Vacuum Leak

[mhochman]

I had my car at the dealer for a rough or miss in the idle. They cant find any problems , they say there are no vac leaks and it idles normal. I purchased a vacuum gauge and checked it tonight. I can watch the needle move very slightly every time the the engine misses. It seems like a leak to me...........

 

[MrShorty]

It's a question of cause and effect.
Scenario A: There's a momentary vacuum leak which causes the engine to stumble or
B: The engine stumbling causes the engine vacuum to momentarily decrease.
From what you've given, I don't think you can tell which scenario you have.
Is the miss consistent enough for a cylinder balance test to isolate it to one cylinder?

 

[mhochman]

Engine miss

Yes I would think so. It does kinda of have a rhythm to it. If I gapped the plug or plugs a little wide could that cause problem?

 

[MrShorty]

Maybe. FWIW I know that when I first changed the plugs on my Explorer, they were worn to a gap of about .07-.08, yet I didn't notice any driveability symptoms. If you know that you gapped the plugs wrong, then you might check them.
I can also say that, when I had a couple of weak cylinders, the only symptom I had was a slight miss at idle. It didn't seem to really be running rough, but I got under the hood and ran a quick cylinder balance test and isolated to problem to #2 and #6.

 

[mhochman]

Can you tell me how you performed that test??
Did you fix it.

Thanks,.....

 

[MrShorty]

Two ways to do a cylinder balance test. Warm the engine up. With the engine idling, pull the spark plug wires one at a time. If pulling a particular wire causes the engine to stumble worse, then that cylinder is good; hook it back up and move to the next spark plug wire. If pulling a particular wire has no effect on how the engine runs, then that cylinder isn't contributing; determine why it isn't contributing. Alternatively, you can unplug the fuel injectors one at a time.

On my engine, I ran a compression test, and found that I had bad piston rings, so i ended up putting a remanufactured engine in.