I went this past weekend to look at a possible donor car for a V8 conversion. It was a black on black '90 780 turbo 105K with a "bad engine." I took a very thorough look at the car and as it turns out, the engine is fine, at least as far as I can tell. I checked all lines and hoses, as well as electrical connections, sensors, etc. The car registers no trouble codes. The car will spew acrid light-blue smoke indicative of burning oil... this spewing is directly proportional to RPMs. It's damn near non-existent at startup, and then gets worse as the car warms up. The owner says it got gradually worse over a period of 2-4 months, and that the problem went away briefly (a couple weeks) when the car was brought in for some sort of maintenance (they couldn't remember, but thought that some gaskets were changed- maybe headgasket, intake/exhaust manifold gaskets, and possibly the rear main) and then got worse again. It also uses oil (duh) at a rate of about a quart per 1500 miles. If it wasn't for the smoke billowing out the back, you would think there was ABSOLUTELY nothing wrong with the way the car runs/drives. It's quiet and powerful, boost levels are normal, no whining from the turbo or problems with engine function from idle to redline.

So, there are 3 ways for oil to get burnt up and exit the exhaust pipe, unless I'm missing something: Down through bad valve guides, through the turbo oil bearing (both by being sucked in through and atomized by the compressor then burned, and by being blown out through the turbine and burning up on the hot exhaust gasses), and up through the piston rings via blowby from bad rings or a clogged PCV system.

Valve guides seem unlikely as I don't think I've seen many such failures discussed here on the board.

I then checked the turbocharger. It spun freely when turned by hand at the compressor section, although I don't know how much that vouches for the bearing's integrity. I pulled off several hoses and found no more than a few drops of oil in any of them, which is not bad considering it's non-synthetic and that the car hadn't been driven in over a year (right?).

I checked compression which was 135-145 on all 4, and it held the pressure too so I'm guessing that the rings are ok. The flame trap wasn't pristine, but it was clean. I was able to blow a bit of low-pressure compressed air through the breather box and observe oil blowing out the filler cap, although the engine compartment was clean and showed no signs normally indicative of blowby / clogged PCV like spewed oil and such. I thought about what I did afterwards and I hope I didn't blow out their rear main or something by doing that... I guess we'll see. Also, I did the "jiggle test" on the filler cap as well as placed my hand over it to check for vaccum. The cap did jiggle, but it's plastic, not metal, and there was a slight vaccum which would sometimes keep it seated.

One more nagging detail. I could hear a hissing sound, similar to a vaccum leak, coming from the vicinity of the intake manifold. I was hoping to find it by spraying carb cleaner all around the hoses, nipples, manifold gasket, etc, but it seemed to have no effect. I was able to isolate the noise to approximately where the head meets the block in between the #2 and #3 cylinders using a garden hose as a pseudo-stethoscope. One more interesting detail, although I don't know what it has to do with it... when I disconnected the idle air control motor, the noise got quite noticeably louder. Reconnecting the IAC did not make it quiet again. Thoughts on this?

End result- I'm guessing it's either the PCV system or the turbo. Since one is much cheaper to deal with than the other, I suggested to the owners that the get a new breather box, flame trap, and associated hoses and see if that solves things. If not, it's probably their turbo? What do you think, bricksters???? Did I miss something?

Thanks very much in advance, once again.

-Jon
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1LT Jonathan T. Belmont-- '91 Coupe 166K 'Gustaf'