The Stovebolt Site Forums The Shop Area Engine Shop Why does my 350 smoke?

I need help from you engine experts! I had a rebuilt 350 put in my 51 3100 about 6 months ago, and although it only gets about 10mpg, it has run very strong and smoke free except for a few ocasions. The first time I noticed it, I was on a 60 mile drive. About 30 miles into it, the truck started smoking heavily for a few seconds every few miles. It seemed to happen more when the engine had a little strain on it, like going up a small hill. The smoke was blue/grey and smelled like oil smoke. 3 hours later, I drove it home with no smoking whatsoever.
Since then, it has done it again a few times. It seems to happen only after I have a hard acceleration at highway speeds, then it may start to smoke a few minutes later. I usually drive it gently, and it never seems to smoke then.
I changed the oil 500 miles ago, and it is still almost full.
What the heck is going on? I'm going back to talk to the mechanic again, but he doesn't seem to have any good ideas about what can cause this to happen.
Thanks for any help!

Shoe,

Any PCV valve or smog hardware on this rig? I don't see it mentioned, so it probably isn't turbocharged!

Were the heads rebuilt when the engine was done? Does it have conventional brakes, or did someone stick power brakes on it?

Stuart

Is the smoke coming out of the exhaust or out of the engine compartment?

I,m thinking PCV valve also. Some of the aftermarket valve covers don,t have shields around the vent holes. Pull the pcv valve out & see if there is oil in the hose. You have to have 2 sorses of vents. The pcv plus a vent cap or tube going to the air filter.

Thanks, everyone.
It does have a pcv valve, no other smog hardware. It does have front disc brakes with a booster, and power steering.
I assumed the smoke was coming out of the exhaust pipe, but I'm not sute. It has dual exhausts, and it seems to come from the right side (passenger side) but all I've ever seen is through the rearview mirror. It does put out a lot of smoke for a second or 2, I think too much to be from the engine compartment.

Shoe,

If the smoke is coming out of only ONE pipe, and they don't have a common connection under the car, you might be dealing with a head issue. Take a look at the tailpipe end and see if one appears more black or oily than the other.

Just for grins, and because they are cheap, ( I guess) replacing the PCV valve might not hurt.

Stuart

Do you have a chrome valve cover that doesn't have a baffle under the PCV port?
Scott

If it has an automatic trans you might check and see if the vacum modulator valve is leaking some transmission oil into the vacume hose going to the intake
Ronnie

i had a similar smoke issue. it turned out to be the intake manifold gasket on one side was allowing oil to get into the exhaust warming port of the intake manifold. tiny amounts of oil from the lifter chamber were going straight into the exhaust header on one side. it doesn't take alot of oil to make alot of smoke. the telltale sign for mine was drops of oil coming out of the collecter joint on one header when the engine was cooling down and parked on a hill. hope this helps. good luck.

Let us know if it an automatic. It sure sounds like a vacuum modulator leak at low vacuum conditions. Have you noted any loss of trans fluid?

Any oil pulled in through the intake would show itself out both exhaust pipes. Shoe has stated this "might" be coming out of one exhaust pipe only.

If this is true, it would lead one to believe that the problem was isolated to that bank.

Stuart

Thanks again.
It does have an automatic trans (350)
I checked the fluid level, and it still shows full on the transmission. Where do I find the vacuum modulator?

48bigtrucks- the valve covers are the stock covers.
garyg- I'll look into the gasket leak, we did have problems with the exhaust manifold gaskets, maybe there are more bad gaskets in there!

The modulator is a round chamber on the rear of the trans. It has a small vacuum hose attached to it. Remove the vacuum hose & if there is any fluid in it it is defective.

I wonder if the rebuilder accidentally got the ring gaps lined up with each other instead of staggering them. That would cause some blow-by in one cylinder and make the smoke only come out of one exhaust pipe.

I checked the modulator, there is no fluid in the vacuum hose.
Phil- I don't think it is misaligned rings, it only smokes for a few seconds at a time, and has only done it a few times.

Shoe,
Pull the valve cover off and check to see if an oil drain back hole is clogged up. If the person that assembled the engine used excessive amounts of RTV "blue glue" when assembling the engine, some of it could have broke off and made its way to the cylinder head where it clogged the hole up. Hope this helps.