Not Again! 0 PSI in Cylinder After Recent Repair to Merc V8 TBI

[GlenDownUnder]

Recently had the top end (cylinder head, valves, new plugs etc.) on our Merc 5.7L TBI redone by an authorised reputable Mercruiser mechanic. The manifolds and risers were inspected and retained as they were still in good condition with no signs of holes etc. Genuine Merc parts, gaskets etc were used were available. The engine is raw water cooled. The bullhorns were replaced about 12 months ago but were reported to be in very good order, with the exahust flap valves working as they should.

Since the repair, the boat seemed to be running well. It was being started at least every 7 - 10 days during this time. It had done about 4 - 5 hours in about 4 weeks, 50% at cruise speeds around 2800 - 3000rpm, the rest at low RPM putting along at 8 knots of less. It had not been pushed any harder than that. It has not overheated and the temperature gauge has been tested and confirmed correct.

Over the past 2 - 3 weeks, the motor become harder to start, but when it started, it ran smoothly with no suspicious noises from the engine. After not being able to start it at all, I pulled the plugs and found one of the tips completed melted with signs of rust around the top of the plug where the tip had been. The ground electrode was still intact but was black. A compression test revealed 0 PSI in that cylinder, the rest of the cylinders were around the 190's.

I plan on doing a leak down test and use a borescope to help isolate the exact problem. Next will be removing the rocker cover to check everything is moving up and down like it should.

But in the meantime I am wondering what the cause could be. Obviously, I don't want (and can't afford) this happening again.

The mechanic suggested the water ingress could have been caused by a backfire. I dont recall the motor backfiring at all but could one backfire cause this?

I've also read that stopping the boat suddenly can cause water to enter the exhaust but surely that isn't likely unless there is very large, sudden stop to force water up the exhaust, past the risers and into one cylinder.

So I ask: What could cause a critical failure in this cylinder so soon after a top end overhaul?

 

[AllDodge]

Incorrect timing, to much advance and detonation
What did the other plugs look like?

190 psi is high, and if correct you should be using premium fuel

 

[GlenDownUnder]

Incorrect timing, to much advance and detonation

Thanks heaps Alldodge, appreciate the response. The timing was checked during the repair and I checked it the last time it ran and it was 8 degrees as it should be. The other plugs all looked good aside from a very slight carbon build up. I was thinking that may have been due to the last few weeks trying to get it started.

The most recent fill-up wasn't premium, just the usual 91 unleaded, but typically I would run it on 98.

 

[makomark]

Is there any warranty on the work done by the mechanic??

 

[AllDodge]

Wonder why the compression is so high, might be a bad gauge, but if numbers are correct wonder if the head was shaved

 

[GlenDownUnder]

Is there any warranty on the work done by the mechanic??

I would think so but they dont seem too interested in helping TBH.

 

[GlenDownUnder]

Wonder why the compression is so high, might be a bad gauge, but if numbers are correct wonder if the head was shaved

I'm not sure if it was shaved, it's a good question to ask them. Are you implying it's possibly a head gasket gone? This was what led to the rebuild, head gasket blown between the two central cylinders.

 

[AllDodge]

I'm not sure if it was shaved, it's a good question to ask them. Are you implying it's possibly a head gasket gone? This was what led to the rebuild, head gasket blown between the two central cylinders.

no, if gasket was gone than pressure would be lower, your reading 190 psi and it should be 160 at most with boring oversize

 

[GlenDownUnder]

no, if gasket was gone than pressure would be lower, your reading 190 psi and it should be 160 at most with boring oversize

Thanks again AllDodge, sorry, I meant the head gasket gone in the cylinder that is now showing 0 PSI.

What could be causing the high pressure readings in the other cylinders?

 

[AllDodge]

Thanks again AllDodge, sorry, I meant the head gasket gone in the cylinder that is now showing 0 PSI.

What could be causing the high pressure readings in the other cylinders?

Preignition causes detonation, and that increases heat

Extra heat blows gaskets, warps heads

Higher pressures are from liquid, water or to much fuel, or less combustion chamber volume

 

[GlenDownUnder]

Preignition causes detonation, and that increases heat

Extra heat blows gaskets, warps heads

Higher pressures are from liquid, water or to much fuel, or less combustion chamber volume

Great thanks for that.

 

[makomark]

The Merc Manual for that vintage (93-97) calls for 150PSI cranking pressure - 190 PSI is way over what it should be...

As far a contributors, using the wrong parts could also be a contributor but, without seeing a list of what changed and to what, it is hard to say...same for the heads going to a machine shop.

still a bit baffled by the inferred "lack of interest" by the shop that did the work...don't know where you are but the better shops around here go out of their way to stand behind their work...

