98 Malibu 2.4 rough idle

Approx 99,000 miles.

Car just started rough idle / miss at a stop sign and or in the driveway. Happens when cold or warm. Almost want to stall, if you kick the idle up to 900 or higher, it sounds fine. No OBD codes.

The cylinders are all within 5lbs of each other at 235lbs of compression. Vacuum is below normal at idle. A shop said it was dumping a bit more gas at idle into two cylinders than it should be and vacuum was a little low and it was missfiring on all cylinders at idle. They tested the O2 sensor and opened and closed all sensors with their test machine.

Their recommendation was a full tune up. I had changed air filter, OEM plugs, new coil and sparkplug boots. New egr valve, fuel control valve, idle solenoid,
o2 sensor, gas cap, and radiator cap.

All the plugs looked good, no foul or burning on old plugs.

The car seems to have normal power when driving and doesn't make any abnormal sounds.

I am lost and would appreciate any suggestions. Would a CAT converter being plugged cause rough idle and low vacuum maybe? If so, any test for it/

Thanks for having a service like this.

Rod


Get a pressure gauge and remove oxygen sensor hook it up, a reading of 1.25 and higher will indciate a clogged exhaust system creating the backpressure.

Or remove the exhaust pipe at the exhaust manifold-what's your ears. Note how the engine reacts.

Vacuum Gauge reading: high reading when engine first started, falling back to 1 or zero and gradually increasing to 15 or 17.

Any certain gauge I need to use?

I had taken out the O2 sensor and listened, seemed to still idle rough. Maybe that hole isn't big enough to make a change.

When you put Vacuum Gauge reading info, does that mean it should do this when I test the vacuum?

What about teh MAP or MAF sensor needing cleaned? I don't think it has a MAF though.

Thanks
Rod