Engine smoking


LawnSite Bronze Member
Hi guys. I'm looking for somewhere to search before I begin tear down.

The history first. Got a '94 K-2500 GMC truck. 350 engine, target motor, 8,000 miles. I picked the motor up used from a wreck for $350.00, so, basically, I stole it.

Bolt all the goodies on, pollution controls are functioning, and the engine smogged ok. It burns about a quart of oil every 800 miles. I stopped driving it until we get this problem handled.

Problem being....At an idle, like at lights, this engine begins puffing smoke, then if you sit long enough, looks as though it foggs mosquitoes until it clears out.

My first thought was try the easy stuff first, and that is why the pollution garbage was checked out first...

I read a tech bulletin a while back where GM had troubles with intake manifold gaskets, so the vacuum would suck oil right out of the valley and do what this motor is doing. The other issue could be valve guide seals. I have not yet pulled compression, which I am assuming will be fine, and will look for plug discoloration when I do.

So, given this history, if this was your engine, where would you begin looking to solve this problem?


LawnSite Senior Member
Parkton NC
Pull a comp check,if non conclusive, have a cylinder leak down test done, check plugs. Then pull the valve covers abd check for excessive sludge buildup. It may be that the oil is not draining back down from the head to the pan. Valve guide seal s are cheap and easy to replace, I'd do it anyway cuz if they aint leaking now they will be soon. It may be the valve guides themselves, they are easily knurled or replaced by a machine shop. But start with a compression check, its a real good diagnostic tool and it will give you a good insight as to the condition of that engine. Post back when ya have more info on this one.


LawnSite Senior Member
thats "pcv". wouldn't want you to check your pvc pipe.


LawnSite Silver Member
some gm target engines that were assembled in mexico were put together with no valve seals. look there first.


LawnSite Fanatic
Memphis, TN
Originally posted by jc1
some gm target engines that were assembled in mexico were put together with no valve seals. look there first.
My first thought was the valve seals also.

I would change the PVC first because you can get one for about a buck. Then have the leak down done.


LawnSite Bronze Member
Just wanted to post staus back..Had a big long "Honey Do list" this weekend...Will probably get to it this week...PC valve was changed, and I even took that a step further. I plugged the inlet on the throttle body with a rubber nipple cover and ran the truck without the PC valve for 20 minutes or so...Thought I may have fouind it, until I hit a light, stopped, light turned green and puiff the magic dragon appeared from mt Flowmaster tail pipes....

I did not think it was the PC valve because we put over $1,300.00 in pollution garbage to get it through Ca smog within the past year...Just about any out of state vehicle from the 90's and before will require this...Not to say there was a malfunction, it could happen, just not likley with this scenario..

Anyhow, I'll let you all know what we find later on...Thank you for all the good input, it shows I was not too aweful far out with my thoughts....


LawnSite Member
Smithfield, NC
If the engine sat for awhile after the wreck, the oil rings may have seized to the pistons not allowing them to expand and contract to clean the cylinder walls. Also they may have been seized to the walls and cracked when you started it. A compression test will not reveal a problem with the oil rings, only with the compression rings. If the engine runs smooth at idle, you probably don't have a problem with the compression rings since it takes around 100psi for the cylinder to fire at idle. Hence, low compression on a cylinder causes misfire at idle. Valve seals may be dry rotted if it sat too long. Could have had rust on the cylinder walls and caused damage to walls and rings when you spun the engine over. Haven't seen a lot of intake gasket problems on pre-96 models but 96 and later vortec engines leak coolant like the Titanic. They have redesigned the gaskets for those. Oh yeah, I'm a GM technician full time and LCO part time. Does it have stock valve covers or aftermarket. Some aftermarkets didn't have baffles and would allow the PCV system to suck the oil out of the engine. Pull the plugs and look for crusty ashy looking substance. If it is only on one or two plugs, you can narrow it down at least.