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1999 Pontiac Grand Am

I think I am experiencing the DexCool consequences on my Pontiac over the last several months. I was unaware of the lawsuit settlement and missed the October 2008 submission date. Please help me figure out what is going on. It has 140,000 miles on it.

I am getting P0105 (Map Ciruit Issue) and P0300 (multiple misfires) error codes. I put ZMax in my fuel and the misfires appear to be stopping. I also found that the map sensor vacuum hose had a hole in it and I replaced that but I am still idling rough. My thermostat housing, which I have replaced 3 times in the last year and a half has a pinhole leak despite my resealing it.

So this is where I am at. The car is fine till it warms up. At that point I hear noise from underneath the car, like the transmission or drive train is rattling, and it accellerates slowly until I hit about 20 miles an hour, then it is fine. Once I have been going for a minute I experience occasional power loss. Just occasional short stabs, but not as pronounced as before I added the ZMax. I am reading a lot about the intake manifold gaskets failing on Grand Ams, but don’t want to dig into it until I am sure that is the problem. I should probably get a new car, but thanks to the economy I was laid off and haven’t found a new job yet. Please help me figure this out.

The rattle may be the heat shield on the catalytic converter has come loose. Wrap a wire, or band, completely around the catalytic converter. That will quieten it, and it (the wrap) will last for years.
Change the fuel filter, and check the fuel pressure with the engine under load.
Remove the spark plugs, and examine them. It’s a good idea to just change them. Here is a picture showing different engine conditions’ affect on spark plugs:
Is there a problem with engine cooling?