Bad piston rings or cross-threaded spark plug?

Post by **findude** » 02 Oct 2009, 06:29

My 1996 850 NA is at just under 100,000 miles and has been using oil and smoking since about 90,000 miles but has lately gotten much worse and will likely fail its next emissions inspection on “visible smoke” alone. At this point I’m putting in a quart of oil every 500 miles or so; there are no oil leaks. There is no CEL and a scanner borrowed from Advance Auto reveals no codes at all, though I still have a problem with monitors being “incomplete” (haven’t been able to successfully complete the “learning” process either). The car also seems to be running rich, but I think that is an unrelated problem.

I did a compression test (average of two, one hot the other still warm)
#1: 220 psi
#2: 216 psi
#3: 214 psi
#4: 196 psi (220 after adding a bit of oil in chamber)
#5: 219 psi

Spec (from Haynes manual) calls for 185-213 psi, with maximum variation of 28 psi. I’m assuming the readings above 213 simply mean my pressure gauge is out of calibration.

From previous experience and what I’ve read, these results condemn the piston rings on the #4 cylinder. However, the #4 spark plug was very hard to remove and reinstall, and it makes me think it may have been cross-threaded at some point—like maybe when the spark plugs were replaced at the 90,000 mile service. It does seem unusual for an 850 engine to have bad rings with so few miles on it, so I’m wondering if it’s something other than the rings in this one cylinder.

If the spark plug is cross-threaded, is it possible that the reduced pressure in the combustion chamber would be allowing more oil past the rings? If so, will solving the cross-threaded problem restore pressure to that cylinder and solve the oil-burning problem? Pouring a bit of oil into the chamber would have coated the spark plug threads: could that be an alternative explanation for the pressure coming up to normal after adding the oil? Would having a machine shop repair the threads solve the problem?

I appreciate any suggestions, advice, condolences, etc.

FCPEURO · Post by **FCPEURO** » 02 Oct 2009, 08:14

If adding oil made the compression go up, it is bad rings. The threads on the spark plug hole would have no affect. Also, the smoke is oil getting by the rings and burning off.

Craigd2599 · Post by **Craigd2599** » 03 Oct 2009, 06:54

try doing a "seafoam" on the top end just in case it's stuck and not broken. The plug was probably so hard to remove because the burnt oil sludge glued it in place. Instead of Seafoam use GM top engine cleaner. There is a good link here on the site about how to use the vacuum tree to do the job. It's worth the $15.00

Post by **wheelsup** » 03 Oct 2009, 22:04

Those are really high compression numbers (even the "low" one) to be burning any sort of oil. I wouldn't expect visible oil burning to happen until 150 or below...

Craigd2599 · Post by **Craigd2599** » 04 Oct 2009, 06:13

Thats a good point Wheels. I'd definitely invest the $15 and do the top engine treatment. a piece of carbon could be wedged in there or something else has the ring stuck. Ya never know

Post by **wheelsup** » 04 Oct 2009, 10:36

Can't a clogged PCV system also cause oil burning?

FWIW I put 1/3 of a can of seafoam into the oil of my wife's VW, ran it at idle for 5 minutes, and it went from burning 1 quart/700 miles to 1 quart/1500 miles. But her compression was low - half were at 160 (good as new) and half were around 95 (!). I haven't checked the compression again but might after I do another round of seafoam treatment.

I might try that GM top engine treatment however.

Oh and my #4 cylinder was also the lowest on my test. I've heard from several others they had the same results. Not sure why.

FCPEURO · Post by **FCPEURO** » 05 Oct 2009, 08:44

The seafoam should help a lot. Just don't run it in the oil for long.

Craigd2599 · Post by **Craigd2599** » 05 Oct 2009, 08:56

when you do the top engine treatment you suck it into the throttle body by feeding it into the vacuum tree. There is a write up here on doing it. Be VERY careful not to overload it or you can hydraulic a cylinder. Use about 1/2 to 2/3 can in the top, pour the rest into the crankcase. Plan on a plug change when you are done, as all the crud clings to the spark plugs. You must change the oil too. After that, I dump a bottle in a full tank twice a year. Thy tell you not to, but I have never had a problem as long as it's diluted in a full tank of gas. You will have quite a lot of carbon smoke when you are doing this so do it outdoors. Once you have fed all the treatment into the car, drive it around the block, accelerating at FULL throttle to about 30mph several times. This will blow the stuff free. You will be shocked how well this helps overall performance.

Post by **findude** » 05 Oct 2009, 19:58

I'm going to try the SeaFoam treatment and change to a heavier oil. I won't get to this until after the long weekend, but I'll report back.

Thanks for all the input.

my ovlov · Post by **my ovlov** » 17 Oct 2009, 02:10

Could be a PVC/Oil Trap/Flame trap issue. With the engine running and your oil cap off, is air being forced out of the oil port? Is there air coming out of the dipstick port?