Just wondering because I’m going to be replacing the rear main oil seal soon because it’s leaking. I’m wondering if it would be wise to first get my compression down to where it’s supposed to be and also check my oil pressure and bring it down too if it’s too high before replacing the seal.
What are your thoughts on this?
98 Mazda Protege 185k
What would make compression high other than heavy carbon deposits?
Anyway, compression and oil pressure have no effect on the main seals.
What you want to do is make sure the PCV system is working properly so the crankcase pressure is not too high.
p.s. Worn, leaky rings could raise crankcase pressure due to blowby.
There is no such thing as excessively high compression. It’s strictly a function of the compression ratio. As for high oil pressure the only thing that would cause that would be a malfunctioning / stuck relief valve in the oil pump which is unlikely. Your leaking seal in probably just due to wear.
What’s the procedure for cleaning heavy carbon deposits? Does the cylinder head have to be removed or could I just use some cleaning solvent poured in through the spark plug holes followed by an oil change?
A few years ago the compression was a little over 200 psi in each cylinder so I bet it’s climbed at least another 5 psi or so. It’s only suppose to be 195 psi/cylinder. The only thing I can think of that could be problematic with overly high compression is that it probably puts a little extra strain on the battery each time I start the car.
Any other reasons why compression should be kept at factory specs? Or should I just leave it alone and keep driving it the way it is?
Try doing a compression test. I’ll concede that heavy carbon deposits could raise the compression readings but, boy, that would be a lot of carbon.
Some use Seafoam or Tekron. Do you have pinging and does the engine run on after you turn it off? I would try either and see if it helps.
A restricted exhaust will cause compression pressure to increase. I have seen 215psi from a well worn engine with a toasted catalytic converter.
If it’s the original seal then it needs to be kept in mind that rubber is 17 years old and a leak is not unexpected.
That could be combined with a seal wear groove on the crank journal and the problem becomes even worse.
If a wear groove is present then a Speedi-Sleeve will be needed to repair the journal surface.
It would also be a good idea to make sure the engine main bearing thrust surface is not worn and allowing the crankshaft to flop around. You could check some of this by grasping the crank pulley and shoving it towards the transmision. Now force it the other way. There should be very little movement. This type of problem is more prevalent with a manual transmission car that has seen some hard driving and heavy clutch use.
As to compression I wouldn’t worry too much about it. If carbon is an issue you could dribble some water into the intake tract (very slowly) while the engine is running. That will scrub it down some. 
I have a feeling the OP was talking about excessive crankcase pressure, most likely due to excessive blowby, and may be confusing it with compression and oil pressure. Excessive crankcase pressure would cause seal leakage and even seal blow-out. Oil pressure and cylinder compression will not do that. The crankcase breathes through the pcv valve, and excess pressure should back flow through the fresh air tube. If the pcv valve and fresh air tube are clear, then the only fix would be an engine overhaul.
My2cents, don’t confuse compression with compression ratios. The ratio is strictly a calculation, the compression in psi is a function of not only the ratio but also how the engine is aspirated and its inherent pumping losses. The compression actually varies depending on the engine operating conditions. A stock engine operating at low rpms with an almost closed throttle plate and no artificial aspiration (supercharger or turnbocharger) doesn’t develop the amount of pressure that an engine operating at higher speeds with an open throttle plate does, or that an engine with a turbocharger or supercharger pushing air in does.
Compression tests confuse the issue, because they check for the cylinder’s ability to compress gasses strictly based upon the mechanical compression ratio without the engine in operation. They don’t give a reading of the pressures reached during operation.
There are some other variables (N2O, anyone?), but that’s the gist of it.
I agree with OK4450 about the fact that seals do fail over time. They’re just rubber and have a metal shaft spinning in them thousands of time a minute. You can figure for yourself based on the mileage and an estimated average rpm and speed how many times that shaft has turned in the rubber seal… it’ll be a whole bunch. And I’m not even counting the absolute fact that even an unused rubbery bit will deteriorate with age.
