Power degrading.

[el_roy1985]

First off, my cars info...

1996 3400/5 speed
3100 injectors and fuel rail
Replaced MAF two years ago
replaced plugs, wires, coil packs, cps and fuel filter last year before it got so bad that it quit running.

The original problem was a strong jerking misfire that seemed like the engine was shutting off, only to be restarted be the clutch "push" starting it. If it happened with the clutch pushed in, it would just quit running. The problem was first apparent on the way back from a 200 mile trip in one direction. At about 100 miles left, it started doing this so badly(continuously), it tore up my clutchmaster clutch plate springs by the time I got home. Prior to this, it had not happened. After it started to happen, it would only be during longer trips. In town drives were just fine. Although, the engine was jerky with it's power output to where it would lag and then pull really hard for a second or two before going back to lagging.

After a year or so of messing around with it, it finally got so bad that it just stopped running altogether. Bumps would seem to agitate the problem, but it was still most apparent during acceleration. It was happening at all speeds and on all lengths of trips at this time. I brought it home, changed the plugs one last time, and it sputtered a few last breaths before not starting again till this year, after I replaced the engine harness. I tried starting it just prior to switching the harness, but it was a no go.

The engine ran GREAT for the first few days. More power then the 3400 ever had from the first time I drove with it in my car. Now it has begun misfiring under hard acceleration. This is a different problem then I had with it before. It registered on the original 3100 PCM as a P0300 (multiple misfire) and I switched that out for a tuned PCM that I purchased off of heavywoody. I figured maybe the tune caused it to run badly and even if it wasn't tuned for my injectors (tuned for '99 injectors I think it was?), I figured it would at least put more fuel into the engine.

Immediately after starting the engine I noticed it seemed to have a better tone to the exhaust and it idled much better. It idled fine before, but now it idled great. Took it for a drive and it seemed to be doing fine. After the engine warmed up, it started misfiring. The problem grew worse throughout the duration of my drive. This computer has not registered any codes though. Is it because of the way it's tuned? I don't understand why it wouldn't throw a code with the obvious misfire situation it is having.

At this time it has also started to do the hard misfire at speeds above 40. Why would replacing the harness get it to run again, but leave the original problem there?

Cylinder pressure on the front bank was even all the way across, prior to getting the engine running again. I skipped the rear bank because of the difficulty, and at the time, I felt that I would at least have a running engine with all three front cylinders at acceptable pressures. I also recently (today) checked for blow-by since it is currently running, but there is none. So without checking the other cylinders (and considering it will run right some of the time) I don't think I have a compression issue.

With a spark tester, the light appears to be consistent without any misses at idle. Random misfires can still be heard through the exhaust though. Fuel pressure was around 40 at idle with the 3100 FPR. I swapped on the 3400 FPR to no avail. I will be checking fuel pressure again later today, but haven't yet since I swapped the FPR.

Before swapping computers, I checked the plugs. They all appeared to have nothing on them. So IIRC, this should mean it's slightly lean? If it was horribly lean, they would be covered in white residue, correct? Would it be possible that they are fowling in this situation? (still wouldn't explain why my spark light was consistent though) If they had fowled, would they still be able to spark under no load (or cooler running temps), in turn burning the fuel off them keeping them as clean and dry as they are when I check them?

Being that the plugs are as clean as they are, I'm leaning more towards a fueling issue at this point. What pressures should I be getting at idle? I will try to get pressures during driving, but I'm not sure if my FPR will allow me to do so.

I'm mostly confused as to why it gets worse, the longer I drive. Once the engine is heated up, the cylinder temperatures should remain stable, shouldn't they? Maybe the fuel pump starts to lose pressure as it warms up after being run for extended periods of time? Would knock caused from low fuel ratio, cause the engine to just shut itself down to prevent catastrophic failure?

The fuel in the tank is probably a year old, but was full the whole time. No stabilizers were used. (yeah, maybe a stupid move on my part) This problem has been around during the fresh fuel days of the freshly swapped 3400 though, so it's not like old fuel started the problem to begin with. I'm mentioning this just in case, but I doubt it has anything to do with my current situation.

The longer I let the car sit (in days), the better it will run when I start it up again. The problem seems to return more quickly every time though. Driving nice and easy seems to prolong the time before the problem starts or even cause it to become a little better if you drive easy after some hard driving.

So does anyone have any suggestions as to what it may be? This is a very frustrating problem, but it needs to get figured out this year. No more slacking on this car, I finally have the time/money to work on it, so it is getting fixed. Hopefully before the last autocross of the year...

