Broken spark plug. Now what?-gctid393294

[Guest]

Long story but need some guidance...

Took a leisurely fathers day cruise. Motoring around 3200 rpm all very well and then a loud bang, and a god awful racket from the engine. Shut down immediately and took tow back to dock.

On inspection found spark plug separated in half. Part still finger loose in block, threads appear undamaged. Other half hanging from wire.

Checked compression on all cylinders all compression good (96 to 118) on all but #6, the one with the blow out. This tested at 50 but would not hold pressure.

Put plugs in and started it up. Motor ran and sounded great.

Getting advise from do not start it and pull head immediately. To don't worry about it now. Do valves in the winter.

People with good mechanical exp believe a piece my be stuck in a exhaust valve and it may either clear or should do no further damage

Boat has less than 300 hours. professional tune up was completed as part of a spring commissioning about 7 hours of running time. No evidence of water in cylinder. The metal fitting around plug appears defective it is not crimped fully.

Any suggestions

[Guest]

Ask around locally for a borescope. There is some kind of damage in that hole, and it's likely not going to get worse if you use the engine, but it's not likely to get better either. It would be good to get a look inside. You may find damage to a valve, or a scratch on the cyl wall from spark plug bits. Lets hope that this winter you can get by with just the head work, and not have to get a new block.

[2850Bounty]

Checked compression on all cylinders all compression good (96 to 118) on all but #6, the one with the blow out. This tested at 50 but would not hold pressure.

This is not a good sign.

96 - 118 psi is very low. The cylinder with 50 psi indicates damage of some sort.

How many cycles did you give each cylinder when you took the readings?

I agree with Doc!

Buy, beg, borrow or steal a Bore Scope that allows photos to be taken.

Keep the failed spark plug. Do not toss it.

BTW, which brand spark plugs?

.

[Guest]

OK - this is from an "old" motorhead with the emphasis on old. I read that the porcelain on the plug broke off and the plug body was found loose in the threads so my first question would be what would cause that? I can only imagine 1 scenario assuming a cast iron head, and that is the plug had not been torqued properly on installation or there was an ongoing overheating of the engine which sometimes can cause a plug to "come loose", but I'd go with the failure of proper installation in this case. Inspection of the failed plug parts could add info - i.e internally was there any damage to the plug electrodes or porcelain? To break a plug internally means damage to the electrodes usually caused by the wrong size plug or detonation but to actually blow the outer porcelain and a loose plug body spells poor installation to me. In that case you might well end up with bad threads in the head and resulting leakage. Since the engine runs OK with a new plug and assuming there is no audible burned valve noise, loss of power, misfires, stumbles, etc you might have dodged a bullet. But I'd get the threads checked by a qualified mechanic who could repair if in fact they are damaged. A borescope inspection certainly is not a bad idea, but again I can't think of a plug failure situation other then the scenario above. That would be my 2 cents.

[Guest]

I agree with captjim.....my first thought was dang he's old ..but yet again...so am I..........but back to the mayhem....I used to race under 2.o ltr canam cars many yrs ago in what seems another life....was the chief mechanic and bottle washer for the team....anyway...we had a similar incident that happened .....we were running in quals and blew a plug...looked like a rifle shot right up thru the deck of the body.....pulled the thing apart and found the threads still in place but the ceramic had separated at an explosive way...we pulled the head and found no damage to the cylinder ..we figured that on install the plug had been torqued wrong by one of the younger guys and cracked the ceramic.....eventually it caused arcing around the boot.....burned thru the core and eventually exploded out the top of the engine......we put the head back on and new set of plugs being careful with the torque specs of the plugs and never had another problem...not saying that is the case with yours...but I am concerned with the low compression of 96#....believe it or not....that can cause piston wrist pin damage from a low compression scenario........

something to think about is to check the hole.....run a race thread thru it to be sure it's clean.....then reinstall a new set of plugs....

:arr arr

[yachtman]

Not that I am against a bore scope but you have already replaces the plug and ran it. So it doewnt really do any good unless you plan on tearing the engine apart now and want to know before hand what you are up against.

Im with rick those compression #'s are way low. If it were me I would start saving my money for a future new engine providing rhe compression test was done correctly.

It was also mentioned why this would have happened. In all my life the only time I have seen that is when the plug wasnt tightened.

I would think that you would want compression more like 120psi +

If thise are infact the real compression #'s you engine is tired.

[Guest]

My experience has been that the compr tester gauge is pretty inaccurate. Couple things to check. Start the engine and warm it fully. While warm, remove all plugs, ground the coil wire to the engine. Open throttle fully. Insert tester sequentially and run about 6 revolutions on each cyl. If there is a relief port on the side of the tester, you can semi-calibrate the meter with an air compressor with a known good gauge.

The borescope is simply to identify where the damage is. If the cyl wall in the block is damaged, it could cause enough blowby to cause trouble. If it's just a valve, no problem. Excess blowby on a boat can be problematic, even though crankcase gasses are vented back to the air intake.

[Guest]

Thanks everyone for the replies. Here are answers to some of your questions. Plugs were confirmed correct with boat shop, don't have the part number with me but they were AC Delco brand. #6 blew apart, what remained installed was finger loose. Engine has never run hot. Again less than 300 hours. I guess that doesn't mean much as @#$$ happens but I'm the only operator and the gauge has never run hot. #8 was also loose. So maybe I am looking at a installation error from my mechanic.

