1990 Chevy Caprice Classic

Car cold starts are fine. After the engine warms up, putting engine under load causes it to stumble/hesitate really bad. More noticible in cold weather then in warm weather. Changed the IAC, thinking that was the problem. Didn't make a difference. The kicker here it there is no computer code showing a problem. Carb , of course, it TBI. Any ideas please? Thanks.

 

Check Grounds, to motor. You should also check out the air temp sensor! Coolant temp sensor! Something is telling ECM, that it's not up to temp. O2 can also be a problem! Stay rich or stay lean.You can ohm all of these and see what is not working. O2 is best tested with scanner, to make sure it switching.
Hope This Helps....

 

does this only occur as you leave from a dead stop or when you are at cruising speed? if from a dead stop...maybe a TP sensor starting to go...the glitch may be too fast for that older controller to "see" the drop out a set a code. they can cause terrible hesitations and it takes a lab scope to see it electrically...even a scanner won't update fast enough. ditto on temp sensors being a possibility...

 

After I put it in drive and push the accelerator down, it starts hesitating and it is hard to increase speed. When I rev it from a standstill, I can feel the engine cutting out as well. This only happens after the engine has warmed up. I can drive away fairly well after the engine is first started and still cold. There are no trouble codes displayed. After the one gentlemans reply yesterday, I did notice on the ALLDATA site in a section different where I looked before there are 2 oxygen sensors in the left exhaust, one at the manifold and one down just before the exhaust "y". A year ago I replaced the one at the exhaust "y" after I did get an o2 trouble code. At that time, I did not see the section that showed the 2 02 sensors. This problem starts to go away in warm weather. Had the same difficulty last winter until it got warm, but couldn't solve it at that time. Got better and forgot about it until this winter. I am a little slow at this so it will take me some time. Thanks for your reply.

 

Old car.

How many miles?

Vacuum drops off under load, and can be related to that somehow.

What all has been worked on replaced in recent history?; air filter, pcv valve, plugs, wires, etc.

Has air cleaner assembly been removed to see if damper that lets in fresh air after warm up opens?

That vacuum lines are not cracked including rubber and brittle plastic tube ones.

Plugs inspected to make sure plugs good and no cracked insulators? Color of plugs to see if running too rich? That wires are good? When engine gets warm, expansion can occur and widen breaks in insulators and wires. Does this car have distributor or not (if yes, more to look at that for like pitted contacts, carbon tracking, etc.). Clean around coil hookups? Etc.

Timing has not mysteriously changed.

Any stray sparking under hood at night, especially after engine has been warmed up maybe after you got back from somewhere?

(The basics.)

Unless a person has a handle on their older car, with the basics, such problems as yours can seem daunting because now days, not only is there the basic stuff to diagnose, but all the emission control related sensors involved that bring up codes. If this was me, I'd be checking out some of this stuff first as that is a good idea anyway.

When cars are under load everything has to be more at optimum.

 

Sorry took so long to reply. I'm checking the MAT this morning. A little over a year ago, I did get an 02 sensor code. At that time I did not see in another section there were 2 02 sensors in the left exhaust, one at the manifold and one right before the exhaust "y". However, when I changed the one at the exhaust "y", the trouble code went away. Then after your reply yesterday, I noticed on ALLDATA I has missed the second 02 sensor and never changed it at the time. Appreciate your help and am working on it slowly but surely.