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Anyone know if it matters if the engine is warm or cold when a compression test is done? Got a line on a used engine that's been sitting a while and I don't think I can get it running where it's located but I can do a cold compression test if that's useful?

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Here is a video file from the Marine Dr on compression and spark test - He clears up the throttle open or closed issue when compression testing but doesn't state cold or warm all though it appears cold...
Video Link

I did some quick research and there seems to be a recommendation towards a warm engine. Everyone does say that the warm engine may change your numbers but it shouldn't change your cylinder to cylinder comparison.

I hope this helps
Scott

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Here is what I have always been told:

Compression should be the same warmed up or cold. If it is less when the motor is warmed up the it means that you may have a worn set of rings. This is because when things are cold they shrink up and seal better however, once warm the gaps open up.

Every compression test my mechanic has ever done on my motors has been while the motor is cold (air temps in the 70 - 80 F range).

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Thanks for the replies Guys, I feel much better about doing a cold compression test now to get an overall idea of the engine condition.

It's a late 80's looper V6 that's been sitting for 6-7 years, which I understand isn't great for the engine (but it keeps the hours low...). Trying to find out if it was winterized before it was stored as I'm not sure it's worth checking into if it wasn't. Price is decent, but without a sea trial, which isn't possible in this case, it feels kind of like buying blind....

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Well, I may or may not be comparing apples to oranges here. I know that when I used to conduct pre-purchase inspections on piston aircraft, I always warmed up the engine before hooking up the hoses. If you have to do a cold compression test, be sure to squirt a little oil in the cylynders before moving the pistons up and down. This holds especially true if the motor hasn't run in 6-7 years.

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That's actually been part of my dilemma. It would be great to get the engine running and burn off (hopefully it's in there) any old fogger, then run the test. If there was no fogger and I spray some in, it's going to up the compression numbers and mask any issues that may be in there. If it was winterized and there's still fogger in there, it'll also up the numbers temporarily giving me a false reading....

If I remember correctly, when checking a car compression that's low, to see if it's the rings vs the valves, a little oil is added to the cylinder and the compression checked again. If the compression increases it's the rings and if it doesn't it's the valves. I assume any fogger oil in a 2 stroke would have the same effect?

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Any fogger would be gone after that long of a period. Squirt some WD 40 in each cylinder. The most important thing is that all cylinders are with 10% of each in comp. numbers. You will have to rebuild the carbs anyway, which is no big deal. Also check the oil in the gearcase too. Here is something I just learned. A friend picked up an old Evinrude `56 from upstate NY, I thought he new something about outboards. when he dropped it by my house in Fl. the first thing i did almost unconsciously was spin the prop. It wouldn`t budge. I made sure it was in nuetral, and it still wouldn`t budge. I removed the fill and vent screws, and nothing. When i took the gearcase apart I found the gears were just a ball of rust. Just something easy to check, but expensive to fix.

Rich

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Good point on the lower unit Rich, thanks. The last engine on the boat was a '87 225 hp Johnson and the compression numbers weren't great when we bought the boat, 85-90 psi (but the whole rig was a good deal) and it didn't last long. So I feel like I have to pay attention to overall numbers as well as variations between cylinders.

What is the most harmful thing about an outboard sitting so long, anyone know? I've always read it's better to keep them moving but not sure why? Oil dripping off the crank etc over time?

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Jeff...I have to put in my 2 cents on what you said about the compression changing when the engine is cold due to shrinking.

I would believe that the compression would be the same with a hot or cold engine. If there is shrinking in the metal, wouldn't the shrinking occur in both the block as well as the pistons/rings? If everything is shrinking (in the cold) or expanding (in the heat) it would be doing so in all parts, not just the block or piston. Therefore the compression would be the same regardless of the engine temp.

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I'm in the final stages of restoring a 1962 13 whaler that came with a 1982 35hp johnson outboard that hadn't been run in 6 years.I squirted some fogging oil in the cylinders let it sit for a couple of days then hooked it up to a battery and cranked it a few times with the plugs out then did a compression check.Most repair manuals will tell you the amount of compression isn't as near as important than if they are within 10% of each other.Mine checked out just fine.

I had to rebuild the carb and fuel pump and hopefully will be starting it up soon.I also changed the lower unit oil which was really black and smelly.