What are good cold compression numbers?

[PiratePast40]

Volvo AQ130D (older pushrod motor). The motor is out of the boat and in the garage. Everything is put back together and ran a cold compression test by turning it over with the starter. All cylinders came up to 115 to 118 lbs quickly and held pressure for a few minutes before I switched to the next one. The fact that they all came up quickly, were close to each other, and held pressure gives me some piece of mind. I could't find the recommended compression numbers for the motor in the Seloc manual. What should I be getting for cold compression on this engine?

Thanks

 

[mkast]

Re: What are good cold compression numbers?

By "put back together" I can assume the rings aren't seated.
Cold compression number don't mean much on an engine that had just been put back together. Try it again after the rings have seated.

 

[Gary H NC]

Re: What are good cold compression numbers?

At least they are all even.
Once it is run and the rings seated test it warm.I would bet the numbers come up a bit..

 

[PiratePast40]

Re: What are good cold compression numbers?

"Put back together" in this case means that I replaced the bellhousing, vibration damper, and associated bearings. I didn't mean that it was completely torn down, guess I should have been more specific - sorry bout that.

But I did paint the oil pan, rocker arm cover, air inlet cover, and dipstick handle - does that count as a rebuild ?

 

[windsors03cobra]

Re: What are good cold compression numbers?

"Put back together" in this case means that I replaced the bellhousing, vibration damper, and associated bearings. I didn't mean that it was completely torn down, guess I should have been more specific - sorry bout that.

But I did paint the oil pan, rocker arm cover, air inlet cover, and dipstick handle - does that count as a rebuild ?

Round here we call that a "DuPont Overhaul" sounds like you got 3/4 of a DuPont Overhaul, shoulda spray bombed the whole kit~n~kaboodle.

 

[Bondo]

Re: What are good cold compression numbers?

All cylinders came up to 115 to 118 lbs

Ayuh,.... She's Tired.....

But I did paint the oil pan, rocker arm cover, air inlet cover, and dipstick handle - does that count as a rebuild

Nope,.... A DuPont Overhaul just ain't gonna cut it...

 

[J. Mark]

Re: What are good cold compression numbers?

You say it held compression for a few minutes-are you sure your compression gauge doesn't have a valve that just captures the highest reading and holds it? Most do.

Cold compression checks are better than nothing, but still not great. If you are talking an 8:1 compression ratio motor, your numbers are okay. If it is a 10:1 you are down a good bit.

Were you having a particular problem before the removal? If not I'd be inclined to leave it alone. If it wasn't running great before . . . now is the time to deal with it.

 

[PiratePast40]

Re: What are good cold compression numbers?

I can't tell from the brochure about the gage if it maintains the highest pressure or not. It just has a pressure release button. That was something that I hadn't considered.

The motor was out of the boat when I bought it. It's clean, shows no signs of leaks, and ran smooth and without vibration on the engine stand. It has the appearance of a motor that had a recent overhaul or at the very least, was well cared for. I don't know much more about the history but it does have relatively new tuneup parts. I suspect the older owner threw in the towel when water was allowed to enter the driveshaft carrier bearing. The resulting action by an unrestrained driveshaft and u-joints probably contributed to their decision to just walk away. So - that's why the motor is out of the boat. Starter should be back from the rebuild shop in a few days (doing the compression test was the last life it had left), the carb rebuilt kits just got here, and new tuneup parts are on the way. Am hoping to finish the hull work in the next few days and install the motor and outdrive. That's why I was doing a compression check - reading up on possible problems and some of the first questions that are asked if things don't work perfectly. I really just wanted to get a baseline.

Soooo -given those circumstances, do those sound like good numbers or is the test suspect because the motor wasn't warmed up or didn't spin fast enough due to a started on it's last legs?

 

[dontask]

Re: What are good cold compression numbers?

Was the cold compression test done with the throttle full open as to NOT starve the cylinders to draw in air. Very important step if you didn't do it. Next was the engine being spun at the normal cranking speed ? If the engine oil has not had the chance to coat the cylinder walls or rings recently then this could be a factor in lower compression readings. If you just rebuilt it the rings are not seated,(the rings take hours to fully seat). Good luck

 

[Don S]

Re: What are good cold compression numbers?

The engines aren't designed to hold pressure, just the gauge. You will NEVER get a cylinder (even with valves closed) to seal tight enough to hold pressure.
Basically, you should be seeing around 150 to 180 PSI compression, even on a cold engine.

 

[PiratePast40]

Re: What are good cold compression numbers?

The engines aren't designed to hold pressure, just the gauge. You will NEVER get a cylinder (even with valves closed) to seal tight enough to hold pressure.
Basically, you should be seeing around 150 to 180 PSI compression, even on a cold engine.

