Johnson GT 200 overheating or???

First the history, I have my brothers 1990 Johnson GT200 and I have had problems with it going into slow mode. First time it pretty much stayed in slow mode for 10 miles, right from the get go, but no alarm. 2nd time it was intermittent, mostly it would open up, sometimes slow mode but never an alarm. I got to thinking electrical interference since there was no alarm (I grounded the temp sensor and the alarm went off, so it works) so I checked the wires and one of the sensor wires was touching a plug so I thought ah-ha, there is the problem. Fixed that, took it out and it ran pretty good, it had a little miss that would come and go, figured that was just a little bad fuel (the boat had sat for a while) or something, but then it went into slow mode after about 10 minutes and this time the alarm went off. I shut it down and felt the heads, they were hot like a really hot cup of coffee, but they didn't burn. The motor wouldn't start back and I was getting worried when I realized that the tanks were hooked up backwards and I had been running on the tank that was now empty. I switched over and it ran in slow mode for 20 minutes back to the ramp, with alarm. I took the water pump out today and it was perfect and so was all the housings. It has a gauge for water pressure and it runs about 3 psi at idle to 13 at WFO. That seems maybe a tad low, but should be enough I figure. It also pee's well. I am thinking either a bad sensor or when the boat was running out of fuel it was running a little lean and got a little hot, but shouldn't it have cooled down? Going to run back out to the lake today with a IR temp probe. Any other ideas? I hope I got all the goodies lined back up putting the foot back on, it seemed pretty hard to get her back up in there.

Re: Johnson GT 200 overheating or???

It might not be going into SLOW, it might be starving for fuel on one side.
A timing light would show if its dropping spark on one bank, also try unplugging the shift assist switch.

Re: Johnson GT 200 overheating or???

Is it possible the alarm was due to a fuel system obstruction? That engine has a vacuum switch T'd into the fuel line right before the fuel pump, when activated it sounds the alarm horn.

Re: Johnson GT 200 overheating or???

Thanks for the replies, the last time out I am pretty sure it was at least thinking it was over heating because the alarm went off and was continuous and idled down to 2500 RPM steady.

I can't find a firm number of what the head temps should be and what the psi should be at the water pump.

Re: Johnson GT 200 overheating or???

when was the impeller last changed? is it peeing, when running, if so could be tstats.

thermostats start to open at 140* should not go over 170*

in my opinion this is a must have on a boat. http://www.harborfreight.com/infrared-thermometer-93984.html

Re: Johnson GT 200 overheating or???

Unless it has been upgraded to the 1993 design, you should have a water control valve in your cooling system that sometimes causes problems when not serviced. It's also very easy for the small holes in the thermostat cover gasket to become clogged and upset the operation of the thermostat(s). I would recommend that you service the thermostat system if it has not been done recently.

Normal operating temperatures should be in the range of 140 degrees F., +/- 15, generally somewhat cooler at cruising speeds than at idle.

To release S.L.O.W. when activated, the engine must cool below 170 AND, be turned off.

A blocking diode prevents the vacuum switch from activating S.L.O.W. so tht only an overheat should cause it to turn on.

Re: Johnson GT 200 overheating or???

tashasdaddy said:

when was the impeller last changed? is it peeing, when running, if so could be tstats.

thermostats start to open at 140* should not go over 170*

in my opinion this is a must have on a boat. http://www.harborfreight.com/infrared-thermometer-93984.html

Click to expand...

I checked the impeller and it looked perfect. I already have an IR thermometer, I plan on taking it out next time. It is peeing fine. I checked the stats and all looked good, but not 100% on that. Thinking about just changing them.

Re: Johnson GT 200 overheating or???

The tell tale lets you know that the water pump is functional, but does not tell you if the water is ciculating through the cooling passages of the engine.

The case that holds each vernatherm can be unscrewed so that you can test the vernatherm.

Re: Johnson GT 200 overheating or???

OK, taken the thermostats out and dumped them in 150 degree water and they opened. I also ran an engine compression test and it was perfect, 90 on each within 1 or 2 psi so I don't think its a head or head gasket. The bleed hole was clear on both stats also. One thing I did notice is the stats didn't seem to seal up really tight when cool, but no oring or anything, so I figure they are all like that. The only other problem I have found is the drive shaft coming through the water pump didn't really seem to be centered. There was no seal in there, so I figured it just pumped plenty of water and what was lost there was negligible so they didn't seal it. Is 13 psi OK wide open and 3 psi at idle?

Is the water control valve on the linkage? I don't know anything about this or how it works.

Thanks for the replies.

Re: Johnson GT 200 overheating or???

I went so far as to pull the heads for peace of mind, they (gaskets) were rotting, but looks like they were sealed where it counted, but of course, maybe something was getting in there that I couldn't see.

solved

solved

I hate it when people start these posts but don't follow up when they find the problem, so here it is. I did a compression test and all was OK, but I still was worried and wanted a peek in the heads. After I pulled them nothing looked out of the ordinary where the head and cylinder meet, but upon further inspection I noticed the gaskets had dry rotted and cracked where it seals the water jacket around the cylinder from the water ports in the head. You would think this wouldn't be a big deal, but it was somehow not keeping enough back pressure or something on the water and not letting it get all the way up to the tops of the heads. Crazy.