compression test question

[tbird175]

I need to check compression on my 1978 175 HP BlackMax. I know that I need to remove all the sparkplugs and prop throttle to WOT position. I will crank engine from solonoid with ignition in off position. Should I attach the muffs to protect the water pump while performing the test? Have I missed any other steps?

 

[Texasmark]

Re: compression test question

Probably wouldn't hurt. My personal preference is to take a piece of uninsulated wire and wrap at least one turn around the threads of every spark plug then connect itto the block to ground the case of the plugs so that the spark has some where to jump and keep the firing voltage at a normal level. This will ensure that your CDI's that fire your plugs will not get over voltaged while you are doing your compression test. Others may argue, but at $100 per CDI new, I don't care to take the chance.

Mark

 

[Faztbullet]

Re: compression test question

If it has a kill switch just "trip" it so it wont fire.....

 

[Chris1956]

Re: compression test question

As long as the ign key is in the off position, there is no need to ground the spark plugs.

 

[tbird175]

Re: compression test question

Thanks for the info. I do plan to leave the ignition switch off to prevent firing. My main concern is will the water pump be damaged by spinning the engine for testing without water being supplied to it.

 

[aussieflash]

Re: compression test question

I never do,though i usually warm engine up on muffs before testing,so the impeller is lubed up.Your only cranking 5 or 6 turns on each plug....i wouldnt worry.

 

[jerryjerry05]

Re: compression test question

No need for water.
Comp can be done cold or hot.
Just make sure all the plugs are out and cycle it untill the guage stops going up.
I use a #10 wire with the ends removed about 3/4"I fold that over and stick it into the plug lead.
The other end I strip and put into a #2 battery connector.Crim that and ground it out on the block.
For about $2 I have a way to keep the CD's good.
Even with the key off it's still a good idea to ground them out.

 

[M9.9]

Re: compression test question

Hi OP and all,
I'm not sure about that particular year or model?, but some of the older Mercs are Ignition ON if the main engine electrical harness is detached from the engine. I suppose my point may be moot since your talking about doing the test with ignion key off.

 

[tbird175]

Re: compression test question

Thanks for your replies.

 

[viper1]

Why are you removing all plugs first?

Sent from my DROIDX using Tapatalk

 

[phebus1]

Re: compression test question

You remove all plugs so that the engine can turn over with the starter motor at an adequate rpm. The compression created with the plugs in creates too much resistance against the starter.