Rebuilt engine smokin' up a storm

Raindem

Veteran Member
Jun 3, 2012
173
Sonoita, AZ
I've got a couple hundred miles on my rebuilt 305. Did all the disassembly/assembly myself. Had it machined .030 over at the local shop. Stock crank, new pistons, mild cam, no other mods. Up until now it had been running pretty good. Yesterday I went for a short drive and I noticed on deceleration I'd get a little puff of smoke out the tailpipe. No big deal, I figured, just needs to be driven more.

Well, I was idling it today in my shop while checking the ATF level. All of a sudden huge clouds, and I mean HUGE, of blue smoke start bellowing out the back. There was no odor and no change in how the engine ran. So I shut it down for awhile and started it back up and the same thing happened. Except now the smoke emits immediately upon startup and does not stop. If I rev the engine it comes out even thicker.

I know I have to tear back into the engine, which I'll start tomorrow. But I'd like some ideas on what to look for before I start. The first thing that comes to mind are valve stem seals. The reason I say that is that I didn't really understand how the ones that came with my rebuild kit worked. They sent me 2 different kinds of bell shaped seals and I wasn't sure which ones to use, if any, so I ordered a set of the OEM o-ring style. Still not sure if I even put them on right. My heads are OEM and are not machined for those special seals.

So besides valve stem seals and the obvious blown head gasket, what else could cause bellowing blue smoke?

Thanks

Curt
 

APEowner

Veteran Member
Dec 9, 2013
646
Albuquerque, NM
Were you by any chance checking the ATF because it mysteriously disappears? If that's the case you might have a bad vacuum modulator on the transmission.

The amount of smoke that you're describing won't be caused by valves seals. Start by checking the plugs and doing a compression check or even better a leakdown test. I wouldn't tare into the motor until I had a better idea of what the issue is.
 

Damon

Veteran Member
Nov 16, 2000
12,939
Philly area
Agreed, it's not valve seals. Way too much smoke for that. The trans vacuum modulator is a good piece of deduction by APE. If the diaphram inside ruptures, it will suck liquid ATF up the vacuum tube and straight into the intake. If that's all it is you're in for a quick and cheap fix to what might look like a catastrophic problem.
 

dale68z

Veteran Member
May 14, 2009
1,407
Peoria Arizona
My guess is a internally leaking intake gasket, sucking in engine oil.

To test.
Block off breathers/ pcv. Attach a vacuum gauge to the oil dipstick with rubber hose. Start the engine. If you see any vacuum on gauge, you have a blown intake gasket.
 

Raindem

Veteran Member
Jun 3, 2012
173
Sonoita, AZ
Thanks for the replies.

I was checking the ATF because it shifts rough, at what seems like too high of an RPM. I did in fact suspect a bad vaccum modulator but had no idea it could actually draw ATF into the combustion chamber. That would certainly explain the smoke. I've heard of people running ATF in their engine to clean it out, and describing how much smoke it makes. Is there a test I can do to determine if the modulator has ruptured?

Dale, I'll run that test right after I eliminate the vac mod as a possible source. BTW, whereabouts in Arizona are you?
 
Last edited:

Twisted_Metal

Administrator
Staff member
Lifetime Gold Member
Feb 26, 2004
31,884
Bloomington, MN
^ Good idea!

The entire vacuum line should be completely dry.

If the diaphragm in the modulator is blown... The inside of the vacuum line will be wet with ATF.
(Easiest to see at the lower, transmission end, of the connection.)
 

Raindem

Veteran Member
Jun 3, 2012
173
Sonoita, AZ
I was so hoping it would be the vac mod, but no such luck. I disconnected the line and plugged the port at the intake. Started the engine up cold. After about 30 seconds started spewing out smoke again. Ugh.

So what's next mostly likely culprit? Oil control rings? Would a single cyclinder burning some oil create that much smoke? Or as Dale mentioned does the volume of smoke indicate that oil is being drawn into all the cylinders?

In the meantime I'll do a compression test and check out the plugs. I have a leakdown tester but I've never used it, so I guess now is a good time.

Thanks

Curt
 

Raindem

Veteran Member
Jun 3, 2012
173
Sonoita, AZ
Here's the compression numbers:

1 - 135
2 - 125
3 - 130
4 - 125
5 - 135
6 - 125
7 - 140
8 - 135

Not perfect, but I don't see any major problem there. All the plugs except #3 were oily. Hmmmm. Unlikely that 7 out of 8 oil control rings would fail at the same time.
 

CorkyE

Veteran Member
Lifetime Gold Member
Nov 4, 2004
11,931
Ringgold, GA
I'm going with intake gasket also, pulling oil from top of engine. But check first that the pcv isn't sucking oil either. Just disconnect and restart the engine.
 




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