HELP!! # 5 Misfire

[andyrud]

2002 Chev Astrovan 4.3 AWD 109500 miles.
Hi Guys,
I really need your help! The van runs GREAT when driving along. But, after idling for a couple of minutes it throws a P0305 code that says cylinder 5 is misfiring. If I drive on highway for hours no codes at all. My OBD 2 reader shows misfiring at idle but as soon as I rev it up to around 1200 RPM it stops misfiring. Below is what I have done to try to correct it.
New Spark plugs and new wires. No help
New ignition coil. No help
New dist. Cap and rotor. No help
Put a different Dist. In, tried that, and didn’t help.
Changed the # 5 fuel injector, that didn’t help. Changed it again, that did not help.
Compression test showed 120 to 130 psi for all cylinders.
Did a leak down test, was in spec.
Installed new crankshaft position sensor, did not help.
Any Ideas and suggestions to what I should do next?!
Thanks in advance,
Andy R

[MountainManJoe]

Welcome to the forum.

Check for vacuum leaks. These engines notoriously start leaking at the lower intake manifold gasket, starting with cylinder 5 and 6.
Re: Intake manifold gasket replacement

[andyrud]

Thanks, I checked for vacuum leaks and found none. Checked hoses, etc. also.

[MountainManJoe]

how did you check? They can sometimes be very difficult to locate. A smoke tester is the most reliable and thorough way.

Also, did you visually confirm there is good spark from the #5 plug?

[andyrud]

I did have a smoke test performed by my shop mechanic. Also, spark is good. No misfire during RPM above 1200.

[MountainManJoe]

Try a noid tester in case it's the injector wiring.
fuel injection balance tester may also be an option on a high level diagnostic tool.
unlikely with such low miles, but coked up valves? maybe take a peak with a boroscope.

[andyrud]

I would not know how to use one. The mileage is mostly high speed highway using good gas and Mobile one oil.
Could it be the Spider? Do you recommend changing out the old one with an updated one?

[MountainManJoe]

Could be. Search the forum there are topics about it.