Low compression 1 cylinder
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=50513
Printed Date: October-01-2024 at 6:22am
Topic: Low compression 1 cylinder
Posted By: jblocal
Subject: Low compression 1 cylinder
Date Posted: March-20-2022 at 6:40pm
| Went out on the boat last week and when leaving dock heavy in gear about 1500 rpm could feel engine surging. Got up on plane and could still feel surging and some power loss ended up turning back didn’t want any problems offshore. Did a compression check all 120-130 with #7 cylinder at 50lbs. So did a leak down test which I never done before and I can hear air inside engine and think it’s coming from intake. So where do I go from here pull the head to get better idea? Wanted to make sure not missing anything and get my self into more work then needed. I replaced the heads few years ago with Gt40p maybe I didn’t get something right. |
Replies:
Posted By: MrMcD
Date Posted: March-21-2022 at 2:55am
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To give good advice we need good information, your compression test is fact and good information, the leak down information can be very helpful if you know if it is coming out your carburetor or if it is coming out your dip stick or PCV valve. Air escaping out the dipstick or PCV would indicate you have a piston or piston ring issue most likely. Air coming from your cooling system would be head gasket. Air coming out the carburetor would indicate a burned Intake valve. Air coming out the exhaust would indicate a burned exhaust valve. No matter what issue you have, with 50 psi in #7 you will be pulling that head off to find the answer but it would be good to know what the leak down told you in advance of pulling the head. Drain the block of coolant before pulling the heads off, it will help diagnose the problem if you don't have water in those cylinders as you pull the heads. Study the parts as you remove them trying to see where your problem is. Wish you good luck on this one. ------------- |
Posted By: KENO
Date Posted: March-21-2022 at 5:06am
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If you want to get a better idea of whether the air is coming from the intake valve like you think, then pull the intake manifold and exhaust manifold off and before you go any farther with disassembly, do the leakdown test again. It'll be easier to tell, since you can look right into the intake port. Same routine for the exhaust valve
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Posted By: jblocal
Date Posted: March-21-2022 at 11:22pm
| Damn ok. Thanks for the help! I’ll let you know what I find. |
Posted By: jblocal
Date Posted: June-05-2022 at 11:52am
Finally got around to pulling head. Air was coming out of the carb on the leak down test. I couldn't find any issues with my untrained eye. Now with the head off what would be my next best step? Take head to local machine shop see if they can find issue or have them replace valves? Cylinder with problem is 3rd from left.  |
Posted By: MrMcD
Date Posted: June-05-2022 at 12:35pm
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You can tilt the head up so the exhaust is down and the intake ports are facing up. Spray a fair amount of WD 40 or any thin oil into that intake port on #7. I have used Diesel fuel, Laquer thinner or acetone for this test also. They wont rust your valve but are thin and will leak out around a valve that is not seated. If you want to test the exhaust roll the had over and try that side. Let is sit at least 15 minutes. It will usually seep right out if the valve is not sealed tight. A very slow seep that only leaves a small stain but is not running out could be OK. You can tap on the valve with the wood end of a hammer and see if it seals after tapping. To have 70 PSI a valve would have a fast leak. Has the engine been overheated and were any of the head bolts loose on tear down? You can also test the valve with high pressure air, 100 PSI blowing on the back of the valve while spraying soapy water on the chamber side of the valve looking for leaks but water on cast iron rusts quickly.
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Posted By: jblocal
Date Posted: June-05-2022 at 12:58pm
| Sprayed WD on intake port #7 and wd was leaking out pretty fast. Tapped on it might have slowed it down little but still leaking. Engine never overheated and all head bolts were tight. A while back my son hit the throttle while flushing boat in yard it reved up pretty high before I got to it. Only thing I can think of that maybe could caused issue. |
Posted By: MrMcD
Date Posted: June-05-2022 at 4:31pm
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It might have broken the valve spring on that #7 intake valve. Easy to remove and check. Over Revving is one of the few things that will fracture a valve spring. If one broke it is probably prudent to change all 16, they were exposed to the same stresses. This is just speculation because we don’t know if the engine hit 5,000 RPM or 10,000 RPM at this point. If you are removing valve springs it is also prudent and the perfect time to replace the valve stem seals. Edit, if your valve stem seals were new in those heads 2 years ago they are fine, Stem seals usually age out after 15-20 years of use, not 2 years if quality parts were used. ------------- |
Posted By: jblocal
Date Posted: June-05-2022 at 5:02pm
| Thanks for the help |