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3.6L 4stroke has low compression, need help diagnosing valves


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IDoSeaDoo
User

Apr 25, 2021, 8:31 PM

Post #1 of 9 (879 views)
3.6L 4stroke has low compression, need help diagnosing valves Sign In

Hey everyone, this isn't really a car, but it is the same type of motor. It's an 07 Suzuki DF250 outboard. It's a 3.6L 4stroke with dohc and bucket-style lifters. I'm getting low compression on one cyl (140 vs 160-170 on the rest). I did a leak-down and it's leaking past the Exhaust Valves. I measured the valve tappet clearance and they good cylinders are the same as the bad ones. I tested the sound each valve makes when I open it manually (cam pointing away from tappet) with a screwdriver and let it close. The good valves make a hollow THUNK sound, while the bad ones make more of a clack sound, like when the cam is pressing on it. I even took out the tappet shim and tested the sound, it was the same. I soaked the cylinder in seafoam over night and ran it at 3000rpm for a few min. No change. I then soaked it in seafoam, valve cleaner and carb cleaner for 6hr and ran 1.5 gal with a can of seafoam in it at 2500 rpm (~20min or so). No change in compression. I haven't tried the Power Tune yet (Quicksilver brand decarb), but that's probably my next move. This motor has only 580hr, but about half of them were run hard, like over 4k rpm. Anyone know what this valve issue from what I described???


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Apr 26, 2021, 2:50 AM

Post #2 of 9 (865 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

? It's an engine OK with me. Water toy with this for an engine, OMG must be outrageous!
IDK but would use a scope to see if a valve is cracked for this.


Easy on SeaFoam - love that stuff too but can cause probs where there were none before too. If looking for magic in a can there are many, most are just junk,


T



IDoSeaDoo
User

Apr 26, 2021, 8:15 AM

Post #3 of 9 (853 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

It's for a 25' Proline boat. It has 4 valves per cyl, and both exhaust valves sound the same.


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Apr 26, 2021, 8:57 AM

Post #4 of 9 (849 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

Sound? You should probably LOOK at them. Your call a scope or take it all apart and make a lot of work over "Knot" (on purpose) knowing.
You said hours which should bel lower like 1/4 of expected life IMO but there's more to things that are watercraft (owned tiny thru yachts) and many other's. Problems were exhaust thru manifolds, wet cooled didn't fare well as that is ambient water for some or most could be salt water + that much faster.
You are good to know compression so far, a lot more than most. The results are disappointing to both of us.


It's hard but suggested a scope first vs taking it all apart for what or why.


BTW, those hours are a lot to me for a toy watercraft I'm UNFORTUNATELY too familiar with owned and did a lot to take break from this crap fixing or diagnosing INSANITY up to ice makers on fly-bridges!
Here to help, let's solve this - OK?


Tom



Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Apr 26, 2021, 12:40 PM

Post #5 of 9 (841 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

Your chances of seeing something that justifies a 20lb difference is about impossible.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



IDoSeaDoo
User

Apr 27, 2021, 6:45 AM

Post #6 of 9 (810 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

I figured as much. My question is IF it's just a little rust or a bit of carbon deposit, could it clear up on its own if I just put the motor back in service? Apparently, the engine sat and developed this issue as a result. I'm hoping to avoid having to tear into it.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Apr 27, 2021, 6:50 AM

Post #7 of 9 (808 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

There are many possible causes like a weak valve spring/sticky valve or piece of carbon stuck on valve. Both of these things could clear on their own if it was from lack of use.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



IDoSeaDoo
User

Apr 27, 2021, 7:27 AM

Post #8 of 9 (804 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

I can't imagine why both valve springs only on the exhaust side would get weak. Could it be from running lean and excess heat or something? My guess is that these valves were open when the engine was last shut off, allowing some surface rust to form on the valve's mating surface. Can that sort of rust work itself out with a bit of run-time? I'm trying to avoid a very time consuming and costly disassembly over something that can maybe solve itself.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Apr 27, 2021, 7:57 AM

Post #9 of 9 (798 views)
Re: 3.6L 4stroke has low compression, need help diagnosing valves Sign In

First off, you can't know the problem is with both valves and even if it is, they were both left open over a long period.

Even the thought of tearing an engine down over a 20lb difference in compression is nuts.

I don't know what your relationship is with this boat but it seems you are pushing for a problem where very little exists. A 20lb difference would be a completely acceptable tolerance in most engines.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.







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