M20 toast? Is a leakdown test worth doing?

[danielmp]

Bought this car in November, 1990 325i with ~170,000. Head was rebuilt with 3 angle valve job at 150,000 miles for unknown reason. I've been driving the car a lot over the last two months and it's been losing a lot of oil, about 1 quart every 600 miles. No oil leaking onto my driveway.

The car was running great, pulls very hard for an m20 all the way to the red line and no signs of smoke from the muffler or oil in coolant or vice versa - but I was definitely worried about the oil loss. Over the last week it was running a little rough at idle but otherwise drove fine. Then yesterday on my way to work it started missing, most notable under load with a significant loss of power. I had changed the plugs when I bought the car (3-4k miles ago) and they looked very old with some mild burned oil residue across the board. I changed them again yesterday evening, 1-5 looked good but #6 was covered with a caked burned oil residue. After the plug change, the car is running great again, no miss.

plugs 1-6:


closeup 5 and 6:


Seems to me that oil is getting past the piston rings in cylinder 6, probably busted rings? I talked to my mechanic before changing the plugs and scheduled a compression and leakdown test for Friday. Do you guys think it's worth paying 2 hours of labor for those numbers or is the diagnosis already clear? I'm thinking I should just cut my losses and look for a used m20 to swap in - adding oil and changing plug number 6 in the mean time as needed.

Thoughts from the experts are appreciated, I'm still somewhat of a noob.

[jlevie]

Compression and leak down tests will tell what the condition of cylinders and valves is. Given that the head was rebuilt and now cylinder 6 appears to be burning oil, my suspicion was that the head job was to fix damage from an overheat. The hottest cylinder on the engine will be cylinder 6 and if the overheat was severe that may have damaged the rings. But it is possible that the oil burning is from an over rich condition on the cylinder (injector problem) or it could be from bad valve seals and/or worn valve guides.

Have the tests done and you'll have a better idea of what is going on and can then formulate an appropriate course of action.

[danielmp]

Thanks Jim, glad you posted the first response as I now have my answer. I appreciate the help and I'll update the thread with compression/leakdown results.

[frolf]

Jim Levie is the best.

[ZM Blue Devil]

Jim Levie is the best.

Quoted for the truth, lucky to have him.

[danielmp]

My mechanic thinks the problem is likely the oil ring in #6. The car doesn't smoke on startup and he thinks that makes valve guide unlikely. He thinks the compression and leak down will not be too helpful cause the compression ring is likely good by the way the car drives and good gas mileage. If the valve guide seals are bad the compression and leak down should be normal anyway right? So if this is a valve guide seal or piston oil ring the compression and leak down are likely to be normal. I told him to do them anyway, hoping I'm not wasting time and $ and will have a better idea with the results. Thoughts?

[MR 325]

You shouldn't be wasting money on any tests, the rings are bad.