M40C cutting out/ surging.

elong0916

Cadet
Joined
Jul 31, 2013
Messages
9
Hello, I have a 2000 Tohatsu M40C I have fixed a couple problems on but still has an issue that I'm trying to figure out.

When running on muffs or in a 50 gal barrel it will start and idle smoothly in neutral or forward gear all day.

When I take it take it out on the lake it does good, runs smooth, goes about 32mph (GPS) at WOT...but after running wide open for about 4-5 minutes it starts surging/cutting out, and usually gets progressively worse until it dies completely. If started back up immediately it will idle roughly and try to die. Once you get it back going it will run ok and if you run it about half throttle it will not have issues but if you run it back wide open it will start surging again after a few minutes.

A little background:

Compression checks good with engine cold AND with engine warm.
I have replaced the water pump housing and impeller (it needed that anyway).
Temperature tested the thermostat.
Has good stream of water from the indicator and water coming out of exhaust idle relief ports.
After running WOT for several minutes you can hold your hand on the head for several seconds so it is not overheating.
Replaced the complete carburetor assembly because I found one of the ears that hold the pin for the float was broken off.
Have had the prop refurbished / repaired at a prop repair shop.

Any ideas of where I need to go with this? I'm thinking that the fuel pump could be having problems possibly or maybe the coils are heating up and then cutting out... not sure if either of these would be a common problem?
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Re: M40C cutting out/ surging.

You can check fuel pump issues by priming bulb when engine wants to die, if springs back to life you have a fuel pump issue. Some related issues concerning your problem.

1-Dried/cracked head gasket. Head gaskets are not eternal, will need to change base gasket if changing a new heasd gasket.
2-Electrical Ignition Coil issues, heats up and misses
3-Plug issues. Are they the ones specified for that model ?
4-Fuel connector internal o'rings hardened, lets air in and engine misses. O'rings are not sold separately, will need to remove them and buy spares at auto shops. Or try another complete fuel line.

It's good to have at hand a spare Ignition Coil, you never know when will fail. Are relatively cheap, order one and test, probably is the direct culprit ?

Happy Boating
 
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pvanv

Admiral
Joined
Apr 20, 2008
Messages
6,500
Re: M40C cutting out/ surging.

You need to test/diagnose the fuel, electrical, and mechanical systems, in that order. Prime suspect would be either a weak fuel pump or vacuum leak allowing air into the fuel system. Next would be the coil and ECU, finally, mechanical issues that occur only when particularly warmed up. I recommend using the Factory service manual, available from any dealer -- including several forum members, even me, (not an aftermarket manual) as a guide.
 

elong0916

Cadet
Joined
Jul 31, 2013
Messages
9
Re: M40C cutting out/ surging.

Thank you Sea Rider and Paul, I have a strong suspision that the fuel pump is my problem, I did try pumping the fuel bulb and it seemed to help. I have placed an order for a new fuel pump and am also going to replace the filter in that mount on the motor block right before the carb. I'm sorry but I don't know the procedure for testing the coil on this engine, if the fuel pump doesn't fix it I guess I will be needing a manual. Will give update once the fuel pump is installed.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Re: M40C cutting out/ surging.

You really need to change just the fuel pump's internal diaphragms that usually wears, baloons with work, that model has a near heavy duty diaphragms compared to less HP Tohatsu models. If changing a compleete fuel pump doesn't solve your problems, try a new fuel line or one that is working well, then go to auto shop and change both connector's o'rings, both have different diam.

If not a o'ring issue you need a special multitester to test coils, personally instead of buying a special tester would invest on a new Ignition Coil which should cost around $ 70.00. The 40 C is a nice battle horse. Post your findings.

Happy Boating
 
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