1989 90 "H.P" johnson overheating

[adamthorpe]

hello everyone I am have a frustrating problem with my 90 hp Johnson the problem is happening when the engine gets warm. the motor just shuts down after about 15 -20 of use and wont start again for about 20 mins and will only run for 5 mins or so. We took the boat to a mechanic and started it on land with the headphones on at witch point we noticed the cylinder head gaskets were worn out and there were also leaking water. We left it there to get the gaskets replaced and have the thermostats replaced as well. This weekend we picked her up and took it for a spin it did have a lot more power but not for long she did it again shut right down. So now what to do . I am thinking the problem could be electrical??? could there be a flaw in a sensor in between the engine and the thermostats I am have no idea if there is even a sensor or not. or could it in fact be overheating dew to the impeller being weak seams to me there there is a good presser being peed out could it be the fuel pump getting warm and shutting down we always have the vent cap on the tank opened up but we did not always. we actually sucked a couple plastic tanks in a few summer ago. Any suggestions would be wonderful I am truly confused and frustrated
thanks Adam Thorpe

 

[HighTrim]

Re: 1989 90 "H.P" johnson overheating

Welcome to iboats.

Are you getting an overheat alarm when this happens? If not, have you tested your alarm to ensure that it is functioning?

 

[jtexas]

Re: 1989 90 "H.P" johnson overheating

Motors with perfectly functional water pumps can overheat, too. If you suspect overheating, for goodness sake stop running it, unless you are intentionally converting it into an anchor.

Is the fuel line still pressurized when it quits running? If so, the fuel pump is fine.

First, what's the compression on this motor.
Second, do a spark test (not a "spark plug" test). get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it's not firing in the hole under heat & compression.

If you can replicate the symptoms on land, or arrange to arrive back at the dock just as it shuts down, do the spark test after it quits.

Let us know what you find out.

 

[adamthorpe]

Re: 1989 90 "H.P" johnson overheating

thanks for your replys
no we never got alarm and no we have never checked it.

also thanks for the good tip for testing the spark ill be sure to do that next time. Iam sure the fuel line is pressurized even hours after. What my mechanic said is that the problem is electrical and to replace the power pack does that seam right to any of you the power pack getting warm and shutting the motor down? we also plan to replace the ignition switch the spark plug leads and the fuel line. thanks

 

[jtexas]

Re: 1989 90 "H.P" johnson overheating

Faulty powerpack can work until it heats up.

The motor doesn't get as hot idling on the muffs, so it's possible that the symptoms won't appear while testing it on land. But even though the motor will idle on the muffs, if the powerpack is malfunctioning it may be unable to throw a spark across that 7/16" gap, so that test might still be of value.

A qualified marine tech should have no problem at all replicating the symptoms in his test tank, and testing each component in the ignition system to pinpoint the cause. If your guy is thinking of replacing parts without testing 'em first, run don't walk as fast as you can in the opposite direction.

If you don't have to reprime the fuel line by squeezing the primer bulb to restart the motor after it shuts down, a new fuel line will be a waste of money. Spark plug leads, if they're old & worn out, won't be a waste of money but they won't solve your problem. Keyswitch is very unlikely. But if it *is* the keyswitch, the motor will start if you disconnect the black/yellow wire (tape it off to prevent stray voltage). Note that if it starts with the kill wire disconnected from the keyswitch, you'll have to use the emergency lanyard to shut the engine off, or else choke it.

In general, I'm opposed to randomly throwing parts & money at a problem. On the other hand, powerpack replacement is quick & easy (bolt off, bolt on, plug it in), and if that doesn't fix it well, there's way worse things than having a spare powerpack on board.

If you are inclined to do your own engine repairs & maintenance, there's really nothing on this motor that can't be fixed by a determined do-it-yourselfer with basic tools and a good service manual. I highly recommend the factory shop manual.

 

[adamthorpe]

Re: 1989 90 "H.P" johnson overheating

What were trying to do is is eliminate the smallest problems first . The ignition switch is sticking and does not seam to be responding to a choke very well. Its a small repair that is over dew. It has been suggested to us that over time spark plug leads get worn and cause resistance there for not causing the strongest spark. I just don't want any one to think that Iam ignorantly throwing parts at the prolbem its just more or less making sure that we can take out any venerable when were troubleshooting.

Do you think its a good bet that because the alarm is not going off when it shuts down that it is not overheating and it would be a better indication that it is the power pack that is causing the problem. of course i do need to check the alarm is infact there and working. Does it sound like were on the right track testing the alarm testing the spark, and if we find that the alarm is there and working and the spark is not jumping 7/16'' would that be indication enough to say it is for sure the power pack. Or if we find the spark is jumping are all bets off or is what were facing here is a spark that will slowly losing current when the power pack gets warm and that's what is causing the motor to shut down so there is still a chance of the problem being the almighty power pack. I plan on making the repair with my dad were both very handy but feel this is a little out of are area of expertise. We would be very interested to here any other opinions/suggestions

 

[jtexas]

Re: 1989 90 "H.P" johnson overheating

ah, troubles with the keyswitch, that's different.

If you can hold your fingers against the cylinder head for a few seconds, then pull 'em away without leaving any skin behind, that's evidence it's not overheating. If compression is good on all cylinders, even more evidence.

I agree with your logic.

 

[adamthorpe]

Re: 1989 90 "H.P" johnson overheating

iam not sure I under stand what u mean by if compression is good. I have no idea how to check fore that. It should be just fine I image we just got the cylinder head gadgets replaced. If it pasts the spark test will that indicate that compression is good? Right

I guess the real question is whats the compression right before it shuts down
when we test take the boat for a test run should we just leave the inline tester have one of us watching it for decreasing in spark that should be the best way of telling if the power pack is overheating a shutting the motor down. or is that a horrible idea and well mess something up? if so i guess the next best thing is to run it when it shuts down hook up the tester then see what were getting when we turn it over.

note were doing this in water not on land

 

[kenmyfam]

Re: 1989 90 "H.P" johnson overheating

You need a compression tester and it has nothing to do with the quality of spark. Read the Top Secret Files at the top of page one for some really useful and valuable information.

 

[jtexas]

Re: 1989 90 "H.P" johnson overheating

When you do the spark test, the tester is clipped to the engine block, not to the spark plug. The 7/16" open air gap is supposed to mimic the amount of resistance in the circuit at high speed.

Compression is what happens when the piston moves into the cylinder, it reduces the volume of space in there, so the air and fuel become "compressed". Good compression means the PSI readings are about the same on all cylinders. It's an indication of the over all health of internal engine parts; an engine with bad compression will never run right, no matter what else you do. Follow the "compression testing" link from the "top secret file".

If the motor's been damaged by running too hot, that will show up in the compression results. It doesn't really matter how new the head gaskets are, if you overtemped the engine, or think maybe you did, check the compression.

 

[ctravis595]

Re: 1989 90 "H.P" johnson overheating

its a shame the fix was never reported as im having similar issues with my 1988(or 89) johnson 48 spl. i dont think the engine itself is overheating but possibly an electrical part. seems the plugs, plug wires and coil packs are new so my last suspicion is the power pack