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Motor Enters Slow Mode

S

Stackfish

New member
Hey Everyone,

I'm a newbie and need some help on a problem with a 1994 Johnson Ocean Runner 150hp that's on my recently purchased Grady 192. The issue is that the motor will occasionally start to miss and buck at 2500 rpm. I've found through experimentation that if I restart the motor a couple of time the problem will not present itself. Recently it seems, the following procedure will "fix" the problem:


  1. WOT the motor
  2. Turn off the key with the motor WOT
  3. Put the motor in the neutral start position
  4. Restart



I know the motor is truly not overheating since the water pump is strong and when it does work I make long runs at high RPMS with no damage to the motor. Another interesting thing that's happening, on two occasions when the motor was running well, it went into slow mode as I trimmed it at around 4500 rpm. The motor have never done this while running at high rpm without me trimming.

Any suggestion on how to further diagnose and correct the issue would be greatly appreciated.

Thanks,
Jimmy
 
Clean all connection in the engine with electrical cleaner and put back together with dielectric grease. Sand all grounds metal to metal. Look for cracked, burnt or bare wires and fix as necessary. Once your sure the wiring is 100% then you can start troubleshooting individual components. Look for a wire shorting to ground at a certain trim angle.
 
I`m wondering if the trim senser is connected to the power pack? I suppose it is possible the senser throws the engine into limp in case of a kickup to keep from a run away motor?
 
No, trim sensor is not connected to PP.

Get a temp gun and put it on both heads when the engine kicks into SLOW. Verify that it truly is NOT overheating first.
Try multiple speeds etc. Measure head temps at various places, concentrating on the area around the temp sensors.
If overheating, correct that issue.

If all good there, then disconnect the white/black wire coming from one of the temp sensors....don't remember which side it is on.
Only one side will have the white/black wire. It goes to the pack.

If it still kicks into SLOW randomly then something is wrong in the pack....or the white/black wire as it enters the area of the pack is chafed
and is shorting to ground occasionally,

If the problem resolves itself with the white/black disconnected, then possible bad sensor. You would have to do more testing on the sensor.
 
Now, you state SLOW mode....your words. How do you know?

This motor has optical ignition, and one of the components is the optical sensor.
This is a known problem part. It MIGHT be a bad optical sensor.

Keep that in the back of your head.
 
@daselbee I'm not a mechanic but since it always happens exactly 2500 rpm has me thinking slow mode. I'm not familiar with the optical ignition, is there something with that which could always fail at exactly the same rpm? Thanks
 
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@daselbee Thanks for the troubleshooting tips, that's what I'm looking for. This problem occurs after a cold pre-dawn start (about 65 degrees air temperature) going about 500 feet at 6mph before powering up, could an engine really be overheating by then? BTW, The impeller output is very strong. I am occasionally hearing a short beep from my dash that may be related to the issue.
 
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It looks like a pressure switch is on the tan circuit the switch is on the vapor separator bracket make sure the tube going to it is in good condition and tight on both ends.
 
The tan wire circuit is what kicks it into slow. From what I see it goes to four components. Both temp switches, vacuum switch and the VRO pump. Disconnect the tan wire from the powerpack and if the issue goes away one of those components is faulty. If you still have the issue the powerpack suspect.
 
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