7.4 MPI won't go over 3000 RPM's

Hifirush

Hifirush

Member
Mar 21, 2014
542
North Padre Island
Boat Info
1998 330 DA, 7.4 MPI with V-Drives, raw water cooled, Westerbeke
Engines
twin 7.4 MPI’s with V-Drives
I need some help from some of the engines gurus! My port 7.4 MPI (EFI) won't go over 3000 rpm. When throttling up, it runs slowly to 3200, then you can feel it skip and drop to 3000 and stay there. This has been an intermittent issue, but now it's here to stay.

Plugs, cap, wires and rotor are 9 months old, and in good shape. Compression is in the 145-150 range. Fuel filter has been changed recently with nothing nasty in the old one.

Unfortunately, getting a Mercruiser mechanic to the boat is about a 6 week wait, so I'm trying to fix this. I don't have access to the computer readers many people have.

I just replaced the MAP sensor but no joy. I had this left over from a rebuild so it was an easy "try".

It seems to retard itself. No loss of oil pressure, temps are good, isles perfectly. It's doesn't have that "good power" once over 1800 RPM throttling up, and then wham3200, skip and drop to 3000.

Any thoughts? Please!?!

Thanks, Matt


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paint flakes in fuel pump
anti-syphon valve at tank
electric solenoid fuel shutoff
fuel pump
collapsing fuel line
 
I recently had a similar issue with my starboard engine....it would start easily and idle fine and seemed to run smooth up to around 4,200 rpm but that was as high at it would go...the WOT rpm for my engines is 4,800 - 5,200.....

mechanic found a bad fuel injector using a diagnostic scanner....after the injector was replaced the engine once again runs within the WOT rpm range.....

cliff
 
Last edited:
Are diagnostic scanners unique for marine engines? Could a fancy Snap-On automotive pull anything?
 
Thanks for the replies guys. The bottom was just cleaned about 3 weeks ago, I have a tiny bit of green fur on the tops of my tabs, but it disappears after a couple minutes. As far as I know, the running gear is clean, the fella who has been doing it reports to me what he finds nothing the past couple visits.

I wish I could get ahold of mercruisers scan tool. You finding your bad injector would prove that.

My only, we'll only dilemma is that boats like mine aren't common down here, it's flats boats and outboards. So the few mechanics that can work on these machines are booked up.

Please keep the ideas coming, I only have time to kill and may just find the issue. Thinking of switching ignition module from starboard to port and see if that would change things. Like I said, or thought I did, it runs to 3200 weakly, then you can literally feel it cut back. It seems to me its electronic. But I'm only guessing...

Thanks again, Matt


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Skuza not sure about a Snap on scanner, I don't know about them really


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Did this start to happen "out of the blue", or was it right after (or recently after) some service/maintenance?

Check fuel pressure at the engine with a fuel pressure gauge. This will tell you a lot right away. Good tool to have around, anyways. If pressure is good (and stays good), move on to other areas. If there is an issue with the pressure, some easy things are below, but a service manual will guide you through other things, too (although I might still check some of things, regardless):

Does your boat have a crossover for the two tanks, or does each tank feed directly (only) to a specific engine? If there is a crossover, try it. If not... Try opening the fuel fill cap for the port engine (calm day... watch for water splashing).

Won't (shouldn't) have paint chips with this style fuel system, but the anti-siphon valve is a possible culprit and can be manually checked.

QUALITY/AGE of fuel in port tank?
 
Last edited:
Check the wires going to and the actual knock sensors. If the computer does not sense the knock sensor it will retard the timing and prevent the engine from going over 3000.
 
JohnGordon said:
Check the wires going to and the actual knock sensors. If the computer does not sense the knock sensor it will retard the timing and prevent the engine from going over 3000.
Click to expand...

Thanks John, this may sound like a dumb question, where is the knock sensor on these 7.4's? That seems like exactly what it is doing...

Matt


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Lazy Daze said:
Did this start to happen "out of the blue", or was it right after (or recently after) some service/maintenance?

Check fuel pressure at the engine with a fuel pressure gauge. This will tell you a lot right away. Good tool to have around, anyways. If pressure is good (and stays good), move on to other areas. If there is an issue with the pressure, some easy things are below, but a service manual will guide you through other things, too (although I might still check some of things, regardless):

Does your boat have a crossover for the two tanks, or does each tank feed directly (only) to a specific engine? If there is a crossover, try it. If not... Try opening the fuel fill cap for the port engine (calm day... watch for water splashing).

Won't (shouldn't) have paint chips with this style fuel system, but the anti-siphon valve is a possible culprit and can be manually checked.

QUALITY/AGE of fuel in port tank?
Click to expand...

Dave, that's also a good idea, I do have a cross over and can isolate that tank, I'll try it!


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Skuza said:
Are diagnostic scanners unique for marine engines? Could a fancy Snap-On automotive pull anything?
Click to expand...

I have been told that automotive scanners will not work on marine engines.....not sure if it is just a connection issue or if the code systems are different....

cliff
 
Hifirush said:
Thanks John, this may sound like a dumb question, where is the knock sensor on these 7.4's? That seems like exactly what it is doing...

Matt


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Click to expand...

The knock sensors are on either side of the engine block near the motor mounts. They screw in to the side of the block. There is a small single dark blue wire running to each. The wire is very fragile and also the connector can get easily corroded. Here is a picture of the sensor.
 

Attachments

  • 805544T%20Knock%20sensor%20572.jpg
    805544T%20Knock%20sensor%20572.jpg
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Thanks John. I’ll be checking them out this weekend!

Matt
 
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