MerCruiser 5.0 MPI pinging/detonation at higher RPMs

[badfish29]

Hello Everyone!
Long time reader, first time poster. So I've owned my boat, 2005 Trophy 2352, for almost a year now and I've been having pinging/detonation pretty much the whole time anywhere above about 3500 rpms. Below that I can run all day with no problems but as soon as I move up I'll start hearing it and if I throttle down it'll go away. I've put new spark plugs in it, the previous owner/dealer put the wrong ones in, and checked compression on all the cylinders. I used my trusty Harbor Freight special compression checker so I don't know how accurate the readings are but I got anywhere from 120-165 which I know isn't great to have that much spread but I'm hoping isn't the issue cause I'd rather not tear the engine apart right now if I don't have to.

I just keep feeling like it's more a lack of fuel issue so I went through the fuel system first. I put in a new fuel filter, checked the screen on the fuel pressure regulator which was fine, and did a pressure test which was about 39 psi at idle. I'm thinking about getting new injectors because I think that's the issue. I noticed 2 of them have been changed and they look like they were pulled from a junkyard. They're painted red which is pretty beat up and flaking off. Definitely not new.

The one trip out I had that the boat ran perfectly was when I put in a full tank of premium fuel, 91 here in good old CA, and had a freshly cleaned bottom. I was able to run full throttle with no issue the whole day but on my next trip out which probably wasn't that much later, maybe a month, it went right back to its old max of about 3500 rpms before starting to ping.

At this point I'm thinking injectors but I did read a long thread somewhere about someone having the shift interrupter switch cause pinging but I'm not sure that's my issue cause I don't have the weird rpm fluctuations he did.

Does anyone have any ideas or things I should be checking that I haven't yet? I've got a lot of engine experience just not on boats and enough tools to be dangerous, haha.

Thank you for any help you can provide.

 

[stresspoint]

i would start by changing the distributor cap , while you are in there check the cam sensor , if its not that expensive replace it , those two things can be questionable at the best of times ,
although the cam sensor usually just dies and gives no readings they have been known to cause erratic spark @ high RPM which is IMO what 'is going on..

your motor also has a crank sensor , that also usually just dies and again they have been known to cause erratic timing if they are not calibrated correctly with the proper tool.

timing must be set with the correct tool , if not done this way you will never get correct timing.

has the motor been checked with a diagnostic tool , , if so were any codes showing ?.

 

[badfish29]

Do you think the distributor, cam sensor or crank sensor would be affected by running premium gas? I find it's got to mean something that the only time the boat ran right was when I first put premium in it unless that it just happened to be that whatever electrical issue didn't act up that time.

I haven't had somebody come to scan the engine yet with a diagnostic tool. The boat is in the water and it would probably cost me as much to get a mechanic to come scan it as it would for whatever part I was hoping would fix it. I figured I'd come here first to see if I could get some advice from people with way more experience than I have.

 

[Scott Danforth]

Pinging is either bad fuel, running lean or bad timing

The crab caps last about a year. They cross - fire and that detonation is eating your engine

 

[stresspoint]

i know is is an expensive exercise to have someone come out to scan the engine , however its your choice to parts cannon the motor or have it scanned and fixed once and for all .
fuel injected motors are very hard to diagnose without a scan tool and
as far as i know there is no cost effective AM tools available for your system ,
so its either pay the man that had to pay for the tool to make it available to you "or" chuck parts at it hoping to get it corrected .
problem is you can not time the motor without the tool as you need to set perimeters , so you could chuck parts at it and still never get it resolved .

as i and Scott have posted , try a distributor cap and if cost effective the cam and crankshaft sensors ( i believe a new crank sensor will need calibrating so a scan will be required anyway), failing to fix with these parts and you are better off biting the bullet and scanning the motor.

 

[04fxdwgi25]

Knock sensor (which protects from this) may be bad.
But constant knock is not common. Bad gas? Low fuel flow, causing lean mixture? Vacuum leak? Timing advanced too far? Cap bad? Bad ignition wires?

As stated above, the scan aint cheap, but cheaper than a box of new parts that won't fix the problem.

 

[Scott Danforth]

but I got anywhere from 120-165 which I know isn't great

and did a pressure test which was about 39 psi at idle.

any reading above 150 psi is suspect as that is the spec from mercruiser for a new motor. anything higher means bad gauge or crud in the cylinders post the numbers

what is your fuel pressure at WOT? 39 psi is on the low side to start with. spec is 43psi +

if your fuel pressure drops any more than the 39psi at WOT. you have a fuel system issue.

 

[Stinnett21]

I have the 2002 version of your engine. The cap/rotor swap out is easy. I buy NAPA ECH RR207 cap and ECH RR256 rotor. Magnetic screwdriver with Torx bits T15 and T20. Take a Qtip with some grease on the end and extract any pieces of crud that may be blocking the screens while in there.

 

[badfish29]

I did think at one point there might be carbon build up causing the compression to be too high and making it ping so I did run a can of Sea Foam through it. There was a far bit of white smoke so I think it did do something. I think I'll get new compression numbers today. I found my local Autozone has the cap and rotor in stock so I might just buy it and see how that goes. While I'm out I'll get the fuel pressure PSI at full throttle and see what that is too.

 

[QBhoy]

run it off a remote fuel tank with fresh fuel and change the distributor cap, if that doesn’t work. Make sure you have the correct iridium plugs in it.

