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Mercury Sport Jet 240 EFI RPM dips out?

S

Shawn3B

New member
SYMPTOM: 2002 Sport Jet 240 EFI experiencing short-duration loss of RPM. The engine never stalls and otherwise runs/operates well. It usually only (suspiciously?) occurs at or above 4000 RPM and along with impact (physical jolt - coming off a wave). It lasts about a second, then resumes normal operation.

see video:
https://www.youtube.com/watch?v=79rj0_iIv0w


More details/example: I will be cruising at 4000-4500 RPM, then the RPMs will dip down to as low as 1500 (ranges from 1500-3000) for only a second, then returns to normal operation. The engine always runs cool (I think I've never seen it above 125f per the sensors), but this symptom appears more common as the engine has been used/run for a while. It is more regular/frequent at higher RPM and with more motion. I purchased a Rinda Tech tool/scanner, the ECM is the worst one with just access to live sensors and data, but all sensors pass.



BACKGROUND:
2002 Mercury Sport Jet 240 EFI (SN: 0E3988xx) with 400 hours. The boat is new to me.


I test drove it on Oahu and did not have this issue. I noticed weak start and was aware the ignition switch was "touchy." After shipping to Maui, I used twice (short trips) without any running issues, but was having (re)starting issues - that was related to the ignition switch. That probably caused the batteries to fail ... both had low voltage and 100CCA (very low). I installed two new (Optima) marine batteries and did other work (new spark plugs, fuel filters). This is when the RPM issue started. (I kept the old plugs and put them back to test - no change.)


I hoped the ignition switch would be the fix... you could jiggle it and get varying voltage to the batteries. It was corroded and would not click on "run." I replaced it, but it had no affect on the RPM issue. However, I now have a higher idle RPM (it idles about 1350 ish now, should be at 1050).


I've checked (and cleaned) all electrical connections. I only found two suspect connections, fixed both... zero change to behavior/symptoms. I still suspect electrical.


The only shop I could find on Maui said they diagnosed a bad TPS (high voltage - after new ignition switch), so I got a known good pull (used) one... zero change. I later got a Rinda TechMate Pro tool/scanner, and both TPS checked out, operate the same.


I had put some Sea Foam treatment in the gas tank just prior to the first maintenance (and fuel filters swap), I have also since swapped two of them again. I installed a new fuel pump also. No change. I then installed a new stator, still no change. I've also swapped all of the temperature sensors, no change.


I can mess with the idle/timing screws and affect this same behavior (make it worse, but so far not make it better). The new fuel pump seemed to make the symptom become less frequent, maybe.





QUESTIONS:
Ideas? What should I focus on now or troubleshoot next?

Could something with the timing have come loose?
How precise are the timing settings?
What is the RevLimiter - can it malfunction?
Can something with the impeller play a role? (I see no damage, not aware of any problems).


The only way I can reproduce this is to take it on the water and run at high RPM (and produce waves if they don't exist), and sometimes it takes a while before it even starts.



Strange observation: it appears that the issue has slowly gotten better (less frequent, longer before it starts after being cold). The first use after new batteries - it was immediate and frequent.
 
Sounds like a problem in fuel systems. Check the fuel filter that screws into the lil thump thump electric fuel pump,rebuild the mechanical fuel pump and test fuel pressure on high pressure pump. Also check all grounds as sounds like salt use mtr
 
I'm 100% sure it's a 240 EFI (0E384500 THRU 0E406399)

I also agree with the salt/corrosion theory ... I did use a spray anti-corrosive that was recommended when I cleaned all the connectors... but I did not remove the grounds on the block or the solenoid, they visually looked clean and stable (were not loose)... but I sprayed them and most everything else. We were expecting to a find a loose connector (with regard to the RPM drop out happening on impact/jolt). I took it out that day and there was zero change. the next trip (a week later) was about 90 minutes and I had no RPM dips (a first - and not expected). I then did one more trip a week later - was out all day, with only a couple RPM drop outs all day. Best it's been since the issue started (drastic change after new battery install). So... I am excited that the behavior has positively changed. It's very much unclear why or what.

I suppose I should remove the common ground connectors and clean, and spray the anti-corrosive on that and some other areas again. I wonder if it "soaked in" a bit and had a delayed effect. Either way, I strongly suspect electrical, it would just be nice to isolate it better. It's unsettling it sort of "magically" went away for a bit... has not been a immediate response to any change. Another person suggested something with common ground too.

... I'd like to understand what systems are capable of producing this symptom (specifically, what component could cause the drop out ... I guess it could be a corroded ground that shorts on being jolted - hull hitting a wave). I'm not boat savvy or a mechanic (not experienced), but am a network engineer; I have that mindset. And this issue has been driving me crazy... at its worst the boat was unusable... would drop out too often. I also wasn't sure the risk of damaging it. I do hope it's as simple as a corroded connection and while it wasn't visually clear, I've fixed it (and will better resolve with another pass) ... and it won't come back.
 
Last edited:
sportjet.jpg

See for yourself
 
Quick update: Narrowed down to a fuel pressure issue. Spec says 40PSI at rail, but gauges report 30PSI at idle/clean running, and when the RPM dips out, the PSI dips down to 15-20 PSI.

I had already replaced the pulse pump, at the time not really understanding there are 3 fuel pumps, 2 electrical... or what the VST was (and there was a pump inside that). During the primer/pulse pump change a few months ago I found a bad fuel line (brittle, cracking on inside - and replaced)... well, after taking off the VST and looking inside, the filter there was caked with black debris (fuel line), and larger (sand grain size) in the VST chamber. (I wonder if the line degraded when I used Seafoam cleaner in the tank?) So new VST pump in, new fuel filters too... and my PSI is steady at 32 PSI so far... no dips, but I haven't had a chance to run it at 5k RPM for too long yet. Hoping I have it fixed, but at least I have it narrowed in. The vacuum pump (between pulse and VST)... I have yet to take that apart to inspect.
 
This is a very helpful thread for me because my 2003 240 hp EFI in my Sea-Doo challenger is giving me similar problems. I have attached the video. Is this similar to what your boat was doing?

 
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