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Burning Oil at high RPM, Low Compression on cylinder 8
Hi there folks. I've got an issue with my '99 C5 M6, 177k miles. It burns oil, blue smoke, whenever I'm at high RPM, low load. For example, If I run it to 5k in second gear, but then let off the throttle and decel in gear, the car will leave a heavy trail of blue smoke. Absolutely no smoke on regular driving and cruise, just at High RPMs. On the street, this happens rather infrequently, but once in a while I'm in a spirited driving situation and I notice the blue smoke. I didn't think anything of it, chalking it up to sucking oil through the PCV system.
Just for reference, My mods are in my profile, but I am running an LS6 valley cover, and have a catch can setup.
I recently installed a GPP Hot Cam, new LS6 valve springs and, and OEM top hat valve seals that were a takeoff from a new LS3 crate motor.
Well I had my first HPDE over this weekend, super excited about it, but after my first three laps, I was black flagged, I was burning oil. They were concerned it was an external leak, but I assured them it was the motor sucking it up, and they let me drive. I continued my next few sessions, all the while leaving more and more smoke, because I'm at a constant high RPM and varying throttle. Several drivers came up to me to talk about the smoke, but I really had no good answers, but one older fella with experience mentioned sucking oil up through the rings and valve seals/guides, especially considering the mileage on my car. Having just done new valve seals, I though maybe I messed something up. Anyhow, drove the car home just fine, she runs very strong with the new cam. Car ran strong all day in fact. Parked it Saturday night and crashed.
Now this morning, I had a little time to give the car a once over. There was only a teaspoon or two of oil in my catch can. What the older gentleman at the track said about oil through the rings had me wondering though. I popped off the valve covers, saw no issues, pulled the plugs, again no issues. Decided to whip out my compression tester. I wasn't expecting to find anything major here, Cylinders 1-7 all between 185-200, Cylinder 8 - 120 - barely. Uh Oh. Triple checked all adjacent cylinders, the results are valid. Has the old mans prophecy come true?
I don't have a specific leakdown tester, but I unbolted the rockers, and then added compressed air to the cylinders with the same kit I used to swap my valve springs. Tested 2, 4 and 6, and no issues, no discernable amount of air leaking out anywhere. Tested #8 (the cyl with the low compression) and I can hear a loud hiss through my exhaust, with my compressor struggling to keep the PSI in check. Interesting, I was sure it was going to leak into the crank case, but looks like maybe I have a bent exhaust valve. Could this be my "smoking" gun?
Since I was this far, I decided to remove the valve spring, get the cyl to TDC and lower the valve and boroscope. I couldn't see anything that was pointing to a bent valve, nor could I see anything that would cause the valve not to seal. I replaced the valve seal with a new spare one I had, but the one I took off looked fine. All pushrods checked out straight also.
I buttoned the motor back up, and she seemingly idles fine. No misfires, all systems go.
Is it possible that a bent exhaust valve could cause oil burning at high RPM, high vacuum? I peaked through my Throttle Body, and did not see any oil pooling in the intake manifold, but there was a glossy oily look to everything, which I guess is expected. I expected to see some heavier oil deposits for the amount of blue smoke I produce at high RPM, high vacuum.
I'm trying to decide on a next step.
Option 1. I can swap the head with a used OEM 853 head for very cheap and quickly, and see if the smoking is gone. join a few more HPDE events this summer.
Option 2. I have a set of PRC stage 2.5 LS6 heads in storage, but I didn't want to add all that power yet seeing as I completed my first HPDE and the added power will do me no favors. And also what if the oil problem is ring flutter, then I'm pulling the block and redoing the job anyway. I also have limited time to do the work and this would likely end my HPDE season, and take me a month to complete. I would take this opportunity to plumb an oil cooler also.
Option 3. Drive the car recreationally as is, no more Track days, but enjoy the summer. Install the PRC heads this Fall and return the car to be ready for the Spring. Still no guarantee that this solves my oil burning problem, and still would need to re-ring the block at a later date.
Is it possible that a bent exhaust valve could cause oil burning at high RPM, high vacuum?
No.
That could be a cause of the compression leak, but not the oil burning.
First thing I'd be inclined to think of, is a lack of thread sealer on the rocker bolts; specifically the intake ones. Reason being, the bolt holes go almost all the way through the casting there, straight into the ceiling of the port, leaving it quite thin; and if a bolt hole has been repaired (drilled), or the heads ported, the threads may be exposed. That creates a connection between the intake port and the crankcase.
