96 Johnson 25hp

I’m getting a 1996 Johnson 25hp 2 stroke that overheats. 3 cylinder, compression was 95 on all three. All I know is that it has been run with mix (I assume the injector was properly disconnected but will check) and that it overheats and shuts off under load despite spitting water well.

Just wondering what your attack plan would be. I was thinking a full water pump rebuild first.

Bore scope the cylinders and see if they are scored. 95 sounds low and my guess is the pistons and cylinders are scored, hopefully not. Net thing I would do is pull off the exhaust plate and see if there is sand plugging up the water ports.

Check the thermostat. They will spit water but the thermostat can still be bad. You don’t see many of these engines, weren’t terribly popular. Just sent one out of my shop with that very problem.

Keep in mind, not all the cooling water used by the engine is ejected through the indicator stream. So it may or may not be getting enough water to cool.

One way I’ve tested to see if there is enough water pumping through the system is to connect the engine to the garden hose “muffs”. The garden hose pressure rams so much water through the system that the water pump is irrelevant to the whole process.

So if the water pump is really the problem, the outboard should run fine on the hose muffs. Be sure to run the outboard long enough, to get the cylinders to temp. If the symptoms persist, then you have other issues:

1. Thermostat.
2. Mud/sand clogging water system.
3. Scored cylinders

I hate to tell you this, but this sounds exactly like the issue I was fighting with a Mercury about 3 years ago. It would start and run, but then overheat and shut down. Tried a lot of cooling system troubleshooting, but should have looked at the cylinders sooner. They were scored all to heck. My guess is someone forgot to mix the gas.

Grouse

How do you know it is overheating? I believe your year should have a limp mode activate before it shuts down.

Overheating is from the owner’s report. It idles fine, he said once it out and opened up it overheats.

I have not completed the sale yet – more interested in the boat, but I would need a motor anyway… Guessing I’ll take it to a boat place to have the cylindars looked at. If the power head is ok, the price is right.

Is 95 low compression? What I find says its within normal range.

Typically over 100 is good, 95 ain’t bad but ain’t that good either. Being even between the cylinders is more important at that PSI.

Water pump would be where I start. Good starting point to lower that price for ya. Question marks deserve discounts! If it idles fine a crappy water pump will make due, but if opened up old fins or completely broke off can cause lack of water getting to the engine causing an overheat. Thermostats if weren’t working typically would do it at idle as well.

Pump and stats both easy diy projects for cheap.

Deuces wrote:

Typically over 100 is good, 95 ain’t bad but ain’t that good either. Being even between the cylinders is more important at that PSI.

Water pump would be where I start. Good starting point to lower that price for ya. Question marks deserve discounts! If it idles fine a crappy water pump will make due, but if opened up old fins or completely broke off can cause lack of water getting to the engine causing an overheat. Thermostats if weren’t working typically would do it at idle as well.

Pump and stats both easy diy projects for cheap.

I just rebuilt a similar motor but a 60 hp evinrude. Was over heated. Blew headgasket.
I’d replace impeller water pump kit and make sure its pumping good water. 95 is a little low should be over 110 at least but like maverick said If all cylinders are sitting the same that’s ok. If it over heated the top cylinder would fry first so make sure you double check that one

So what would y’all recommend if the boat shop says the cylindars are not scored? I can do pumps and stuff myself. I want the boat and motor – but with no motor it complicates things.

Just because it was over heated I would do a head gasket. Real cheap and easy. Then water pump. Might as well clean the carbs half ass, and then I bet she is good to go

I go step by step after couple years now of brutal engine fixing ADD. If it ain’t broke don’t fix it.

If not scored, I replace water pump #1. See what kind of condition it was in, if it’s fins are really warped bingo. If it’s missing a fin or two, bingo, but also the need to backflush the system with the stats out and pump off.

While you have the stats out you can put those in hot water with a thermometer and make sure those are popping off normal.

Who knows it may be none of those things and the brown/white or black/white wire leading from the cylinder that triggers the limp mode may just be tripping out when the cover goes on lol.

First thing is preventative maintenance that very well may be the culprit. Then move on to other things if needed

My 2 cents.

It “shuts off” despite spitting water?

1. Does it completely shut off, as in not running at all anymore?

Or

2. It goes into limp mode?

If while it’s idling and running, and is peeing water, how hot is that water coming out the pee hole and how is the the flow?

If it’s peeing water normally, really question if it’s actually shutting down from getting to hot or if there isn’t a wiring or ignition issue somewhere?

Replacing an impeller is not a bad idea, but it “could” be working properly.

Sounds like the owner is relaying info to you and that’s tough to go off of for a 3rd party to accurately diagnose. And what he’s telling you is his interpretation of the symptoms, which could be totally inaccurate. Not by his fault but just not knowing outboards well enough.

Good luck with it.

Thanks for the replies – I have decided against this purchase. Despite the cheap price, having to rent a trailer multiple times to see if the engine is salvagable is an expensive pain. Also seems that these are not very well loved engines to begin with.

The search for crappy pontoon continues….