 

[Roadking57]

What ever happened? Any more info? I read that one cylinder is zero. Which cylinder, and is it the same as first time? If it is the same cylinder could have something going on with maybe broken rings on that cylinder?

 

[GlenDownUnder]

Still unresolved I'm afraid. Yes the reading was zero. Couldn't say for sure if it was the same cylinder but last time it was definitely caused by a failed gasket between two of the middle cylinders (either 3 & 5 or 4 & 6). It was a very obvious failure.

Will do a leak down test first then we're going to pull the rocker cover off and see if that reveals any problems. Then it'll most likely be the head off again. Still open to suggestions.

 

[kghost]

If you confirmed flatness of block and heads and feel good about new head gaskets and hopefully used New head bolts, the only few possibilities I see would be,

Sever crack in head or block

Sticking valve (open)/bad/broken valve spring(s)/ roker arm not adjusted correctly.

If any of the valve are bent or have heavy build up where the stem meets guide or guide is to tight for some reason...

 

[GlenDownUnder]

If you confirmed flatness of block and heads and feel good about new head gaskets and hopefully used New head bolts, the only few possibilities I see would be,

Sever crack in head or block

Sticking valve (open)/bad/broken valve spring(s)/ roker arm not adjusted correctly.

If any of the valve are bent or have heavy build up where the stem meets guide or guide is to tight for some reason...

Finally got around to fixing the motor and replying to this and let you know that you were correct.

We had valve that was stuck open (only by a few mm) and likely a small crack in that head. Highly possible they were linked. The stuck valve appeared to have evidence of rust on the stem. We're unsure where came from. The dry joint exhaust manifolds and risers seemed to be in reasonable condition and had no obvious leaks but have been replaced given they were approaching 7 years of age.

The boat sits in a pen where the stern/transom is subjected to boat wash as other boats pass by. There are flappers in place and they seem to be doing their job.

Is it possible this boat wash could have found it's way through the propellor exhaust, up the bullhorn, past the flappers, up the exhaust hoses into the riser/manifold and then the cylinder? If so, is there a way of covering the propellor to stop the boat wash?

 

[louc]

There are a few things here…your initial failure of a blown HG between the center cylinders suggests other perhaps not yet investigated issues:
Possible lean fuel mix; clogged injectors, fuel system restrictions (anti siphon, pick up tube, tank vent)
Engine not propped properly not reaching spec for max RPM, engine labors pulling the boat up on plane, this overheats valves esp on the center cyls on the V8 where the exhaust ports are right next to each other…
How were the exhaust manifolds checked? Manifolds water tested? Or with acetone?
And with respect to your last question, has the static water line ever checked? There is a set dimension that the top of the elbows should be above the static water line. If not water can back up the exhaust under certain conditions.

 

[GlenDownUnder]

There are a few things here…your initial failure of a blown HG between the center cylinders suggests other perhaps not investing issues:
Possible lean fuel mix; fuel system restrictions (anti siphon, pick up tube, tank vent)
Engine not propped properly not reaching spec for max RPM
How were the exhaust manifolds checked?
And with respect to your last question, has the static water line ever checked? There is a set dimension that the top of the elbows should be above the static water line. If not water can back up the exhaust under certain conditions.

Thanks Iouc,

The initial blown head gasket cause was unexplained but it appeared to happen slowly over time. Fuel pressure has been checked and found to be within spec. I'm not sure how else we can check if it is running lean?

I'm unsure whether the props are correct, but ths has never been raised by the mechanics who have worked on the boat. I'll do some digging (Diving) on this to try and work it out.

Exaust manifolds were only checked visually, but by two separate Merc mechanics. I know this isn't sufficient to rule them out as the problem but they have been replaced. I may have them pressure tested at some point just for the benefit of diagnosing any future problems.

Good call on the static water line, where would I find those dimensions? Other than upgrading the exhaust manifolds/elbows to dry joints, the boat and engine haven't been modified from the orginal spec, so I would hope these are correct. If it helps, the static waterline is just below the flappers. I've attached a video that shows what happens when there is significant boat wash caused by a passing boat. This happens once or twice per day.

 

[jimn]

I'm not sure if it was shaved, it's a good question to ask them. Are you implying it's possibly a head gasket gone? This was what led to the rebuild, head gasket blown between the two central cylinders.

Did they use gasket sealer for the heads? That's a good way to cause a failure between cylinders.

Are you testing compression wet, or dry? Remove the fuel pump fuse before doing it again (if you didn't do that before).

 

[GlenDownUnder]

Did they use gasket sealer for the heads? That's a good way to cause a failure between cylinders.

