91 GMC pickup running rough

Hello all, we've got a 1500 with a 350 in it with 240,000 miles. It fluctuates about 200 rpm's at idle and cuts out/hesitates when getting into the throttle for quick response. I changed engine oil today and the exhaust was hot, but the catalytic converter was scorching hot, 15 minutes later after truck cooled down, I felt exhaust pipe and it was real cool but converter was still fairly warm. Do they generally stay hotter than the rest of the exhaust or is this the possible problem? I've changed fuel pump, air filter, spark plugs, and distributor cap/rotor.

Anyone know where to find a good used duramax? 2500 4x4 auto extended cab or 4 door with less than 100,000 miles for 15 to 20 grand.

Thanks

if it still has the original cat. and that many miles it is sure to be stopped up. universal cats are cheap enough that it won't be too hard with a welder but most people gut them or have a muffler shop put a new one on. I have seen people put straight pipes in place but here in Ga. the emission laws will prob. move farther south so I just keep them if it is required.

Check the distributor. remove the cap and check for a white powder buildup. To check the strength of the pickup coil; disconnect the tan/black wire, start engine and with a timing light see if the timming advances when engine rpm goes above 1700 rpm. If no advanced the pickup coil is weak. also check for wear in the distributor shaft.

would a plugged cat make it run rough? It seems sometimes that at an idle it smells rich.

you have a stopped up catalatic converter

Check O2 sensor first. Is there a lot of soot in pipe or black smoke? Another indication. Never seen a plugged CAT. Cut a few open. Please post what happens. Dave

A catalytic converter that is working right gets pretty darn hot, and the ceramic substrate will hold the heat for a while. In my experience with a plugged catalytic converter, the engine would start and idle more or less normally, but the engine absolutely would not rev up to any RPMs and was way down on power. The car would not attain highway speeds and the engine got hot when I tried to go fast at all. When I decided it was a plugged converter, I unbolted the converter from the exhaust system and then tried to run water through it. The flow through the converter was almost nothing when I filled the intake end with my garden hose, and I wondered how the car had run at all. I "improved" the flow through the converter with a large screwdriver that I poked through the ceramic inside, and almost all the ceramic ended up falling out. Then I bolted the empty case back into place in the exhaust system and drove the car another 2 years. It picked up several miles per gallon gas mileage and would rev freely.

However I live in an area where so far we do not have to have our vehicles tested for emissions. I would not have dared to remove the guts from my catalytic converter if I thought it would have to be examined, since the case became nothing more than a large tube. We were really bucks down in those days, too.

Your 91 GMC probably has throttle body fuel injection. After all those miles, the injectors may be worn out. Or you may have a vacuum leak somewhere that is leaning out a cylinder or two. Do you get any trouble codes? It might pay to have the problem analyzed professionally. I sure would not advise removing the converter and/or destroying it unless a plugged converter is what the experts come up with.

By the way, 240K miles is kind of amazing. You must be doing something right...most folks get lots less before they have to put an engine in almost any gas pickup. Good luck!

Take the air cleaner off and take hold of the throttle body and twist it to see if it is loose. They are know to loosen up and run bad. If it is loose just take it apart and replace the gasket,1 hr job. The cat will run about 1200 deg.

Is that a TBI engine? If so, replace the gasket between the throttle body and manifold. I think you've got a vacuum leak.

Ha, I had it analyzed professionally and they said it was the fuel pump and injectors, replaced them at 600 dollars, ran somewhat better for a week then terrible. It is a TBI and as for the miles its just a chevy. It has had 1 tranny, brakes a few sets of u joints, just had original spark plugs changed a month ago Ill check this throttle body. Thanks for the good input.

I"ll vote for the tbi baseplate gasket. If its bad enough the brakes will feel spongy too from lack of vacuum.

If your cat is bad, it's because your engine has been running too rich. Find the real problem, then if the cat is plugged up, fix that also. Of course, fixing the cat problem depends on if they sniff your exhaust how much you want to spend.

Seen alot of problems w/the fuel press reg on these eng. Hangs on the side of the injector assem. under the plenum. Can be replaced sep. Also if you just finished a long trip a small piece of carbon may be stuck in the e.g.r. (cleanable). Problem 1 may or may not set an 02 code, prob. 2 should set an e.g.r.code. If it has good mid-full throttle takeoff, conv. should be ok. let us know.

I had this same problem with my 91 and it turned out to be $14.00 water temp sensor. it seems that they leak by and foul up the sensor and then this causes the ecu to call for choke. I think if you change it out it may solve your problem
mike

I had this same problem with my 91 and it turned out to be $14.00 water temp sensor. it seems that they leak by and foul up the sensor and then this causes the ecu to call for choke. I think if you change it out it may solve your problem
mike