90 DeVille Rough Running Problems

I have a 1990 Deville with 200 k and it has been running rough when accellerating, Seems to be good when cruising. Before this problem, the car ran perfect. I had it in 2 garages and they ran diagnositics and did not get a problem reading. It is getting worse and would like to suggest to the mechanics what to check for or replace.

What you describe is usually do to bad spark plugs, wires, cap or rotor. When were they last replaced? It could be due to other factors but I'd check those first.

What you describe is usually do to bad spark plugs, wires, cap or rotor. When were they last replaced? It could be due to other factors but I'd check those first.

Thank you, I checked and that was done about 1,500 miles ago.

I think I solved the mystery. It was a combination of things. Along with the tune up the last time (1,500 miles ago) I had the oil changed. The shop put in 5 W 30 oil. The lighter oil allowed a slight lifter knocking. The engine noise sensor was picking up on this and changing the timing. This caused the engine to run retarded and this contributed to using a quart of oil. (First time it ever needed adding oil between changes) The lower oil added to the knocking and more timing change.

I added a quart of straight 30 weight oil and that stopped most the engine noise. The car runs Great again. I only had one time that the car repeated the problem. It was going from a 45 to 55 mph speed zone up a fairly steep hill.

It was a different shop that figured this out for me. This shop has my confidence now for future business.

The lighter oil allowed a slight lifter knocking. The engine noise sensor was picking up on this and changing the timing.

It was a different shop that figured this out for me. This shop has my confidence now for future business.

The 4.5's used in those Devilles did not have knock sensors. So it must have been something else!

Not till the Northstar were there knock sensors, the whole story with the oil deal is a stretch at best. Good info Caddydaddy