Compression test questions

TisMe · Post by **TisMe** » 18 Oct 2021, 15:53

patcal wrote: 18 Oct 2021, 15:35
Maybe I will buy another one redo the test dry & wet this time.

Perhaps a good route to go, it's not fun buying redundant tools but eliminating the variables of a faulty tester is worth it. O'Reilly's offers loaner tools, and I think AutoZone does as well. Give them a deposit and you get your cashmoney back when returning the tool.

Do you know when your engine seals were last replaced? Leaking intake cam seals will typically show oil on the timing belt, or the bottom of the housing- but I believe an exhaust cam seal can give you sneaky leaks. Unless you're completely missing the seal like I was, but that provides a much worse leak and easily shows its evidence.

patcal · Post by **patcal** » 18 Oct 2021, 17:00

I'm going to rent one from O'reilly's tomorrow. I will run the wet & dry test & we will see. Will update when I am done. Thanks.

ignatz · Post by **ignatz** » 18 Oct 2021, 19:20

My 2001 V70 2.4T uses about a quart every 1,200 miles or so. Plugs looked oil free after 50,000 miles and zero oil leaks and no smoke. My PCV was done about 4+ years ago and that didn't improve oil consumption. I just keep track of my mileage and add oil when necessary.
Yours is worse than mine, but for me driving 12,000 miles a year.......that's 10 extra quarts of oil a year. I just deal with it.

Post by **RickHaleParker** » 19 Oct 2021, 01:53

patcal wrote: 18 Oct 2021, 10:48 Any thoughts on those numbers? Thanks.

The difference between the highest and lowest is 22% of the highest. (( 160 -125 )/160) * 100% = 22%.
That is too much of a difference. Should not be more then 15%. Some people will say 25% but I think that is way out there.

Do a wet compression test see if the low ones come up. You should do a dry and wet test at the same time. Changes in Atmospheric pressure, Humidity and Temperature will change the reading.

Soaking the cylinders in Seafoam then blowing out the carbon might bring the low ones back up. Rings might be a little sticky. Getting the carbon out might free them up.

ignatz · Post by **ignatz** » 19 Oct 2021, 04:44

I too should try a piston soak to see if the oil consumption improves.

Post by **RickHaleParker** » 19 Oct 2021, 05:45

ignatz wrote: 19 Oct 2021, 04:44 I too should try a piston soak to see if the oil consumption improves.

Stuff rags down the spark plug wells and spin the engine to push out the liquid before installing the spark plugs. Prevents Hydrolock.

patcal · Post by **patcal** » 19 Oct 2021, 08:45

How much Seafoam should I use per cylinder? How long should I let it set?

Post by **erikv11** » 19 Oct 2021, 09:04

patcal wrote: 18 Oct 2021, 10:48 Well I finally got that compression test done. I only did a dry test. The numbers were:

#1=155
#2=135
#3=160
#4=135
#5=125.

Number 5 seems a bit low. Plugs looked OK but they were NGK & I had the Volvo plugs on hand. Any thoughts on those numbers? Thanks.

I'd be sure to focus on technique here (disregard if you already have!) before making inferences. Are these the numbers for checking each cylinder once, or a couple times per cylinder? I do the latter - check number one, relieve pressure, check it again. Make sure you are getting a consistent value before moving on to number two. It's easy to not spin up the same amount of pressure every time and get misleading results.

An old tester shouldn't matter if you are using the same tester on every cylinder and looking at the relative difference, but no harm in getting a new tester.

Post by **volvolugnut** » 19 Oct 2021, 09:29

erikv11 wrote: 19 Oct 2021, 09:04
patcal wrote: 18 Oct 2021, 10:48 Well I finally got that compression test done. I only did a dry test. The numbers were:

#1=155
#2=135
#3=160
#4=135
#5=125.

Number 5 seems a bit low. Plugs looked OK but they were NGK & I had the Volvo plugs on hand. Any thoughts on those numbers? Thanks.
I'd be sure to focus on technique here (disregard if you already have!) before making inferences. Are these the numbers for checking each cylinder once, or a couple times per cylinder? I do the latter - check number one, relieve pressure, check it again. Make sure you are getting a consistent value before moving on to number two. It's easy to not spin up the same amount of pressure every time and get misleading results.

An old tester shouldn't matter if you are using the same tester on every cylinder and looking at the relative difference, but no harm in getting a new tester.

I agree with these comments.
volvolugnut

Post by **RickHaleParker** » 19 Oct 2021, 20:44

erikv11 wrote: 19 Oct 2021, 09:04 An old tester shouldn't matter if you are using the same tester on every cylinder and looking at the relative difference, but no harm in getting a new tester.

Good point!

Relative difference is what matters. Absolute measurements will vary with Barometric pressure, humidity and temperature. That is why the accuracy of the gauge is not too critical. Consistency matters more then accuracy. You can achieve consistency by using math. Take multiple readings, weed out obvious flukes then calculate the average.

Evaluate everything against a Datum. In the post above I used the highest reading as the Datum. Another way is to calculate the average of all the cylinders then plot the deviations of individual cylinder from the average in terms of percentage.

I use the highest reading above as the Datum because, it will be the one closest to ideal. Comparison will give an idea how far from ideal the rest are.