Caprice hesitation and stalls..again..

Yeah, I know there is quite a few threads here about cars that hesitate and stall, but my take is slightly different:

My 92 Caprice Classic is a bone stock original 25k absolutely flawless car. It`s like brand new, and I do suspect it`s been sitting a lot, but does also appear maintained.

When I picked up the car at the port and drove it home the car did hesitate. On the trip over here the tank was about empty, so I figured maybe the gastank had condensated water in it. I poured in a few bottles of condensated water removal stuff, and the car actually gradually improved. I drove the car for 10 hours back home with no more problems.

Now the car has been sitting a few months, and the problems I have now leaves me clueless:

The car starts perfect when cold. Runs and idles perfect. Drives and picks up perfect. After 5 minutes driving it will start to hesitate, and gradually stop. In other words after it has warmed up. The real weird thing is that is cranks funny too.

After the car sits for a few hours and cools off, it is just perfect again. Drives normal, idles normal and cranks normal.

I did try to pull any codes from the OBD, but there was no fault codes.

I am used to play with older cars..that make sense to me...but this one leaves me baffled.

Any suggestions??

Thanks,

You said 25000 miles so are the plugs,wires,fuel filter and other tune-up items all still original?Could there be a restriction in the exhaust that is clear when cold but begins to slowly restrict as you drive it?

My guess is that most of the tune-up items are from the car is new. Since the car is normal at cold, my thoughts are that these items are OK. Whatever is causing the problem must be heat-sensitive??

I don`t know what could restrict the exhaust when hot. Also, the strange cranking..it`s just like trying to start a car with the timing way off...

Have you touched the timing at all?When you picked it up at the port,you said it ran fine except for some hesitation and you have only driven it for about 10 hours since?Were those 10 hours all short trips or did the engine have time to thoroughly warm up?I am thinking its the catalytic convertor but the other members will chime in shortly.

Check the ground at the thermostat. GM decided that it made sense to put a ground in conjunction with two different metals and water.

The computer only uses the sensors when it goes into closed loop, about 5 minutes after initial startup. Another culprit might be the ignition module in the distributor (heat sensitive). If you replace it use heat sink compound, not dielectric grease.

The PCV valve is a seemingly innocuous part, but can cause strange conditions if not working correctly. Check the port in the TBI as well.

If it was the catalytic converter, you would have the symptom at all times. I had one clog, and it did not matter if it was in closed loop or not, it would run even more doglike than normal. I was lucky to get 45MPH for top speed.

Last, check the operation of the EGR valve.

Yeah, my gut says check the EGR and PCV first. It's definitely not a clogged cat, because as Fred says, when they go, you lose EVERYTHING speed wise.

Fred helped me with my car running rich.
That stupid ground at the thermostat was the culprit. I cleaned the ground and ran better.
I ended up replacing my entire cooling system now it runs great.

Onto my opinion....
I have had a similar issue with a past vehicle. It ran great when cold, but once warmed up it would run like crud. Eventually the issue got worse and would just stop running altogether.
It turned out to be some part inside the distributor. I dont know the part exactly, it was 20 years ago.

Any views on the weird cranking? When the problem occurs the cranking is just like when you have totally wrong timing...

I would look at the ignition module in the distributor. Another part you can consider is the ESP module on the relay center at the rear of the engine compartment by the antenna. The two of them control the spark, and timing.

I just can`t believe it: I cleaned the ground at the thermostat - and the car stills hasn`t had any problems...

crossing fingers here...gonna go for another test drive..I will keep you updated..

That was that. It was the ground at the thermostat. Drove the car for about an hour and performed a few WOT`s too. Everything works as it should.

Amazing really, that such a small detail would cause such a problem. I guess the sensors require proper ground to function.

Thanks Fred for great input!!

The ground you cleaned is for the O2 sensor, and the computer. It was nice of GM to put a ground with two dissimilar metals right next to water. At least it makes sense to me anyway, Ha.

Here I am again: after cleaning the ground on the thermostat and having the car run fine for a while, the problem has reoccured. I cleaned the ground several times real good, and still get the hesitation after warm up. Car is fine when cold.

Now I have starting the sensor changing routine. BUT, when I changed the fuel filter today, the car ran better than for a long while. And it ran nice for a long while (30 min), until the symptoms reappeared. I was even so confident that the car was fixed - I sent a text to my buddies that all was good...shouldn`t have done that!!

Anyway, car has 25k miles, and has probably been sitting a lot. OBDI does not throw any codes, so that indicates fuel related. I am suspecting maybe crud in the tank, but the old filter was not clogged, and I blew thru it easily.