Try just pulling all those and putting the Loctite/Permatex "hi temp auto thread sealer with PTFE" on the threads.
I haven't checked the stock heads that are on my motor for thread depth, but I checked the rocker threads on a ported set I have in storage and sure enough they go right through to the intake opening. Never woulda thought to use thread sealing loctite, but will now.
I'll report back next time I get a chance to check the thread depth on the head in question. I can't believe it's pull that much oil through there, but ya never know.
I can't believe it's pull that much oil through there
You might not believe how little oil it takes to make a YYYYYUUUUUUUUUUJJJJJJJJJJJE mosquito fogger.
I hope it's something that simple. Pretty much about HAS TO be.
Don't use "Loctite" as we commonly speak of it. Loctite and Permatex are both brand names of the same company, and for their various products, they assign them to one or the other brand name. Maybe even both. The stuff in question is really just pipe dope, except, specialized for oil & gasoline resistance. RTV of any sort for example will DISSOLVE in gasoline.
I have a 99 LS1 that had a lot of miles and drag strip laps on it that was consuming oil. I researched into it and found that high rpm, low load situations cause ring flutter and that allows oil into the combustion chamber.
I can't prove it had ring flutter, but I tore that engine down once I swapped it and had no broken ring lands or rings, but the rings were worn out. I did a leak down test on it before it was pulled out and I had some cylinders with as much as 18% leak down, so yours could just be tired.
So, finding it to believe that my exhaust valve is actually bent, I did some more digging. I boroscoped the cylinder/piston and there was no smoking gun there. I then brought it up to TDC and removed the exhaust valve spring so I could get a good look at the exhaust valve on camera. I spun the valve around and around and all looked good from the top of the valve. I then kind of lapped it with my fingers, trying/hoping to feel something out of round, first against the piston and then against the valve seat. All felt fine. Well I put on a fresh valve seal and reassembled. Checked the threads of the rockers, and all were blind, no protrusion into the intake port.
Ran another compression test and it's healed! Compression back to 185. That puts it in line with the other cyl at 185-200! I dunno, maybe it was a tiny piece of carbon build up or something causing the valve to hang, but changing the orientation fixed it?
Took it for a ride, and it still burns oil on decel from high RPM. I think I'm going to order a new set of valve seals. The valve seals I'm using now are take offs from a 0 mile LS3 crate motor, the top hat design. My old valve seals were the older 2 piece design. Any difference if I'm using beehive springs?
OK so an update, several nights ago I replaced the questionable valve seals that I had in my motor. They were OEM, but I did not know the condition of them, and I'm bowing smoke with them, so out they went. I added some new hat style BTR valve seals. I used compressed air because I had it and was familiar with it, and is just quick. Swapped them all, the old ones looked ok, no gnarly ones. I measured the opening of the old one and they were about .310". The valve stem itself is 0.315". The new valve seals measured 0.300" on the button. I figured, Aha!, these new seals will surely keep oil away.
Well on to a test drive the next day...still smokes on decel. Same as before no change. Took several laps around the block, just to make sure, and the smoke didn't get better on decel from a high RPM.
So now I fix my sights on the PCV system. Despite me running the latest LS6 valley cover, and using a catch can, and also an oil separator on the clean side, maybe this is the culprit? Glancing past the Throttle body opening shows no pools of oil in the intake, just a slight coat of oil on things. Anyways, isolated the PCV system for a few pulls, and no better, still smoking on decel.
So now I'm fearing it's the rings, despite again, all my cylinders showing good compression now. I thought well maybe I can isolate a cylinder, so I disconnect a coil and injector, one cylinder at a time and went for a pull around the block. And despite me doing this 8 times for each cylinder, The smoke was always the same, so I cannot pinpoint this to any one cylinder.
So now I'm at a bit of a loss what to do next. I want to be absolutely sure that the problem is in the rings before I give it a poor mans rebuild. Anything else I should try?
Maybe a 40W oil?
I should mention I am running a new Melling 10295 stock volume, hi-pressure oil pump. Not sure if the higher pressure is causing me any issues?
Just an update, over the course of the last week, in a bid to try and find some clues, gave the pistons a soak with Berrymans B12. Also ran it in the crankcase at idle. It's an old school aggressive treatment but it's know to free up stuck oil rings. It took several days to do the soaking process, alternating the cylinders. Followed all this with an oil change today (came out inky black) and decided to run some 0W-40. Last time I was at the track oil temps were in the high 270s, so the thicker oil would likely be ideal, but not too thick where it can't be street driven.
