5.8L Ford engine with low compression on 2 cylinders

I have a 5.8 liter Ford engine in a 1995 Four Winns (22'). I just did a compression check and 6 of the 8 cylinders are in the 130-140 psi range. Two of them are low, one at 75 psi and the other at 90 psi. Both are center cylinders, but not next to each other (one on each bank).
Are there known issues with this motor that could make my root cause investigation easier, or could it be anything (rings, valves, etc.)?

I squirted some 50 weight oil in the two low cylinders. The 90 psi cylinder did not increase. The 75 psi cylinder increased by about 15 psi.

Thanks.
Bill

Re: 5.8L Ford engine with low compression on 2 cylinders

Time to do a leakdown test to determine what EXACTLY is wrong, before you take it apart.
Any indication of water on the spark plugs when you pulled them out?

Re: 5.8L Ford engine with low compression on 2 cylinders

The plugs all looked good, the only exception being the rear two cylinders having a bit more oil on the threads and the porcelain was a bit oil. Not black oily, just not dry like I would have expected. However, the rear cylinders were not the ones that were low on compression. The low cylinders were 3rd from the front on the port side and 2nd from the front on the starboard side (sorry, don't know the cylinder numbering on a Ford).

I am hoping that since the oil did not help my compression numbers, that the problem most likely lies in the cylinder head and not the piston rings, but it seems odd to me that I would have a bad cylinder on each side. Thats why I was wondering if these engines have a known problem, like carbon coking on the valve seats or something.

I did notice an occasional drop in power the last time I had the boat out. It seemed to come and go and it seemed like it had a weak cylinder or two at idle, which is why I ran the compression test.

Any help would be appreciated.

Thanks.

Re: 5.8L Ford engine with low compression on 2 cylinders

Do a leak down test... $40 bucks and a air compressor is all you need.. It could just be so many things at this point.. Could be heads, could be rings, could be valves, could be something else.. Guessing is all you can do other than a leak down test

Re: 5.8L Ford engine with low compression on 2 cylinders

Well guys, I wound up doing a leak test, but did not have a "leak down tester" so I just used my compression tester hose and put 100 psi to the cylinders to listen for leakage from the intake (throttle body) or rings (PCV). I listened for exhaust out the prop, but did not really hear anything there for any of the tests. However, I did hear leakage in both the PCV and throttle body. I thought I had figured out it was an intake valve leak until I checked one of the good cylinders and it made more noise than the bad cylinders.

So, I reinstalled the plugs, started the engine and ran about a can of carb cleaner through the motor as well as about 16 ounces of water in an attempt to clean any carbon in the system. When I was done, I retested the two weak cylinders as well as the strongest one. The strongest one stayed at 140 psi, while the two weak cylinders came up to 130 and 120 psi.

So, at this point I am hoping it was just a carbon issue and will run some more cleaner through it the first time I take it out next year and do another quick test.

Thanks for your responses and assistance. I appreciate it.

Bill

Re: 5.8L Ford engine with low compression on 2 cylinders

noturns said:

I thought I had figured out it was an intake valve leak until I checked one of the good cylinders and it made more noise than the bad cylinders.

Click to expand...

did you have the cylinders at TDC compression stroke when you performed the leakdown test??

if not, the results are meaningless

Re: 5.8L Ford engine with low compression on 2 cylinders

Yes, of course you have to check with piston at TDC, otherwise the valves would not be closed. Thats what I did in all of my testing.

Thanks.

Re: 5.8L Ford engine with low compression on 2 cylinders

Wow!!!!! Stranger things have happened.. That one is strange

Re: 5.8L Ford engine with low compression on 2 cylinders

Was there a running issue that prompted you to do a compression check? (just curious) I also have a 5.8 with compression in the 130-140 range warmed, I first checked it during the winter months in an unheated 30's degree garage and numbers were much lower, I know it won't make much differenence on a new motor but it does on an old worn one, could that maybe be the difference in yours that the motor was nice and warm from your decarb job, by the way I'm a believer in the water injection ever since I saw when a slightly blown head gasket seeping water into the cumbustion made the entire chamber valves and all looking like brand new! (steam cleaned) of course she drove the car for a month before I got to it.