Hey guys and gals,
A friend of mine has an early 90's Pontiac Grand Prix. I know, I know. I tried to sell her my Bronco. Anyway, it starts up and runs fine, and around town it's fine. But when you take it out on the highway for a while, and then take your foot off the gas, it dies. Sometimes it will start right back up, and sometimes it will not start. Sometimes it will start, but it will idle really rough and then die again. You let it sit for a while and it starts right back up and can be driven.
To the best of my knowledge there is no oil in water, or water in oil so I'm thinking the heads are fine. A "backyard mechanic" friend of hers said the engine is on it's last leg and to just buy a new engine.
I'm thinking it kind of sounds like a bad catalytic converter though. Any thoughts from any of you on this idea? I have seen (only once) a cat go bad and clog the exhaust so bad the car wouldn't run. My thinking here is that when the car is moving there is enough pressure to keep the exhaust flowing well enough to continue to run, but when she lets off the gas there is too much back pressure and the thing just dies. Once it sits for a little while, the pressure releases, everything cools down and it can run again. Any help here would be much appreciated as I don't want her to spend $1500 and find the engine was fine all along.

 

What controls the idle? Carb, efi? Idle control units do this if they are fubar.

I'd run a blue book value check on a pos like that before I put a briggs and stratton in the trunk let alone a new engine up front......

Also, I think there is a pressue check method for the cat, you pull the oxygen sensor and measure pressure there. Someone here described that once, and the pressure with a good cat is pretty low. Sorry, can't remember more.

 

> I don't want her to spend $1500 and find the engine was fine all along.

GMs are well known for clogging their cats, a good shop can test for back pressure.

> A "backyard mechanic" friend of hers said the engine is on it's last leg and to just buy a new engine.

Have him stay in the backyard and spend the $100 and have a real mechanic do a compression test before any engine work is done, if you are not up to doing the test. Unless it is obvious and the car exhaust looks like a crop duster just went by, I would never replace an engine without a compression test or water in the oil and valve covers.

For $1500 all you will get is a used junkyard engine because it will take at least 15 hours to replace just the engine, never mind swapping stuff from one engine to the other.

On one of my Chevy cars, I welded flanges onto the cat so I could drop it and replace it with a test pipe right after the emissions test because it would clog within a month being on the car. Even though it was in excellent running condition and threw no codes. GM cats have been this way ever since they went to the three stage pellet design, prone to clogging.

 

A rough and ready way to test for a blocked cat is, with engine cold, start it up. Reach underneath and feel the temperature of the inlet pipe to the cat and then the outlet. On an unplugged cat both pipes should be around the same temperature. If the outlet pipe is cool then the cat is probably plugged. Is it throwing any codes?

 

Check the EGR system as well. My '94 4.3L Chevy would act the same until the EGR controls were fixed (for free, due to emissions warranty).

Jim

 

Clean the idle air control and the entire intake system they got a stepper motor if it's
the motor I am thinking of and it get's notchy with age and crudded up. Check the
pvc and clean it to. The egr in those like was said get nasty and need to be taken
apart and cleaned to remove any crap in them, the line that goes to them also needs
to be cleaned. Get the car up to temp and take it for a good ride 5 miles then bring it
to the driveway and pull off a vac line at least a 1/4" one and let it run (keep rpm about
1200-1600 rpm) for about 3-5 minutes. This will make the engine run lean and really
get the catcon heated up and if it is fouled up doing this a few times test driving after
each time may clear up a partially clogged convertor and o2 sensor!!

 

Thanks for all the posts guys. I'll be looking at it this week sometime.

 

My sisters focus had a similar problem, turned out the screen on the fuel pump was plugged. I also had a customer come in to where I work last week with the same problem on the same year/model car. He asked for a fuel pump and hasn't come back yet complaining that I sold him the wrong part and asking for different parts, so you might want to check out the fuel pump as well.