Smoke AFTER decel, good compression, new turbo, 2000mi since rebuild

[87witmoreboost]

I have about 2000 miles on my stock rebuilt 7M. My motor had 95K on it and ran excellent. I'm almost sad I touched it, but I wanted to do a MHG and that involved surfacing my block, so while it was out I did new NPR pistons, rings, and all new bearings (rod and crank). I still had a nice cross hatch on the bore, so I went with stock size pistons (regrets...). I checked ring gaps upon build and all were good. I disassembled my head for cleaning and surfacing, and before re-installing valves I lapped the valves with a compound. It of course got brand new valve stem seals upon re-assembly.

The car went back together and fired up with no serious issues. I am running an HKS sport turbo with that was just rebuilt. I noticed that I am burning a lot of oil. The car billows blue smoke after decel (and ONLY after decel). Nothing on startup, nothing under accel, nothing at idle, etc. If I leave the car in gear as I slow down, I am guaranteed to get a breeze of blue smoke after I stop. If I decel on a hill, the second I get back on the throttle, I puff a cloud out the exhaust. I am running the original PCV system which includes new hoses. I did a compression test, petrified to find low compression, but my compression was fine (148 +/-2 in each cylinder). I tested the compression again and got the same results.

These signs really point to rings, but I don't see how I can have good compression? Do I have good compression rings and bad oil ring(s)? My car is finally built how I want it (Lex/560's/Walbro/AFPR/HKS sport turbo, etc), but I have this problem that I fear is in the very guts of my motor.

I'd really appreciate some tips from anyone with experience of these problems on where to go from here.

Thanks!

 

[S.A. supra]

Check your turbo. Just cause it was rebuilt doesn't mean it won't leak.trust me I know all to well.

 

[87witmoreboost]

The turbo was rebuilt through Aaron TWICE. I thought it was the turbo and sent it back.

If I had a spare turbo, i'd try it. I do, however, trust the rebuilt as Aaron has a real solid reputation and is a great guy to work with.

 

[IwantMKIII]

Sounds like a poor valve seal. Mine started to go after only 15K miles after my rebuild. Its very very intermittent for me though so I simply ignore it for now, not a huge deal.

 

[bluepearl]

What method was used to break the rings in. Were the cylinder bores honed? Are the ring end gaps spaced correctly around the piston? Did you pull and inspect the plugs directly after a smoking incident?

 

[S.A. supra]

Wouldn't bad valve seals smoke on start up also?

 

[Nick M]

The very high vacuum from decel is "sucking" the oil in the chamber from the pan past the incorrectly assembled(or defective parst) shortblock. The turbo can leak that way also, but I will take his word about rebuilt and checked.

 

[IwantMKIII]

Wouldn't bad valve seals smoke on start up also?

As nick said, the vacuum is stronger during decel and occasionally has the ability to 'suck' out the oil past the seal. My car does not smoke on start up ever, but after a long decel. and a hot motor I can smoke quite a bit during my stop at lets say a light. I'm not sure why this happens so intermittently, so far the only conditions it seems to repeat in are very hot day/motor and after hard decel, but there have been plenty of hot days where it doesn't do it so i'm not sure what exactly causes it vs. not. I'd say it happens maybe once every other week, and if it repeats, it repeats during the same day or time im in the car.

 

[87witmoreboost]

What method was used to break the rings in. Were the cylinder bores honed? Are the ring end gaps spaced correctly around the piston? Did you pull and inspect the plugs directly after a smoking incident?

Which method? Nothing too special on breakin. Everyone has an opinion on that and opinions are like a$$holes. I did lots of "non-constant RPM" driving. Didn't rev too high for 1000mi or so. Oil changed at 500mi, then 1500mi. Cyl's still had original crosshatch with no scarring at all. Ring gaps were clocked properly and wouldn't fit right in the cylinder if they weren't.

Plugs have never been checked IMMEDIATELY after a decel, but they are always relatively clean.

Wouldn't bad valve seals smoke on start up also?

Yes. It can't be that or it'd smoke on startup, I assume.

The very high vacuum from decel is "sucking" the oil in the chamber from the pan past the incorrectly assembled(or defective parst) shortblock. The turbo can leak that way also, but I will take his word about rebuilt and checked.

Since we're already technical here, be more specific about "incorrectly assembled shortblock"

 

[bluepearl]

So, I can assume you did not hone the cylinders? Why won't the pistons fit the bores if the ring end gaps are not clocked properly? What type of oil was used during the break in? I wish you the best, good luck on you problem.

 

[IJ.]

So, I can assume you did not hone the cylinders? Why won't the pistons fit the bores if the ring end gaps are not clocked properly? What type of oil was used during the break in? I wish you the best, good luck on you problem.

This!

 

[87witmoreboost]

This!

i DID hone. I use a 400grit stone hone. It was just enough to de-glaze.

 

[Nick M]

Then there is a problem with the oil control rings. It is that or the hone. Do you understand the negative pressure when the throttle closes? Pressure tries to equalize. If the air can't come through the throttle, it will try and pull elsewhere.

 

[bluepearl]

Did you check the bores for out of round and taper? How bout piston to bore clearance?

 

[Supra_Villan]

The turbo was rebuilt through Aaron TWICE. I thought it was the turbo and sent it back.

If I had a spare turbo, i'd try it. I do, however, trust the rebuilt as Aaron has a real solid reputation and is a great guy to work with.

easy as pulling the dp to check for oil residue on exhaust housing for peace of mind. Is your PCV routed correctly? This can cause smoke also, at least i think so. Someone correct me if i'm wrong.

 

[S.A. supra]

easy as pulling the dp to check for oil residue on exhaust housing for peace of mind. Is your PCV routed correctly? This can cause smoke also, at least i think so. Someone correct me if i'm wrong.

Yep, drop the down pipe, and also check the turbo to intercooler hose. It's pretty easy and it should of been the first thing checked.

 

[87witmoreboost]

I checked hoses and they are dry. Yes, I have the original PCV system. Yes, I understand the vacuum issue under off-throttle decel and have from the start. No, cylinders didn't appear to have a taper (and the car never did this on the original bottom end - recall, I only rebuilt because the block was out anyway to be decked)

I suppose with good compression, the oil-scraper rings must be the problem.

 

[S.A. supra]

Did you check the down pipe for oil?

 

[87witmoreboost]

Did you check the down pipe for oil?

Yup, and it was dry as I'd expect after turbo rebuild.

I guess my oil rings are not seating well on cyl, oil is getting past them, and then burning the first time my throttle is cracked. This might be the first time I wish we didn't have piston skirt oil jets on the turbo blocks.

 

[S.A. supra]

sound like it.