Oil cap off, hiss & idle jumps??

Nixon1 · 12-14-2003, 07:42 PM

This happened fairly recently and I was wondering if you guys have any explanation for it?

The car was idling and I was curious about something.... So I decided to see exactly how much pressure was being built up in the engine even though the PCV system SHOULD be operating just fine. So I pulled my oil cap off to let it vent. When I pulled it off, I heard a loud hiss of air escape around the cap, and the engine idle shot up from 800 to about 1300 rpms. It didn't sound too good, so I put the cap back on, but it still idled high and displayed a check engine light. I can't remember what the code was when I pulled it, unfortunately. But anyways, after a few different starts and at different points in time, I tried to duplicate this scenario but I couldn't. Never hissed again and never threw another code, or jumped the idle. So, what the hell could have happened??

Thanks all.

12-14-2003, 08:39 PM

It sounds like excessive blow by, but you should be able to repeat the test every time.

Do you feel positive pressure when you put your hand over the oil fill?

Nixon1 · 12-14-2003, 10:30 PM

I don't know, I'll try that test tomorrow.

But what I can say is the motor has only about 10,000 miles on it maybe.. When it was first assembled, it burned a quart of oil in the first 500-1,000 miles, and continued to burn for around another 1,000 miles at a lesser rate until it finally stopped. I thought that was a very abnormally long amount of time for piston rings to seat. I have a strong feeling the motor may not have been broken in hard enough, hence why the rings took so long to seat.

12-14-2003, 07:42 PM	#1
Nixon1 Registered Member Join Date: Jan 2003 Location: Palm Bay, FL Posts: 432	Oil cap off, hiss & idle jumps?? This happened fairly recently and I was wondering if you guys have any explanation for it? The car was idling and I was curious about something.... So I decided to see exactly how much pressure was being built up in the engine even though the PCV system SHOULD be operating just fine. So I pulled my oil cap off to let it vent. When I pulled it off, I heard a loud hiss of air escape around the cap, and the engine idle shot up from 800 to about 1300 rpms. It didn't sound too good, so I put the cap back on, but it still idled high and displayed a check engine light. I can't remember what the code was when I pulled it, unfortunately. But anyways, after a few different starts and at different points in time, I tried to duplicate this scenario but I couldn't. Never hissed again and never threw another code, or jumped the idle. So, what the hell could have happened?? Thanks all. __________________ '91 Mustang GT 5.0 AOD Daily Driver Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI 13.73 at 103.8, ~2.2 60'

12-14-2003, 10:30 PM	#3
Nixon1 Registered Member Join Date: Jan 2003 Location: Palm Bay, FL Posts: 432	I don't know, I'll try that test tomorrow. But what I can say is the motor has only about 10,000 miles on it maybe.. When it was first assembled, it burned a quart of oil in the first 500-1,000 miles, and continued to burn for around another 1,000 miles at a lesser rate until it finally stopped. I thought that was a very abnormally long amount of time for piston rings to seat. I have a strong feeling the motor may not have been broken in hard enough, hence why the rings took so long to seat. __________________ '91 Mustang GT 5.0 AOD Daily Driver Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI 13.73 at 103.8, ~2.2 60'

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12-14-2003, 08:39 PM	#2
It's Exgmguy Guest Posts: n/a	It sounds like excessive blow by, but you should be able to repeat the test every time. Do you feel positive pressure when you put your hand over the oil fill?