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Back pressure blown off oil cap and shot out my dipstick!

B

beaurogel81

NAXJA Forum User
Location
Madison, MS
I have an 89 XJ with the 4.0 and the engine wont turn over. back pressure built up so bad that it blew off my oil cap and shot out my dipstick. white smoke filled the garage. smelled mostly like exhaust and a little bit of gas and oil. cut off my muffler (doesnt have cats) because i thought maybe it was clogged. didnt solve the problem.. what do you guys think?
 
crushed/plugged exhaust pipe elsewhere? What about a bad PCV? Do you have gas or other contaminates in your oil? Proper oil level?

Not real sure, just trying to help throw some ideas out for you.
 
exhaust pipe is clear. not sure about the PCV. no gas or contaminates in oil. yes, proper oil level.. so im kinda lost on this one. been working on it all night.. heres a pic of my jeep by the way.
l_504ed3b5b28d9ef10efe37421675632a.jpg
 
:doh: I just pulled my head out of my a$$......run a compression test. If you have excess blowby (or a hole in a piston) it will overpressurize the crankcase. This will cause your engine to make relief points(i.e. the dipstick tube and oil cap). Your numbers should all be within about 20% of each other.
 
Yep, a compression test is in order.
 
x3 on the compression test. could be a excessivley worn piston, or any number of other things. did it just start doing this one day out of the blue or has it been sitting for awhile?
 
it just happened after a trail ride last week. and im thinkin the blow by too.. would it leave an oily mess that covers half my air filter? smells like weed eater gas?
 
beaurogel81 said:
it just happened after a trail ride last week. and im thinkin the blow by too.. would it leave an oily mess that covers half my air filter? smells like weed eater gas?
Click to expand...
Yes it would. That's supposed to be the air intake for the crankcase vent system, but when you get excessive blowby, it goes out that way instead of in.

I'd take a compression test first, and see what that tells you, then work from there. Even with a clogged crankcase vent it shouln't blow the dipstick out, because the intake hose usually passes out the excess.

I bought an 89 once at a junkyard that wouldn't start. I thought that since the crank position sensor was definitely bad, the problem might be there, and didn't check the engine out thoroughly. Replaced the CPS and still no start. I did a blowdown compression test and almost put the dipstick through the hood. Turned out it had also been run out of oil or something, and had 60 pounds compression in all six holes! Instant parts car.
 
so would it be worth fixing this motor or doing a motor swap? ive got a 4.3 Vortec 6 but i dont know if its possible without 'extensive' fabrication.. thanks for all the input guys! good to be part of the XJ crowd!
 
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