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thehackmechanic

blow by, horrible leakdown numbers, but not burning oil

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In addition to sorting out the lean running issue on the tii, I'm also trying to wrap my head around the issue of blow by. Compression is quite respectable (about 160 +10 in all cylinders), but a leakdown test showed a mind-bendingly-large 30% leakdown in all cylinders, with the leak coming past the piston rings. Pulling off the oil filler cap while the car is running unleashes a lot of air, confirming the leakdown test. However, the car burns no oil, at least not to the eye. I haven't driven it enough to know if it's consuming any. The guy I bought the car from said the previous owner had it stored for about 12 years.

The blow-by is not oily. There isn't gunk in the valve cover outlet, the breather tube, the air cleaner, or under the oil cap. There is, however, a LOT of black varnish coating the valve train. A coating of carbon is visible with an inspection scope at the top of the piston, but when I pulled one of the intake plenums off to look at an intake valve, I expected a horror of carbon, but the valve looked pretty clean.

I've bought cars that sat for a long time, and I always associated "stuck rings" with visible oil burning, and assumed that when the burning stopped, the rings had become "unstuck." Here, the combination of good compression, no visible oil burning, atrocious leakdown, and hurricane-like blow-by has me a bit flummoxed. Which of the three rings being stuck fits this symptomology?

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Every M10 I have been around pumps a lot of air from the filler cap. These cars have no PCV system. That air itself does not mean blowby from bad rings. My motor since new pumped out enough air to bow your hair. (ha I am a poet). the motor is running good, has good compression, does not smoke, then just drive and enjoy.

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Guest Anonymous
Every M10 I have been around pumps a lot of air from the filler cap. These cars have no PCV system. That air itself does not mean blowby from bad rings. My motor since new pumped out enough air to bow your hair. (ha I am a poet). the motor is running good, has good compression, does not smoke, then just drive and enjoy.

How many M10 engines have you been around and do you know what a leakdown test is?

The stock M10 engines all had positive crankcase ventilation. Every stock M10 valve cover has a venting tube from top of the valve cover to the intake. It serves to reduce unburnt hydrocarbons and vent the crankcase.

___________________

Hack, if your leakdown numbers are obtained with a cold engine and your compression numbers are with a warm engine, you could be dealing with large ring gaps or irregularly shaped and/or tapered cylinder walls. Reaching operating temp and normal expansion might be enough to seal things up. But with your readings, something else seems amiss.

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Both the compression and leakdown numbers were obtained with a warm engine. Both tests were repeated. The first time I had the valve cover off and verified TDC by looking at cam lobe overlap of corresponding cylinder (e.g., #1 is at TDC when lobes of #4 are overlapped, etc). The second time I just ball-parked it by looking at the rotor position relative to the notch on the dizzy for #1. When it's off because a valve is open, you know it -- you get 100% leakdown in that cylinder. Leakdown tester is a brand new OTC 5609, presumably not a piece of garbage. Has two actual pressure gauges that read real PSI, as opposed to one for inlet that reads PSI, and another that reads calculated leakdown percentage. I set the inlet pressure from my compressor to 90PSI, and read about 60PSI on the second gauge on all cylinders, yielding 33% leakdown. If a valve is open (ie, trying #4 when #1 is at TDC), the second gauge reads very close to zero, as anticipated.

I suppose I should try it on a known good car to verify the leakdown tester.

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Guest Anonymous
Every M10 I have been around pumps a lot of air from the filler cap. These cars have no PCV system. That air itself does not mean blowby from bad rings. My motor since new pumped out enough air to bow your hair. (ha I am a poet). the motor is running good, has good compression, does not smoke, then just drive and enjoy.

How many M10 engines have you been around and do you know what a leakdown test is?

The stock M10 engines all had positive crankcase ventilation. Every stock M10 valve cover has a venting tube from top of the valve cover to the intake. It serves to reduce unburnt hydrocarbons and vent the crankcase.

___________________

Hack, if your leakdown numbers are obtained with a cold engine and your compression numbers are with a warm engine, you could be dealing with large ring gaps or irregularly shaped and/or tapered cylinder walls. Reaching operating temp and normal expansion might be enough to seal things up. But with your readings, something else seems amiss.

I am an anonymous coward and a douche

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Guest Anonymous

Until you differentiated the hot vs cold, your numbers reminded me of using exotic high compression forged pistons. They were very problematic until warmed up and were never particularly reliable over the long run. With respectable compression numbers, it may be that one of the two compression rings is stuck or worse, but the likelihood of that happening on all cylinders is like MikeS revealing what really is in Hangar18.