I also agree with Busted that it’s crankcase pressure that pushes oil past worn seals, and the Positive Crankcase Ventilation (PCV) system is prudent to check out. If you’re going to change the rear seal, that’s only prudent. Might’s well check everything out.
You need only worry about the compression if it’s too low. That’d mean there’s too much combustion pressure blowing by the rings pressurizing the crankcase. It doesn’t sound based on what you’ve written that that’s the cause of your seal leakage.
‘‘I also agree with Busted that it’s crankcase pressure that pushes oil past worn seals, and the Positive Crankcase Ventilation (PCV) system is prudent to check out.’’
The fresh air tube is that short little hose that connects the PCV valve to the intake manifold? So the entire PCV system is just the PCV valve and the fresh air tube?
‘‘Try doing a compression test’’
I was planning on it. Maybe I’ll do one tomorrow and post the results.
‘‘Do you have pinging and does the engine run on after you turn it off?’’
Nope. The engine runs/sounds perfectly fine. Starts right up when the key is turned and shuts down immediately after I turn it off.
‘’ I have seen 215psi from a well worn engine with a toasted catalytic converter.’’
I’m almost certain my 3-way catalytic converter is toast. So certain that I recently bought one - just haven’t installed it yet. And it’s funny you say 215 because that’s about where I was guessing the psi is at. Hmm… maybe on to something here.
Nice posts BTW. Especially ok4450 and tsm. It wouldn’t surprise me in the least if I found out you guys had written college level automotive instruction manuals at some point in your lives.
I often forget how much science is involved when it comes to engines but then I come to this site and start reading.
The fresh air tube is the hose from the air filter housing or intake track before the throttle body to the block or valve cover without a valve. The PCV line runs from the PCV valve to the intake manifold. This creates a circuit that should keep the engine block from pressurizing.
@the same mountain bike - I was referring to the BASIC compression of an engine as measured during a compression test when no other factors enter the picture, not the compression during operation.
Finally got around to testing the compression.
1: 215
2: 220
3: 220
4: 220
I love how 3 of the 4 cylinders are identical and that #1 is only slightly different. I’m under the impression that the closer the cylinder pressures are to one another the healthier the engine? My only concern is that this engine is only suppose to be @195 psi. Maybe that’s just what it should be at when the engine is brand new. Don’t really know. I’m going to replace the 3-way catalytic converter and do another test. Should be within the next couple of weeks.
Don’t pull the catalyst yet. Remove the up stream O2 sensor and check compression on the most convenient cylinder. If the compression drops it would indicate a restricted exhaust but not necessarily a cat.
Mystic, thanks for the compliment. I just last year retired from a college. I’ve written quite a few college courses, including developing an entire technical degree program in cooperation with other faculty (me being the lead), but not on the subject of automotive. I guess I just developed a knack for explaining things.
My2cents, I got it now. Apologies for the misunderstanding. No disrespect meant.
I find it hard to believe that a car that runs fine could possibly have an obstructed exhaust system such that it would cause high cylinder compression readings.
I’ll just reiterate that compression testing of similar engines even with high miles have shown pressures in excess of 200 PSI and it was all completely normal.
My opinion is that should be dancing in the street to have that kind of pressure at 185k miles.
The alternative would be having 85 PSI and the engine is toast…
If you’re really and truly concerned about this you might consider purchasing a vacuum gauge. These are cheap, easy to use, will last a lifetime, and can tell you a lot about what’s going on with an engine.
A gauge is 20 bucks; converters are considerably more.
@the same mountainbike - Appreciate that. I guess my post was a bit sparse and unclear. So - no prob.
Yes @ok4450, a vacuum gauge could confirm or eliminate the possibility of a restricted exhaust. I certainly hope that the OP doesn’t replace the cats based on the possibility that they are failing.