6V92 DDEC 625hp Engine Lag Issue

We are having an engine lag issue with our port engine. We have Detroit Diesel 6V92 DDEC 625hp engines. The problem exists only when we try to get the boat up on plane. The starboard engine does just fine coming up, but the port engine starts lagging at about 1,300 RPM and only makes it to about 1450 RPM until the starboard engine get the boat up on plane and then the port engine finally comes up and the boat runs fine thereafter at cruise speeds, but if you come back off of plane, the same thing will happen again trying to get the boat back up.

By way of background, the current and existing problem started only after we left the boat in the Bahamas this summer with some Bahamian fuel in it. The boat sat in the Bahamas for 45 days in the middle of the summer without being turned on or ran. When we went back to finally run the boat for a fishing trip, it idled fine but would not pass 1000 RPM when we put it under loan (went out to run it). I slowly was able to work some of this bad fuel out of it by reving it up in neutral and finally, the RPM’s started climbing. I was then able to get 1500, then 1800 and so on until the starboard engine came back. The port engine never really recovered and is doing what you see now. Since then, we had the fuel filters replaced (racor and secondary), the fuel tank polished (although this is new fuel), all fuel lines inspected and blown out, all injectors were pulled and sent to RPM Diesel for inspection (one was rebuilt because it did not atomize), and the turbo was pulled and inspected along with all other air sections of the engine. We also switched the fuel box/computer (ECM) and other sensors with the Starboard engine and nothing seemed to solve the problem. All of the stats say the engine is running fine, but clearly something is wrong, and it all seemed to start after that fuel issue in the Bahamas, but the entire fuel system was checked and cleaned out and nothing seems to be coming up wrong.

Can anyone offer any advice?

There are two SIMPLE tests that should be performed before going any further.

The first test is a simple fuel pressure test. It should be done with a reliable gauge and the two engine's fuel pressures should be compared to one another for reference. Somewhere in the 60 to 70 psi range at no load is what I would want to see.

The next test is called a fuel SPILLBACK or volume test. These engines should spill about 1 gallon per minute depending on the size of the restricted fitting. This test MUST be done on the tank side of that fitting.

If you don't know that the fuel system can deliver these values reliably, it really doesn't matter how much work was done...you'll still be guessing.

Many times after a system has been "bugged up" like yours with algae, the filters may get plugged quickly if any debris was missed in the cleaning process. This necessitates changing them out a couple of times to get things cleaned up.

The fuel return lines are sometimes not cleaned very thoroughly due to accessibility issues and that can cause you problems. Any biological left there can cause new growth to occur and screw things up again quickly. I trust you are running a good algaecide in those warm, "watery fuel" climates?

Anyway, until I KNEW the fuel performance specs, I wouldn't be throwing any more time or parts at them.

Good luck.

I did neglect to mention one VERY important aspect of the "spill" test. When performing this test, it should be done in a container of clear, clean fuel so that any air entering the system will show up as bubbles. When the end of the test line is submerged in the container, NO bubbles should be seen. If you do see bubbles, you have found your lagging power issue. Find the source of the air entering the suction side of the system and repair. Your troubles will then, likely, disappear.

Good luck.

jgmo - thank you for your input. We have an engines fuel pressure monitoring system that shows the both engines in the low 60s psi, which is normal for these engines. The fuel pressure sensor is on the fuel return line. We have also done the fuel spillback test. It has good flow and no bubbles that I was aware of. Since we had the fuel filter systems completely changed (went from factory to racor) and had the fuel polished and all lines cleaned, I assumed that all bad gas was eliminated, but it is possible that it has re-manifested in the tank or some other area. We also had the fuel pump tested for aeration. The boat is actually entering "smoke control" mode on the DDEC. This issue is driving us crazy…let me know if you have anything else that you can think of? Thanks.

Ok, so far so good. Your fuel system appears to be working properly. So....

...on to the turbo!

Your post has a KEY word in it IE: LAG.

This could very well be a turbo lag issue and if I were there as the technician I would proceed with a turbo boost evaluation. The problem is, unless you have the tools and the DDEC manual with the HEXADECIMAL charts for doing so, it's not easily accomplished by the average skipper. If you are working with a qualified, well equipped tech though, he should be able to assess this performance area and give you a go/no go on the turbo and the boost sensor.

I have to sign off for now but ask all the questions you want.