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-   -   Can I have a bad ring but not burn oil? (http://www.m3forum.net/m3forum/showthread.php?t=334991)

graphite9 Sun, Sep-19-2010 05:51:04 PM

Can I have a bad ring but not burn oil?
 
So, Hello all im new to this forum. I recently picked up a 98 m3 with 157k, super clean for $4500. Problem is, it has one cylinder with 90psi. All others were 175-180. Thats why I got it so cheap. The prev owner said he was driving it, and it started running rough. He pulled over and it sounded fine at idle. Got back in the car thinking maybe it was too cold. Continued driving, to notice it felt like it was missing. Immediately went back home and parked it. Pulled the plugs, all looked clean. Towed it down to the shop and ran a compression test. Cyl 2 was abnormally low... 90. But the car doesnt smoke at all. No coolant in the oil, or vise versa. Is it possible you could drop that much in compression and not burn oil? I have budgeted for a valve job, but if its a piston im screwed!

THanks for any input.

Maginnovision Sun, Sep-19-2010 10:13:41 PM

There are many reasons a cylinder could be low. If you're doing a valve job you'll be able to check every possibility. Valve job should include pressure testing the head and decking it. You'll be able to see if the gasket decided to blow as well, which will have to be replaced anyway. Just because coolant and oil aren't mixing doesn't rule anything out. The mixing of the two is an indication something is wrong, not mixing doesn't mean there isn't a problem. If it is a bottom end problem you should be able to see that as well. I'd say go ahead with the valve job, and you should be able to figure out what's wrong. If nothing else, the shop SHOULD have done a leakdown test after the compression test. If they didn't, or didn't suggest it, don't go back there.

M3F1RST Sun, Sep-19-2010 10:17:26 PM

I was going to indicate the same thing, there could be valve issues or headgasket issues that aren't apparent, it doesn't have to be a bad ring to lose compression. If you do a leakdown test you can usually tell where the air is coming from to determine the issue.

graphite9 Sun, Sep-19-2010 11:03:46 PM

thanks for the input guys. Yeah I'm suprised they didn't do a leakdown test. Im just praying its not bottom end. The car has higher mileage, but I do know it was dealer maintained for its life, and it 100% stock. Is it very common to have a head gasket go, and not have fluids mix? And I was under the assumption that overheating was the cause to blow a gasket. (on a n/a engine). My mechanic is coming over to do a leakdown on it today(I hope). I have done major engine work on hondas, subies, and vr6's, but I'm new to BMW's. I really hope its just a head issue. If the leakdown indicated the bottom end, Im going to look for a good used engine, rather than having it rebuilt. If its a valve, I'm going to do a valvejob/HG, and all assc parts. How long do the chains normally go? My mechanic is super anal and wants to do it all. But in my opinion, if they are in spec, Id like to leave em...Thanks again for your input, Im just praying to the bmw gods its a HG, or burnt valve.

salty Mon, Sep-20-2010 12:44:42 AM

Squirt oil in the cylinder to isolate wether it is the valves or rings. (if the pressure goes up it is the rings) If it needs some rings, it might need a rebore as well. Bottom end refers to the crank and bearings, which is not related to compression issues. Good luck. I'd be going with a used engine if anything is terminal.

graphite9 Mon, Sep-20-2010 01:36:31 AM

well, I just did the oil trick. I dumped an oil bottle cap into the problematic cyl. Think It might have been too much because it jumped to 275...EEK. Im gonna wait till my mechanic brings his leak down tester over. I did notice a tad bit of oily looking substance on the plug when I first pulled it before the tests. But, It could be all the unburned fuel the in the cylinder. Does anyone know where the best source for used motors is? Seems like they run about $3500 with 80-150k. If the head needs a valve job, I can have it machined, and all associated parts+labor for under $1,700. If its the bottom, I'll part out the engine and grab a good donor.

egebhardt Mon, Sep-20-2010 02:33:48 AM

Nice. A big jump with oil in the cylinder. That's good news. Maybe a valve issue after all.

I put in a call to a www.bmrparts.com (Sacramento, CA) a month ago because my engine burns a quart of oil every 1,000 miles, which is not as bad as a lot of them I know of.

They wanted $2,500 for a used engine with 210psi, guaranteed. $100 shipped to anywhere in the Bay Area of CA. I did not purchase it and scrapped the whole idea.

salty Mon, Sep-20-2010 02:52:29 AM

A big jump in the pressure with oil implies rings not valves.

egebhardt Mon, Sep-20-2010 03:37:26 AM

Quote:

Originally Posted by salty (Post 4946572)
A big jump in the pressure with oil implies rings not valves.

Oh. Duh. My mistake.

Braymond141 Mon, Sep-20-2010 03:47:17 AM

Quote:

Originally Posted by graphite9 (Post 4946488)
well, I just did the oil trick. I dumped an oil bottle cap into the problematic cyl. Think It might have been too much because it jumped to 275...EEK. Im gonna wait till my mechanic brings his leak down tester over. I did notice a tad bit of oily looking substance on the plug when I first pulled it before the tests. But, It could be all the unburned fuel the in the cylinder. Does anyone know where the best source for used motors is? Seems like they run about $3500 with 80-150k. If the head needs a valve job, I can have it machined, and all associated parts+labor for under $1,700. If its the bottom, I'll part out the engine and grab a good donor.

An oil cap is certainly not that much. 275psi? That would lead me to believe you have a faulty tester.

Redo that test with your mechanic around. Make sure the engine is warmed up when you do this test.


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