Engine Oscillates at 3K RPM - Like it is running out of fuel

M

mark guthrie

Member
Aug 25, 2009
101
St. Pete, FL
Boat Info
2001 280 Sundancer
Engines
4.3L with Alphas
This is a 280, with twin mercruisers. The port engine is having the problem, the starboard is fine.

Here are the symptoms of the port engine:
- Starts fine
- Running up to 1,500/2,000 rpm, fine
- Accelerating to get on plane, around 3K - will run fine for about 20 to 30 seconds - then the RPM starts to oscillate, dropping 200, then up 100, then down/up, getting slower each time - this is repeatable, every time
- Left to continue, it will die. Alternatively, pulling back to around 1k when it starts to happen, it will not die
- Does not immediately want to restart, but given 30 seconds or so, it will start fine

Seems like a classic fuel starvation issue, but where to start?

I am thinking the best action is to swap parts with the starboard? The difficultiy is that both engines have to have a load, 3K plus to reproduce the problem.

Suspects:
- Fuel filter/water seperator
- Low-pressure fuel pump
- High-pressure fuel pump
- Fuel pick-up, but I am thinking there is one pick up for both engines, but I need to check

Btw, the problem came on fast. For the first 10 minutes of the ride, both engines were running great. Then the problem started and as first it was hardly dedectible, for several minutes (one of those things where you think it might be your imagination), then the RPM started oscillating a lot.

Any input is appreciated.

Thanks, Mark in St. Pete, FL
 
You say High pressure pump so I am going to assume MPI engine with "cool fuel"?

If yes, cool fuel units have a history of internal paint peeling and clogging the injectors and pressure regulator valve.

Many posts on here about cool fuel issues, search on that.
 
Thanks guys. The engine is throttle body fuel injection, so standard intake manifold but fuel is injected. 2001 280 Sundancer.

I don't know what CF2 is? If I do, that rules out being a bad low-pressure fuel pump? and paint chips?.

Yes, a fuel pressure test would be good, but I don't have the equipment to do that. Was trying to troubleshoot myself for the easy stuff. Just put two engines in and replaced an outdrive.

And not sure what cool fuel system is...I guess a fuel cooler? Pretty sure I don't have that.

Thanks for the brain power!
 
hughespat57 said:
You say High pressure pump so I am going to assume MPI engine with "cool fuel"?

If yes, cool fuel units have a history of internal paint peeling and clogging the injectors and pressure regulator valve.

Many posts on here about cool fuel issues, search on that.
Click to expand...

I search for some posts but didn't find any...I will look again.
 
Look up your parts diagram using your engine number. mercruiserparts.com

Airtex E11004 is a good replacement pump for under a $100 for a CF2
 
Last edited:
A6BF391A-9A63-4239-9CD8-B0044916EFCA.png

I think this is the adapter needed to hook up a pressure gauge on the TBI motors.



Tool Aid SG 37650 General Motors Inline Adapter https://www.amazon.com/dp/B0002SR544/ref=cm_sw_r_cp_api_glc_fabc_9Oq1FbJZDETNG
 
scoflaw said:
Look up your parts diagram using your engine number. mercruiserparts.com

Airtex E11004 is a good replacement pump for under a $100 for a CF2
Click to expand...

Thanks. I found one on https://www.fuelpumpfactory.com for $250. I was ready to order it and then someone suggested I check the screen on the input side. I did and it was not even visible, so many particles had accumulated.

So I cleaned out and plan to test today. And I did not order the pump.
 
Problem solved. The screen was the issue.

I was ready to order a new low pressure pump. Then one of the mechanics that I use suggested checking the screen on the input side of the pump.

Sure enought was a lot of particles. Cleaned it out and it worked fine.

Thanks for the input.
 
Greetings and Happy New Year! I have similar symptoms, after full warmup, engine--2003 5.0MPI-- very intermittently and very briefly drops RPMs or cuts off. If it cuts off, it does seem to restart right away. This is all done on water hose cooling water, I don't trust it to go into the ICW waterway in this condition. Did you solve your problem by cleaning the screen at the fuel cooler (part # 35-805406, for my engine)? Background story; while doing maintenance and cleaning and running engine on earmuffs/hose, hose came loose and engine overheated, trashing the impellor. Since due for annual service anyway, took boat to local shop for repairs and maintenance. After service, engine would not warm up to normal 160 degree temps and water pressure reading was way high (43 PSI, I think) AND engine would cut out intermittanly, as it still does. Took it back to shop, they replace water pressure sensor, cleaned impellor debris from thermostat & it's housing. Now water pressure and temps seem normal (actually temps a bit low, around 150-155 degrees). Engine still momentarily drops RPMs or cuts off AFTER full warmup. Wondering if all the overheating resulted in paint chips or other debris in fuel screen? Shop trying to tell me it's either ECM or engine wiring harness, which doesn't make sense to me. Ideas/suggestions welcome, but also wanted to confirm which screen or filter I should check.
In advance, thanks very much.
 
BigD@LKN said:
Greetings and Happy New Year! I have similar symptoms, after full warmup, engine--2003 5.0MPI-- very intermittently and very briefly drops RPMs or cuts off. If it cuts off, it does seem to restart right away. This is all done on water hose cooling water, I don't trust it to go into the ICW waterway in this condition. Did you solve your problem by cleaning the screen at the fuel cooler (part # 35-805406, for my engine)? Background story; while doing maintenance and cleaning and running engine on earmuffs/hose, hose came loose and engine overheated, trashing the impellor. Since due for annual service anyway, took boat to local shop for repairs and maintenance. After service, engine would not warm up to normal 160 degree temps and water pressure reading was way high (43 PSI, I think) AND engine would cut out intermittanly, as it still does. Took it back to shop, they replace water pressure sensor, cleaned impellor debris from thermostat & it's housing. Now water pressure and temps seem normal (actually temps a bit low, around 150-155 degrees). Engine still momentarily drops RPMs or cuts off AFTER full warmup. Wondering if all the overheating resulted in paint chips or other debris in fuel screen? Shop trying to tell me it's either ECM or engine wiring harness, which doesn't make sense to me. Ideas/suggestions welcome, but also wanted to confirm which screen or filter I should check.
In advance, thanks very much.
Click to expand...

Your problem sounds different than mine, but checking the screen is easy, quick and cheap. Mine had some kind of particles completely covering it. They were easily removed. It was suggested to use carb cleaner, but I didn't even need to do that. My symptoms were repeatable, idle and up to about 2k rpm, no issue, but going above 2.5k is when is would start.
 
Thanks.
It was the screen in the fuel cooler assembly, correct?
Item # 14 in this diagram? https://www.mercruiserparts.com/bam/subassembly/31849/8093/60
I wish I could borrow or rent a scan tool to check for codes in the computer.
Weird and intermittent symptoms are the hardest, especially when other hands have been involved and you're not sure if you're getting the whole story.
 
BigD@LKN said:
Thanks.
It was the screen in the fuel cooler assembly, correct?
Item # 14 in this diagram? https://www.mercruiserparts.com/bam/subassembly/31849/8093/60
I wish I could borrow or rent a scan tool to check for codes in the computer.
Weird and intermittent symptoms are the hardest, especially when other hands have been involved and you're not sure if you're getting the whole story.
Click to expand...

No, I don't have a fuel cooler (2001). I have a low pressure pump that is mounted to the bilge. I think they changed the design.
 
Hi, on the first sea trial with our's, we had a very similar problem. Turned out to be the fuel/water filter separator. After about 2000 and could not get it over 2600. Acted like it was bad fuel. Never quit but wouldn't go! Good Luck.
 
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