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Old 05-13-2018, 12:26 PM   #1
moustacio
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Default Lower Compression on Cylinder #3

Hi everyone,

Would like to ask your opinion about my compression test results on my b230ft in a 250K + mile 88' 740. The car sat for 3 years before I stated driving it again 2 years ago. Lots of blowby, no piston slap.

Not sure on the accuracy of the gauge I used but I got ~120psi wet and dry in cylinders 1,2,4 and ~80 psi dry, 120 psi wet on cylinder number three. I performed the test correctly, however only ran the car for a few minutes before starting (temp gauge maybe 1/8 of the way from cold). Last compression test I performed, maybe 30K miles ago, gave me ~135psi on all four and the car was at operating temperature.

Other than the poor results the car runs and drives just fine, idles a little rough when first started, with decent power and boosting to about 10psi. Last couple of 180 mile highway journeys at 75mph gave me ~26mpg and I average ~22mpg with combined driving.

You think I'll be alright continuing to drive to work and that?

I'm hoping it's stuck piston rings that I can free up. What do you think the chances are that the number 3 cylinder has broken ring lands or rings or just wore faster than the rest? Or, what's the chances that cylinder #3 has stuck rings?

I don't have the time, money, or garage to open up the engine, hence trying to get informed opinions. Thanks for any good or bad news you may have for me.
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Old 05-13-2018, 01:13 PM   #2
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Drive it, keep your eyes out for a replacement engine and start saving.
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Old 05-13-2018, 01:26 PM   #3
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I used to have a Saturn, some of which had issues with oil rings getting clogged up with carbon. A popular remedy for that was to soak the pistons with Marvel Mystery Oil overnight right before an oil change.

http://www.saturnfans.com/forums/showthread.php?t=80118
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Old 05-13-2018, 02:22 PM   #4
moustacio
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Hi and thank you,

Just picked up a bottle of MMO before writing this. I'll give that a go next weekend with fingers crossed, and follow up with a fully warm compression test.

How much time do you think she has doc?
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Old 05-13-2018, 03:32 PM   #5
EivlEvo
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What did/do the plugs look like?
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Old 05-13-2018, 03:43 PM   #6
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No amount of tuneup in a can will remove carbon from behind the rings, if that is the problem.
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Old 05-13-2018, 04:49 PM   #7
moustacio
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Hi and thank you,

Spark plugs all look good and I wouldn't be able to tell one from another. No oil fouling on #3 or the rest. Maybe a bit lean running but I've heard that's typical? with ethanol in the gas.

Forgot to mention that vacuum readings are low with what I assume is the stock T cam. I get maybe 12inHg on startup, 15inHg at operating temp, and not much more on deceleration. It varies but I'd say around 17inHg with foot off the gas.

Also I've never felt a misfire.

Thanks again for any additional information
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Old 05-14-2018, 03:33 PM   #8
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Do a leak down test on #3, if a lot of air comes out of the oil fill cap with the cap off the rings are stuck to the piston. The reading of the leak down test should be 20% or better for a good motor. Also check the valve lash. Listen to the intake opening as well as the exhaust pipe, to see where air is leaking.
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Old 05-14-2018, 11:45 PM   #9
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Thank you for the response.

Valve clearance was within specification when I checked about 4,000 miles ago while changing the gasket.

I will definitely ask some of my neighbors to see about an air compressor for a leak down test. Thanks again and I'll post back with any updates.
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Old 05-15-2018, 12:47 PM   #10
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if it's not running poorly or fouling plugs i wouldn't worry about it at all.. i have a 91 240 with 5/16 of crankshaft endplay, and i wouldn't have known if i never would of did the timing belt
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Old 05-15-2018, 09:58 PM   #11
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Hi and thank you for the bode of confidence.

I really love this car, even with its problems and shot interior. I'll hopefully get some more diagnostics done along with some piston soaks in the hopes of freeing possibly stuck rings. I know it's cheaper to buy a junkyard engine but hopefully I will continue to drive the car until I can do an engine rebuild.

