92 Firebird misfireing issues

447 Views 35 Replies 4 Participants Last post by ChaseK, 2 mo ago

ChaseK

Discussion Starter · 2 mo ago

Ok I bought a 92 Firebird 3.1 around a year ago and it wasn’t running when I got it. The P.O. Had replaced the entire distributor, plugs, IAC, and fuel filter trying to fix a misfire and one day it just stopped being able to crank and he sold it to me. I fixed the no crank and got it running again and am in the middle of trying to figure out the misfire.

The problem I’m having has some really weird symptoms I’m going to list everything I have noticed but it might not all be important
1. A really bad misfire at idle that gradually gets worse as the car warms up
2. If I fluctuate the throttle around just above idle the misfire gets significantly worse sometimes causing the car to die
3. While it’s misfiring if I open the throttle just the slightest bit the car immediately revs up to 3000rpm
4. If I unplug the tps the car immediately revs up to 3000rpm
5. It has a much higher fuel consumption while misfiring
6. If I let the car get up to operating temp the tach starts jumping around (like up to 5-6k rpm) even though the rpm is staying relatively consistent
7. Check engine light with code 33

I fixed a few vacuum leaks throughout the car. That didn’t change anything. Then, after some research I replaced the ICM and I finally heard the car run “normal” I drove it around a bit and nothing seemed bad I parked it and disconnected the battery to clear the check engine light. Later that day I started it back up and it was back to its old habits. I went back into the distributor to look and see if anything was different and it wasn’t so I put it back together and started it back up. It ran great like a normal car for about 2 months during that period I changed the plugs again and wires with them. randomly a couple days ago it started again and ever since it’s been off and on running good and bad I haven’t been able to figure out what causes it to switch between running good and running bad
Any advice would be greatly appreciated and thank you to anyone who read that whole thing

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ZumpTA

· #2 · 2 mo ago

You didn't mention any stored codes when you got the car, but mention clearing the codes. I'm assuming the Check Engine light is on again. If so, there are stored diagnostic codes waiting for you. All you need to get at them is a paperclip or piece of wire as prior to 1994 it was possible to "flash" the codes on the instrument panel.

Here is the procedure as documented by shbox.com; let us know what you find:

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ZumpTA

· #5 · 2 mo ago

Seems to mean the same thing as on my '93, which it should being OBD1.

Meaning: Manifold absolute pressure (MAP) sensor – circuit malfunction
Causes: Wiring open circuit/short circuit to positive, hose blocked/leaking, MAP sensor, ECM

You need to use a Factory Service Manual to run through the diagnostic flow-chart for this code and determine which cause fits the 4 potential causes fit. I know '93 is a different animal, but the same diagnostic steps should apply. I'm sure the "voltages" are the same. If not, there's "generic" instructions all over the internet for diagnosing MAP issues; but the flow-chart in the '93 manual should be much closer if not an exact match to the procedure you need to follow. I'd start there unless you have a '92 Factory Service Manual handy. Chilton & Haynes have nothing on the Factory Manuals.

Failed MAP system can most certainly cause your symptoms and negatively impact fuel economy.

Here's the link to the 4th gen manuals, best I can offer; grab the '93 manual and go to page 6E3-A-64

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Looking around, I did see mention of MAF being a potential cause as well. Something about it reporting more air entering the system than possible. Just for grins, you might want to put a multi-meter on the MAF and see if it's within spec too. They didn't put a MAF sensor on the '93, so I don't know the first thing about it. The '94 manual might have the information and measurements you need to test it though.

Good Luck!

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ZumpTA

· #7 · 2 mo ago

ChaseK said:
I think the map was also replaced by the PO as well though

Frankly, doesn't matter if he did. So many electronic replacement parts these days have a short service life, erratic operation, or are DOA. I say these days, it's been a good 15/20 years, but the last 10 have been hardest on Quality / Reliable electrics. Unless you personally purchased a reputable brand from a reputable source, always question it. Even if you bought the best brand from the best retailer, there's still no guarantee you've got a good component with a long service life on your hands, just much better odds.

Pretty much all production of everything has either moved to China, or if it was previously in China, offloaded to a "cheaper" production facility; most likely sub-contracted to Africa. Africa is quickly establishing itself as "China's China" for manufacturing after all.

Most of the "Genuine Delco" stuff on Amazon and eBay is counterfeit now too. The real "New Old Stock" items are very few and far between at this point.

The Duralast electronics from AutoZone are still of reliable quality, as are their Duralast Gold mechanicals.

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ZumpTA

· #14 · 2 mo ago

1993TransAm said:
Might you have a stuck valve or two?

Good call. I'm now wondering if the "new distributor" has a faulty ICM. Without knowing where it was sourced from, "new" doesn't carry much weight. Did they buy the $300+ Pertronix or equivalent distributor? An MSD? Or did they cheap out with the $78 Cardone unit.

With these things sitting around $300 from reputable manufactures and a brand new Cardone unit sitting alongside them at 25% the cost, and no-name units selling for even less; I have my suspicions of what's installed, and I certainly wouldn't trust it until I verified it. Even if they got the $178 Summit Racing "Special", I'd question it.

Given the intermittent nature, I want to lean towards "electrical". In my experience, mechanical failures are rarely intermittent, but, I'm not an "internals" guy, so; Yeah.

I'd personally be doing diagnostics on the "new" ignition components, specifically the ICM that came with this new distributor.

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