So I have a 85 firebird v6, 2.8. It ran low on oil and partially locked, and I had to get it towed home. Changed the oil and filter, new oil added with some engine restore and a little seafoam for extra lubrication, and it starts and runs. The problem is, the idle starts to go down after it gets up to temp, and it’ll start dropping so low that it almost stalls before revving back up. If I drive it at temp, it’ll drop idle and die at stop lights, and really slow crank until it starts again. I’m confused because it’s clearly not seized, but it runs like it’s got sticky pistons. Does anyone know why it’s doing this, and how I might fix it? I haven’t driven it since the last time it kept stalling, to try to minimise further damage, but it’s my daily and I’m having to rely on my friends truck

 

When you ran it low on oil, the pistons wore badly, really really REALLY badly, to the point that you've now got excessive piston to cylinder clearance. And so you'd think, if the clearance is too large, why is it getting tight? Well, when it runs the heating caused by the very hot combustion gasses leaking past the pistons causes them to expand and get stuck in the cylinders. Let it cool off, pistons shrink a bit, and it'll run again till it gets warm and the pistons begin binding again.

Sorry bud, it's new engine time, start here

https://www.thirdgen.org/forums/v6/6...-do-3-4-a.html

 

Now you have an excuse to V-8 swap it...

 

How low on oil was it? If it was about empty, you probably smoked the bearings, surprised it's not knocking.
if it was maybe a quart low, probably no big deal.
Have you checked fuel psi?

 

Main bearing failure will not cause knocking but will put a huge resistance into the rotating mass making the engine harder to turn over.

Bad main bearings like that need more than a simple bearing replacement. Sounds like you need a new engine. It wil be cheaper in the long run.

 

The OP describes the engine getting tight when it comes up to operating temperature, his exact statement;

"The problem is, the idle starts to go down after it gets up to temp"

"If I drive it at temp, it’ll drop idle and die at stop lights, and really slow crank until it starts again."

If it starts normally when cold, and gets tight when hot, it's the piston/cylinder fit that is compromised.

Bottom line = new engine time.....

 

For future reference, SeaFoam is NOT added lubrication. If is a mixture of light oil, petroleum naptha, alcohol, and water.

https://www.greenpartstore.com/asset...afoam_msds.pdf

Research the constituent CAS numbers and it will become apparent that the product is, at best, very thinned-out oil with a high percentage of solvents.

 

If it was a new engine with improperly sized pistons, sure they could expand enough to stick.
old engine with miles on it, no, its not the pistons.

need to know how low it was on oil, the fuel psi when its hot and the inj ohms when hot and cold. A quart low isn't the same as empty.
If the engine is mechanicly fine, it sounds like something electrical is breaking down under some heat.