1998 DT 140 EFI dropping off at high RMP..

[btspac]

Hello all, any help with the following issue would be much appreciated! I have a 1998 DT 140 EFI on my 21'Bayliner Trophy. I've used it and maintained it for 7-8 years without a single issue. Last year, I took it out after de-winterizing it and half way through my coastal trip, it started dropping suddenly from 5000rpm t 2800-3200 (no Oil or other indicators or warnings). Note: some fuel was left in the tank but had stabilizer added. Sat for 5 months

Initially, I suspected bad gas, added fuel stabilizer again and added fresh fuel. (although on top of existing fuel). Problem persisted. More info: Starts with no problem. Idles smoothly. Runs without missing (hooked up to the water hose) and is responsive to short revs. Under load, it runs great up to 4100 RPM or so. I can drop the throttle and it will hit 5200-5400 RPM and shoot out of the hole but then bog down shortly after planning (10-15 seconds later). If I'm careful to not exceed 4200 RPM or so...it runs fine on plane or plowing. It's as if the 4100 RPM mark is the magical point of no return.

To further explain, I noticed that my fuel line bulb was collapsing when it would bog down. Not always but often. So here is what I did--> We pulled the inline fuel separator/filter. It was packed with crap. I replaced not only the filter but housing. I replaced the hose and bulb from the filter to the motor. I pulled the mechanical fuel pump and found that one of the smaller inner diaphragms had failed. Bought a NEW one and replaced it. Replaced all the fuel lines under the cowling because one was weak enough I was concerned about it collapsing. We then pulled all the injectors, cleaned and inspected them. Put them back in. Pulled the VST (it was actually super clean). We cant tell if the fuel pressure regulator is working or not. I suspect that this is some sort of plunger or diaphragm inside. And it clearly uses a vacuum hose connected to the throttle body.

The fuel injector pump comes on with the ignition. I have not been able to test the PSI at the fuel rail...need to get a gauge. It spurt out pretty strongly though. No gas coming through the Pressure regulator. Oil sensor appears to be working. Oil is being pumped and used. Plugs look good.

I initially switched off of my main tank and to a portable but nothing changed. However, I have NOT done that since the above work. I added seafoam to the Bayliner tank and ran it through for a hour at home then ran almost all of the 1/3 tank I had left of fuel. As I said, the thing starts, idles and runs great up to 3000rpm. If I gradually plane the boat without exceeding 4100-4200 it does fine. I can run all day long at 3200-3600 or so. Also, the higher the RMP, the faster it bogs. Turning the boat or if the motor lifts slightly out of the water, causing higher RPMs, albeit not from the throttle...it still results in bogging down. HELP!!! Is this the fuel injector pump? Fuel pressure regulator? I don't think it is the ECM but I have no idea...advice?

[Solarman]

1998, dt140, bogs at higher rpms

You mentioned the mechanical pump (low pressure), did you replace the entire pump, or just the diaphragms (repair kit)? There are small check valves under the diaphragms that have been fragmenting, and pieces even lodging/blocking the fuel going through the lines to the needle valve in the vst, restricting fuel flow.
Second, the high pressure pump has 2 filters. One is located at the bottom of the vst,small cone shape inline going from vst to HP pump, and another filter after the HP pump going to the pressure regulator. Did you clean both of these?
Your problem sounds fuel related. And i'm guessing you did clean the filter going to the low pressure pump (mechanical pump)? The collapsing bulb would be caused by the blocked fuel separators, or blocked vent line on the main tank.
Let us know what you find.

[btspac]

All excellent points. I did replace the entire mechanical (low pressure fuel pump.) I did clean the fuel filter leading to the low pressure pump. I also cleaned the inline cone shaped filter leading into the vst. I did not know about the filter between the high pressure pump and regulator given that I missed that and that this may be ethanol/bad fuel. Today I will re-clean ALL the filters empty the vst and run off a brand new portable tank and fuel...see what happens. I checked the vent line on the main tank before but will do so again. Ever see one of those lines collapse during use? I'll isolate the tank issue by moving to the portable. Thank you! Stay tuned.

[btspac]

You mentioned the mechanical pump (low pressure), did you replace the entire pump, or just the diaphragms (repair kit)? There are small check valves under the diaphragms that have been fragmenting, and pieces even lodging/blocking the fuel going through the lines to the needle valve in the vst, restricting fuel flow.
Second, the high pressure pump has 2 filters. One is located at the bottom of the vst,small cone shape inline going from vst to HP pump, and another filter after the HP pump going to the pressure regulator. Did you clean both of these?
Your problem sounds fuel related. And i'm guessing you did clean the filter going to the low pressure pump (mechanical pump)? The collapsing bulb would be caused by the blocked fuel separators, or blocked vent line on the main tank.
Let us know what you find.

Also, I should correct my statement. It was not a diaphragm but one of the smaller inner CHECK valves in the mechanical fuel pump thst failed. And yes the filter leading to the vst had crap in it. I suspect this was from the check valve. I will look at the filter between the vst and regulator and change fuel...I'll post later today.

[Solarman]

vst needle valve

Did you replace the needle valve in the vst? Or, clean it? Did you remove the seat for the needle valve? Have heard of fragments of those check valves lodging in there.
It seems you are getting fuel from what you described, but could be slight restriction from full flow that's causing the problem.
Let us know.

[btspac]

No, I didnt. I am pulling the vst off as we speak and checking that. I did not pull the needle valve out, from underneath the bulb. Dumb I know. I did not replace it obviously. That cone in-line filter leading into the vst had crap in it again, so bad fuel is certainly part of this issue. I can'it find the filter between the injector (high pressure pump) and regulator though. The path is from the injector pump right into the bottom of the fuel rail. The only other filter I see is within the intake side of the pump and is the internal screen. Am I missing something?

[btspac]

***Solarman***

Thank you! Your comment about the check valve piece being stuck in the needle valve housing was correct! There I found that dang piece that had broken off. I am going to test it on the river but it must have been preventing full fuel flow. I'll let you know but thank you in advance!

[btspac]

Did you replace the needle valve in the vst? Or, clean it? Did you remove the seat for the needle valve? Have heard of fragments of those check valves lodging in there.
It seems you are getting fuel from what you described, but could be slight restriction from full flow that's causing the problem.
Let us know.

Great news! The motor is back to normal. Thank you for your advice Solarman. I should have removed the needle valve and seat. Sure enough, the mechanical pump was the entire culprit. We knew it was shot, because one check valve set a piece down stream. After replacing the pump and cleaning filters I knew better than to keep going when we didn't find that piece. Once you mentioned the seat, I took it apart this AM and there was the plastic fragment. It was on the input side of the needle valve and was blocking the inlet to the VST. Would have never seen it if I didn't remove the seat. Took it out on the river this AM and it ran perfectly. Thanks again!

[Solarman]

dt 140, running again

Glad to hear its ok. We here at the forum appreciate hearing alls well again. Hopefully, we saved you from spending many dollars.lol
Take care and share your experiences here if you can.
And go boating again!!

[btspac]

In fact, you did. I was about to purchase a new high pressure fuel pump and pressure regulator in order to "hopefully" fix the issue prior to our vacation starting tomorrow. So, in this case, it saved money and a vacation I'll be chasing Chinook tomorrow!!