New pinging sound under load 5.7l 250 HP Mercruiser

[PDW911]

Looking for some ideas/guidance

The motor is a 5.7 250 HP Mercruiser with an alpha drive (boat is 12 years old but a new engine was put in a year and 1/2 ago....so it is under warranty (??)

Coming in from a fishing trip off shore suddenly started hearing a rattling/pinging noise. Actually thought is was a loose beverage can lying against engine box.....slowed to idle, noticed nothing and went back up on plane.....noise started again....backed off, shut down...checked all fluid levels etc and looked for loose parts..tightened flame arrestor...started it up, went on plane and noise started again...backed down to 2300 RPM and noise subsided...limped back into dock...couldn't reproduce with gears in idle..even up at 4500 RPM

Called mechanic...he went for ride..said it was blown spark plug so he changed all plugs and rotor/cap this week. Went out yesterday and noise returned at 2800 RPM under load...back to dock....no noise in idle even at 4800 RPM...mechanic went for ride again...he's kinda stumped...we didn't have stethoscope to see if it was under valve cover...definitely a motor noise, not outdrive

Noise is definitely a tinny pinging noise (gets louder as you up RPM under load), not a real "clunk" and only happens under load.....valves, cylander...lifters???..why under load only or is this a drive train issue

I appreciate any input...not looking for any one to solve this, but want to have some ideas what it might be before mechanic starts doing exploratory surgery

 

[Scott Danforth]

Detonation would be my first thought. Could be running a bit lean?

 

[Bondo]

Noise is definitely a tinny pinging noise (gets louder as you up RPM under load), not a real "clunk" and only happens under load.....valves, cylander...lifters???..why under load only or is this a drive train issue

Ayuh,..... Welcome Aboard,.... Sounds like Detonation, yer motor consumin' itself, probably from a lean condition,....

Pull the fuel filter, 'n check it's Contents for water, 'n crud,....

Surprised yer mechanic didn't understand it, nor see any evidence in the spark plugs he took out,....

 

[PDW911]

In fairness...I think he was aiming this way...he kept fiddling with carb settings while underway....and I had just gotten fuel a couple of days before....on a near empty tank...maybe all the rock and rollin' jostled up some water (stinkin' ETOH....I should be consumed, not in fuel).......not being a mechanic...why detonation under load and not while at idle??

 

[Bondo]

.why detonation under load and not while at idle??

Ayuh,... Without a Load, very little fuel is necessary to rev to the moon,.... No lean condition,....

Heavy load, max fuel needed but not there, equals a lean condition, equals detonation,.....

 

[keith2k455]

I agree. Did the problem just start? Did your "mechanic" check your timing to make sure our isn't out? Are you sure you didn't get some bad fuel?

 

[Rick Stephens]

In fairness...I think he was aiming this way...he kept fiddling with carb settings while underway.

A carburator is only adjustable on the outside for idle settings. You cannot change any other power range. Your mechanic was mostly wasting your time trying to adjust the carb for a high rpm ping.

A carburetor is one big massive compromise. Each carb is most efficient to run best at one power setting - full open and full airflow. All other throttle plate settings are where the compromise is and lots of engineering goes into fixing lean or rich problems at all the various less than full open settings. At idle, the air fuel mixture screws set how rich. At all other throttle settings the ports, venturi, jets and needles set the fuel air mixture. If the only place you are running lean is at fairly high cruise, then probably the main jets or mains in combination with some other port, even in the secondaries, is probably plugged up. That is all it takes - and at high RPMs also means the ignition is advanced to maximum, giving the most likely time to detonate. And the most hazardous to your engine as high speeds, lots of heat, running lean and spark advanced - made to order to burn a hole in a piston.

Rick

 

[PDW911]

makes sense..thanks for the responses..I know he was coming back to check compression, timing etc..I'll be sure to ask about fuel filter and checking carb

Fuel probably isn't bad...this guy sells a lot of gas and I haven't seen a barage of customer's at his fuel dock complaining....if it is fuel related I am thinking more in line with water or sediment in my tank..who knows, I know he (mechanic) will change water separator

Is this spontaneous event just a case of "stuff happening"..the boat ran fine the two previous days and I didn't notice anything going out, only half way in

As far as damage, since I throttled back should this limit the chance of blowing the piston?

 

[Rick Stephens]

You did right. Pulling power down cools everything and probably is good enuf to keep everything primo. Don't borrow trouble - running with a lean situation or too far advanced can burn a hole fast. Powering down fast prevents that from happening.

