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Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10620
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 2004 90 hp Johnson, J90PLSRR. It runs fine until I

Customer Question

I have a 2004 90 hp Johnson, J90PLSRR. It runs fine until I hit 3000 rpm when the SLOW, rev limiter, kicks in. No alarms, no lights. At dock in neutral at 3000 its the same, more like a misfire but I'm not in gear. Any ideas where I should be looking? I recently had a sinc and link done at my local dealer. Thanks, Mike
Submitted: 7 months ago.
Category: Boat
Expert:  Tan replied 7 months ago.
Hello. Mike. My name is***** locate the SLOW cause disconnect the tan wire at the top of each cylinder head. Does engine run better? Then unplug the VRO pump does engine run any better? If none of these change how the engine run you are not in SLOW mode. We need to check spark and compression on all cylinders. The compression should be 90+ psi with no more than 15% difference between cylinders. The spark must jump a 7/16" gap on a s[park gap tester. Then finally run the boat and squeeze the primer bulb when boat is at full throttle. Does engine run better? If no change push the choke at full throttle. Does engine run any better. Post back with questions and findings.
Customer: replied 7 months ago.
OK, I let her warm up. Throttle at 3100 rpm. Disconnected tan wires (temp sensors) one at a time. Disconnected VRO. No changes, still missing. Tried pumping fuel bulb, no change. This is a rebuilt powerhead, new last July. Been running great. First time on the water this year.
Customer: replied 7 months ago.
New plugs and Thermostats also
Expert:  Tan replied 7 months ago.
How about spark and compression?
Customer: replied 7 months ago.
Didn't get to them yet. Pouring rain. May take awhile to get a compression tester
Expert:  Tan replied 7 months ago.
That's Ok I'll be available all day.
Customer: replied 7 months ago.
This is.my first time using this service. What happens if I can't get until.next weekend. Gotta.go to work :)
Expert:  Tan replied 7 months ago.
Thta's fine the question will stay open till we finish with your issue. Just post back when you get the chance to do the tests.You can post back just as you have been. My hours are 8 am- 6pm PST every day.
Customer: replied 7 months ago.
Will do. Thanks
Expert:  Tan replied 7 months ago.
Your welcome talk later.
Customer: replied 7 months ago.
Tan, are you online? I have some new information about my motor. Mike
Expert:  Tan replied 7 months ago.
Hi Mike yes I am.
Customer: replied 7 months ago.
Spark and compression look good. I've replaced the power pack. Same problem, the SLOW kicks in over 3000. I noticed a couple things though. It runs fine over 3000 until it warms up. I get a HOT alarm and the SLOW kicks in. New thermostats, no change, new temp sensors, no change. Plugs are new. Does this help?
Expert:  Tan replied 7 months ago.
When was the last time you had the water pump serviced?
Customer: replied 7 months ago.
Last July. It's pissing strong. I removed the Thermostats and run the motor. Looks good on both banks. Back flushed also.
Customer: replied 7 months ago.
Also stove tested the stats. Passed!
Customer: replied 7 months ago.
This motor has an air intake temp sensor also. Somewhere on the throttle body. Seems unlikely but could that cause the problem?
Expert:  Tan replied 7 months ago.
Ok the only things that will put the engine in SLOW mode are no oil or temp. When the engine acts up have you disconnected the temp sensors in both heads and the VRO pump. If it still reduces rpm then you have another issue. Have you verified spark when the engine looses rpm with a timing light?
Customer: replied 7 months ago.
Last week I let the motor warm up. Reved it to 3100 and disconnected the 2 temp sensors and VRO one at a time with no change noticeable. I haven't used a timing light.
Expert:  Tan replied 7 months ago.
This test has to be done on the water with boat under a load.
Customer: replied 7 months ago.
What rpm?
Expert:  Tan replied 7 months ago.
At the rpm the engine is acting up at. If it only goes to 3100 at full throttle this is where you need to do the test. If the engine isn't acting up the tests won't show us anything. In neutral on a hose isn't when it is acting up is it?
Customer: replied 7 months ago.
FYI my boats in the water at my place in Delaware. Timing light is home in Maryland 3.5 hours away. Have to wait for that test
Expert:  Tan replied 7 months ago.
We have to verify spark and that it isn't a temp or oil issue at the engine rpms the engine is acting up at. That's fine I will be here.
Customer: replied 7 months ago.
Ok, thanks...more to come next Friday.
Expert:  Tan replied 7 months ago.
Ok Mike. Remember do this when the engine is acting up check spark on all cylinders with timing light. If spark is good then disconnect the temp sensors and the VRO pump. Then run engine. The timing light test you want to do while engine is acting up just watch the flash of the timing light. You should have a steady flash with no dead spots.
Customer: replied 7 months ago.
OK
Expert:  Tan replied 7 months ago.
Thanks talk next week.

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