Question about Compression testing

[sibnai1]

Hi, it was suggested that I do a compression test on my baby to diagnose a "running hot" issue, my questions are:

Added 07/05/15 I have to correct the statement above, it was suggested that I do a compression test after "running hot" (200* +) to make certain that I did not damage the engine or head gaskets. Added 07/05/15

1) Do I have to put the "muffs" on and run the hose while conducting the compression test??

2) Should I take the gas lines off of the carb while conducting the test??

I understand that I have to disable the ignition and unplug (after numbering) all the plug wires and remove and label the plugs while tumbling the engine but didn't know if the impeller had to be wet when I did this. Please advise and thank you all in advance.

 

[src]

If you can arrange it, a leak-down test can give a more accurate picture of what's going on in the engine.

 

[suss1173]

i don't think you need to remove every plug wire if you just remove the center one / or main one from the distributer / coil. Hopefully someon else will chime in to confirm this though.

Regarding muffs, you don't need them b/c your not running the motor your just turning it over. It's more like just tapping the starter so the piston in the cylinder you have the compression reader screwed into can come up and make pressure (or hopefully make pressure) in that cylinder chamber.

 

[sibnai1]

If you can arrange it, a leak-down test can give a more accurate picture of what's going on in the engine.

thanks for the tip

 

[sibnai1]

i don't think you need to remove every plug wire if you just remove the center one / or main one from the distributer / coil. Hopefully someon else will chime in to confirm this though.

Refarding muffs, you don't need them b/c your not running the motor your just turning it over. It's more like just tapping the starter so the piston in the cylinder you have the compression reader screwed into can come up and make pressure (or hopefully make pressure) in that cylinder chamber.

I thought I read somewhere that the impeller if spun dry for something like < 2 mins is subject to damage, hence the question, thanks for the tip on the coil wire, makes sense!!

 

[suss1173]

I would agree the impeller should not be spun dry, but you are only tapping the key to build compression in that cylinder. You will be holding the key or a few seconds at the most. Than you will get your reading unscrew the gauge and than repet the process on remaking cylinders. The key will not be held long enough to get the impeller hot.

hopfully btdocter, or others will chime in. He is usually very helpful with this kind of stuff.

 

[Hifirush]

If you can arrange it, a leak-down test can give a more accurate picture of what's going on in the engine.

This is good advice. However, if this is not possible, you should do all the cylinders. The way I do it, take out all the plugs, remove the coil wire, (block if you can, it still will try to charge the system, that bolt will surprise you!) and have someone turn the engine over, letting it turn over 2-3 times to get a decent reading. If there is a huge difference between cylinders, then you may want to have that leak down test. A leak down test will let you know specifically where the "air" (compression) is going, i.e.. past the rings, through the gaskets, the valves, etc.

Hope it something easy to fix.

Matt

 

[sibnai1]

I would agree the impeller should not be spun dry, but you are only tapping the key to build compression in that cylinder. You will be holding the key or a few seconds at the most. Than you will get your reading unscrew the gauge and than repet the process on remaking cylinders. The key will not be held long enough to get the impeller hot.

hopfully btdocter, or others will chime in. He is usually very helpful with this kind of stuff.

funny enough btdoctur is the one that suggested the compression tes

 

[sibnai1]

This is good advice. However, if this is not possible, you should do all the cylinders. The way I do it, take out all the plugs, remove the coil wire, (block if you can, it still will try to charge the system, that bolt will surprise you!) and have someone turn the engine over, letting it turn over 2-3 times to get a decent reading. If there is a huge difference between cylinders, then you may want to have that leak down test. A leak down test will let you know specifically where the "air" (compression) is going, i.e.. past the rings, through the gaskets, the valves, etc.

Hope it something easy to fix.

Matt

I sure hope so too thanks for the best wishes!!!!!!

 

[Ten Years Gone]

edit

 

[NorCal Boater]

During a compression test the motor is only turning at cranking speed and really not fast enough to cause significant damage to the impeller. If you feel better with the muffs on....put 'em on.

Sib, the fact that you are asking this many questions about this tells me you might want to have a shop do this for you. No disrespect intended but you want to make sure you get proper readings.

 

[Lazy Daze]

Flip your kill switch "off" (you know to open the throttle, right?) and don't worry about the water. Obviously it can't hurt to put the water on, but there's really no need. As mentioned above, the engine isn't spinning anywhere near even just a normal idle speed. No worries. When your hands are cold, do you rub them together nice and slow?

