Ok, now this one has me a bit stumped...

I have recently had all kinds of ignition problems... which have been cleared up with a new distributor, coil, and ign. module. Timing is good, as are the plugs and wires.

Now I am noticing a slight stumbling of the motor when cruising at a steady speed. It feels almost as if it misses occasionally. It cannot be heard, and there is no movement of the tach needle, but I can feel it stumble a little and pick up. Acceleration is great, and its idle in park is perfect. It seeems to do it at speeds over 40MPH, as I cannot notice it at speeds any slower.

Now does this still sound like a possible ignition problem? I cannot imagine there is anything else in the system that could possibly be bad, but since I have had so many problems there, I wonder if I missed something.

Or does this sound like a seperate problem? Any thoughts on it? Thanks all!

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John
AOLIM: Fformula88

1988 Fiero Formula
2002 Jeep Wrangler Sahara

Possibly caused by the a/t? Perhaps the lock-up is ocassionally slipping and kicking back in? Thats my best guess at the moment.

So you've done a the right things and it still misses under load. What wires did you put in and how well did you route them? I know this sounds strange but even new wIreS can leak voltage under load. Do you still have the heat shields on the spark plug ends?
Remove them. Do a lights Out Test and rev the motor. Pay close attention to the areas were the wires are routed close to the block or other ground source. Hope this helps!

Wires are routed fine, and there is no visible arcing that I can find. Auto tranny problem is a suspicion of mine too, but so far I have concentrated on going over the ignition system again to be sure I caught everything.

I'll send this one TTT one more time... why has my Fiero forsaken me?

I have a simalar stumble on mine to. I'm thinking mine is the throttle position sensor. The very beginning of the sensor is the parts that gets the most wear, so I'm thinking that very first light push on the accel isnt regeistering on the TPS, so the ECM doesnt know I'm giving it more throttle, but the butterfly is starting to open, leaving me with no timing advance, no additional fuel, but with more air.

Good article on Throttle Position Sensor (TPS).
http://clubs.hemmings.com/clubsites/fieroswest/Tech%20Scene/Idle/Tech012002.htm

Thanks Pyrthian, I think I will check out the TPS next. I did check the resistance last summer in relation to a poor idle problem, which now I am convinced was my distributor problems, and the sensor seemd to be operating ok. I'll definatelty check it out again.

Even if the resistance readings seem ok, I think I will swap in the TPS sensor I have in the 85 SE to see if that clears up the problem or not. The SE is running great, so I would assume its sensor is ok. It also has 20,000 fewer miles, which is a lot less wear on the sensor.

The article posted by CC is good too! Thanks for the reference!

Wish me luck!!

if your tranny is anything like the '84 2.5l autos, i think the stumbling is just the way the tranny is.

I read in my pontiac manual that the tranny would have a tendancy to surge and buck a little at speeds between 40 and 60 km/h.

whether they say that cuz it's true or if it's just their quick "fix" (like adding "check engine oil" sticker behind gas door to solve oil leak problem in 84) is beyond me.

Haven't checked the TPS yet, but wanted to get back to amikin:

The auto is in fact the identical transmission as the 2.5's have. I don't remember seeing a reference in the manual about it, but I may have missed it. It would be interesting to find that!

I do not think its a normal thing. It is something new I have noticed since replacing most of my ignition system. Its also at speeds a little faster than what your manual states. Your Km/H would roughly translate to speeds between 25-35 MPH, where as my symptoms seem to start at 35 MPH and go up the scale.

Thanks for the thought... I am going to have to go thru the manual now and see if it has such a reference!!

I'll have to go read the manual, cant remember where i saw it but i know i read it somewhere.

i'll let you know what i find out

Why don"t you disconnect the tcc lockup solenoid & then road test.

I would attack the MAP sensor next. They can get flaky, and generally show up symptoms when the engine isn't idling.

Here is the page out of my manual I was referring to. I highlighted the section.

The speed range does go higher than what I had said previously, but if the car didn't do it before it may not be the cause of what you're experiencing.

I know my car has behaved that way ever since I bought it last year.

If this is an auto trans, like Fiero Norm suggested, the tcc lockup solenoid could be starting to act up. Disconecting it and driving without it for a while will tell.

Thanks for the scan. I does indcate that it may be a normal problem. Although I will remain suspicious and continue checking around as I have not noticed it before.

Today I am thinking of testing the car with the TCC unplugged, andgoing over a couple of the other sensors.

Thanks again for everyones help and advice! Wish me luck!!

quote Originally posted by amikin: Here is the page out of my manual I was referring to. I highlighted the section. The speed range does go higher than what I had said previously, but if the car didn't do it before it may not be the cause of what you're experiencing. I know my car has behaved that way ever since I bought it last year.

I've noticed that my 88 GT (auto) does this exact same thing at about 40mph. When I mentioned it to my Dad (previous owner and also worked at Pontiac on the Fiero program)he stated that it was a normal condition. Apparently there were some bugs in the ECM/PCM that the engineers never ironed out before production.

Here's a thought.

You said you recently replaced the ignition system. Maybe the ECM was compensating for something in the ignition system before that the problem wasn't evident. Changing the sytem out may be making the ECM run "normally" which in turn has brought out that bug Turbotoad mentioned.

Hope you get it sorted. I'm going through some "pinging" headaches and know how frustrating it is when your car isn't running as it should.

quote Originally posted by turbotoad: I've noticed that my 88 GT (auto) does this exact same thing at about 40mph. When I mentioned it to my Dad (previous owner and also worked at Pontiac on the Fiero program)he stated that it was a normal condition. Apparently there were some bugs in the ECM/PCM that the engineers never ironed out before production.

Would that also then affect the manual trans? I have the exact same thing happening in my '86 GT with 4spd man.

Another thing to try might be to unplug your tach filter and see if it still does it. But if I had to guess, I would say it is kinda normal.

My 88GT does something like that every now and then.

The car might have been doing it all along, but after all your ignition troubles, you have probably become hyper-sensitive to every little thing that your car does, I know that I am like that. The next thing you know, you start replacing things when you don't even have a problem

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A bit of an update...

I haven't touched the transmission yet. I decided to play around with the ignition a little more, and checked my timing setting. I decided it was a little off, and corrected it, but that didn't touch the symptoms. Also, it seems that the timing mark on the pully jumps around a lot, making it a little difficult to get a good reading.

It could be normal, but I doubt it. I took the car for a longer drive last night, and decided to try it with cruise control to take varying pressure from my foot out of the equation, figuring it will hold the pedal steady better than I can.

This stumbling is very pronounced with it on, as long as the car does not have to go up any incline. If the road is flat, or has a slight down hill pitch, it feels like the car rolling on slightly rippled pavement with this stumbling. It is more pronounced in the 40-50 MPH range, but I did not have much of an opportunity to get above those speeds for any distance. The roads I was on were also in great condition, paved within the last couple years, and as smooth as glass.

I still think something needs to be sorted out with it. I'll get back to it again this evening, test out a few more items.