Part throttle bog in 3500 Fiero (OBD1)

[caffeine]

Lately I've been experiencing a part throttle issue with my engine. It seems like anytime I'm giving it less than 15% throttle it wants to bog (feels like it lost spark advance). It also seems to run normally if given lots of throttle (accellerates fine). When I let off the throttle I can hear lots of popping from the exhaust. I've done a datalog and the TPS, MAP and spark advance all seem to be normal. My instinct tells me this has to be a problem with the ICM, coils, CPS, ECM, or wiring. At first it was intermittent but the last time I drove it happened the whole time. I double checked the external trigger to make sure it was still set to 0. Grounds appear to be fine.

Any ideas?

[1988GTU]

If its not a vacuum line and the resistance on the coils/icm test out fine, check fuel pressure and also look at the injector spray pattern.

[caffeine]

If its not a vacuum line and the resistance on the coils/icm test out fine, check fuel pressure and also look at the injector spray pattern.

How do I go about testing the icm and coils? If it was a fuel problem I would expect worse performance at WOT, not better. And as for vacuum leak, it was idling a bit high today but when it was happening before it was usually almost stalling out with a hunting idle.

[caffeine]

Just cleaned off the tip of the crank sensor (had some metal shavings on it) and reset the ECM and its running like normal again. We'll see how long that lasts...

[caffeine]

No issues this morning but yesterday afternoon the exhaust was popping pretty loudly on decel with some lag/bucking when going from zero throttle to part throttle. According to my last datalog the tps and map respond instantly when I push the throttle. Still seems to have full power at WOT though.

Edit to add: it's always had some lag when goin from 0-part throttle but I didn't really give it much thought until now.

[LZeppelin513]

If you have a spark plug wire unplugged it can act like that. Maybe re-check the plug wires, pretty basic but I thought I would throw it out there.

[caffeine]

If you have a spark plug wire unplugged it can act like that. Maybe re-check the plug wires, pretty basic but I thought I would throw it out there.

I was thinking the same thing; checked my plug wires and coil grounds and all seem to be fine.

Yesterday I noticed it was mostly bogging around hard corners (also stalled once yesterday after going around a corner) so when I got home I yanked on all the engine bay wiring with the engine idling to check for loose connections but couldn't find anything making a change. Only wiring left that could affect it is the ECM wiring in the interior. Also I noticed when it bogs momentarily the tach jerks up and down. I might change the spark plugs (didn't install new ones during the swap) but other than that I think think the only other things that make sense are coils, ICM, or interior wiring.

I also think there could be something up with the IAC but I don't see how that would cause the other issue.

[pocket-rocket]

If the tach is jumping when it does it it sounds to me like it's definitely a problem in your ignition system. I would be looking at the condition of your cps, cps wiring and icm. I would also be looking into where the shavings on the cps are coming from.

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[LZeppelin513]

If the tach is jumping when it does it it sounds to me like it's definitely a problem in your ignition system. I would be looking at the condition of your cps, cps wiring and icm. I would also be looking into where the shavings on the cps are coming from.

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Agree with above, tach jumping -> check CPS, CPS connector, any splices along CPS wire, etc.

[caffeine]

If the tach is jumping when it does it it sounds to me like it's definitely a problem in your ignition system. I would be looking at the condition of your cps, cps wiring and icm. I would also be looking into where the shavings on the cps are coming from.

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Shavings on the cps were from when I was doing some clearancing for the alternator after installing poly trans mounts. So that's not really a problem. I could understand how shavings on the tip of the cps could be an issue though. Anyway, my cps wiring hasn't changed from before as far as I can tell, so I'll see about replacing the icm and cps.

[pocket-rocket]

Just remember, when an icm goes bad they normally don't work at all or stop working when hot. Same goes for the cps. Either way, I would start with the cps since it's cheaper. This kinda thing I'd why I keep spare ignition parts around (when I find them cheap in the buy/sell area), makes it easier chasing problems. I hope you find the problem

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[caffeine]

Just remember, when an icm goes bad they normally don't work at all or stop working when hot. Same goes for the cps. Either way, I would start with the cps since it's cheaper. This kinda thing I'd why I keep spare ignition parts around (when I find them cheap in the buy/sell area), makes it easier chasing problems. I hope you find the problem

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I'll try the cps and then making a new twisted pair since its not difficult. I too would like to have a spare ICM/coils so I might pick those up as well anyway. The ones I used came with the motor I bought and I don't know how many miles are on them. I've already put about 8,000 kms on the swap as well.

[caffeine]

I'll also mention that sometimes when it loses power at idle the check engine light comes on like it just stalled but only for a second. And other times it simply stalls. No codes though. Just earlier today when I started it the idle was erratic (bouncing up and down) but after idling for a minute or two, it suddenly revved up and then stalled. When I started it again it was idling normally. And it seems to make no difference whether it's hot or cold. It only seems to make a difference when I've just driven over a big bump in the road or taken a turn aggressively.

[LZeppelin513]

It only seems to make a difference when I've just driven over a big bump in the road or taken a turn aggressively.

You had to do quite a bit of rewiring for the 7730 and new motor. A loose splice or connection would cause sporadic open when the wire is jostled like over bumps or sharp turns. After I did my swap, I had a bad sensor connector that I had picked up from the junk yard. I eventually tracked it down by shaking and tugging on all the wiring while the engine was running while listening for the symptom to develop. I would pay extra attention to the ignition wiring.

[LZeppelin513]

Make sure your positive line from the battery to the power distribution is good. That wouldn't cause your drivability issues but it could cause the stalling. The two may not be the same problem.