2004 Grand Prix GT 3.8L intermittent near-stall/stall while driving: P0107/P0420

[ek245]

I'm trying to track down an intermittent stalling/near-stalling issue with my 2004 Pontiac Grand Prix GT. It's the naturally aspirated 3.8L Series III 3800 (non-supercharged) with about 245,000 miles.

The car has only completely stalled once. The other times, it acts like it's about to stall but catches itself and recovers.

When the problem occurs, it can happen both while maintaining a steady speed and while accelerating. The tachometer drops down to around 1,000 RPM, and then the engine catches itself and comes back up.

The one time it actually stalled, the engine cut off completely and the tachometer dropped straight to 0 RPM.

There isn't much warning beforehand. It doesn't seem to gradually lose power; it feels more like the engine suddenly loses RPM and then either recovers or, in the one instance, completely dies.

I've had the same problem several times now.

The first time, I brought it to a shop and they had the car for 11 days. They weren't able to reproduce the problem. They did some sensor cleaning, but apparently didn't find a definite cause.

I got the car back and had it for about 12 days before the same issue happened again, so I brought it back to the same shop. They aren't charging me for the diagnostic this time because it's the same issue returning shortly after the first visit.

The only trouble codes I've personally seen are:

• P0107 — MAP sensor circuit low input
• P0420 — Catalyst system efficiency below threshold

Both codes have been present both times I've brought the car in. I haven't yet asked the shop for the complete list of codes they pulled, including pending/history codes, which I realize I should have done.

I'm trying to figure out where I should have the shop focus their diagnosis. Since the engine RPM sometimes drops dramatically but recovers, and the one time it actually stalled the tach dropped straight to 0, does this point toward a possible intermittent loss of the crankshaft position/RPM signal?

Could the P0107 be related to the near-stalling, and is simply cleaning the MAP sensor enough to rule anything out, or should the MAP circuit, wiring, and connector be tested?

I'm not looking to just throw parts at it, especially since the shop hasn't been able to reproduce the problem.

What would you guys check/test first on a 3800 with these symptoms?

Thanks!

 

[showme]

I don't know if you figured this out yet, but I will say that we were having problems on a 2008, and I changed the MAP sensor and spray cleaned the MAF sensor, and it cleared a lot of things up. If you do install a new MAP sensor (VERY simple), I would suggest installing a new pcv valve, which sits directly under it, just to make sure it's not clogged with oil. They're cheap and easy to change.

Secondly, my daughter's Ford Expedition started going into limp mode, but as soon as she would pull off the road and shut if off, it would work fine again until next time she drove it. I scanned for codes and the only code I got was something about catalyst efficiency, also. I called my diesel mechanic who does work on my Silverado's Duramax, asking him if he thought she needed new catalytic converters, and he said no, he didn't think that was it due to the fact that if it was a catalytic converter problem, it wouldn't just start running right after just turning it off and back on. Since he doesn't do diagnostic work anymore, I took it to a local mechanic who did a diagnostic test on it, and he told me it looked to him like the information was pointing to a throttle body problem. So, I installed a new throttle body and it hasn't had any trouble since then, which was about 2 months ago. My grandkids had a problem a few years ago on their '08 Grand Prix and I installed a new throttle body on it, and it's been running well ever since. Hope you've got it taken care of, but if not, maybe this will help. Good luck!