2003 Johnson 70 temporary power loss

Robert Kemp

Recruit
Joined
Aug 20, 2018
Messages
4
I bought a 24' pontoon last April with a 2003 Johnson 70 4 stroke with fuel injection. About 15 minutes after putting it into the water the check engine light came on indicating an overheating issue but temp gauge was fine and engine felt normal. It worked perfect for 2 months and that CEL never went off. In July with a full boat at full throttle after a while I got a power loss almost like it dropped a cylinder until I backed off the throttle. This was sporadic at full throttle for 2 more months but it wasn't a ski boat so I figured that I could live with it. In September when I was taking it to the ramp to take it out of the lake for the season it began the power loss at 1/2 and 1/4 throttle. I drove right from the ramp to a boat mechanic where I left it. A few phone calls over the winter suggested that he was looking at the ECM but was not sure. I ended up taking it from him and getting a hunch that he didn't really do anything to it all winter as he charged me nothing. So here I am. Anybody experience this? I'm wanting to diagnose this and thinking I'd start with spark. Is there a tool that would allow me to visually see if all spark plug wires are getting spark at full throttle while actually using the boat on the water? I already bought a brand new boat so I figured I'd play mechanic with this one for a while and maybe give it to my son. Thanks
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Yes sure get a timing light, you can use that while on the water to check spark
 

Robert Kemp

Recruit
Joined
Aug 20, 2018
Messages
4
Thanks. Its at a certified dealer now. I'm just thinking ahead and expecting the worst which to me would be no definitive answer without many hours at $120 per hour.
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Thanks. Its at a certified dealer now. I'm just thinking ahead and expecting the worst which to me would be no definitive answer without many hours at $120 per hour.

Tell them to keep in touch with time used, i always say what will you do in first hour etc (before i started doinfg my on work)
 
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