Air leak confusion

[MrScott]

Hi All,

I suspect my SC (CIS) has a large air leak -- it's idling low and rough, down on power.

Covering the air intake completely with my hand has no effect on idle. I squirted soapy solution around the injectors, intake runners, etc with the engine running and didn't notice any bubbles or change in idle.

The strange thing is if I remove the oil cap the idle drops noticeably. Doesn't that contradict the "plugging the intake with my hand has no effect" test?

I'll try the positive pressure test next, just wanted to verify I'm heading in the right direction.

Thanks,
Adriano

[Flat Six]

Cracked airbox?
Check POV?
Rubber boots (intake runners)?

Sure it's not ignition-related?

[MrScott]

I checked the pop off valve, rubber boots, injector o-rings and all other usual suspects with soapy water. They seemed fine, but I don't think the "soapy water" test is conclusive.

I swapped in a spare distributor cap and set of wires, no change.

My first step was to rule out ignition so one by one I disconnected the wires and listened for a change in idle. All dropped the idle noticeably except #6, where it dropped but not as noticeably -- the engine still ran reasonably smoothly. I thought it might be possible #6 has less of an effect on engine balance due to firing order?

[boyt911sc]

Originally Posted by MrScott

Hi All,

I suspect my SC (CIS) has a large air leak -- it's idling low and rough, down on power.

Covering the air intake completely with my hand has no effect on idle. I squirted soapy solution around the injectors, intake runners, etc with the engine running and didn't notice any bubbles or change in idle.

The strange thing is if I remove the oil cap the idle drops noticeably. Doesn't that contradict the "plugging the intake with my hand has no effect" test?

I'll try the positive pressure test next, just wanted to verify I'm heading in the right direction.

Thanks,
Adriano

Adriano,

If you wanted to check the air leak/s (??) in your engine using the bubble test, you need to supply pressurized gas in to the system similar to the pictures I posted. A running engine would not exhibit similar results (bubble test) because the engine creates vacuum instead and the aqueous soapy solution would not increase your engine idle speed because is not combustible like a starter fluid.

Tony

[MrScott]

Tony,

Got it. Thanks. I was thinking two things:
1) soap would bubble or at least move visibly around a vacuum
2) enough water would create a temporary seal

I also tried with starter fuild but I was reluctant to spray it liberally on a hot, running engine.

I'll see what I can work up as far as a pressure setup