 

[badfish29]

I made a video of the distributor movement. It seems to just be the head of the distributor. The shaft that goes down into the block doesn't move at all. Is this normal movement or should I replace the whole distributor?

 

[badfish29]

I pulled a couple of plugs cause I'm still suspicious of the 2 changed fuel injectors. The first picture is of the cylinder with the changed injector. It looks suspiciously clean like maybe the injector isn't even working? The second image is of a cylinder that looks to have an original injector. It has deposits on it like there's at least something going on in the cylinder.

 

[QBhoy]

I made a video of the distributor movement. It seems to just be the head of the distributor. The shaft that goes down into the block doesn't move at all. Is this normal movement or should I replace the whole distributor?

Hi. There is no adjustment on the distributor to speak really. The timing is done by the ecu. On the distributor, for initial set up only, there are two lines marked on the base. As long as you are between these lines, the position is set and the ecu takes over from there. That said and not an official answer…I’ve found on the rare occasion…that there is a slight difference to be made by only slightly rotating the thing my a mm or two only. After the 13mm securing bolt is slackened off below. But I’d be certain that’s not your issue at all. Given that you are pinging as low as those rpms stated. Your issue is likely to be either fuel quality or the actual distributor cap itself. Trust me when I say that this is now absolutely thee most notorious and infamous issue that these mpi engines have. Don’t trust the appearance of the cap at all. It can look great and still be the source of your issue. I’ve been droning on about this issue for about 7 years now. Just in the last few years there, it’s now officially became a very well known thing. Most prevalent after winter storage too. If that’s not your issue now…it certainly will be soon. Be very careful installing the cap too. Very fragile and easy to crack the thing when tightening it down. Need to make sure it’s home and seated before tightening the two screws down.

 

[QBhoy]

I pulled a couple of plugs cause I'm still suspicious of the 2 changed fuel injectors. The first picture is of the cylinder with the changed injector. It looks suspiciously clean like maybe the injector isn't even working? The second image is of a cylinder that looks to have an original injector. It has deposits on it like there's at least something going on in the cylinder.

View attachment 376450
View attachment 376452

All symptoms of the infamous diz cap. Honestly. If it’s not the cause. It won’t be a waste of money buying one anyway. Because it will be soon.

 

[QBhoy]

Just read your original post again there. Just about the compression test results. Cheap gauge or not…the variance shown may be a concern there. You may have something else going on there after all. Hopefully it’s just as a result of variances of a cold engine or fuel washing and losing the seal of the rings, being optimistic!

 

[badfish29]

Is it normal for the cap to be able to rock back and forth like it does? I'm trying to decide if I should just get the cap and rotor or a whole new distributor.

 

[Lou C]

I don’t think there should be any movement of the distributor or cap. Something’s worn or not right there.

 

[stresspoint]

something going on there with that dizzy that don't look right , can you try to post a pic or vid of where the moving originates from.
you may need to remove the dizzy to inspect this will mean it needs to be clocked again , that is no problem because you will know everything is correct.

 

[tank1949]

Hello Everyone!
Long time reader, first time poster. So I've owned my boat, 2005 Trophy 2352, for almost a year now and I've been having pinging/detonation pretty much the whole time anywhere above about 3500 rpms. Below that I can run all day with no problems but as soon as I move up I'll start hearing it and if I throttle down it'll go away. I've put new spark plugs in it, the previous owner/dealer put the wrong ones in, and checked compression on all the cylinders. I used my trusty Harbor Freight special compression checker so I don't know how accurate the readings are but I got anywhere from 120-165 which I know isn't great to have that much spread but I'm hoping isn't the issue cause I'd rather not tear the engine apart right now if I don't have to.

I just keep feeling like it's more a lack of fuel issue so I went through the fuel system first. I put in a new fuel filter, checked the screen on the fuel pressure regulator which was fine, and did a pressure test which was about 39 psi at idle. I'm thinking about getting new injectors because I think that's the issue. I noticed 2 of them have been changed and they look like they were pulled from a junkyard. They're painted red which is pretty beat up and flaking off. Definitely not new.

The one trip out I had that the boat ran perfectly was when I put in a full tank of premium fuel, 91 here in good old CA, and had a freshly cleaned bottom. I was able to run full throttle with no issue the whole day but on my next trip out which probably wasn't that much later, maybe a month, it went right back to its old max of about 3500 rpms before starting to ping.

At this point I'm thinking injectors but I did read a long thread somewhere about someone having the shift interrupter switch cause pinging but I'm not sure that's my issue cause I don't have the weird rpm fluctuations he did.

Does anyone have any ideas or things I should be checking that I haven't yet? I've got a lot of engine experience just not on boats and enough tools to be dangerous, haha.

Thank you for any help you can provide.

I am assuming u have a Chevy V8 with Thunderbolt 5 with Knock Sensor. If so, make sure sensor is functioning CIRRECTLY.

 

[dubs283]

I am assuming u have a Chevy V8 with Thunderbolt 5 with Knock Sensor

Video posted by OP clearly shows the hei/flat cap style distributor commonly found on merc mpi engines

Based on the decent amount of corrosion on external areas of the engine seen in the video one would guess the distributor cap itself is suspect. As far as the movment seen on the distributor I'd verify the clamp bolt is secure and if continues to have movement removal and inspection of the complete distributor assy would be next