Got it all buttoned up, went out for a rip, and still smokes. Unchanged.
Not sure what else I can check. Decided to pull the Intake Manifold, maybe there's a clue? A little bit of shine when you look into the the IM through the TB hole, but no oil pooling per se. Notice the oil around the seals of the IM and intake ports. Oil and soot made it's way pretty far up the runners of the IM. Long story short, I still can't figure out if it's the heads that are worn out, or the rings are worn out. Does anyone have any insight from reading all of this?
To sum it all up:
Smokes on decel from high RPM.
177k mile LS1. GMPP Hot Cam (-0.5 degree overlap)
LS6 PCV system, with a catch can on the dirty side, oil separator on the clean side
Replaced valve seals, twice, once with OE hat style seals, second time with BTR hat style seals.
Tried isolating a cylinder, unplugged fuel and spark from each cyl one at a time and went for a ride, still smoked each time
Compression test is all OK now, regained compression on cyl 8 by reseating the exhaust valve.
I have not performed a leak down, but what will that show me at this point?
Any advice is appreciated. I have a new set of heads that could possibly go on, that could eliminate the top end from the equation. While more power is always fun, I am very comfortable with the power I'm currently making as I learn from beginner HPDE courses. But I would hate to do the work only to have to pull the motor anyway. What else can I try to possibly pinpoint my issue?
A bit oil collecting around the seals I had this intake squeaky clean roughly 1500 miles ago. So to have this level of carbon and oil on the intake port is certainly interesting. This intake valve is open, is that wear on the stem? As I rotate the crank, and all other intake valves open, they all seem to have a similar looking wear spot when coming down and out of the guide.
Hello,
Time to get out the bore scope again. Check the condition of the cylinder walls... glazed, scored, or what ever. More important look at the carbon build up on the top of the pistons. Typical bad-oil using rings is that the carbon is washed clean around the edge of the piston from oil being sucked up passed the rings.
A cylinder with bad rings can pass the compression test because the "extra oil" will seal the rings during the test.
Also. how much play, looseness did the valve to guide have? Excessive wear there will cause oil use no matter what the seals look like.
I had this intake squeaky clean roughly 1500 miles ago. So to have this level of carbon and oil on the intake port is certainly interesting. This intake valve is open, is that wear on the stem? As I rotate the crank, and all other intake valves open, they all seem to have a similar looking wear spot when coming down and out of the guide.[/QUOTE]
Thanks for the replies gentlemen. I hadn't updated to this post but I did pull the valve springs off earlier this week to give the valves a wiggle test, and all appear/feel to be tight with no issues. I checked both the intake and exhaust valves. I have a spare set of disassembled heads with little miles on them, just for comparison, and they feel exactly the same way with the valve in the guide. The spare set of heads I have measured, and the stems and guides are in spec. I'd be shocked if the valve guides have any more than .001-.004" of clearance on the installed heads, they feel very secure in the guide.
I'll attach pics of my boroscope adventure. I certainly can't tell much of a difference, except a couple cyls are a little dirtier than others. My camera is not that great, it's a bit chunky, with a limited focus range, so to scope the condition of the walls is asking a lot, I can get it to focus and look at one specific spot just fine, but that's all. I can still see the crosshatch in the cyls.
Cyl 2 Cyl 4 Cyl 5 Cyl 7 Cyl 8 - This one looks the worst, and the carbon on top is a bit gooey Cyl 8 again, that mark is where I rested a valve against it.
Thanks for the pics......
Kind of looks like oil is cleaning off the edge of the pistons.
You should get a second opinion before surgery, I am not a real doctor , I just play one on TV!
Experts come on in !
Most pics look dry on top, if they are using oil they should look wet. That is why I think you get a second opinion.
Thanks for chiming in again. I attached a video of all 8 cylinders via my boroscope. Due to my camera I cannot see the entire piston crown at any one time, mostly just looking at the upper half of the piston. But all cylinders seem to have a cleaner edge, enough so that I cannot tell one or two pistons are definitely the issue. Maybe some expert eyes will see something that I cannot see.
07-23-2024, 09:38 AM
Burning Oil at high RPM, Low Compression on cylinder 8 
Sorry for the late response 