But that would not explain the low compression around the block. Maybe it is operator error on my part. Using a borrowed, old gauge, on a cold engine, no oil added, and not many revolutions. I guess I will re-do the test.

I will get the threads looked at. A auto mechanic friend said he would help me with a Leak Down Test. Again I guess to help identify where the damage may be.

Can't say I am happy to hear that my engine sounds old and tired with such little use, and what I thought I was doing to give her good care.

Others in the know that have looked at the plug thought that the crimp was not correct. I guess this would be very rare and not likely to get the attention of Delco. We will see how receptive the mechanic is to my insinuation that it is installation error.

So I seem to be on the right track for finding the problem. I guess I am just praying it is in the valves and not deeper. Hope summer is not slipping away....

[Chief_Alen]

100 psi is absolutely the min. compression. Anything less the engine needs rebuilding, or scrapping. Whoever installed the plugs messed up.

[SomeSailor]

Just pull the head and be done with it. It's a couple of hours round trip and you'll have your answer. I wouldn't run it though until I knew.

[2850Bounty]

I agree with Mike that the cylinder heads will eventually need to be removed.

However, if you are curious about a leak-down test, now is the time to perform one.



See Tech Tip #3 here for the procedure.

Mark your harmonic balancer off in increments of 90* from the OEM TDC mark....., and follow the firing order after testing #1 cylinder.

This will remove the guess work from the equation.

.

[SomeSailor]

It sounds like there is nothing to leak down from.

I'd not turn the motor anymore than I had to at this point. You're just banging up the valves and scoring the piston walls. As soon as compression dropped the motor needed to be opened up. I would only mess with the heads in the boat because they'll make the block lighter to lift out. You will be pulling that motor, so it's really moot when you remove the heads or how much hand-wringing you do in the bilge.

Don't waste your summer if you can afford to get started.

[Guest]

I would say with 96 - 118 psi the motor is junk.. order a reman long block. Should be cheaper than playing with motor in the boat.

[Guest]

jnicholas wrote:
Thanks everyone for the replies. Here are answers to some of your questions. Plugs were confirmed correct with boat shop, don't have the part number with me but they were AC Delco brand. #6 blew apart, what remained installed was finger loose. Engine has never run hot. Again less than 300 hours. I guess that doesn't mean much as @#$$ happens but I'm the only operator and the gauge has never run hot. #8 was also loose. So maybe I am looking at a installation error from my mechanic.

But that would not explain the low compression around the block. Maybe it is operator error on my part. Using a borrowed, old gauge, on a cold engine, no oil added, and not many revolutions. I guess I will re-do the test.

I will get the threads looked at. A auto mechanic friend said he would help me with a Leak Down Test. Again I guess to help identify where the damage may be.

Can't say I am happy to hear that my engine sounds old and tired with such little use, and what I thought I was doing to give her good care.

Others in the know that have looked at the plug thought that the crimp was not correct. I guess this would be very rare and not likely to get the attention of Delco. We will see how receptive the mechanic is to my insinuation that it is installation error.

So I seem to be on the right track for finding the problem. I guess I am just praying it is in the valves and not deeper. Hope summer is not slipping away....

300 hours on an engine and all cylinders showing low compression - I vote for a "bad test" as opposed to a ruined or tired engine. Plus you found another plug loose in addition to the one that blew out - that to me confirms my suspicion about bad installation of plugs. I know from experience that getting plugs in on a marine engine can be an issue as the exhaust manifolds can make getting a straight shot at the plugs a problem - you have to use a u-joint adaptor and some care with the plug socket. Sometimes the angle sort of "locks up" the u-joint adaptor at just the right angle and you think the plugs tight - but it isn't. On those plugs that are a problem I always pull the socket off, rotate the rachet a bit and re tighten to make sure. But install on those plugs also makes it easy to crack or break the porcelain.

Unless you are missing parts off the plug body of the blown plug, I think you just blew a plug and may have no internal damage. How about taking what you know to the marine shop that did your tune up and plug install and get them to assist with a leakdown and or compression test to verify any possible internal damage.

Sounds like you already ran the engine after replacing the plug. If so, and you had some kind of internal damage, you're engine is already toast. Since you noted no issues with running, I'm gonna go with a simple though not excusable bad plug installation.

[Guest]

captjmh wrote:
300 hours on an engine and all cylinders showing low compression - I vote for a "bad test" as opposed to a ruined or tired engine. Plus you found another plug loose in addition to the one that blew out - that to me confirms my suspicion about bad installation of plugs. I know from experience that getting plugs in on a marine engine can be an issue as the exhaust manifolds can make getting a straight shot at the plugs a problem - you have to use a u-joint adaptor and some care with the plug socket. Sometimes the angle sort of "locks up" the u-joint adaptor at just the right angle and you think the plugs tight - but it isn't. On those plugs that are a problem I always pull the socket off, rotate the rachet a bit and re tighten to make sure. But install on those plugs also makes it easy to crack or break the porcelain.

Unless you are missing parts off the plug body of the blown plug, I think you just blew a plug and may have no internal damage. How about taking what you know to the marine shop that did your tune up and plug install and get them to assist with a leakdown and or compression test to verify any possible internal damage.

Sounds like you already ran the engine after replacing the plug. If so, and you had some kind of internal damage, you're engine is already toast. Since you noted no issues with running, I'm gonna go with a simple though not excusable bad plug installation.

I think the cause of the problem is the person that changed the plugs last. Spark plugs don't come loose, if properly installed. Griff