Are you testing compression wet, or dry? Remove the fuel pump fuse before doing it again (if you didn't do that before).

Thanks Jimn,

No, there was definitely no gasket sealer used. Genuine Mercury gaskets each time.

Always doing compression testing with the throttle open and fuel pump disconnected. I actually unplug the cable that powers the fuel pump as that is very easy to get to.

 

[kghost]

Finally got around to fixing the motor and replying to this and let you know that you were correct.

We had valve that was stuck open (only by a few mm) and likely a small crack in that head. Highly possible they were linked. The stuck valve appeared to have evidence of rust on the stem. We're unsure where came from. The dry joint exhaust manifolds and risers seemed to be in reasonable condition and had no obvious leaks but have been replaced given they were approaching 7 years of age.

The boat sits in a pen where the stern/transom is subjected to boat wash as other boats pass by. There are flappers in place and they seem to be doing their job.

Is it possible this boat wash could have found it's way through the propellor exhaust, up the bullhorn, past the flappers, up the exhaust hoses into the riser/manifold and then the cylinder? If so, is there a way of covering the propellor to stop the boat wash?

You dont say which valve was stuck open. If it was the exhaust that could be the source of the water. Dry joint or not, water is in the exhaust stream and some could be sucked back into head. Or, The crack is deep enough to be into water jacket in head and is leaking a very small amount of water over time. Thats my guesses......

 

[louc]

Thanks Iouc,

The initial blown head gasket cause was unexplained but it appeared to happen slowly over time. Fuel pressure has been checked and found to be within spec. I'm not sure how else we can check if it is running lean?

I'm unsure whether the props are correct, but ths has never been raised by the mechanics who have worked on the boat. I'll do some digging (Diving) on this to try and work it out.

Exaust manifolds were only checked visually, but by two separate Merc mechanics. I know this isn't sufficient to rule them out as the problem but they have been replaced. I may have them pressure tested at some point just for the benefit of diagnosing any future problems.

Good call on the static water line, where would I find those dimensions? Other than upgrading the exhaust manifolds/elbows to dry joints, the boat and engine haven't been modified from the orginal spec, so I would hope these are correct. If it helps, the static waterline is just below the flappers. I've attached a video that shows what happens when there is significant boat wash caused by a passing boat. This happens once or twice per day.

when the compression test was done, and you had the spark plugs out, what did the porcelain insulator for the center electrode of each spark plug look like? Was it bleached white? Or light tan/brown? Because if they were bleached white, the engine could be running too lean, despite good fuel pressure. Could be dirty injectors....

the wide open throttle RPM test is very important, every boat should have this checked to make sure the prop pitch is correct. If the pitch is too great, and the engine never reaches its specified WOT rpm, the engine will be laboring every time it pulls the boat up on plane, which overheats intake and exhaust valves especially and is a common cause of blown HGs on these engines. On my personal boat when I bought it, I had that problem, it had the correct prop for a V8 but not for a V6 which is what it is. So I replaced a 14.5x19 prop with a 15x17 prop and the boat reached the specified WOT rpm (4600-4800 for a 4.3 4bbl OMC Cobra), as it should. Running a boat with too much prop pitch is like forcing a truck to start out in 2nd gear towing a trailer, very bad.

The static water line test is described in Merc shop manuals, it's a bit tricky to measure but certainly is worth doing. Sometimes you wind up having to add spacers between the manifold and the elbow.

As far as the actual compression #s I agree they are too high, if the gauge is to be trusted. I have found that what I get on my engine (1988 4.3 V6) is right about what's listed in my OMC factory shop manual, 160-170). And it was the same with the OEM cyl heads which I replaced in 2017 with a set of reman heads due to blown head gaskets, cracks from a previous overheat and corrosion from 15+ years of salt water use. The reman heads gave the exact same compression test readings as the OEMs. If too much is taken off a reman head to make it flat and level, it could raise the compression test readings.

The thing with changing the cyl heads is the job requires attention to detail. You must very carefully clean the block deck surface and also, those cylinder head bolt holes must be cleaned out, or else the bolts will have too much friction to get accurate torque readings. I remember spending hours doing this, with thread chasers, solvent and compressed air.

checking exhaust system for water intrusion with an inspection camera

reman heads installed on the old 4.3

also about the exhaust manifolds, I found that I can check them for water intrusion by removing one of the pipe plugs in the elbow (there is one on top for a sensor for exhaust temp, and a couple on the sides used for a one way valve to reduce vacuum in the exhaust on Vortec engines) and with an inspection camera you can see if there's rust inside or not, very easy to check this....