1990 EJ22 4EAT rough idle after warm up, code 24

[notnowles]

I am having a problem with a rough idle after warm up, which sets code 24 (Idle Air Control Valve, I believe). It will only do it if I let the car idle in P or N, never in D or R.

 

In D or R, the idle speed steps up and is very smooth.

 

By shorting the plugs one at a time, I have figured out that it is the front cylinder on either bank that is missing when the rough idle is present, and when this is the case, the “Check Engine” light appears.

 

I sprayed “Intake Cleaner” into the air inlet hose of the IAC valve, but it seems as though the valve is working, because the car idle speed does step up and down as I believe it should.

 

I am suspicious of a vacuum leak under the manifold, but I don’t hear or see anything that would be a clue.

 

I have been living with it for a while, because the idle is fine as long as the car is in gear. However, if I let the engine run in P or N, the idle speed will remain at just under 1500 for a bit, then abruptly drop to just below 1000, at which point the missing and check engine light appear.

 

I have a used IAC valve which I could clean and install, but again, it seems to me that the one in the car is working, despite the setting of code 24.

 

Any thoughts/suggestions are welcome and would be appreciated!

[naru]

You might be right about the vacuum leak.

 

I would swap IACs.I don`t think code 24 would set just because of low rpms,but,I may be wrong.

 

Nothing more profound at the moment,sorry.

[notnowles]

Naru, thank you for the suggestion to swap IACs.

 

I think that's a good idea.

 

I will get over the dealer next week and get a new gasket so I don't run the risk of creating a leak where there wasn't one!

 

And I need to clean up the extra to give it its best shot at helping me.

 

Thanks again!