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Tracking compression in high mileage engines #5132193 06/12/19 10:23 AM
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WA1DH Offline OP
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Out of complete curiousity, has anyone tracked compression loss versus mileage on a high mileage engine? My Ford Escort has just reached 245k miles. I do a plug/wire change around every 50k mi and started the habit of doing a compression test while I have the plugs out. Here's what I noticed:

200,000 mi:
Cyl 1 - 170 PSI
Cyl 2 - 170 PSI
Cyl 3 - 170 PSI
Cyl 4 - 180 PSI

245,000 mi:
Cyl 1 - 155 PSI
Cyl 2 - 155 PSI
Cyl 3 - 150 PSI
Cyl 4 - 170 PSI

Those are all dry compression numbers. I read that Ford specs 180-190 PSI new on these engines and 100 PSI is the rebuild threshold. If there's any linearity to the drop I can estimate around 380k miles before I hit the rebuild PSI. It currently burns a quart of oil every 4,000 miles.


1995 GMC K1500 5.0 V8 148k mi
1999 Ford Escort 2.0 I4 240k mi
Re: Tracking compression in high mileage engines [Re: WA1DH] #5132202 06/12/19 10:29 AM
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Imp4 Offline
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When it comes to old cars and beaters, when I mash the right pedal and it accelerates reasonably and with no hesitation I figure the compression is just about good.

You'll not catch me doing a compression check on at 20 year old Escort unless I'm desperate.


2001 Mitsubishi Galant 2.4 - 165k+
Grandma's creampuff...
Re: Tracking compression in high mileage engines [Re: WA1DH] #5132218 06/12/19 10:55 AM
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ecotourist Offline
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Very interesting results..

There was very little decline in compression right up to 200,000 miles, then quite a loss from 200,000 to 245,000. My bet is that the compression will fall off rapidly from here. That little bit of "looseness" is likely to become self perpetuating.


2000 BMW 528i 5MT M Sport
2007 Honda Accord EX-L 4Door V6 6MT
Re: Tracking compression in high mileage engines [Re: WA1DH] #5132221 06/12/19 11:01 AM
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Trav Offline
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Compression loss is not generally linear as can be seen by you own numbers #4 lost 10 psi wile others lost 14 and 20 psi, there are many factors that can effect compression.
Wear is just of them but when oil consumption low the bores and ring tension are usually good enough to hold decent compression, carbon deposits in the upper rings can cause sticking and loss of tension and rotational movement. rings with this condition can do some really weird things, both ring end gaps can actually align and this will have a greater loss. The valves also play a role but a leak test will be needed for best results.

Engine temp and ambient temp can both effect the reading, taking a test in Dec at 30f on a cold engine will have lower numbers than one in July at 95f cold engine, try to test a warm engine not cold as parts seal better warm or hot and try to keep the ambient temp range close between testing.
What I do if I start seeing things like this is do a top end overnight piston (or even the whole weekend) soak then run a can of Berrymans Chemtool in the oil at idle only for 15-20 min then do and oil change and use a can of rislone for a full OCI or 16oz Kreen for 1K then retest.

I did one not long ago with 150 - 165 on all 4 and 1K later it had 210 on all 4.

Edit: When doing a compression test make sure its WOT only anything else will produce lower readings, a leak down test can better help determine where the problem lies.

Last edited by Trav; 06/12/19 11:04 AM.

ASE L1, Master. Deutsch Meisterbrief.
Re: Tracking compression in high mileage engines [Re: WA1DH] #5132240 06/12/19 11:29 AM
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I'd imagine you're going to have some other fault making it non-cost-effective to repair before the compression loss does.

Heck if it were here where there are salted winter roads, the rusted through unibody would be unsafe to drive by now.

Last edited by Dave9; 06/12/19 11:29 AM.
Re: Tracking compression in high mileage engines [Re: WA1DH] #5132264 06/12/19 11:55 AM
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addyguy Offline
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For an engine with 245k miles, I'd say these results, including the oil consumption, are pretty darn good.

If you like doing compression tests, it might be interesting to add Restore oil additive to the engine, run it for 5k, and see what your numbers look like after.

They advertise it by showing before-and-after compression tests. Yours would be an interesting real-world test.


2010 Kia Soul 2U - 2.0L I-4, auto; 146k miles.
Castrol GTX HM 5W-30, STP blue bottle; Fram TG 9688.

2009 Pontiac G5 - 2.2 I-4, auto, 121k miles.
Oil/filter TBD
Re: Tracking compression in high mileage engines [Re: addyguy] #5132309 06/12/19 12:54 PM
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Trav Offline
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Reports have been made of bearing damage using that stuff, I never used it but I wouldn't risk it.


ASE L1, Master. Deutsch Meisterbrief.
Re: Tracking compression in high mileage engines [Re: WA1DH] #5132408 06/12/19 02:55 PM
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atikovi Offline
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Just normal variations on the testing method could account for the differences shown between the two tests. An extra engine revolution or two could raise the reading 5-10psi. How much voltage and the condition of the battery could influence cranking speeds and psi readings. Oil and engine temperature too.

Re: Tracking compression in high mileage engines [Re: WA1DH] #5132570 06/12/19 07:07 PM
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gfh77665 Offline
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A perfect candidate for Restore. Proven effective to increase compression.

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