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I have a 1999 pontiac bonneville sle. About 120000 on it. It was giving me problems for several months,when you accelerated from a stop and or pushed the gas down more than half throttle it would backfire (through the carborator I think) . At one point while testing I held the throttle down and after maybe 1 minute the catalytic got red hot. It finally gave me a code for random engine multiple misfire. And a code for cylinder number 6. My local autozone told me that it was probably a coil pack that wasn't firing right.I bought a new one and the car ran great for about a month.and then one morning it will not start..it turns over but that's it. It seems to be getting gas and spark.(by I used the old school spark plug on the block test. And it seems to be ok??.. any ideas anyone?? I also checked the ground bis on both side inside the car and they look good. I will take any ideas or suggestions!!!!... please help!!!! Also my #1and 2 plugs had alot of dark carbon on them. Enough you couldn't see any white on them..(those were the only 2job plugs I looked at.)

THese cars do not have carbuerators. When has the Upper intake Plenum and lower gaskets been changed? How old are the plugs and wires? We need more info to assess the issue. I do not know anything about catalytic converters so someone will chime in on that shortly.

I'm sorry. I meant throttle body. The plugs and wires only have less than 15000 on them.. that was actually the first thing I replaced when it started sputtering. I have put about 45,000 on the carand to the best of my knowledge the upper or lower has never been changed. I was reading that it seems to be a fairly common problem with that 3800 motor. Is there any way of checking before I tear it all apart?

Are you losing coolant

I am losing some coolant. A very minimum amount, maybe an inch from the overflow every 2-3 months

It's possible the cats plugged since it's getting red hot. Or is the exhaust crushed or bent somewhere?
Is there a lack of power?
While it's parked and running have someone push the gas pedal and you check to see if the flow of exhaust coming out the tailpipe feels like it's enough.
There are others here who would know better then me. I was just throwing that out there because I've had plugged cats in other vehicles and those were just a few of the same problems.

Check the fuel pressure regulator vacuum line. It should not smell like gas. If it does, or has gas pouring out, replace it.

Also, what brand and type of spark plugs are in there? If they're Bosch spark plugs, replace them immediately with Delco, Autolite, Denso, or NGK plugs. 3800's are known for having a violent dislike to Bosch spark plugs.

** update**** after loosing coolant on a regular basisfor many months. I realized that my coolant is almost all gone!! I also borrowed a pressure gauge( compression checker) I tested cylinder number 16 and 3. I seem to only get about90 psi on cylinder 1. I have about 110 on number 3. It is also backfiring through the carburetor (I think) and leaving a strong smell of unburnt fuel and also some white smoke after the backfire. Anyone have any suggestions on what to do next??

http://www.pontiacbonnevilleclub.com/fo ... =53&t=1061

http://www.pontiacbonnevilleclub.com/fo ... =53&t=3591

Read those. As stated before, there isn't a carburetor on these cars, you're referring to the throttle body.

Definitely sounds like your uim and lower intake gaskets took a crap. What 00Beast posted should help you out 100%. The longer you wait the more damage your engine will take. Better be safe than sorry

Don't overlook the cat though, regardless of the intake issues you find. The fact that your cat was getting that hot may have damaged the internals. If they burn off and come loose they can become a restriction in your exhaust and cause very noticeable performance issues. Start simple by tapping a hammer or something solid against the cat and listen for any internal rattling noises. That is an easy sign of cat issues.

Totally skimmed over the cat part. That needs to be replaced.

To me, a red-hot cat indicates an over-rich condition. Besides your possible intake coolant leak, I would suggest a possible bad fuel pressure regulator. Any check-engine light? Any codes pulled?

Wouldn't a plugged cat run hotter than normal though?

I would think that this might explain why he has experienced the "backfires". With a plugged cat the exhaust gasses can be backing up in front of the cat and the pressure builds up in the system, causing the "backfire" to happen after 1/2 throttle is released.

This is of coarse on top of the UIM and LIM from the sounds of your coolant leakage. While doing the LIM and UIM, I would check the fuel pressure regulator out as descibed earlier.

Just my 2 cents.

The misfire under a load or during heavy accelleration sounds like a possible crank position sensor to me. I too am interested to know if he has any codes. If there's a question about the cat, simply disconnect it at the downpipe and see if it throttles up better. You'll upset your neighbors, but it should be a pretty quick way to see if this is the problem. Does this car have a tachometer? A good indication of a bad crank position sensor is when the tach stops working or becomes erratic when the misfire happens.

Oldman wrote: Does this car have a tachometer? A good indication of a bad crank position sensor is when the tach stops working or becomes erratic when the misfire happens.

That's a good tip right there.

When my '99 went down (no start) it had no codes, and as it turned out, the crank position sensor pickup was clogged with crud. Used a garden hose with a small nozzle to squirt behind the balancer for a coupla minutes, slopped up the connection with dielectric grease, plugged it back in, and it fired right up.

Might be worth a try. Cheap enough.

Also, in my experience, a car running lean will burn the cat converter, as well. Whichever it is, rich or lean, there's definitely a fuel mixture problem there.

Bing wrote:Wouldn't a plugged cat run hotter than normal though?

I would think that this might explain why he has experienced the "backfires". With a plugged cat the exhaust gasses can be backing up in front of the cat and the pressure builds up in the system, causing the "backfire" to happen after 1/2 throttle is released.

This is of coarse on top of the UIM and LIM from the sounds of your coolant leakage. While doing the LIM and UIM, I would check the fuel pressure regulator out as descibed earlier.

Just my 2 cents.

That was my point about the cat.
You hit the nail on the head. IMO

Change the upper plenum gasket. Same thing happened to my 99SLE. I was loosing coolant and car was loosing power at mid throttle. I don't think you will need a new plenum.

No, you definitely need to replace the plenum, whether it's leaking or not. It's not if it will fail, it's when.