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92 Firebird misfireing issues

442 Views 35 Replies 4 Participants Last post by  ChaseK
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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If the O2 sensor is “hovering right around 450mV”, it may not be working. Or the ECM is not going into closed loop. The ECM should go into closed loop when the engine is warmed up, and the O2 sensor appears to be warm enough to start working.

A cold O2 sensor (less than 600°F) does not work, and reads 450mv, which is the bias voltage supplied by the ECM. In closed loop, the O2 sensor should be moving rapidly back and forth over a range from slightly rich to slightly lean. The ECM use the INT (integrator, or short term fuel correction) to toggle the A/F ratio back and forth between rich and lean so the catalyst converter will work.

When it did warm up you indicate the BLM went to 156. That means the O2 sensor was reporting “lean”, and the ECM determined it needed to add 22% extra fuel to maintain the 14.7:1 A/F ratio. That is a serious “lean” condition.
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Do you think that this all could be because of a bad fuel filter ? Also if I remember correctly the P.O. Had changed the injectors a couple years before he sold it and now that I think about it had said they were out of some ford motor. Could that be the problem that they are the wrong injectiors or do you think it is something else. Thank you for the time you have sent trying to help me figure this out
Check the fuel pressure. Check it at idle, with the fuel pressure regulator vacuum compensation line attached. Record the pressure. Disconnect the vacuum line and the pressure should rise. Record the pressure. Then, using a fuel pressure gauge with a long hose, tape the gauge to the windshield and take the car out on the road. Take it up to ~4,000 RPM (if it will do that) at wide open throttle. Record the lowest pressure while at WOT. Post the results.

If the injectors are the wrong flow rating, it could be causing the problems. The Ford/SVO injectors are flow rated at a different pressure, which changes the flow rating when they are operated at the typical GM pressure. Some of them have a colored top - red, yellow, etc. - that indicates the flow rating.
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The injectors look like they have white or gray plastic on the fuel rail side and yellow on the intake manifold side I am trying to send a picture of one but is isn’t letting me attach it.

As for the fuel pressure test here are the results of that

Key on prime - 43 psi
Idle with vacuum - 40 psi
Idle without vacuum - 47 psi
2k rpm partial throttle - 42-44 psi
4K rpm WOT - 48-50 psi
Fuel pressure is a bit high, just barely within the GM upper limit at idle, a bit over at WOT.

But the fuel filter is not your problem.
I can’t get the website to let me post the injector picture but to my eyes they look gray or white on the top with yellow at the base it’s kinda hard to see them with the way these intake manifolds are. Do you think it is the injectiors or could it be the computer telling them to fire less or for shorter periods because of the information it is receiving ? Also not sure if this would affect the air flow all that much but the car does have what looks to be a home made cold air intake with a spectre air filter. Another thing I should probably mention is that while the engine is missing the intake noise is much louder then when it isn’t misfiring
It's the top color. I believe they are::

gray = 14 #/HR

yellow/orange = 19 #/HR

blue = 24 #/HR

red = 30 #/HR

dark blue = 36 #/HR

Gray sounds small, but I have no idea what the spec is for your 92 3.1

Do they look like this:

Product Font Screenshot Technology Engineering
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That looks like them I will see if I can find what the spec is for them
Motorcraft injectors in a 3.1L V6...


Rochester Multec injectors should be black.
It was a popular fad…… specially in the LT1 engine. I used Ford SVO injectors for a while because the stock GM Multec injectors would not operate reliably at pressures above 60 PSI. The NOS 5176-EO dry nitrous kit I was running raised the fuel pressure to 85-90 PSI to flow the extra fuel required when the nitrous was activated.
Well, now Fred, that's different. 😄
From what I’ve seen online the 2.8 and the 3.1 both came factory with 15lb/hr injectors I couldn’t find the exact spec in the factory manual but that probably is because I’m not very familiar with using the factory service manuals. Also I think ford ones are rated at a lower pressure.
Generally the Ford injectors are rated at 2.7 bar (39.15 PSI). When they are operated at GM's typical 3 bar (43.5 PSI) they flow 5.4% more than rated. Assuming the 14.0 is correct, they would flow ~14.8 #/HR. That's close enough that the ECM can make small adjustments using the long term fuel corrections (BLM). You would see the BLM at ~130.

As far as I have seen, the injector flow rates are not in the factory service manual. For the LT1 V8 used in my 1994, the injectors are called out by GM as 24 #/HR, but if you go into the PCM program, the factory tune shows them as 24.9 #/HR.

The injectors don’t appear to be the problem, unless the O-rings on the manifold end are damaged or missing.
First post you indicated replacing the ICM cured the problem for a while. Could be a faulty ICM, excessive heat soak, etc. An auto parts store should be able to test the ICM. If you do find someone to test it, make sure they get it good and hot while testing it, so any problems with excessive temperature show up.
When I changed the ICM it still misfired for a day or two then it cleared up for about a month or so and it started acting up again sometimes it will run great for a week and others it won’t run for 5 minutes without dying I will see if my local advance can test it for me.
I think I might have made some progress. I took off the intake and noticed that the loud noise was coming from the IAC. I stuck my finger in the hole to try and block it a little the car instantly cleared up and ran much better. I put an old IAC I had laying around on and it now isn’t missing but the throttle still responds a little weird and the idle will start at around 900 rpm slowly creep up to around 1300 rpm and then it will snap back down to 900 for a full cycle of that it takes about a minute. Also the block learn is all over the place ranging from 116 to 156 and the O2 follows it around 330mV on the low end and 550mV on the high end but the check engine light is now off and it is pulling much better vacuum
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