It sounds like you know your way around LDTs, but gauges can be off, but? Do you remember dropping them from a three-story building? lol Even without the gauges, a good ear is sometimes a decent substitute, still the idea of trying it out on another vehicle or trying another LDT makes sense.

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Hack what I meant earler is M10s all have a lot of air that pumps out of the oil filler cap so that really is no syptom of anything bad. But 33% leakdown is not good. I do not know much about that portion but you may want to wait for Marshall to chome in. He is pretty good at this stuff

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Guest Anonymous

Rob,

Have you tried using a vacuum gauge as a backup? It sure beats hooking up the compressor!

Plug for Rob.

ps: To the other poster, Rob has been hacking for decades and if you join BMWCCA, you can read his column. He also has a running blog, and, dare I say it, a pending book release.

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Given that the engine sat for a long time with out running, I would bet on stuck rings. Try some Marvel Mystry oil in the cylinders and let it set over night. You will get a lot of smoke when you start it in the morning. After that I would run it up to 6,000 RPM for as long as you can get away with it. That should get everything moving. Of corse your other option is to rebuild the motor. G

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Guest Anonymous

newbie here but if using compressed air for leakdown couldnt valve condition affect numbers, call me a rookie but the air has to go somewhere?

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Rob,

Have you tried using a vacuum gauge as a backup? It sure beats hooking up the compressor!

Plug for Rob.

ps: To the other poster, Rob has been hacking for decades and if you join BMWCCA, you can read his column. He also has a running blog, and, dare I say it, a pending book release.

Lewis I guess this may be directed at me and I am not sure why. I showed no disrespect. He asked a question on a public forum so we call can offer ideas and opinions. I have read his articles, I know who he is. I am sure if I had somehow offended him he would have said something. I kindly threw in what I thought as have others.

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Rob -

What are your long-term plans for this tii? Do you have room to perform an engine rebuild? I like the idea of just driving it, changing the oil and keep monitoring the performance until you decide to rebuild it. In my case, the tii had sub-par compression numbers on 2 cylinders and the KF pump was leaking oil.

jim

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@uniformed, Lewis, Nbristow--> Just because I write a column doesn't mean I'm not a 2002 guy just trying to figure things out like everyone else. I'm not a pro so there are certain holes in my knowledge, and I'm very candid about them, which is why I, like everyone else, post questions here. I've had 25 2002s but have, in fact, never paid any attention to the amount of air coming out the oil filler cap until this one, so I can't say it's better or worse than any other 2002. All comments are appreciated.

@Jim--> "What are your plans for the car" is in fact THE question. I was letting my left brain churn through this last night. My plans are to: Own it forever, sort out the lean-running FI issue because I want to understand them, drive the snot out of it when the weather is dry and keep out of the moisture when the weather is not, NOT restore the body unless the right rear shock tower gets scarier, and NOT drop thousands into an engine rebuild when the car runs well and isn't burning oil. Like the lean-running issue, I want to understand how a car with good compression and no oil burning can have such horrible leakdown numbers. Hopefully the answer is "it doesn't -- you're doing the test wrong or the LDT is foobar'd."

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@uniformed, Lewis, Nbristow--> Just because I write a column doesn't mean I'm not a 2002 guy just trying to figure things out like everyone else. I'm not a pro so there are certain holes in my knowledge, and I'm very candid about them, which is why I, like everyone else, post questions here. I've had 25 2002s but have, in fact, never paid any attention to the amount of air coming out the oil filler cap until this one, so I can't say it's better or worse than any other 2002. All comments are appreciated.

@Jim--> "What are your plans for the car" is in fact THE question. I was letting my left brain churn through this last night. My plans are to: Own it forever, sort out the lean-running FI issue because I want to understand them, drive the snot out of it when the weather is dry and keep out of the moisture when the weather is not, NOT restore the body unless the right rear shock tower gets scarier, and NOT drop thousands into an engine rebuild when the car runs well and isn't burning oil. Like the lean-running issue, I want to understand how a car with good compression and no oil burning can have such horrible leakdown numbers. Hopefully the answer is "it doesn't -- you're doing the test wrong or the LDT is foobar'd."

Rob my apology if I offended you. You had stated it had a lot of air coming from the oil filler cap in the original post. I was just saying I don't think that is a symptom because all the 02's I have been around seem to do that. You have owned 25 02's..I have owned one so obviously you know quite a bit more than me but I was just trying to help. I have no idea who Lewis and Uninformed is and its to bad they or he messed your post up

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