[3X00-Modified]

Replied on 60degree.

[wicked-irocz]

I'm thinking you could have one or more fuel injectors going bad. Check the ohms across the terminals. Must be over 10 ohms but anything under 12.8 is very suspicious. Your knock sensor could also be failing. Do you have something you can datalog with or someone who could do it for you?

[el_roy1985]

Got a hold of a scanner that reads the sensors, so here are some values I obtained from it. (I still have to go through the injectors to get readings off of those, this is all I had time for today.)

Load Value - 54.9%
Coolant - 186
STFT - Bank 1 - 0%
LTFT - Bank 1 - 13.2%
Manifold Pressure - 27.6 in.hg
Engine RPM - 3723
Vehicle speed - 50 mph
Ignition advance - -24.5 deg
Intake air temp - 78
Maf flow rate - 14.3lb/min
Throttle - 99.4%
Bank1 - sensor1- 02S Output 0.895v
Bank 1 - sensor1 - STFT 0.0%
Bank 1 - sensor2 - 02S Output 0.445v
Bank 1 - Sensor2 - STFT 99.0%


Load Value - 6.2%
Coolant - 190
STFT - Bank 1 - -2.3%
LTFT - Bank 1 - -3.9%
Manifold Pressure - 8.4 in.hg
Engine RPM - 1780
Vehicle speed - 35 mph
Ignition advance - -36.5 deg
Intake air temp - 89
Maf flow rate - 1.2lb/min
Throttle - 5.4%
Bank1 - sensor1- 02S Output 0.325v
Bank 1 - sensor1 - STFT 2.3%
Bank 1 - sensor2 - 02S Output 0.445
Bank 1 - Sensor2 - STFT 99.0%


Load Value - 2.7%
Coolant - 192
STFT - Bank 1 - -0.7%
LTFT - Bank 1 - -1.5%
Manifold Pressure - 8.7 in.hg
Engine RPM - 904
Vehicle speed - 0 mph
Ignition advance - -28.5 deg
Intake air temp - 99
Maf flow rate - 0.5lb/min
Throttle - 0.0%
Bank1 - sensor1- 02S Output 0.080v
Bank 1 - sensor1 - STFT 0.0%
Bank 1 - sensor2 - 02S Output 0.445
Bank 1 - Sensor2 - STFT 99.0%

Load Value - 2.7%
Coolant - 190
STFT - Bank 1 - 0.0%
LTFT - Bank 1 - -7.0%
Manifold Pressure - 3.3 in.hg
Engine RPM - 2631
Vehicle speed - 35 mph
Ignition advance - -15.0 deg
Intake air temp - 102
Maf flow rate - 0.4lb/min
Throttle - 0.0%
Bank1 - sensor1- 02S Output 0.005v
Bank 1 - sensor1 - STFT 0.0%
Bank 1 - sensor2 - 02S Output 0.445
Bank 1 - Sensor2 - STFT 99.0%

I guess what I notice right away is all the timing it is pulling. Still have to learn what normal 02 readings are, so not too sure on whether those readings are good or not. I did get a code this time, it registered a P0300 (random multiple misfire). I had this code before, so hopefully it tells some of you more then it tells me along with these parameters.

Thanks in advance for any help you can provide, I hope this takes me one step closer to getting this car running correctly.

[3X00-Modified]

Explain to me where you see pulled timing? 24* at WOT is average levels for a untuned ECM...

You need a full datalog... individual snapshots are the hardest to follow and the hardest to watch anything change on.

You need to get a tool where you can log from leaving the line till when you let off the gas and watch everything that happend from inital start to end at 25ms intervals... thats the only way you'll find it.

Going by your snapshots, the ONLY thing I see is the 13% fuel trim, but thats positive, so thats not as bad as a negative one... .895 at the o2 sensor isn't bad. For a rough estimate a 900 to 912 is about a 12.8 to a 12.5:1 A/F ratio... at least it is on mine. All narrowband O2 sensors differ though so yours may need to read 930 or 940 for it to be right... But the only way to tell for sure is to have a wideband.. BUT going by those snapshots I see nothing wrong.

[el_roy1985]

That's a negative 24*, or is it still normal that it's negative. Maybe I have things backwards?

This is the tuned ECM I got from heavywoody. I mentioned that in my first post.

I guess I thought the 02 was reading very lean. I didn't realize they made such a big difference from sensor to sensor. I will try to get a hold of a data logger(or just take it somewhere), this was just all I had for now so I was hoping it would be of some help.