And yeah I had no reason to perform a compression test other than to see the state of the motor before buying shocks and struts. But as these are safety items and the car runs and drive fine I will go ahead with these repairs.

Thanks again everyone for your thoughts and suggestions.

I run 15w40 in the summer here, would it help/be wise to run 20w50?
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Old 05-16-2018, 08:08 AM   #12
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stuck with 15w40 , all that really thick oil is good at is wearing out the oil pump drive lol, it's good in racing engines but i don't run it on the street
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Old 06-08-2018, 10:47 PM   #13
moustacio
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Hey everyone,

Thought I would give an update and see if anyone has an opinion on if my situation sounds like damage to the piston/rings or badly stuck piston rings.

About a week after I first posted I did an MMO soak and ran a hot compression test on only cylinder 3 with all other plugs in and forgot to open the throttle body. Got 100psi.

Did another hot compression test today with a different (and assumed inaccurate) gauge. Got around 70 psi on #3 and 90psi to 100psi on the rest.

So it seems over the different compression tests that cylinder 3 is around 30 psi lower than the rest.

I also ran a caveman type of leak down on #3 which resulted in air escaping past the piston and into the crankcase

Thanks again everyone
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Old 06-09-2018, 10:05 AM   #14
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I’d run it and not even worry. How much oil does the car use? Engines are soooo cheap for these cars that it really doesn’t matter. I’ve found that these redblocks do not give a single f*** about anything. They can be run without coolant, oil, with a blown hg (for a LONG time), with a thrown rod etc. I’d just drive it..
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Old 06-09-2018, 01:01 PM   #15
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^ agreed.
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Old 06-09-2018, 03:04 PM   #16
moustacio
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Hey thanks guys I appreciate it.

looks like I will keep up with maintenance as usual and try not to worry about the engine.

Promise ring: the car leaks so much oil out the rms that I don't know how much oil is getting burned up. I would guess not that much strangely enough. I never see smoke out of the tail pipe will idling or from my mirror while driving. And the plugs look normal so I don't know.

Maybe it's the case that the oil control rings are in working order buts the compression rings aren't doing there job.

What would y'all think about sealing the rms up? I hate leaving oil everywhere I go but if it will just pop out again from so much blowby maybe I shouldn't. I have the crankcase vented somewhat well now. I have an 2nd 15mm line from the breather box and another from the block distributor hole going to a catch can. Also, the breather box and drain are clear and freely flowing.

Anyway, I appreciate the help and support with my ailing motor
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Old 06-09-2018, 03:44 PM   #17
PromiseRing
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Clean and RTV the living crap out of it. Just milk this motor for a while longer lol
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Old 06-09-2018, 03:45 PM   #18
EivlEvo
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I'm a little confused about your question... if the RMS is leaking, and you're inclined to service it, why wouldn't you?

If you're implying that sealing the motor up better will increase the blowby of your pistons... I mean sure, but also there's potential that it can help as well. Garbage compression or random leaks can mess with mixture and all that which can gum up the works.
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Old 06-09-2018, 04:18 PM   #19
moustacio
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Hi and thanks again guys,

I'm a confusing fellow

I was asking if I was to put in a new rear main seal, would it just start leaking oil again due to the crankcase pressure. But it seems like that isn't the case so I'll go ahead with the fix when I can find a garage to work in.

Thank you
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Old 06-09-2018, 06:40 PM   #20
PromiseRing
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Quote:
Originally Posted by moustacio View Post
Hi and thanks again guys,

I'm a confusing fellow

I was asking if I was to put in a new rear main seal, would it just start leaking oil again due to the crankcase pressure. But it seems like that isn't the case so I'll go ahead with the fix when I can find a garage to work in.

Thank you
Oh sorry, I misunderstood your question so ignore my prior advice.

I don't know anything about the RMS repair except that it's probably wise to use an OE volvo seal unless you like pulling transmissions.
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