Slight stumble runs warm - Third Generation F-Body Message Boards

lincolnpark773 · 05-18-2007, 02:53 PM

I have the stumble when my 3800 II (60K miles) is fully warmed up. I mean slightest, it only happens on light to medium acceleration. The motor tends to warm up quickly relative to the temp outside (Chicago) and doesn't cool down as fast as I think it should when the fans run. Like most GM cars they work great when they are new but a few years later they run hot and the fuel economy isn’t what it used to be even with a tune up. It idles and runs smoothly. Here’s what I did and the results. I usually run Shell gas. The fuel pump was tested statically (at idle) and came up good (I think it was 60 or 65 psi), the fuel regulator was tested here again statically and was 40 or 45 what ever was considered normal.

Change: New plugs, wires and air cleaner:
Result: This helped but only eliminated half of the stumble.

Change: Took car to shop, mechanic ran car on cigar box sized Snap On hand held analyzer. He told me the water temp sensor was bad. After thinking about it I decided the sensor had to be shorted, open or out of range.
Result: Even more of the stumble eliminated but a little still there.

Change: Incoming air temp sensor replaced. I measured the new and old and got identical resistance measurements so I knew this wasn’t the problem, but I wasn’t about to bother to return a $15 part.

Change: Mass air flow sensor replaced.
Result: Less stumble but not eliminated, constant pinging regardless of fuel quality or temp all but gone.

Change: Throttle position sensor changed. Old unit had three times the resistance of new one at idle and different again at full throttle. I knew this think was messed up.
Result: Here again. Even less stumble, better fuel economy completely eliminated all pinging even in the worst conditions with 87 octane gas.

Change: Manifold absolute pressure sensor. Here again different resistances. I checked if with an ohmmeter while using a vacuum gauge to simulate idle vacuum.
Result: Better still but still the slightest hesitation.

Change: New radiator. The old radiator was .625” thick @ 24fins/inch. Replaced with 1.125” same fins/inch.
Result: Ran cooler but on hot days 90 degrees plus the fans run constantly and the temp gauge is at 225 degrees. The factory has the fans turning on at 212 degrees and off at 204 or so, so this tells me the system is operating beyond capacity at 225.

Change: Exhaust manifold oxygen sensor.
Result: Better fuel economy, runs even smoother. Still tends to heat up the instant you pull up to a light or get off the highway. This tells me the radiator is the same temp as the engine. In other words nearing capacity. I would think on a 60 degree day the system would have some reserve. Even my wife’s Saturn could easily pull the temp down and keep it there once the fans turned on even on 100 degree plus days.

As far as I know I only have one more sensor to change, the oxygen sensor on the catalytic converter.

The way I see it the only other possibilities are:

Dirty fuel injectors?
Bad catalytic converter?
EGR valve?
Intake manifold leak?

I’m not really knowledgeable about how this motor controls spark timing and if that could be an issue or is the cam timing could be off and causing headaches.

Any insight would be appreciated.