2000 Optimax 150s both overheating after spark plugs change. Need help

J

JPTL

Seaman
Joined
Sep 20, 2003
Messages
52
Original OptiMax,150s on my Whaler Conquest. 415 hrs. Ran fine last season.
This season I decided to change the plugs. I went for factory recommended NGK PZFR5F-11. It had IZFR5G plugs in it. When I took them out, they all had a pretty heavy coating of black soot on the electrodes.
I figured a plug change would make these engines run better, but at this point it hasn’t. They’re both throwing the overheat/throttle down alarm (Smartcraft). It seems to come on at about 172° at about 1750 RPMs. Both engines are throwing the alarm, which is a real head scratcher for me. I’ve researched the correct plugs for these engines and I come up with both the new ones that I installed and the ones that were already in it, also getting mixed details about them being interchangeable or a plug change in the last 20 years.
Where should I start?
 
dingbat

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
17,244
I
JPTL said:
Original OptiMax,150s on my Whaler Conquest. 415 hrs. Ran fine last season.
This season I decided to change the plugs. I went for factory recommended NGK PZFR5F-11. It had IZFR5G plugs in it. When I took them out, they all had a pretty heavy coating of black soot on the electrodes.
I figured a plug change would make these engines run better, but at this point it hasn’t. They’re both throwing the overheat/throttle down alarm (Smartcraft). It seems to come on at about 172° at about 1750 RPMs. Both engines are throwing the alarm, which is a real head scratcher for me. I’ve researched the correct plugs for these engines and I come up with both the new ones that I installed and the ones that were already in it, also getting mixed details about them being interchangeable or a plug change in the last 20 years.
Where should I start?
Click to expand...
One plug is platinum, the other is iridium. MGK says they are interchangeable with the only difference being the platinum is gapped to 0.044 the iridium is gapped to 0.032.

Fouling typically is not a plug problem on an injected motor. One plug would be an injector. Fouling on all holes, on both motors is more than likely a fuel problem.

Replace fuel filters and run off a separate fuel supply to see if that clears up the issue. If your not running 10 micron fuel filters, you may need to have the injectors cleaned as well
 
J

JasonDom

Recruit
Joined
Jul 20, 2026
Messages
2
I agree with the OP, stop chasing the plugs. Both numbers are correct for that engine. The timing of your plug change could be a coincidence.

Watch water pressure when the alarm hits - low PSI possibly? Check the impellers or consider the complete water pump rebuild kits and new thermostats, two per engine (stainless 143 degree ones).
 
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