Lean Misfires at 2700-2800 rpm

[badass3000]

Originally Posted by Mr4

... He tested the spark and found that in the bad range it misfired randomly, not on the same cylinders.
...

that scares me

[ProwlerGT]

Originally Posted by Mr4

I just bought a 92 3000GT VR4 for my son, and it seems to be suffering from this exact problem (misfiring in the 2-3k rpm range). The person I bought it from just had it serviced for it's 60,000 mile service (done at ~67000 miles) and states that the car was running beautifully. She drove it for another 4000 miles after the service and said it was great. She parked it in her garage and didn't drive it for almost a year. When she decided to sell it she noticed that it had a bad hesitation, backfiring and the like in the 2k to 3k rpm range. A friend though she should change the fule filter since it had been setting so long without running. That did not seem to fix the problem.
We came along and bought the car, thinking the problem was simple and could easily be fixed. We took it to our friend, a good mechanic, and he spent 3 days solid only finding one problem with the idle motor, which was showing problems, including a vacuum leak. Changing that did not fix the misfiring in the 2-3k range. He tested the spark and found that in the bad range it misfired randomly, not on the same cylinders.
Bottom line is that I have a great car that runs like crap and after seeing this site, I am wondering if it can ever be fixed. I am planning to take it to the dealer, who has better analyitical equipment than my friend to see what they can find.
Has anyone solved this problem???
Please help.
Mike

If you go to the dealer I can almost 100% guarantee you they are going to want to replace the ECU. Don't let'em. They will replace it with an ECU with the same faults that will reappear later on. Send the ECU off for repair and have'em work the whole board... capacitors, traces, drivers, etc... put it in writing. Sometimes though this doesn't resolve it for everyone, but most of the time it does. To see what has fixed other's 1st gen VR4 problems click the hesitation thread in my signature.

[Roybatty]

Originally Posted by Mr4

I just bought a 92 3000GT VR4 for my son, and it seems to be suffering from this exact problem (misfiring in the 2-3k rpm range). The person I bought it from just had it serviced for it's 60,000 mile service (done at ~67000 miles) and states that the car was running beautifully. She drove it for another 4000 miles after the service and said it was great. She parked it in her ...

Look up intake pressure test - there is a member that sells a kit - numerous threads - or if you are industrious there is info links to build your own. A common intake leak is the O-ring connection between the rear IC tube to rear turbo - but it could be anywhere. We are all boost addicted here and it tends to loosen all the intake tubing, clamps etc. So be prepared to tighten all your clamps well. GL

[UCSB VR4]

Originally Posted by TurboBob

do the ECU reset again, but unplug your O2 sensors so the trims can't learn.

See if that changes the re-occurrance of the symptom.

Bob

This ABSOLUTELY did the trick for me, except I unplugged the O2s without resetting the ECU (just to record any changes before/after resetting the ECU). Now, my car drives like an absolute champ, so I'm not even going to bother resetting the ECU... the idle RPM doesn't fluctuate up/down/up/down, the acceleration is smooth through the entire RPM band, and when letting off the gas in neutral the rpms step down controllably instead of bottoming out at 300-400 RPM (almost stalling the engine).

I don't suppose there are any ramifications to unplugging the O2s beyond freezing your fuel trims (which actually improves performance on my car). The only thing is that my ARM guage is practically useless now, but what a small price to pay, I never used it anyways.

I actually enjoyed driving to work today, the problem is gone.

- Jeff

EDIT: I wanted to also mention that the BRAND NEW ECU I swapped in had no effect on this problem, prior to the fix implemented above.

[DoctorDex]

Originally Posted by Mr4

He tested the spark and found that in the bad range it misfired randomly, not on the same cylinders.

Mike, do you mind asking your mechanic friend how he tested the spark and determined it was missing randomly? I'm curious to know, because I'd like to test on my own car.

[DoctorDex]

Originally Posted by UCSB VR4

I don't suppose there are any ramifications to unplugging the O2s beyond freezing your fuel trims (which actually improves performance on my car). The only thing is that my ARM guage is practically useless now, but what a small price to pay, I never used it anyways.

I'd recommend switching to speed density with wideband tracking. Driving with your O2s disconnected is going to cause you to run ridiculously rich during cruise. What you'll pay for in gas for a couple of months would easily cover the cost of the speed density setup.

[badass3000]

or just plug up the o2 sensor and cut the wire before it goes to the ECU, that way your ARM works yet the ECU trims are still froze.

[ProwlerGT]

Dex - have you tested your Crankshaft/Camshaft position sensors per the manual? O2 sensors?

Interesting idea: What if you could put the car up on all 4's (or on a 4 wheel dyno), set the cruise control at the problem RPM, and turn on the datalogger for every data value you can log, and record the log for a good bit..... perhaps this would lend more of a pattern in the data results than you are seeing now. If you put it up on all 4's I'd suggest having someone inside the car as a safety measure to cut off the ignition in case the unlikely even it would fall off. What we're missing here is more observation "DURING" the problem time. If you can increase the amount of time you have to observe the problem perhaps something NEW will pop up or be seen.

Honestly if I didn't know any better I would say this is starting to sound like a programmer error in the code on the board of the 91's. I wonder if the issue would go away if you would switch your car to an earlier year ECU 92 or 93 or higher if thats even possible; probably a lot of work.

[DoctorDex]

Originally Posted by ProwlerGT

Dex - have you tested your Crankshaft/Camshaft position sensors per the manual? O2 sensors?

Tried this already, they check out.

Originally Posted by ProwlerGT

Interesting idea: What if you could put the car up on all 4's (or on a 4 wheel dyno), set the cruise control at the problem RPM, and turn on the datalogger for every data value you can log, and record the log for a good bit..... perhaps this would lend more of a pattern in the data results than you are seeing now. If you put it up on all 4's I'd suggest having someone inside the car as a safety measure to cut off the ignition in case the unlikely even it would fall off. What we're missing here is more observation "DURING" the problem time. If you can increase the amount of time you have to observe the problem perhaps something NEW will pop up or be seen.

I'm going to try getting my hands on an oscilloscope first. This would be a good test to do while using the oscilloscope.

[ProwlerGT]

Originally Posted by DoctorDex

Tried this already, they check out.



I'm going to try getting my hands on an oscilloscope first. This would be a good test to do while using the oscilloscope.

I've been trying to locate a fairly affordable oscilloscope myself on Ebay.