Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

[rustyscrew]

Summary: I experienced a small explosion sound from the engine followed by a puff of very black exhaust. The engine then was stuttering a barely able to idle as though it wasn't running on all cyliders. The pop was preceded by a whining sound that that sounded like something overpressuring.

Engine: Mercruiser D-tronic 7.3L (300 hp ) diesel

Details:
I had been running at 2900 rpm for about an hour and a half until the sun came up and then pushed the rpm up to 3600. It ran for about 20 minutes after increasing the throttle and then I heard an overpressuring sound come from the engine so I throttled back and it ran rough for a second and then was fine. Then I ran again and about 10 minutes later I heard the overpressuring sound again followed by an immediate pop (explosion) sound and black exhaust billowed out. The engine then stuttered and ran rough at idle as though it wasn't firing on all cylinders. I checked everything I could think of. No fluids leaking out, no water in the oil, no water in the fuel seperator, and coolant was full. After firing it up a second time it ran just fine. So I kept the rpm's below 3300 for the rest of the day and it ran fine for another 13 hours.

Could this be a turbo problem related to high rpms? The engine is rated for WOT 3600-3800. I shouldn't have been running it there probably, but seems like it shouldn't have caused a problem.

We had some water and crud in the fuel a little bit ago - perhaps its related to this? Injectors?

Thanks for your help.

 

[KrisHall]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

I have not dealt with 7.3's in a marine application...But have extensive school of hard knocks with 7.3's in Ford Powerstrokes.... (International T-444-E )

Turbo? Ummm no.
Water in the Injection Driver Module? Possible.
Timing Chain? Maybe
Starving for fuel with clogged filter and water? could be.
Fouled CPS sensor - good suspect.

Leaky injector(s) with or without blown seals dumping massive amounts of fuel/motor oil at high RPM...High Probability...Hope it is not this because those jectors are not cheap.

If it is the injectors hope it is just blown seals...they can be replaced inexpensively.

 

[KrisHall]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

Might also check the wire harness...those injectors (at least on the powerstrokes ) are 110 volt electric solonoid over hydraulic...chafed wire could cause mis fires.

 

[rustyscrew]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

thanks for your thoughts. I like the theory of fuel filter clog. I didn't see water in the bowl but there are some particles that I think are residual from some work that was done on a hole in the fuel tank. In fact this was the first run on it since it came out of the shop for loss of power at higher rpms that turned out to be fuel starvation (clogged filter). I wonder if at low rpms the flow was sufficient, while at high rpms a vacuum was set up in the fuel line between the engine and filter. If it was enough and cracked a seal and pulled in air, that could explain the sound I heard before the pop.

How does this sound? I like this scenario because barring injector damage it may just require scrubbing the fuel and replacing the filters often for awhile..... cheap comparatively!

( I ran another boat with twin D-tronics. The fuel had a bunch of water in it one time. It worked for awhile afterwards, but eventually the injectors gave out. Ultimately the engines became real lemons and were replaced.)

thanks

 

[KrisHall]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

Could very well be.

An old timers remedy for cleaning the injectors is to mix a little auto transmission fluid in the fuel every now and then. Gives the jectors a little lube.

By the way, Ford sets the red line on the 7.3's at around 3700RPM and Yellow around 3200RPM...

And the honey spot for fuel economy is 1500-1700 and shifts out around 2000 under normal conditions.

Food for thought.

 

[QC]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

The 7.3 D-tronic is indeed turbocharged. Also, I don't think fuel filter/water issues will cause black smoke, more likely white. Black smoke is caused by too much fuel for the available air. Just like might be encountered if a boosted hose or clamp between the cold side of the turbo and the aftercooler blew . . .

Like #14 or 16 or even #8 . . . All speculation at this point, but boost would be highest at the highest RPM in a marine application, also the spooling up and "pop" sounds seems consistent. Again, total speculation, this thing needs to be diagnosed properly or you will be throwing parts at it.

Also, automotive efficiency recommendations are fairly meaningless for a marine application . . . Recommended sweet spot RPM, for example, just doesn't translate. My 1996 5.7L Suburban cruised real nicely at 1500 RPM and 50 MPH, but you would never run a boat like that. Similar discussion with the diesel as we still don't have any gears . . .

 

[rustyscrew]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

Thanks. I will check for any obvious damage to those areas. By "boost" - are you referring to the turbo boost?therefore the greatest pressure in the turbo would be at high RPMs....

On the fuel efficiency comment: Your right. This engine pushes a planing hull. 1500 wouldn't get it on plane so in this case it would yield a very low fuel efficiency since it would be plowing a lot of water.

 

[QC]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

Thanks. I will check for any obvious damage to those areas. By "boost" - are you referring to the turbo boost?therefore the greatest pressure in the turbo would be at high RPMs....

Yes. Marine engines are only at full load at full RPM or when struggling to get on plane and only if they're truly struggling i.e. full load. In a truck engine full load can (and does) happen at a variety of different speeds so turbo "boost" or more correctly Manifold Absolute Pressure (MAP), can actually peak at 200 -300 RPM below rated RPM. Again, since a Marine engine doesn't put out its rated power due to the propeller load "curve" until it's highest possible RPM (makes total sense if you consider that a prop is a fixed load, faster requires more horsepower) then, yes, turbo boost is highest at max RPM under load.

What you described sounded a lot like a trucks aftercooler rubber hose blowing off. And since there would then be a dramatic loss of boost, but the full load fuel rate stays the same you get what? Black smoke and lots of it, at least temporarily.

Again, without proper diagnostics i.e. service manual and test tooling, this is speculation . . . Good luck!!

 

[Lee.Salinas]

Re: Loud pop and stuttering engine in Mercruiser D-tronic 7.3L

I have the identical problem with my 7.3L D-tronic. Did you ever resolve the issue?