PROM in whatever replacement you get.)

The location of the ECM in Fiero can make it run at the upper limits of the thermal range. This isn't real goood for them. It is pretty common to see over heated ECMs start to do wierd stuff like this before they actually croak. (Before you ask, I'm still looking into solutions for the ECM heat issue. I don't have any detail yet.)

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11-Sept-01, The day the world as we knew it ended.

After putting the stat back in I'm mostly O.K. Bucking and stumbling are gone but I still have an idle fluctuation.

TPS voltage at idle is .675V at the sensor and about the same as it goes into the ECM. It is still setting code 24. Drives fine. I made the TPS adustable by enlarging the mounting holes so I could rotate it 5 or so degrees. Made no difference. I swapped in a known good MAP sensor and IAC. No difference.

I've always heard that the ECM is the least likely component to fail but it's starting to look like that might be the case. I'd test the VSS but don't have an oscope or scantool. Guess it's time to seek professional help!

UPDATE:

After chasing the funky idle and backfire on acceleration under load I finally found out what was wrong. No lower o-ring on the fuel injector. Countless hours searching for vacuum leaks, bad wiring, bum sensors, manifold cracks, low fuel pressure and weak ignition it turns out to be a @#*&!! $0.10 O-ring! I'd like to find the !@%*3A who was too stupid or lazy to put on a S%$#@( little O-ring when they replaced the injector.

Reminds me of a computer I had to repair once. One of our clients had a PC that suffered from odd malfunctions, never the same thing twice. My predecessor had built the box and everytime they would call with a Blue Screen of Death he would advise them to reboot. My first week on the job those folks call with a problem. I go see. They were running WinNt (eeew) so naturally I start looking for some OS or driver problem. Not finding one I start looking at hardware. Cracked the case and...you know those thin sheets of *conductive* foam that new motherboards are packaged with to protect them from ESD and physical damage? Well, he installed it. Right behind the motherboard. My jaw hit the floor and rolled under a table. No wonder the thing was flaky, every pin sticking out of the back of the board was shorted together by conductive foam. We really ought to be allowed to kill people who are that profoundly stupid. Really, we should.