Weird problem on '02 3.4L V6 auto

Rod you may know the answer to this one but it has me stumped.

Ok, first in neutral there is a 4000 rpm rev-limiter. Now car forums state this is to protect the engine, which in my mind is absurd. If an engine redlines at 5500 then it is regardless of load because the damn thing is still turning 5500 RPMs. One guy in the forum said something that made sense. He said GM put it there to prevent people (idiots) from neutral dropping the tranny at high RPMs. I had a friend that did this in an '89 Pontiac Grand Am and very soon his tranny went to crapola. So what is the 4000 RPM neutral rev limiter for? And if this is so important then why the hell does my Mitsubish Eclipse, former Pontiac Grand Am, and Eagle Talon allow you to rev to the red in all gears? Seems to me redline is redline - just don't go over it and besides the stock computer won't let you.

So is GM just protecting warranty costs for replacing drivetrains due to misuse and neutral drops or is this blown engine argument valid? Being a car tuner and having seen 3000 RPM stalls, I say the former is a bunch of hogwash.


Now for the problem, hopefully unrelated to the 4000 rpm rev limit. As you begin to accelerate the car does not want to shift correctly. If you lay into the gas it will stay in gear and just rev, but the rev's will never go over 4000. On the speedo, the redline is 5500 RPMs. I know the fuel cut-off for this car is 6000 RPMs. Why the hell is it limiting at 4000?

Second, when you merge onto the highway you must actually release gas pressure to get the tranny to shift correctly. Then the slightest change in pressure makes it seem like the tranny can't decide what gear to get into. Most of the time it bogs the engine so bad it accels like a turtle.

Third, if you lay into the gas on the highway it seems to kick out of OD, downshift, downshift and accel with no problem. However if you floor it, it goes back into the aforementioned gear that just sits there at 4000 RPM with no acceleration whatsoever.

Now if the tranny wasn't shifting and that was the problem, the car would rev out and continue revving until it reached redline at which point the computer would fuel cut. Similar to never clutching and shifting in a manual. No shift, just RPM and no power because you are out of the power band for that gear. So my idea is that the computer is limiting the rev at 4000 and thus the fluid pressure in the tranny is not adequate enough to shift correctly. Now it is possible that the tranny is downshifting too far and the engine actually does not have enough horsepower to push the revs over 4000 RPM in that gear.......but I highly doubt it. It's a Monte Carlo 3.4L V6 and while it's not the 3800, it still has about 165 to 175 hp. That's enough to wrap every gear out.
Shouldn't an auto tranny wrap every gear out if you keep the gas on the floor? It will wrap, shift, wrap, shift, etc..


Could a faulty rev limiter cause these types of tranny problems?
I've never seen this. I got into my Eclipse and ran the crapola out of it (it's a stick BTW) and it fuel-cuts at 5900 RPM. But the engine is solid, doesn't leak, and performs well even in high RPM ranges. I guess that's the diff between GM and Mits eh.

Please help anyone.

I only ever drove a car with an automatic transmission once - never again.

Is RoD still here? I cannot find his name in the members list. Great you went and banned him didn't ya. C'mon fess up.

RoD gave up this forum a while ago. He lurks over at ES sometimes though.

Non-car person here. But it sounds similar to an issue I had once that was due to spark plugs that went bad. Anyways, it's a cheap quick check.

Quote:

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Originally Posted by PJYelton

RoD gave up this forum a while ago. He lurks over at ES sometimes though.

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Scubasteve!

On my older car its a physical govenor that spins weights out to deside when to shift, but on new cars its a computer. Sounds like something is wrong with the computer that tells the car to shift and when.

This is the conclusion I'm coming to as well. I just found out prior to meeting my wife, her car was struck by lightning while in the garage of her parents house. The computer was replaced and so the current one should be covered under warranty.

If there was something wrong mechanically, it would not act this strange and could be diagnosed a bit easier. This is like a 'bug' in the car and since there is no Check Engine light, it's leading me to believe the brain is dying or dead.

On the trucks I build at work the computer tells the tranny when to upshift and downshift. If you connect the wires incorrectly to the tranny it will do exactly the opposite and probably destroy the tranny and differential ring gear as well as the pinion gear.
I suspect the computer simply sends a signal down the wire which activates the shift inside the tranny.

The 4000 RPM limiter really has me quite surprised. I've been tinkering with cars for a long time and the only damage I can see that might occur is that in neutral the engine will rev a lot faster and so the limiter might miss the limit thus over-revving the engine. But still most of these engines can be over-revved for a long time before any serious damage happens. A lot of the cars are tuned down and the RPM's limited just to prolong the life of the car and to cut down on warranty costs. Some of these 4's and 6's and even 8's can rev really high for a long time. I've heard reports of Ford, GM, Toyota, and Mits over-revving for 8,10,12 and even 24 hours before the engine blew. The rev limiter on my Mits is far diff than the one on the Monte as well. Mine will jitter at 5900 to 6000 rpm and you can def hear the rev limiter kicking in. Hers will actually bounce between 4000 and 3500 RPM which is quite strange. It doesn't even sound like a normal rev limiter does.

I say computer as well and so if it's covered, maybe this won't be so bad after all.

UPDATE:

It was a catalytic converter that was totally clogged. This was causing increased exhaust back pressure and thus forcing the computer to fuel cut the engine way before the redline. The computer did have a code in it since the O2 sensor behind the cat was not matching (according to internal specs) the O2 sensor readings on the intake manifold. However, this code did not trip the light which is stupid because driving the car like this would have eventually destroyed the engine. GM extended the warranty on their OEM cats to 10 years, 100K up from 8 years, 80K. It actually is a factory recall on this engine and so all repairs were free. Thanks GM.


The 4000 RPM limiter in neutral is a standard GM practice and it is simply to prevent idiots from neutral dropping the tranny at extremely high RPMs.