2002 Pontiac Montana Cuts Off At Higher RPM’s

2002 Pontiac Montana 3.4L OHV had oxygen sensor code but replaced it and cleared. Head gaskets were leaking as well so replaced (milled heads, pressure tested, valve job, replaced 4 bent intake valves), also replaced plugs, wires, coils and ignition module and O2 sensor, cleaned throttle body. Car cuts out at a specific RPM in relation to how warm the engine is, when cold there is no problem. As it warms, the max RPM goes down, when fully warm max rpm can be as low as 1000, let it cool and max rpm will be higher until it warms again. Feels ignition related and not fuel related.

Condition can be created by driving or by brake torquing in gear (power stalling)or, if warm enough, while in neutral – i know gm’s are limited in free rev rpm to about 3-3500 rpm. Does not throw any misfire codes or any other codes at this time. It is possible to drive car right up to the rpm point where it cuts out but go over that point and it cuts out and tach goes nuts bouncing around – I assume it is losing Tach signal. If you mash throttle, it bucks like it wants to run but only up to a point, cuts out, then re-fires, cuts out , etc. like it had a rev limiter only much harsher. If you then get out of the throttle and let rpm’s drop, it runs again but only up to that specific point – it all seems heat related and ignition related. The warmer it gets, the lower the rpm maximum is.

A couple quick notes before I get to your actual problem. First of all, the heads on the 3.4 V6 engines are NOT capable of being milled. If they were off bad enough to need milling, you should have replaced the heads. This will lead to more problems for sure. And you also say that you replaced 4 bent intake valves? If something catastrophic enough happened to bend valves, which I’ve never personally seen on these engines, that as well would have warranted head replacement, if not the entire engine!

Now, on to your problem. From what you describe, my first thought would be a clogged catalytic converter. If plugged bad enough, as it warms up it chokes off the exhaust flow so badly that maximum rpm comes down further and further until it barely runs. You can attempt to check this by disconnecting the exhaust pipe at the back manifold so it exits before entering the converter. If your problem doesn’t act up after running for a while, you know you need to replace it. If that’s not the cause, I would look into an ignition component or module/sensor failing as it gets hot. This could be anything from an ignition module, to a bad coil or wires, or even a crankshaft sensor or camshaft sensor. A scanner would come in handy to actually watch the live data stream of the sensors and see what is falling off. But I would really check that catalytic converter for sure. Good Luck!

Pontiac 2005 Montana

 It developed a miss on acceleration and set error code P0335, crankshaft position sensor circuit ”A”. I found that the wire for #4 cylinder had been rubbing on a hose and was worn about half way through. I replaced all wires and plugs. Miss was gone and ran great but still sets the P0335 code.

2 questions:
1. Which crankshaft position sensor is the ”A” one? Behind the pully or the other one?

2. Have never seen the crank sensor go bad after 3 pontiac vans and 900,000 miles. Could something else set this code. Could the ignition module or coil pack go bad because of the bad plug wire?
It also now only stays running on the 2nd attempt to start. Fails the 1st and always starts on the 2nd try. (Not a fuel related problem)

Crankshaft Sensor A is the one located behind the crank pulley. Arcing ignition wires or other secondary ignition components such as a cracked coil can set the code P0335. You might want to check that area first before replacing a sensor. Also, at times the wiring down to the crank sensor has been known to become chaffed on the block and cause a shorting condition. If all wiring checks out ok, and there are no other ignition failures that you can detect, you will most likely need to replace the crankshaft sensor. Rarely, the vehicles computer is to blame for the code but without proper checks it is really hard to diagnose a bad PCM without knowing you have a good sensor and wiring to it, and no other ignition problems.