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 Crusader 502 Drops RPM
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Author Previous Topic: Tool  to remove VP inner propeller Topic Next Topic: Fogging a 1999 MerCruiser MPI  


RO# 362

Posted - Oct 19 2020 :  19:05:49  Show Profile  Reply with Quote
1999 Crusader 502 fuel injected. Starboard engine will sometimes drop down to around 2000 rpm's. Increase throttle no change. Lower throttle to idle and then back up again and it's fine. No problem with port engine. Separate tanks.

Engines have 700 hours. This is not the first season that this has happened. But it's happened three times this summer over 50 hours of use.

When this happens all guages read fine. Isn't there some sort of sensor that will lower rpms in some conditions....?

Advice welcome.....

Homeport: St. Louis


RO# 842

Posted - Oct 19 2020 :  20:28:57  Show Profile  Reply with Quote
Something limiting fuel supply?


Homeport: Gulf Coast FL Go to Top of Page


RO# 16045

Posted - Oct 20 2020 :  05:07:22  Show Profile  Reply with Quote
Are you getting an alarm that you can hear? Friend of mine has had similar issues the last 2 years with the same engines. In his case it was an alarm from the transmissions, one was a little low on fluid. The engines are going into limp mode when the rpm's drop
Good luck

87 Mainship 36DC
2006 22' Angler/225hp E-Tec
2021 Nauticstar 211 Hybrid/150hp Yamaha

Homeport: Ashtabula Oh - Punta Gorda Fl Go to Top of Page


RO# 25569

Posted - Oct 20 2020 :  08:12:18  Show Profile  Reply with Quote
Check or replace anti-siphon valve. Cheap an easy first step to eliminate.

Homeport: St. Clair Shores, MI Go to Top of Page


RO# 1159

Posted - Oct 22 2020 :  01:01:02  Show Profile  Reply with Quote
The drop to 2000 rpm may be a good clue the ECC/ECM is going into a safe mode to protect against engine knock , or engine or transmission overheat. For instance, ...any water in your fuel filter/ water separators, or are you possibly running on old unstabilized gas?

An engine scan should reveal any of those problems if they exist (or did since the codes were last erased. ) Especially if a fuel pressure test at higher rpm confirms pressure remains to spec. .

But , FWIW, on random ver infrequent occasions over a 2 yr time period, on one of my Crusader 2003 5.7L MPI engines would drop several hundred rpm at cruise speed for maybe 2-10 seconds then come right back up to my 3400 throttle setting. Nothing showed on the repeated Rinda scans of the MEFI ignition systems and it finally was found to be due to a very hard to notice, slightly bent pin in the fuel pump relay wiring connector. Straightened the pin.... and fine ever since.

If you can do it in time, next time you experience that drop you could have someone quickly open the engine hatch and start wiggling harness wires to see if rpm jumps back up as the circuit closes again.

Couldn't hurt to check the dist cap, rotor and all coil/plug cable connections too.


Homeport: The Vineyard Go to Top of Page


RO# 362

Posted - Oct 24 2020 :  18:32:41  Show Profile  Reply with Quote
I have never had an alarm go off when this happens. Cap / rotor / wires / plugs have all been replaced. It normally runs great but occasionally this happens.....

Homeport: St. Louis Go to Top of Page


RO# 1159

Posted - Oct 24 2020 :  20:10:09  Show Profile  Reply with Quote
You may not have an audio alarm when the engine reduces to 2000 in "safe " mode due to knock or similar, and definitely would no have an alarm if there is a fuel pump electrical issue including pump overheat cutout), unless the engine stops or rpm drop so very low the low oil pressure alarm goes off.

The following would be tough for you to use effectively since you had it happen only 3 times in 50 hours ( similar to my fuel pump relay connector pin problem .)
To rule out the fuel pumps , install a fuel pressure test gauge to see if pressure stays to spec or drops noticeably when the problem manifests.

If you aren't going to scan the engine for recorded events of knock or other problems that reduce RPM to safe levels , to see if it is bad quality gas you can plumb in a clean portable outboard tank filled with fresh new gas right to the LP fuel pump inlet and go for a run to see if RPM are then reliable at cruise speed.

But even at the dock idling in N you could wiggle engine harness wires to see if the pump (or anything else ) cuts out and back on.


Homeport: The Vineyard Go to Top of Page


RO# 33981

Posted - Oct 25 2020 :  20:57:01  Show Profile  Reply with Quote
Swap fuel lines at the gas tank. See if the problem changes to the port engine.

Homeport: Lumberton,NJ Go to Top of Page
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