Performing Compression Test
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=3966
Printed Date: May-04-2024 at 8:15pm
Topic: Performing Compression Test
Posted By: joed
Subject: Performing Compression Test
Date Posted: June-24-2006 at 7:35am
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Just want to make sure I've got this right, plus a few questions.
To perform a compression test... 1. Remove all spark plugs 2. Disconnect wire from coil 3. Put throttle in WOT position 4. Test each cylinder by cranking engine about 5 revolutions 5. Record numbers If I get a low number on a cylinder squirt 1 teaspoon of oil in sparkplug hole, crank over once or twice to distribute oil and recheck cylinder. Questions... Do I perform this check with engine hot or cold? I've seen both recommended. I don't need to be hooked up to fake a lake right? Because I'm not "running" the engine? I've read to stay clear of open spark plug holes. Why? Will there be fuel squirting out? Well... that's about it. Any feedback would be GREATLY appreciated. Thanks for all your help. Joe |
Replies:
Posted By: Bradley950
Date Posted: June-24-2006 at 9:36am
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Joed,
1. Before you begin, make sure the battery is fully charged. 2. Full the fuel injection fuse.( If its Injected) This cuts power to the injectors so they dont spray gas all over the place. 3. Unplug the coil connector, you could get a nasty shock if you don't. 4. Remove all 8 plugs and examine them carefully for signs of a problem. If your valve seals are bad in any cylinder, the plug will be fouled to some degree. Signs of oil on the plugs could indicate a ring problem. 5. Screw the compression gauge into the plug hole 6. Push the throttle wide open to open the throttle plate, and keep it at WOT (wide open throttle) for as long as youre cranking (if you didnt pull the MPI fuse, there would be a bit of a mess, now). 7. Turn the ignition to the ON position, and hold it there for a count of four revolutions (do not turn the key on-and-off four times). On each rotation, the gauge needle will jump slightly less than the previous one. It may take more than 4 revolutions to get a stable reading; crank for up to 10 seconds or until the needle stabilizes, whichever comes first. Note: If your battery is weak, the engine will turn over slower and give you lower readings. 8. The last number the gauge jumps to is the compression for that cylinder. 9. Repeat for each cylinder. All a "wet test" entails is a bit of oil in the low cylinder. 10. Add one capful (1-2 tablespoons) of oil to the cylinder with low compression. 11. Let it sit for a minute to allow the oil to flow down and coat the sides of the cylinder and piston. If there are any gaps in the rings, the oil will seal them for the next few minutes. Crank the engine for 4 revs, remembering to floor the throttle. 12. If the reading improved significantly (more than 30 psi), your rings are probably worn. 13. Replace the plugs, preferably in the same cylinders they came out of. You may want to clean or re-gap them first, or put in new ones. A dab of anti-seize on the threads would be a good idea. 14. Remember to put the MPI fuse and coil connector back. Youre not going anywhere without them! ------------- Brad Miller bradley950@yahoo.com |
Posted By: joed
Date Posted: June-24-2006 at 10:49am
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Thanks Brad.
A couple questions. Do I perform this check with engine hot or cold? To just get a relative test I'm assuming I can do it cold? I've read to stay clear of open spark plug holes. Why? Will there be fuel squirting out? Should I protect the surrounding area of the boat? I don't have EFI. Thanks for the clarification. Joe |
Posted By: M3Fan
Date Posted: June-24-2006 at 2:38pm
| I've never heard of taking all the plugs out- just one at a time... |
Posted By: Mercrewser
Date Posted: June-24-2006 at 2:45pm
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Not sure about being warmed up, and I cant see how it would work if only one plug was removed. They all have to be out. Doing it while the engine is warmed up may be better. ------------- 1969 Chris Craft Commander SS #39 |
Posted By: joed
Date Posted: June-24-2006 at 4:28pm
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Everything I've read states to pull ALL plugs, then test one cylinder at a time.
I did my test cold just to get a relative comparison. Here's my results: 1 - 120 2 - 130 3 - 140 4 - 140 5 - 130 6 - 125 7 - 120 8 - 140 I thought #8 was bad because of oil fouled plug but those results do not seem to comfirm that. Joe |
Posted By: jimbo
Date Posted: June-24-2006 at 4:59pm
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Definately have all the plugs out. It should be done with the engine warm but those numbers look ok.
I saw the picture of the oily plug you posted. Any chance that plug wasn't firing? like maybe a loose wire at the plug or the cap? You put in new plugs, right? I'd run a tank of fuel through it and check the plugs then. I think you're ok. |
Posted By: joed
Date Posted: June-24-2006 at 5:04pm
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Thanks Jimbo...
I did have a bad wire... you can read about my recent puzzle in the backfiring thread. Any input would be appreciated. Joe |
Posted By: Bradley950
Date Posted: June-26-2006 at 12:59am
With those #'s you should be good to go. Are you happy with your results? What brought on the test?  ------------- Brad Miller bradley950@yahoo.com |
Posted By: David F
Date Posted: June-26-2006 at 10:38am
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The main reason for pulling all the plugs is so the engine will crank a bit faster (i.e. no compression) during the compression test. If you are only trying to make sure the cylinders are all within relative tolerance to each other, then I think either way is fine. Just make sure you test each cylinder the same way.
Make sure you ground the coil wire to the block. Avoid overheating the starter. Allow it to cool every so often. ------------- |
Posted By: jo-e90
Date Posted: July-20-2014 at 9:50pm
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I wonder how the Fresh water pump would like this dry test after all the dry cranking? ------------- 1997 Sport Nautique |
Posted By: JasonS
Date Posted: July-20-2014 at 10:06pm
| I'd hook the water to it since you are cranking the engine over. I have seen a dry run impeller and it ain't pretty. |
Posted By: SNobsessed
Date Posted: July-21-2014 at 1:41am
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Or loosen the belt. How's it going Dan? How do you like your Sport? ------------- “Beer is proof that God loves us and wants us to be happy.” Ben Franklin |
Posted By: MrMcD
Date Posted: July-21-2014 at 2:05am
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Testing compression with all spark plugs removed is important, it takes a huge compression load off the starter, you will be doing a lot of cranking and all plugs need to be removed for this test so why would you wish to test with the plugs in the other 7 holes. Pull all and you will get better results and your starter and battery will perform better in the test. If you are fouling one plug with good compression you probably have either a Valve Stem Seal failure or an intake manifold sucking oil into that cylinder past the gasket. If the cylinder had a bad plug wire it can certainly foul out the plug. Hope you got it figured out. |
Posted By: jo-e90
Date Posted: July-21-2014 at 11:45am
Again, it would be good to add to the list to loosen the impeller pulley belt and return the throttle to neutral, especially if you forgot to move the throttle back to neutral when staring the engine back up like I did. Glad I removed the belt during testing but hooked it back up before starting and ROAR! because I forgot about the throttle.  Having a good summer Snobsessed!! Sport runs great but have it up for sale looking for a v-drive and hidden ballast. ------------- 1997 Sport Nautique |