1989 Mercruiser 5.0l won't start when hot

[BryanKipp]

My 1989 5.0l Mercruiser with the Thunderbolt ignition is having starting issues when hot. I replaced the starter, battery, plugs, cap, rotor, and carburetor. The motor runs at about 200 degrees at 3000RPM. it will start immediately when cold or if I start it right after shutdown, but if I let it sit (I. E. Swim break) for 30 minutes, it just turns over but will not fire. I've seen lots of posts saying possibly vapor lock. I'm wondering if it could also be a failing ignition module?

 

[Bondo]

The motor runs at about 200 degrees at 3000RPM.

Ayuh,.... Yer Overheatin' it Big time,..... 'bout 160? or so is as high as it oughta get,...

Pull the lower unit, 'n replace the raw water pump impeller 1st,....

 

[alldodge]

Agree, 200 is to high

Need to find out if your not getting spark or fuel.

When it won't start, can you pump the throttle and see fuel go down the carb?
Have you tried starting with the shifter in throttle only mode and open the carb some?
Have you replaced the fuel filter this season?

Don't see vapor lock, that happens with fuel injected motors

 

[BryanKipp]

I replaced the water pump entirely so it's good. Replaced the fuel filter while on the water today. So I'm guessing new thermostat. Also, there is a neoprene line the appears to come off the fuel pump and connects to the back of the carb. When I replaced the carb, that line was too short to reach the fitting. Could that potentially cause this issue?

 

[alldodge]

I replaced the water pump entirely so it's good. Replaced the fuel filter while on the water today. So I'm guessing new thermostat. Also, there is a neoprene line the appears to come off the fuel pump and connects to the back of the carb. When I replaced the carb, that line was too short to reach the fitting. Could that potentially cause this issue?

No, that is the fuel pump rupture tube. If the fuel pump diaphragm ruptures, the excess fuel will be pushed up that tube and into the carb so the boat doesn't blow up

 

[flipbro]

Is your choke working properly? And you should verify your temp gauge is correct. Get an hand held IR gun.

 

[BryanKipp]

The choke appears to be working correctly. It is closed after pumping the throttle when cold and wide open when warmed up. I haven't had an answer to whether the ignition amplifier might be failing so it doesn't function properly when the engine is hot. That's really the only component I haven't changed. Also, with the engine running hotter than normal, would engine timing have a possible effect there? I'm going to replace the thermostat, but am really trying to nail down any other potential issues.

Thanks for all your help!

 

[alldodge]

The choke appears to be working correctly. It is closed after pumping the throttle when cold and wide open when warmed up. I haven't had an answer to whether the ignition amplifier might be failing so it doesn't function properly when the engine is hot. That's really the only component I haven't changed. Also, with the engine running hotter than normal, would engine timing have a possible effect there? I'm going to replace the thermostat, but am really trying to nail down any other potential issues.

Thanks for all your help!

Incorrect timing can cause heat to increase, but in a non-HP motor it isn't enough to notice. To find out if its the module, coil or other, your going to need to test. Suggest start with seeing if fuel is going down the carb when throttle is pumped. If you have fuel, then check the ignition. This is for a TB-V which I assume you have (do not know your serial number), but is also valid with TB-IV

 

[flipbro]

Engine timing will cause your engine to run hot. It will also cause hard starts. I would suggest you check timing.