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.
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.