150 to 180 PSI compared to 115 - OUCH!
When the starter comes back, I'll run it again. Hopefully at a higher cranking speed. The throttle was open to allow full air in flow. But since it had sat for several weeks without an oil pan, the cylinder wals could have easily gone dry and the pump not started doing it's thing yet.

I can hope for better numbers but from what both Don and Bondo have said, I probably won't be winning any drag races with this motor. Will the fish mind?

 

[Don S]

Re: What are good cold compression numbers?

Will the fish mind?

Probably get bored and leave, while waiting on you to there.

 

[dontask]

Re: What are good cold compression numbers?

Please post your compression numbers after you get full cranking speed for the compression readings. I'm curious to know if it made any difference. But also make sure you have a good fully charged battery..

 

[PiratePast40]

Re: What are good cold compression numbers?

I'll do another test and post the results when I get the starter back from rebuild in the next few days.

 

[PiratePast40]

Re: What are good cold compression numbers?

Probably get bored and leave, while waiting on you to there.

Either that or I'll only be able to catch slow fish :redface:

 

[J. Mark]

Re: What are good cold compression numbers?

Personally, I would pull the intake and the heads and have the heads checked by a shop. It is not at all uncommon to have a valve or two in an older motor that just don't seat properly. When I rebuilt mine I had to replace 6 valves that were too far gone to regrind. I got cold compression figures from as little as 60 on one cylinder (which also had a blown head gasket), 90 on the adjacent cylinder, to between 135 and 150 for 6 of the 8 cylinders.

Pulling the heads will allow you to clean carbon from the pistons, inspect the cylinder walls for obvious damage, have the heads rebuilt if necessary and put fresh top end gaskets on. For very little time and not too much money you can have some piece of mind.

If you haven't put a new gasket on the oil pan, it might be worth pulling the pan back off, removing a main bearing cap and looking at the bearing for signs of wear. Just make sure to retorque it properly. If you have put a new gasket on, but not run it, you should be able to remove it and reuse it.

 

[PiratePast40]

Re: What are good cold compression numbers?

Your advice is well taken. Pulling the pan isn't a big deal and checking a main bearing sounds like a good idea as long as the motor is hanging from a hook in the garage. I'll wait until I get the started back and recheck the compression and also check after a few squirts of oil in each cylinder to see if there are any changes. I can certainly do pressure tests if there are apparent problems but don't want to make this a research project.

In all reality, if it's either valves or rings, it's time for a complete overhaul. But on the other hand, if it's just a litle tired then I'll run it this year. My planned use is fishing the mountain lakes and reservoirs so going out to sea isn't really an issue.

 

[J. Mark]

Re: What are good cold compression numbers?

Valves being bad is a cheap fix. Most machine shops will rebuild a set of heads for under 200.00 around here. Even though the valves may be worn, the rings and bearings tend to last longer than the valves in many american made V-8's. Valve seats take a tremendous load and beating in a boat motor.

I have seen many motors that needed valve jobs, where the bottom end was still very sound (back when hotrodding cars was a past time) I've seen main bearings that had 150,000 miles on them that looked perfect. Bearings fail, but in my experience, that is usually the result of somethig more than normal wear.

I agree a research project is out of line for an older boat motor. There are some cheap and relatively easy things you can do though.

Pull the valve covers, turn the engine over by hand and measure pushrod movement to verify the cam has no bad lobes.

Run a fresh compression test

Squirt some oil in the cylinders and rerun the compression test

If the numbers come up a bunch you need rings-time for a rebuild. If they don't change more than a little bit you could look at the valves or do a leak down test, or just go ahead and pull the heads for a visual inspection.

 

[PiratePast40]

Re: What are good cold compression numbers?

Got the rebuilt starter back and pulled the carbs to rebuild them as well. Squirted some WD40 in the cylinders and cranked it around for several 15-20 second runs to make sure things are seated and lubed.

Numbers are 150, 150, 130, and 120. Waited for about 20 minutes to bring the battery back up and got close to the same results.

Guess I'll be pulling the head for a valve job. I've heard that the AQ130's are basically the same as the Volvo B20 but with a different cam grind. Does that mean that any competent automotive machine shop can do a valve job?

Thanks for any advice.

 

[PiratePast40]

Re: What are good cold compression numbers?

You're right - I was over reacting. These results are starting to point toward a complete rebuild but will need to look closer. The first tests were done after the motor sat for about 6 weeks. It was only run the one time and on an engine stand for 5 to 10 minutes. Prior to that the motor sat for (as far as I know) about 2 years. My feeling is that the cylinders were completely dry for the first round. I'll run another test tomorrow and also pressurize to see where I hear the leaks - upper end or lower end. I'll also pull the pan and check a main bearing cap to get an idea on how they're looking.

My thoughts are that if it seems to need a complete rebuild andit's operable, I'll run it this season and then either have this motor rebuilt over the winter or see if I can find another block and heads to send to a shop for rebuild.

The initial indications are sure pointing to the lower end though.