5.7 mercruiser dead on 2 cylinders - need some opinions and advice

[vetting]

Had a bilge pump fail on me while I was washing down the boat after a long fishing trip. The back of the boat dipped enough to cause water to come back up through the exhaust riser and then cracked the head. I had water in only one cylinder (#6) as a result and we found which one it was by disconnecting the spark plug wire from the coil to the distributor. We then took one spark plug out at a time, cranked the motor over to see if water would shoot out, compression tested the cylinder, put that spark plug back in, and then went onto the next one. The crack was right in the water jacket next to the valve.

Replaced the head and all of the gaskets. Boat ran great for 2 weeks without many issues besides just some some minor electrical issues on the coil that was due to old connectors on the purple wires. The only thing that I did notice was that the WOT speed dropped from 29 to around 25-26 mph.

Last Saturday we had a 8 hour fishing tournament. Was cruising back to the harbor at 3500rmps and started to get some chatter in the motor and it went away if I decreased the rpms a bit. Didnt think anything of it and thought it was just the rockers that needed a readjustment. Then began to lose power and the speed dropped down to 15mph from 25mph. At that rate we werent going to make it to the cutoff for the tournament so I throttle back up and got it back up to 25mph but then the temp started to climb. I got up to 200 degrees but nothing over that. I limped it back into the harbor and then it wouldnt idle for more than 5 seconds. Of course I'm in a line of 30 boats to drop the cooler off for the weigh in. I had to blip it in gear and then throw it in neutral and rev it to 1000rpms to keep it running - that was not fun doing that 20 times. Limped it back to the slip and then started to investigate the issue tonight.

Pulled one plug at a time and did a compression test. Before the new head was swapped on - compression was 140 to 150 on all cylinders.

The head was replaced on the 2-4-6-8 side.

Plug #8 was not even hand tight and the electode was slightly melted. 4 was also loose and 6 wasnt much better. All 3 plugs had white soot on them from blowby and 4 was dark.

1-150
3-150
5-140
7-150

2 -150
4- 5
6 -120
8 - 0

So how bad do you think it is? What would have caused the plugs to work loose or were they not installed correctly? Could the lack of compression be just bad valves/rockers or do you think I actually melted the pistons?

 

[biggjimm]

The pic isn't that clear but that plug looks like the threads have been crossed. Might just be lack of quality/clarity of the pic but take a close look at it.

You could have had the valve train improperly adjusted & bent a valve on the low comp. cylinders??? I had that happen on an old truck (327 Chevy) & for some reason one of the lifters hadn't pumped up all the way when I adjusted the rockers & first time it seen higher rpm's that lifter pumped up all the way & valve smacked the piston dome. Or so that's what Comp. Cams tech said happened.

 

[vetting]

The plug wasnt cross threaded - the picture just looks goofy since the plug was so burnt up.

 

[Bondo]

Ayuh,..... Looks like piston bits on the spark plug,....

My guess is, ya burnt it up from Detontation, probably caused by a lean condition,.....

 

[tpenfield]

Looks like the head is going to come off again. . . Something got messed up on the replacement. You can maybe get a look inside the cylinders with a borescope/ inspection camera to see if it look like valves or piston issues. I am also wondering if you are getting water in on that side of the engine ? See if the head bolts are loose. . .

 

[vetting]

Ayuh,..... Looks like piston bits on the spark plug,....

My guess is, ya burnt it up from Detontation, probably caused by a lean condition,.....

So what would have caused a lean condition on that side with the newer head?

 

[vetting]

Just a thought - is it possible that the low compression on 4 and 8 were due to the other plugs being loose on that side?

 

[vetting]

Also, if it was due to an improper valve lash adjustment - that would mean that the rockers are too tight correct?

 

[tpenfield]

If your compression tester could not thread in to make a tight seal, then possibly . . . but you would probably get some reading or the the spark plug and/or tester would just blow out of the hole.

If you had improper valve adjustment, then possibly, but you could even have piston damage with the valves that much out of whack . . .

The thing is that you said the boat initially ran fine after the rebuild, except not quite as fast.

you could try backing the rocker arms out a bit so the valves stay closed then hook some compressed air to those cylinders when they are at BDC / or even TDC to see if they hold air You won't need much psi to tell . . .

 

[vetting]

Took another look at it last night. I didnt have my compressor hose with me so I just backed off the rockers on #8 and did a compression test. It got up to around 110psi. I then went ahead and readjusted all of the rockers back to spec and #8 went back to 0psi. So what damage would that indicate?

 

[Bt Doctur]

what spec are you using to adjust them?

 

[vetting]

Putting it on TDC on 1 - adjusting the intake/exhaust rockers (listed in the manual) to 0 lash - where u cant spin them anymore by hand and then doing another half turn. Then turning the motor over one full turn to adjust the remaining rockers.

 

[Bt Doctur]

try it this way, r5otate the motor until an intake valve opens and closes, that puts that cylinder on compression stroke. adjust both valves to 0 lash plus 1/2 -3/4
move to the next cylinder, rotate motor until the intake opens and closes..etc................................................

 

[vetting]

try it this way, r5otate the motor until an intake valve opens and closes, that puts that cylinder on compression stroke. adjust both valves to 0 lash plus 1/2 -3/4
move to the next cylinder, rotate motor until the intake opens and closes..etc....................................... .........

Is there a way to determine that with the intake still on?

 

[Scott Danforth]

I always start with #1 cylinder at TDC (timing mark), then rotate the crank 90 degrees and adjust the next one on the firing order.

 

[vetting]

So turns out the rockers were all too tight from the beginning. To fix, I replaced all of the plugs that were fouled, then backed off the rockers about 4 turns and got some compression on the 2 dead cylinders. I then warmed up the boat to operating temp with the valve cover off on that side. I then was able to back off the rockers another 2 full turns on average before they started to clatter with the motor running. Then turned them back down to where they stopped clattering and turned them another half turn or so. I didnt do another compression test yet, but its purring like a kitten at 600rmps at idle and in gear with 50 psi of oil pressure. So what I think what happened is that they were all too tight from the beginning and once I did a long WOT run, it heated the oil enough that the lifter cleared out and the lifter finally filled the entire way. Then the push rod was putting too much pressure on the valve and they werent closing all the way hence the 0 compression. Would also explain why the plugs all looked like they were super dark and why everything heated up. Its just a matter of any damage was done running it for 2 weeks with the extremely tight valve train. I'll run it like it is for a few days, see how it runs, and then do another compression test.