bad idel on a 1992 buick road master

I have a 1992 buick roadmaster with a 5.7 v8 in it i know it have a noisy lifter but when it is a operateing temp the idel gose up and down and it runs realy rough but at cruseing speeds its fine. i tested all the sensors and stuff they seem fine i just cant figure out wahts going on pleas help

 

Couple of things to look at.

Check for vacuum leaks and make sure that the egr valve isn't staying open.

If the cap & rotor is old, change that. Yeah....it's not alot of fun.

Take a look at the injector spray pattern. If it's not a fine mist, new injectors might be in your future. When I changed my Dad's on his '93, it made it run like a new engine. At the very least, run a bottle of Techron (by Chevron) in the gas tank when filling up.

If you have access to a scanner that will show you what the sensors are telling the ecm, keep a close eye on how quickly the coolant temp. sensor reacts. Same goes for the O2 sensor readings, cross counts, etc.

If the throttle body is dirty, give that a good spray. If so, undo the IAC valve and give that and the passageways a good cleaning.

 

'92 should be a Chevy tbi 350. So cap/rotor isn't a big deal to do.

Check for vaccuum leaks

Change fuel filter

Check for correct battery/alternator voltage

At idle the motor spins lower rpms so the charging system will put out lower voltage. That means that the fuel pump gets less volts and in turn puts out lower preasure. As the rpms come up the alternator spins up and everything comes back up to normal. That's the easiest thing to check and will get you a baseline on where to look next.

 

I have a 92 Roadmaster, the previous owner had a simliar problem, the intakes are know for leaking at the heads.

 

i just put an o2 sensor in yesterday. didnt do any thing I do have coolant leaking from the intake but im replaceing the lifters anyway so i gess when i put all new gaskets on i will see if fixes the problem.

thanks for the help
-jake