How much blow-by is normal?

I have a 90 wit hthe 4.0, and it seems to have an enormous amount of blow by.

At the same time I think my PCV valve is pooched also.

At any given time I can take my oil cap off and I can barely see my rockers. If I take off the oil breather tube(not PCV), at idle it shoots smoke easily a foot.


Truck only has 226,000KM on it, it's still basically new :laugh3: To me this doesn't seem normal...

All 4.0's have a lot of blowby. Call the dealer and get new CCV hoses and fittings. They're not that expensive. Also, check your air filter and make sure it's not oil-soaked.

I just recently out a cone filter on, and that hose is just hanging until I figure out what to do with it.

clean the 2.6 mm orifice on valve cover also the vacuume line that runs to your intake manifold(carb cleaner works well). There is no P.C.V. this style is a C.C.V. system (controlled vacuume leak, basically)

anony91xj said:

All 4.0's have a lot of blowby. Call the dealer and get new CCV hoses and fittings. They're not that expensive. Also, check your air filter and make sure it's not oil-soaked.

Click to expand...

Blowby is caused by combustion gases "blowing by" worn piston rings into the crankcase. The crankcase becomes pressurized and the result is leaking gaskets/seals, oil getting blown back into the air filter, and oil getting blown out from the dipstick tube.
The crankcase can also become pressurized if the CCV system is blocked and is unable to scavenge the crankcase. Clearing the CCV system might help but if you're still getting a lot of blowby, it's time to do a wet & dry compression test of the cylinders. This will confirm if there's ring/bore wear that'll require an engine rebuild.

I collect mine in a gallon jug. The 16 oz bottle wasn't big enough for long trips. Still pulls hard at trail speeds, but it's a slug on the freeway.
Jeep on!
--Pete