Mercruiser 4.3L engine. Last trip out last season engine seemed like it wasn't firing on all 6 cylinders. Too busy to deal with it then so I winterized and put it away for the winter. I changed the oil/filter in engine and gear lube in outdrive in the fall as part of winterizing. I noticed that the old engine oil may of had fuel mixed in. Started it up last weekend for 1st time this season. Very noisy lifter. Never had that occur in the past. New oil also seemed contaminated with fuel. Dip stick showed about 1 quart overfilled. Pulled plugs. All seem OK except #1 cylinder seemed more carbon fouled/ blacker than the other 5. Compression is at 140 on 5 cylinders(room temp engine)but #1 was reading 35 to 60 PSI. I shot a few squirts of oil in #1, compression up to 90. I was thinking ring problem, either frozen or broken. Fuel getting dumped in to oil. Hoping problem is due to collapsed lifter or bad intake or exhaust valve. That's a much easier fix since I won't need to pull motor. I can't see how fuel is getting into crankcase if it's a head problem. Any chance it's something other than bad ring seal in #1? I will try to do a leak down test on Saturday.
Low Compression in #1 Cylinder?
- Thread starter seachunk2
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A vacuum test is likely a lot easier . It's value is highly underrated.
Try "scenario #4 " for rings and several after that for various valve issues. Sounds like rings.
http://www.secondchancegarage.com/public/186.cfm
Try "scenario #4 " for rings and several after that for various valve issues. Sounds like rings.
http://www.secondchancegarage.com/public/186.cfm
GeorgeKohler
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- Feb 3, 2015
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I've always been told that if the compression increases with a few squirts of oil in the cylinder, then it's a ring problem. If not, then a valve problem.
Removed the intake manifold and saw one of the pushrods for the #1 cylinder bent. I think is was for the exhaust valve, which was probably frozen. Removed both heads and brought them to the shop, along with the intake manifold,for a overhaul. No point in doing only one head. The other push rods seem ok. Once I had the engine opened up it was obvious there was water getting in.Looks like the exhaust manifold/riser was leaking, which froze the valve. I was a bit surprised since they were only 5 years old and I only get out about 5x a season. I fresh water flush the motor after use but the salt water and the fact that I don't run the engine often enough probably made things worst. So between the overhaul of the heads and purchase of a new set of exhaust manifolds/risers I should be fine, albeit a good $1K+ poorer.
That's good you found an obvious problem to fix. Have any idea why compression improved when you gave #1 a squirt of oil if it was a valve issue? Did the water cause cylinder rust and sticky rings? Or was the normal oil seal at the rings compromised by gas and water dilution before the squirt.?
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- Sep 28, 2002
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You need Marine gaskets, due to raw water cooling. I used Fel-Pro.
Other tips…having done the same job 2 years ago:
Find out if the head bolts are still usable, sometimes due to corrosion in the threads (raw water cooling, salt water) they will not clean up well enough to torque properly
Get a thread chaser and clean the bolt holes in the block really well
Make sure the deck surface of the block is as clean as you can get it
When you get the cyl heads back, if raw water cooled in salt for a long time, ask the machinist if there is any risk of rust through behind the valve seats. this sometimes happens after 10-15 seasons worth. On my heads, the machinist found cracks in the center cyl exhaust seats and the cooling passages were eroded due to raw water cooling corrosion so he advised replacing with good reman heads which is what I did.
If you get new head bolts keep in mind you have to use a sealer on the threads like Merc Perfect seal or Permatex Aviation
When installing the intake gaskets use a bit of sealer on the front water transfer ports, esp if there is any pitting on the intake or cyl head water ports. If you have the older non-vortec engine with the 12 bolt intake, it helps to use Permatex Aviation on both ends of the intake gaskets to keep them from sliding around when you lower the intake onto the cyl heads. The front and rear parts of the intake are sealed with a bead of RTV.
I would one by one pull out the lifters and check the cam lobes to make sure none are scored or worn down. On mine it has a roller cam and the cam lobes looked like new. When re-inserting the lifters coat the lobes with clean engine oil. Return the lifters to the bore they came from.
Then there are the jobs of clocking the distributor and adjusting the hydraulic valve lifters.
Other tips…having done the same job 2 years ago:
Find out if the head bolts are still usable, sometimes due to corrosion in the threads (raw water cooling, salt water) they will not clean up well enough to torque properly
Get a thread chaser and clean the bolt holes in the block really well
Make sure the deck surface of the block is as clean as you can get it
When you get the cyl heads back, if raw water cooled in salt for a long time, ask the machinist if there is any risk of rust through behind the valve seats. this sometimes happens after 10-15 seasons worth. On my heads, the machinist found cracks in the center cyl exhaust seats and the cooling passages were eroded due to raw water cooling corrosion so he advised replacing with good reman heads which is what I did.
If you get new head bolts keep in mind you have to use a sealer on the threads like Merc Perfect seal or Permatex Aviation
When installing the intake gaskets use a bit of sealer on the front water transfer ports, esp if there is any pitting on the intake or cyl head water ports. If you have the older non-vortec engine with the 12 bolt intake, it helps to use Permatex Aviation on both ends of the intake gaskets to keep them from sliding around when you lower the intake onto the cyl heads. The front and rear parts of the intake are sealed with a bead of RTV.
I would one by one pull out the lifters and check the cam lobes to make sure none are scored or worn down. On mine it has a roller cam and the cam lobes looked like new. When re-inserting the lifters coat the lobes with clean engine oil. Return the lifters to the bore they came from.
Then there are the jobs of clocking the distributor and adjusting the hydraulic valve lifters.
Thanks LouC and the same to all others. As it turns out, the heads were not repairable due to the excessive corrosion so the re-builder provided replacement heads. Cost me a few bucks more for the core charge. I ordered a Fel-Pro gasket set and will be installing the heads and manifold by next weekend.Right now I am on the hunt for new Mercruiser manifolds and risers (elbows?). The last set were aftermarket HGE brand. I want to give OEM a try since my track record with aftermarket is 3 years to replace. The Mercruiser manifold part is 99746A17. The riser part is either 8077988A03 for a hot dipped ceramic coated or 807798Q03 for whatever E coating is. There's a $70 difference between the A03 vs Q03 riser. So I am trying to determine if the A03 version is worth the extra $140 approx. However, it appears that the only manifold is 99745A17 and I can't find a definite description as to whether that is hot-dipped ceramic coated or E coated. If the manifold is E coated then I'll reason that it doesn't pay to get the ceramic coated riser. I'm looking in to this but if anyone is familiar with these finish coating, please let me know your thoughts. Happy Father's Day to all the Dads!!!