 

[sibnai1]

During a compression test the motor is only turning at cranking speed and really not fast enough to cause significant damage to the impeller. If you feel better with the muffs on....put 'em on.

Sib, the fact that you are asking this many questions about this tells me you might want to have a shop do this for you. No disrespect intended but you want to make sure you get proper readings.

I've done compression tests before but on that contraption that has 4 wheels, never on a boat hence the trepidation and the questions, the thing that scares me most is the stuff that I dont know, thank you for the advice ........................... no disrespect taken, I can tell from your other posts that you are being thoughtful and trying to help .........................

 

[sibnai1]

Flip your kill switch "off" (you know to open the throttle, right?) and don't worry about the water. Obviously it can't hurt to put the water on, but there's really no need. As mentioned above, the engine isn't spinning anywhere near even just a normal idle speed. No worries. When your hands are cold, do you rub them together nice and slow?

as always thank you for chiming in!!

PS: I like the cold hands analogy

 

[sibnai1]

I beg to differ.

I have it on good authority that an engine idling for 30 seconds without water will shred the vanes off an impeller. I realize the OP will not be "running" the engine, but the friction occuring inside that steel cup is the same regardless of engine speed.

Why not start with good habits, and take the extra time and effort to hook up the water. It doesn't need to be a geyser, just enough flow to keep the impeller lubricated.

Just my opinion...:huh:

I like the personal experience: "I have it on good authority" and the "good habits".

So since there is really little to no time involved in hooking up the muffs and water supply and it can't hurt ................... it means it can only help, well maybe, lol!!! Thank you for your advice.

I'm not trying to start a war, I'm just looking to develop good habits and asking for advice and real world experience so that I can enjoy many many years of trouble free service from my baby.

 

[NorCal Boater]

I've done compression tests before but on that contraption that has 4 wheels, never on a boat hence the trepidation and the questions, the thing that scares me most is the stuff that I dont know, thank you for the advice ........................... no disrespect taken, I can tell from your other posts that you are being thoughtful and trying to help .........................

That said....

Label the plug wires and remove them and the coil wire
Remove all the spark plugs
Put the muffs on
Inset the compression tester in each plug hole
Crank the motor with the starter enough to get the reading
Right down the readings
Return everything the way it was
Open a beer

You said you are doing this because your engine is overheating? Do you have evidence of water in the oil? Have you checked the other causes of overheating? Impeller, recirculating pump, exhaust manifold and elbows? Any of these items in poor condition could cause over heating however, in the case of the circ pump they usually just leak when they fail.

Let us know what you find.

 

[sibnai1]

That said....

Label the plug wires and remove them and the coil wire
Remove all the spark plugs
Put the muffs on
Inset the compression tester in each plug hole
Crank the motor with the starter enough to get the reading
Right down the readings
Return everything the way it was
Open a beer

You said you are doing this because your engine is overheating? Do you have evidence of water in the oil? Have you checked the other causes of overheating? Impeller, recirculating pump, exhaust manifold and elbows? Any of these items in poor condition could cause over heating however, in the case of the circ pump they usually just leak when they fail.

Let us know what you find.

no water in oil, new impeller this season, circ pump not weeping or peeing into bilge. the good Doctur suggested I do a compression test to help diagnose my running hot issue. CORRECTION added 07/12/15 the good Doctur suggested I do a compression test because the engine ran hot and he wanted me to check and see if I had done any damage) I still have to check the thermostat and the exhaust/risers but I get to play with a gauge when I do the compression test so I figured I'd start there. She runs hot at speed but will idle indefinitely. When I changed the impeller at the start of the season she ran great, then the last time I took her out 2 weeks ago she started misbehaving as described.

 

[Lazy Daze]

Well, shoot - that sounds like a failing impeller, no? Possibly sucked in some sand and it damaged the impeller or housing.

 

[NorCal Boater]

Well, shoot - that sounds like a failing impeller, no? Possibly sucked in some sand and it damaged the impeller or housing.

I agree. I don't see where a compression issue would be causing this. If you don't have one, get a laser thermometer and check manifold and elbow temps. But I agree with Dennis....sounds like an impeller.

 

[sibnai1]

Well, shoot - that sounds like a failing impeller, no? Possibly sucked in some sand and it damaged the impeller or housing.

I changed the impeller this season because she was running hot when I went out the first time, then she ran great after the impeller change now she does not run so great, she is running hot again, I don't think I ingested sand could milfoil do the same damage as sand?? I did not think impeller cause it only has approximately (9) nine outings on it.