backfire blows off PCV and Oil Fill Cap - Advice? - Blazer Forum - Chevy Blazer Forums

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Old 02-08-2010, 10:18 AM
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Unhappy backfire blows off PCV and Oil Fill Cap - Advice?

hey, I'm trying to troubleshoot my 1989 s-10 Blazer. It has the 4.3L V6.

It won't start. When I try to crank it it turns over and eventually there's a backfire from in the engine - not in the exhaust. Last night when I tried it I had the air filter off and as it backfired flames shot up out of the carb, the PCV valve blew out, the oil fill cap blew out and smoke started coming up out of the engine block. Hmm. I'd really like to troubleshoot this one a bit before I give up and call a mechanic but I'm not sure where to start. Any suggestions? If you need more info just ask.


P.S. Could this be a result of a fuel leak in the plenum I've read about here on the forum?

Last edited by radiobiz; 02-08-2010 at 10:22 AM.
Old 02-08-2010, 12:30 PM
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Thats not good... I would be checking compression on each cylinder to start with. Or its sparking before the valves close so the fire travels up the intake, but that doesnt explain the pcv blowing out. Do a compression test with your coil wire off so the fire doesnt happen. I would also stick a pencil into the spark plug holes to make sure the piston is moving if you get a zero compression on the test.

What started all this? Did it die out when running, or just wouldnt start on day?

You have an 89 so I dont think you have the notorious CPI system that some of us have been troubled by.
Old 02-08-2010, 12:44 PM
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The fire out of the throttle body sounds like the timing is jumping a tooth, not sure about the PCV valve and oil fill cap blowing out though .
Old 02-08-2010, 01:32 PM
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The only time I ever had anything like that happen to me was on my '80 pickup. The accelerator cable frayed because I was goofing around and jammed the pedal to the floor and got stuck at full throttle. I was headed straight for my parents garage door, which was closed, and all I could was push rthe clutch and shut'er down. I had a similar backfire which blew the PCV out with such force that it dented the hood.

Trapping gas/emissions under the valve covers and them somehow igniting would definitely blow the pcv, and if the backfire occured while the valves were in the right position and/or worn I could see a pressurization of the oil causing the oil cap to fly as well.

Is or was the pcv in good working order prior to the problem?
Old 02-08-2010, 01:44 PM
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I had a blocked port on the TBI that would prevent any crankcase ventilation and I was blowing out the PCV and Oil out of the dipstick tube. I did not realize what was going on untill I rebuilt the TBI unit and found the port totally plugged. I do not know if this has any bearing on your situation though.
Old 02-08-2010, 02:25 PM
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A little background. Basically it hadn't been running for a year. As a birthday present to myself last month I got her running again.

I replaced the distributor cap and rotor (the original problem as it turns out), put in new plugs and wires, changed the oil and filter, new fuel filter and air filter and ran some fuel injector cleaner through it. After all that it ran, but rough.

2 days later it started stalling at redlights (not sure if that's related or not). I got it home and it stalled in the driveway. I tried to crank it, no go. Pumped the gas and cranked and that's the first time it did that blow out the pcv valve and oil cap mess.

Well I've been busy for a couple weeks so I haven't messed with it. Last night I tried to crank it to move it into the garage and boom it did it again.

You guys have given me a couple of starting points. Thanks very much.
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1989, 43, backfires, backfiring, blazer, blew, cap, chevy, engine, oil, pcv, plug, spark, start, tbi, valve, vortec

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