Weird 3.1 Hesitation... can't figure it out!

[toadson]

Ive been dealing with all sorts of problems with my Fiero after swapping in a 3.1 and ecm from a 1990 Lumina. My Fiero has a 5 speed, so I had the computer tuned for a manual tranny. For almost 6 months, the car ran really rough and lopey. It turns out the IAC was stuck or something... because after I removed it, cleaned it, and reinstalled it, it started to run normal. I also had a wandering idle issue to deal with, which turned out to be my tps sensor. I replaced the TPS with another used unit, and the wandering idle was gone.
The only problem I have now is a slight hesitation. While the car is sitting and idling, I can give it a quick, light tap of the throttle, and the engine will suck air, start to die, then rev up. It happens fairly quick, but is very noticeable. It also happens during normal driving, inbetween shifts. It will almost kill the car shifting into 2nd gear. The thing I dont understand, is that the car runs fine when you give it a lot of gas.... either sitting in neutral or while driving. This hesitation only happens at light taps of the throttle and during normal driving.
I found another used TPS and tried it out..... nothing but a wandering idle resulted. Ive also switched 4 different MAP sensors, none which helped the problem. I do have a scanner however, and compared MAP readings under various conditions with another buddies car, and everything looked normal. Something that made me wonder though, is that my car is throwing a code, which says the MAP voltage is too high. Ive also tried other IAC's, to no avail. I cant think of anything else that could be wrong to cause this hesitation, but I would love to get it figured out so the car can be driveable! Thanks for any insights

[kflo 93 gt]

Check all of your Vac lines, ALL of them. I'm almost garaunteeing one of them has a hole in it. Check under the upper intake manifold where the vac lines run between the runners, a lot of times on the 3.1 they will shift and get smashed pinched.

[toadson]

re

I went ahead and checked out the vaccuum lines, but saw nothing wrong with them. Ive disconnected the battery, and it still hesitates. I also filled it up with premium, but of course it didnt change anything. Im still lost haha.

[Superdave]

sounds like it may be running a bitch rich...



maybe swap out the FPR...

[TazMan]

Just wondering. You said you changed the tps and the wondering idle was gone, but when you tried a third tps it came back. Did you check for the hesitation with the third TPS?

[toadson]

re

I would say its running rich for sure - according to the scanner it runs rich at idle until you rev it up. The exhaust also burns your eyes (no catalytic converter) when you stand behind it. As far as the hesitation with the third tps - yes it was still there.

[Superdave]

yah, could be the FPR or a bad injector(or a few bad ones).

while you have the plenum off check all the injectors, should all be around 12 ohms. I'd swap the FPR out just for good measure either way.

[toadson]

re

I checked the fuel pressure tonight. At idle I have 35 PSI, and it revs up to 45 or so when I rev it up in neutral. The fuel pressure also rises when the car hesitates, so Im assuming its not a fuel related problem. I thought maybe the fuel pressure was dropping when I blipped the throttle, but it rises instead. Does this sound fishy to anyone or does it sound like something besides a fuel problem?

[toadson]

re

I put on a new fuel filter tonight with wishful thinking, and of course it didnt help at all. if anything, it made it hesitate worse. The old filter was past its due, because the fuel inside of it poured out the inlet and was a very dark/brownish color. I double checked the fuel pressure. With just the key in the ON position and the engine not running, it has about 40 PSI. During idle, it has 35 PSI and when you rev it up, it reaches up to 45 PSI. Also, my canister purge solenoid was capped off, and I decided to hook it up. It made no change to the hesitation at all being hooked up. Im stumped.

[Mach 5]

clogged return line maybe?


Jarek

[toadson]

re

I dont think the return line is clogged, since the pressure builds up when you block it off. If it didnt change, I would suspect that. I got my hands on a tech 1 scanner and did an idle relearn using it. It still has the hesitation of course. Im sending the chip back to the guy I had program it. The o2 sensor voltage drops extremely low when it hesitates, and he says he can probably fix it with some tuning. Im going on vacation in a few days anyway, so by the time I get back it should be ready to go.

[SappySE107]

who is programming it?

[toadson]

re

Sinister Performance did the tuning. He has a good reputation at Pennocks Fiero Forum, and for the price I paid it was well worth it. He isnt charging me to fix the hesitation either, so in my opinion you cant beat it. The website is www.gmtuners.com if you want to check it out.

[SappySE107]

Yeah, being cheap can't be beat...

[91z24build]

Any new updates? Just wandering cause my car does the exact same thing (completely stock 91 z24 3.1 5speed) It has done that ever since I have had it, I'm not too worried though its getting ready to have a new engine and fully programable EFI sys. put in it, Its just a pisser trying to drive when you first start it. It seems it dissipates from very bad to decent after it warms up.