Can I test compression on an engine stand?

[clockwatcher]

My 1989 Merc 350 Magnum with 650 hours is on an engine stand in my garage while I replace the stringers and floor in my boat. I didn't check the compression before I took it out. Is it possible to check the compression without warming it up or would it be a waist of time?

Also, the engine ran ok when I removed it, but it is 21 years old. The temperature usually read 170-180 mark with a new impeller and housing last time out.

I plan to replace:

• circulation pump,
• a few cooling hoses,
• rebuild the carb,
• replace plugs and wires,
• exhaust gaskets,
• starter,
• and repaint.

Any suggestions on other things to check or replace before I put it back in the boat?

Thanks in advance.

 

[zbnutcase]

Re: Can I test compression on an engine stand?

Ideally it should be warm, but an engine in good mechanical shape will have good readings regardless. A leakdown test might be easier with an engine thats on a stand, however.

 

[HT32BSX115]

Re: Can I test compression on an engine stand?

Howdy,

Yes, you can easily do a compression check "on the stand". doing it cold will give you a fairly good indication of the compression knowing that a low cyl might get a little better if the engine is run a while.

If the temp read "high", it could be due to clogged manifolds/risers, hoses, previous impeller fragments obstructing flow etc AND/or a defective temp sender/gage. If you have an oil or PS cooler it could be partially clogged with debris or impeller fragments.

Since you have the engine out, you can easily inspect all the cooling system components for obstructions.

Engine over heats are almost NEVER caused by circulating pump problem. Unless it's screeching, leaking or the metal impeller is corroded completely away, you won't get much benefit from a new one.

I would suggest you add a drain hose to the pan drain to make it easier to drain oil out of the engine!

I put one on mine and clamped it to the port riser.

Now I use a Moeller Vac pump to remove oil.

--18 strokes on the handle and the oil comes out easily hot or cold!

--No more standing on my head to fish a slimy hose out of an oily bilge!


Cheers,


Rick

 

[KJSmitty]

Re: Can I test compression on an engine stand?

Yep, a leak down would be the way to go - and it would tell you more of what you might want to do while the engine is "out." Without having a starter wired up a compression check would be unachievable - and of course you have the lack of a warm engine issue as well.

Something that may give you more life/function is a valve job/new springs and possibly valve train etc. Rings might be good yet many times a valve job can really bring an older motor back to life. A leak down will indicate whether valves and/or rings are getting weak - or neither!

I'm sure you are already yet definitely change the thermostat as well.


Cheers

 

[clockwatcher]

Re: Can I test compression on an engine stand?

Thanks for the quick replies. A leakdown test gage is on the menu. I'll try the compression test also. I have a thread-in compression gage and a long breakover bar to turn the crank.

HT32BSX115,

I'll order new senders. This motor has 2 on the housing. I'm friends with the PO and there's a real good chance he installed the wrong parts. My exhaust manifolds and risers are fairly new, maybe 40 hours in fresh water. I'll look them over. Last year I removed the thermostat housing and wire brushed it in muriatic acid. It had a lot of rust scaling. I suspect the block and heads probably look the same. Is there any trick for cleaning the engine cooling passages?

I have an oil pan drain hose hanging on the bench ready to install. That'll be a little slice of heaven.

By the way, what plane is in your avatar? I thought it was a C-5 at first.

 

[HT32BSX115]

Re: Can I test compression on an engine stand?

I'll order new senders. This motor has 2 on the housing

Those 2 senders are most likely a regular sender and a normally OPEN switch that goes to ground at a particular high temp. (alarm)

By the way, what plane is in your avatar? I thought it was a C-5 at first.

It's a McDonnell C-17B. I flew the C-141 for about 18 years and the C-17 for another 3 years or so before I retired.

 

[clockwatcher]

Re: Can I test compression on an engine stand?

Those 2 senders are most likely a regular sender and a normally OPEN switch that goes to ground at a particular high temp. (alarm)



It's a McDonnell C-17B. I flew the C-141 for about 18 years and the C-17 for another 3 years or so before I retired.

I thought the 2 might be for a dual helm boat? I'll figure it out.


Retirement. What a concept. Being in Washington I thought you might be on the manufacturing side. I do airframe engineering in Fort Worth.
Thanks for your time and help.

 

[HT32BSX115]

Re: Can I test compression on an engine stand?

Retirement. What a concept.

Yeah.....I just retired from one (part-time) job so I could do the other one full time!!