Rick

 

[Bondo]

I know he (mechanic) will change water separator

Ayuh,.... Make sure it's Contents are examined for evidence, 'n not just tossed in the trash can,....

 

[hnt]

I would suggest a compression test. I had two loose spark plugs on #4&#6 on my 5.7LXefi The head overheated around the plugs and blew out the head gasket between #4 & #6. I had the exact same symptoms as you described. Just a thought

 

[PDW911]

Thanks for all of the input..I've yet to hear from the mechanic, but given the holiday and his work load but I will advise...I know he indicated he wanted to do a compression test and check the timing

 

[PDW911]

Well..thanks for all the input...bad news, starboard head blown. We repowered the boat two years ago, so the engine was supposedly in warranty....Mercruiser looked at it and said "over heat"...no warranty.....funny I didn't have any heat related issues during the past months...we did take up some mud in late June...the alarm went off and we shut down and then limped back in with engine cooled. Mechanic cleaned and replaced water pump, drive pump and put in new exhaust "flappers" ?? due to damage from heat (???)

Options are re-power again (ugggh) or new heads (far less costly)..mechanic indicates no issues with cylinders except some carbon from the cross over...he is recommending "go with the heads"

We are both curious why this type of damage appeared now...I ran the boat pretty hard from early July until the "pinging" started...no heat issues and no signs of [FONT=&quot]poor performance until this sudden onset and even with the noise I didn't lose "performance"

Could this early heat issue suddenly crop up to this problem almost two months later??

thanks[/FONT]

 

[bruceb58]

What do you mean by running it really hard?
Out of curiosity, what was your WOT RPMs?

 

[PDW911]

4000 ish...obviously it went up and down based on where I was, water condition etc...did some off shore fishing, so I ran about 8 - 10 miles out a couple of times...you know how that is...2500 then 3500 then 2200 etc...I think one trip it was flat so I was able to get up RPM...but I always try to keep them 4k-4200 MAX...a lot of bay trips inshore fishing..shorter trips

I think my son used it during this time and took his kids tubing.....high stress I'm sure

We ran a 2-1/2 hour boat parade in mid July (after heat repair) at near drift speed (2000 +/-)

In bay I hit 4K or so coming in...odd 4200 where I could get up speed and on plane...generally most local stuff is 3K but we have a lot of no wake areas so we obviously drop down so that is how I noticed noise disappaited under 2800 when it first happened...I backed down and noise went away...I went up and noise returned...so we backed down and returned to port....no heat alarm no issues with performance that I could feel; but obviously I didn't "peg" the tach once we thought something was amiss

That is why I am wondering...it wasn't like boat sat idle from late June until week before labor day

 

[PDW911]

Sorry..I meant wondering why the issue NOW if it was related to something in June...obviously could be a separate issue...but we had no indication until it starting to make noise

I'm startin' to think I should adopt my mechanic :>

 

[Bondo]

Ayuh,.... It may, or may not be connected to the overheat in June,....

The pingin' is Detonation,... That's why you've been asked the questions We been askin',....

Detonation is induced by advanced timin', which I don't believe is the issue,...

Or by a Lean condition, which is Why I asked 'bout the Contents of the fuel filter,... a tiny bit of water could be the culprit,...

Or, what yer rpms are at Wot, not how fast ya run it,...
If it'll only pull 4200 rpms at Wot, it's luggin', 'n a smaller prop might be needed,...
That motor is happiest when it Can run 4800 rpms at Wot,.... light chop, light load, flat out gittin' it,....

Pingin',... Detonation is in fact, localized overheatin', hot spots in the cylinders, as the motor consumes itself,....

 

[bruceb58]

Sounds like he could go higher than 4200. My bet is he wasn't lugging.

I'm startin' to think I should adopt my mechanic :>

The fact that your mechanic was trying to adjust mixture while it was running tells me you need a NEW mechanic.

What did the plugs look like?

 

[hnt]

I would ask around to find a good engine shop and take the head(s) to them to be evaluated. There is a good chance yours can be reworked at a fraction of the cost of new ones. Cast iron heads can often take an overheat event without much damage (as opposed to aluminum heads). You might get lucky and get by with minimal head work and new head gaskets. Regardless you want to find the source of the overheat and fix that.

 

[keith2k455]

I would also make sure that the oil and water passages in the block are flushed along with the drive cooling passages. Make sure the thing can stay cool next time and see if there's anything else causing the overheat.