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2G Smoke during engine braking

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Turbodhi

Proven Member
115
10
Oct 18, 2015
Olathe, Kansas
So I recently rebuilt my turbo (big 16g) and the car STILL smokes when I downshift and coast. The turbo wore a hole through the oil deflector and a decent groove into the thrust bearing so I chalked it up to that. But after a rebuild it's still happening. Any ideas? And I'm going to be checking that I didn't toast the turbo immediately after rebuilding it.
 
I would think that smoke from a turbo oil leak would be worst at high rpm, as the pump spins faster and puts out max oil pressure. Smoke during engine braking sounds like classic engine internal oil control issues, such as valve stem seals. High intake vacuum (less than atmospheric) during intake stroke pulls oil in from the head or cylinder walls (which are at atmospheric pressure). In a non turbo, this subsides under wider throttle, since that by nature allows more air to enter the intake and relieve some of the vacuum. In a turbo it actually builds pressure above atmospheric, so the airflow from intake would perhaps even end up building pressure in the crank case (which should be released by the pcv valve). Check compression to assess ring condition and pull valve cover to inspect stem seals, in that order to go from easier to more involved diagnoses.
 
I've replaced the stem seals about a year ago but I wouldn't put it passed this car for them to be bad already. And my boost gauge is telling me I have 20-22 in/hg of vacuum. Last I checked my compression was good which would explain the good vacuum numbers. If my valve seals/guides are bad I can see how the high vacuum would suck oil through.
 
How long since the rebuild? I installed a different used turbo on my Autozam and it billowed thick clouds of smoke for weeks until I ran seafoam through everything. It has been perfect for the 2500 miles since.

As mentioned, oil smoke during engine braking is one of the telltale signs that the oil seals are bad. High oil pressure and low air pressure gives it the greatest differential across any seals. Turbo seals, valve seals, head gasket. Before you go to town with the seafoam, pull the spark plugs and see if they're showing signs of oil in the combustion chamber. That can help rule out a lot of things.
 
It would surprise me that valve stem seals would go bad that fast, unless of course the valve stems themselves are heavily worn. Your high vacuum will try to suck air in from the wherever it can, so there may be some other place it’s leaking in. I’ll ponder more on this puzzle.
 
How long since the rebuild? I installed a different used turbo on my Autozam and it billowed thick clouds of smoke for weeks until I ran seafoam through everything. It has been perfect for the 2500 miles since.

As mentioned, oil smoke during engine braking is one of the telltale signs that the oil seals are bad. High oil pressure and low air pressure gives it the greatest differential across any seals. Turbo seals, valve seals, head gasket. Before you go to town with the seafoam, pull the spark plugs and see if they're showing signs of oil in the combustion chamber. That can help rule out a lot of things.
It's been about a week since the turbo rebuild. My oiling system is my next project to tackle.
 
It would surprise me that valve stem seals would go bad that fast, unless of course the valve stems themselves are heavily worn. Your high vacuum will try to suck air in from the wherever it can, so there may be some other place it’s leaking in. I’ll ponder more on this puzzle.
In the car's defense, it has 200k+ on the odometer. It wouldn't surprise me if my valve guides are tired.
 
New seals on wore out guides will last about 2 weeks max, and it will start smoking again because it wiggles. Been there, done that.
 
During downshifts/high vacuum/throttle blade closed you should be seeing higher cylinder/manifold vacuum, during which time oil and crankcase vapor can be getting drawn past the rings or intake valve seals & guides due to the high vacuum.
 
Leaky valve guides present themselves more at start-up than anything else (unless its like a hotdog down a hallway). The valve will rest and allow drops of oil to run down past the seal/worn guide into the combustion chamber, thus creating the famous burnt oil and smoke at start up. If its the valve guides, it would be doing it at start no matter what and MAYBE coasting if its bad, but definitely at startup. And 90% of time will saturate your spark plug with oil as they are about 1/4" from each other. That being said-

I'd look at the easy things first:
Turbo seals
bad PCV system
spark plug check

Turbo seals: pop off you downpipe and look for oil
PCV: pop off you connection at the IM (or TB, or entire IM) and look for oil. And, for the love of god, put a real check valve in that line.
spark plug: I won't insult your intelligence by explaining.

In both cases it should be very apparent.
 
It's going to be tricky to check for oil in those locations considering I JUST rebuilt my turbo. It had burned a hole through the oil deflector plate which was sucking oil through. Everything got a coat and I can't afford the down time to strip it off the car and clean it all 100%. This thing is my daily. That being said, I cleaned some so I can peek to see if it looks like there's anything new. And I double checked the turbo it should be good considering it was rebuilt a week ago.
 
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