RB26 smoke during 3rd gear WOT only

[Lopin18]

So, a friend called me to see a GTR with a built RB26 that has been smoking hard under WOT load , it made 480hp, 360tq with the last turbo (precision). on the dyno and they saw smoke coming out of the exhaust housing and then the exhaust pipe.

Before that, they thought the turbos were the problem. new turbos, same thing, precision single turbo, same thing.

So i went to help, checked compression (all good around @ 170psi), did a leak down, found cyl 1 and 4 giving a loss of around 10-12 psi @ 74psi down to 60 and 70psi to 54, other cyls had 2-4psi loss. This is my only clue.

* Checked PCV valve, its fine, plugged it just in case

* Took valve cover hoses off (oil catch can has no crank return nor atmosphere vent, its basically a closed box)

* Oil return seems adecuate, no rough path to the crank

Then, we tested the car, hard wot 1st,2nd and 3rd gear, it only smoked on 3rd gear WOT at high rpms (6500+) and it was a lot of smoke, it blocked the lane behind us. Oil came out of the disconnected valve cover outlets and fell on the turbo and block. As expected. we have to retest with the hoses pointing to the intake side but since they DID see smoke coming out of the tailpipe on the dyno, i can only hope that the PCV being completely closed off could have done something internally with its built pressure on the dyno and burnt oil inside the combustion chamber. I saw some wetness on the pistons from the plug hole, saw no oil or sign of it on the downpipe and exhaust housing.

 

But being realistic... i fear that something is wrong either on the pistons or valve train area. I thought it was the PCV system because i saw the closed box and wondered about the PCV valve etc.But seeing the compression fine but a bad sign on the leakdown. So this left me curious.

 

I thought a cracked piston ringland or a bent valve or bad valve seats would show at all RPM ranges, to different degrees, but this engine only did its smoke screen on 3rd gear, WOT, after 6500 rpms. Any ideas? experiences? i cant open this engine for fun as its not mine so im left with the sad state of not knowing for sure.

 

 

 

Edited August 23, 2020 by Lopin18

[Ben C34]

Explain more about the catch can what you mean by closed box. Give a diagram of how that is plumbed

[GTSBoy]

46 minutes ago, Ben C34 said:

Explain more about the catch can what you mean by closed box. Give a diagram of how that is plumbed

You're fishing, aren't you?

[Lopin18]

Both hoses from the valve cover go to the oil catch can, PCV valve is joined with one of those valve cover outlets.

The catch can itself has 2 inputs from the valve cover outlets. NO outputs on the catch can, meaning no return to the crankcase, no vented filter, no outlet to the compressor pipping.

 

So basically pressure just builds up, theres nothing venting or allowing the crankcase pressure to go anywhere (just the PCV valve, which will be closed during boost)

[r32-25t]

Yeah that’s going to be your issue, if the crank case pressure has no where to go it’ll find somewhere, usually straight out the turbo seals 

[Ben C34]

32 minutes ago, Lopin18 said:

Both hoses from the valve cover go to the oil catch can, PCV valve is joined with one of those valve cover outlets.

The catch can itself has 2 inputs from the valve cover outlets. NO outputs on the catch can, meaning no return to the crankcase, no vented filter, no outlet to the compressor pipping.

 

So basically pressure just builds up, theres nothing venting or allowing the crankcase pressure to go anywhere (just the PCV valve, which will be closed during boost)

Well that is completely f**ked.

[Lopin18]

Neat, i was thinking so much i didnt remember that the oil return line from the turbo is another path for pressure to build up. Thanks for the feedback. On monday they will trst the car again to confirm, ill reply back then

in the dyno they noticed the valve covers leaking pressurised air, along with oil, but the valve covers were a bit loose, i can imagine the pressure buildup was enough to cause what they saw...

 

 

[R_34]

I'm surprised the cams and crank seals didn't let go/popped off already.

[Chris32]

I don't really understand how such a simple thing can be so misunderstood!

Either block your PCV and vent the rocker covers to atmosphere into a CATCH CAN - with a hole at least 1" in diameter for each hose going into it so pressure can escape, preferably with a filter/s to catch some of the oil jizz

Or keep your PCV valve stock and install a SEALED OIL/AIR SEPARATOR r that goes between the rocker covers and hose into the turbo inlet

They are the only two options that exist for it to work properly

[Lopin18]

 U would be surprised how many people dont take crankcase ventilation into account.

They tested the car today, no more smoke. So hopefully its just working the crankcase ventilation. 

 

Thank you btw, i had missed some concepts including the turbo oil return line.