Rough idle when A/C is OFF

[5speed]

Hey everyone,
So my baby is having another issue. Whenever the car idles with the air conditioning OFF, the whole engine start shaking every second or two. It's enough of a shake that the whole car shakes. No CEL light comes on, but it sounds like it's almost misfiring. My mechanic just cleaned the IAC and I just cleaned the MAF + the air filter. Ran a tank of seafoam and have been running 93 octane gas for the last few tanks. Also tried disconnecting the battery for 20 minutes and even 2 hours, still no luck. Unplugging the MAF also did not fix the issue.

The only thing that stops it right now is turning the air conditioning ON, which smooths out the idle perfectly. I've seen tons of threads about this online, but nobody ever seems to answer back with what fixed the issue. As always, any help is appreciated.

 

[trombonedemon]

Hey everyone,
So my baby is having another issue. Whenever the car idles with the air conditioning OFF, the whole engine start shaking every second or two. It's enough of a shake that the whole car shakes. No CEL light comes on, but it sounds like it's almost misfiring. My mechanic just cleaned the IAC and I just cleaned the MAF + the air filter. Ran a tank of seafoam and have been running 93 octane gas for the last few tanks. Also tried disconnecting the battery for 20 minutes and even 2 hours, still no luck. Unplugging the MAF also did not fix the issue.

The only thing that stops it right now is turning the air conditioning ON, which smooths out the idle perfectly. I've seen tons of threads about this online, but nobody ever seems to answer back with what fixed the issue. As always, any help is appreciated.

Start with the basics and check spark plugs for moisture and vacuum leaks around the Intakes and throttle body. If you can rule out fuel and air, the next steps would be firing of the spark plugs.

 

[wmburns]

+1 on above. Especially the part about moisture in the spark plug well.

A couple of things to add. When the battery is disconnected, the PCM forgets all idle trim settings and must re-learn them. So if disconnecting the battery to resolve an issue may actually make it appear worse. At least for the time it takes the PCM to re-learn.

Which brings up another possible theory. Loose battery terminals. Let's say the battery terminals are loose and making intermittent connection. Each time the cables come loose, the PCM resets. This starts the re-learning process over.

Guess what? A rough idle is EXPECTED after a PCM reset.

What is the idle with the AC off and with the AC on?

Inspect the AC clutch. Look for a worn out pressure plate. Very common problem.

 

[5speed]

The idle is perfectly smooth with the ac on. Once I turn it off it becomes rough and the engine has a slight misfire. If I accelerate to 1500 rpm the shaking and misfire still occurs.

 

[5speed]

Ok so I checked around for vacuum leaks. One of the IAC bolts was loose and it had a slight leak there. Tightened it and there is no more leak but the issue remains the same. I couldn't find any vacuum leaks anywhere else.

Gonna have my mechanic look at sparks next - I don't have the tools to get to them. Although if it was a spark issue wouldn't it show up with the AC on and off?

 

[wmburns]

Did you look at the AC clutch?

Check the acc belt and all pulleys for excessive run out. Look at the acc belt tensioner. Is it constantly moving?

Look at the main crank damper. Is it wobbling and altering the belt path (run out)?