[igottogonow]

I have a 1993 jeep grand cherokee 4.0 liter with 161,000 miles. For some reason while it is idling, it starts to smoke out tailpipe after it gets hot. I've checked PVC, engine compression avg 125psi. The smoke is blue and seems to be getting worse. About a year ago it was involved in a wreck, not bad but it did cause some front damage, I later found out it cracked the power steering bracket and cracked the exhaust manifold which I had repaired by welding it. Someone told me this could have caused it to start burning oil because of the cold air getting in where the crack was, I've had to repair this crack several times over the past year. Any truth to the cracked exhaust manifold theory?

[CaptainRich]

Blue smoke is oil. White smoke is coolant. Black smoke (or sooty tailpipe, for that matter) is too rich of a fuel mixture.

Not likely caused by the collision.

Failures like this don't show up in compression testing. The oil is drawn past the intake valve when the valve is normally open anyway.

The valve guide seals become brittle over time. There is little anyone can do to stop this from happening. But, it's not a major repair, like honing and re-ringing the engine.

Good luck!

(editted) btw: your comp readings are on the low side:
Compression Pressure 827-1034 kPa (120-150 psi)

.

[bnjroo]

Previous owner of 94 grand cherokee 4.0 200k. Current owner of a 93 wrangler 4.0 with 190k. Had the same issue after changing oil and using 5w-30. Idling hot in the summer especially would get blue smoke from exhaust. Drained the 5w-30 and put in 10w-40, and the problem went away.

Be sure you're using the correct weight oil for your vehicle/mileage. Thinner oil will tend to slip by the rings on high mileage engines.

Beyond this, I'd consider a new pcv valve and possibly a ring job (CaptainRich is right: your compression is kind of low but at least it's even, so you can rule out head gasket problems.)

PS if you don't want to spend 600 smackers on a factory exhaust manifold, you can get one for less than 100 online. I bought mine from 1aauto.com and I haven't had a problem with it at all. And no, no way a cracked exhaust manifold will cause that.

[TxGreaseMonkey]

Blue smoke indicates oil is being burned. Characteristic of bad valve seals is blue smoke, when you start in the morning and upon acceleration. If it smokes consistently, then it's likely ring-related. Compression of 125 psi is not very high.

[igottogonow]

I know its been quit a few years but I did replace the valve seals and my problem went away. Guess what the Jeep is still going good. 2 kids through school drove it and now my 30 year old son uses it for his fishing car except when I need it for snow. Yhanks for the reply.

[TxGreaseMonkey]

Thanks for the update.