1993 Force 150 diagnosis

[HotTommy]

Some of you may remember the Force 150 I rebuilt last year. It's been doing well for a while except for those flywheel bolts that broke for no apparent reason a month ago. That problem was fixed and I've put about twenty hours on it since then. Today I encountered a new problem with three symptoms. I'll get out the FSM tomorrow and start troubleshooting it, but I'm interested in hearing where the old heads here think I should look first. Here are the high points.

It started easily cold and ran well for about three of my usual one hour use cycles. I usually tow kids on the tube for 15-20 minutes (80% to 100% throttle) before stopping to swim for 40-45 minutes. When I went to start the engine after a swim break, I noticed that the engine wanted to die as the throttle moved forward to engage the propeller. This was not normal. On the third attempt, I rapidly shoved the throttle forward and managed to keep the engine running at a fast idle. I pushed it up to pull the tube and everything seemed normal. But about 4-5 minutes later I felt the engine lose power as if I had pulled the throttle back. I pulled the throttle back to idle and the engine died. I shut off the key while I retrieved the people from the water. A few minutes later I turned on the key and heard what I believe was the overtemp horn. I did not restart the engine. I got a tow to the marina from a passing boater and have the boat and engine back home where I can work on it.

I suppose the impeller could have failed, but it is only one and a half seasons old. That and the fact that I did not enjoy reassembling the lower unit the last time I did that job makes me want to check other things first if they are likely or informative. So my question for the old guys is this. If this were you engine, where would you start to diagnose the problem?

 

[pnwboat]

Make sure the motor turns freely. Check compression to see if the pistons/rings in good shape.

 

[jerryjerry05]

Like PNW suggested a comp test.
The carbs air screws readjust.

The fuel system. The choke or enricher make sure it's not feeding too much fuel into the system.
Pinch off the hose to the carbs and see if that makes a difference.

 

[Frank Acampora]

Tommy: I have the same overheat problem with my 150 and it is not the impeller. She will cruise all day long, but after idling a while then shutting it down, the overheat alarm sounds and will not quiet until a LONG run. I have changed the overheat sender and believe it or not added a pisser so I can see if I am getting water to the top of the block and head.

The impeller is old but good Unfortunately I have not used the boat yet nor changed the impeller. HOWEVER: Since this is a new problem and since the impeller is older It will be changed as part of regular maintenance.

I suggest that you change your impeller too--cheap enough insurance. You don't need to disassemble the lower unit, just drop it from the leg and remove the top of the pump. Four bolts, simple. However, it helps if you have an extra set of hands when re-attaching the lower unit.

Check and clean the spark plugs. Check compression ratio on all five cylinders and check carbs. Then try troubleshooting the ignition.

 

[HotTommy]

Update: I checked the compression and #3 and #4 were very low. I pulled the head and can see signs of damage to the #3 piston and the #4 piston, cylinder wall and head. The damage looks light enough that new pistons, rings and a hone could make it right again. BUT ..... (rant follows)

I'm right back where I started with this engine a year and a half ago. I bought it with one damaged piston/cylinder and fixed that. I spent a lot of time and money rebuilding and replacing fuel, cooling and ignition systems to get it to run right. And now it fails with two damaged pistons, each running off a different carburetor. I'm a logical person and I understand that mechanical systems don't last forever and parts sometimes fail. But I have serious doubts about the fundamental design of outboard motors in general and this design in particular. I see no reason why the engine portion of a boat motor should be less reliable than the engine of a car. I feel like the manufacturers of outboard motor engines are doing what General Motors did with cars back in the 1950s and designing their products to last just long enough to pay off the loan. That leaves me feeling like I will never get this engine to the point where I can count on it to start and run every time for two or three years. .... I don't know if I want to keep playing this fix-run-break-fix game anymore. If I thought a new engine would be both more reliable and require less maintenance, I'd consider one even at the outrageous prices they ask. But from what I've heard elsewhere on this forum, that is hardly the case. .... So for now, I'm just going to sit for a while and let my anger and frustration die down a bit. Then, when I'm a bit calmer I'll decide whether or not to keep messing with outboard motors.

 

[Frank Acampora]

Tommy: I know it can be frustrating! My first experience with Chrysler engines was with a 1977 Chrysler 90. It was supposed to be a running engine but I spent a whole season putting pistons in it. I finally bought a new 85 block and built it. I still have that engine and it runs very well.

The stock pistons had semi-keystone rings. These tapered rings do tend to get caught in the exhaust ports and break--I don't know why the manufacturer continued to use them. But, since you do need to replace two pistons, if you decide to keep the engine, replace all five with Wiseco. They have a ring that has only a 2 degree taper and are very reliable.

Right now, due to time and money constraints my 150 has all five stock pistons. I bought it for 300 bucks with two bad pistons and since I already had two good stock pistons from a parts engine I put them in. But believe me: If it ever breaks rings again, all five will be replaced with Wiseco.

 

[HotTommy]

Well, its been a month and I've calmed down some. I'm leaning toward replacing all the original pistons next spring and trying again. But first I'd like to get a better understanding of the chain of events that caused the most recent failure. Here's how I see it. .... In a matter of an hour of running time, the engine went from normal operation, to hard-to-start with a poor idle, to loss of power and shutdown. As I said before, I discovered damage to the #3 and #4 pistons/cylinders. As these two cylinders run off different carburetors, it was not likely caused by a lean condition. As I did add the proper oil mixture and none of the other cylinders were damaged, it was not likely caused by a lack of oil in the fuel. Frank A. says the design of the original pistons/rings makes them prone to failure, but it seems too much of a coincidence for two to fail independently and simultaneously. So here's what I think happened. .... I think one of the piston rings failed as Frank speculated. With four other cylinders working normally, that allowed the engine to seem normal except for a little difficulty starting and at idle. Then, while running at speed with one failed piston/ring, it heated the block enough to cause the piston/ring in the adjoining cylinder to fail.

Do you guys think that sounds like a reasonable failure scenario? I want to have a good understanding of what happened so I can fix the root cause of the problem rather than just replace broken parts.

 

[scout-j-m]

That seems possible. Did you ever change the impeller though so you you could confirm it wasn't failed and the culprit of an initial overheat which started everything?

Also, out of curiosity, when you rebuilt the motor did you have to get the cylinders bored or did you just hone them? And did you chamfer the intake and exhaust ports? I am also in the final stages of my 70HP rebuild and this particular thread is making me nervous lol. I got the machine shop to chamfer the ports so I'm hoping that will help keep them from catching even if it gets a little hotter than it should in there.

 

[HotTommy]

scout,
No, I haven't checked the impeller yet. But there is little reason it should have failed so soon. I'll note that the overheat warning occurred after the engine failed, not before. If the impeller had failed I would have expected to hear the warning first. ... The original problem when I bought this motor was a failed piston ring on #1. It had scored the cylinder lightly. I honed the cylinder myself and chamfered the ports in that cylinder myself. So far that cylinder is still ok. ... Cylinders 1&2 are fed by a single carburetor, so it seems unlikely that the #1 ring failed for anything other than the design flaw noted by Frank A. If I do rebuild this engine next spring, I'll be replacing all the remaining stock pistons.