4.3L Mercruiser dieseling / run on

[b18apwr]

I know this topic has been beaten to death, but I just want to get a little clarification from the experts. I have a 2001 Mercruiser 4.3L, 2bbl Mercarb, Alpha 1 Gen 2. I, like many others, experience run on. In my case, I don't have to run the boat very hard at all to experience this. My timing is right, I'm running 93 octane, just performed a full tune up (see below), but normal running for just a few minutes - and not near WOT or under load - results in a run on / dieseling of my engine. I put my boat in the water for the first time this season over this past weekend. I was hoping my full tune up would have solved the issue, but it didn't.

My question is about the idle speed. At times, when I return my throttle to neutral after running, my idle seems to be higher. I did notice that it looked like the linkage wasn't always returning to rest against the idle cam. I've turned the barrel on the throttle cable a few more turns to add some extra pressure to hopefully rectify this. I haven't had the opportunity to see if this works. Now, back to the question about idle speed. I've seen accounts where the boat should be in gear on the water and the idle should be set as low as possible to keep the boat from stalling. However, the manual never mentions setting it in gear. I have a shop tach and have set my idle to 650 in neutral in the water. Was this wrong? Should I have done this in gear instead?

I bought the boat last fall and only got to take it out twice. Over the winter lay up, I replaced the plugs, cap, rotor, fuel filter, impeller, drive oil, and engine oil. The timing is dead on and the boat operates at an indicated 150-160 degress dead on. Any suggestions on how else I can beat this run on / dieseling issue?

 

[cooter2506]

Re: 4.3L Mercruiser dieseling / run on

Idle should be set in gear in the water to 650-700. As far as your dieseling usually an over heat will cause that but from what you are saying its not over heating. Maybe float in carb is not set right or is heavy with fuel intrusion. Maybe fuel is dripping down carb and lettig it run until it stops dripping. Might be time for a carb rebuild. Others will be along to help aso

 

[b18apwr]

Re: 4.3L Mercruiser dieseling / run on

Thanks for that info. Other than the run on, the boat runs fantastically well. I plan on putting it in the water tomorrow to work on this. I'll reset my idle with it in gear. Im going to try to lower it as far as I can without it stalling on me. Could it be carbon build up as well? I see a lot of people say that is it and a lot that say it isn't. A decarb is something I haven't tried yet.

 

[Don S]

Re: 4.3L Mercruiser dieseling / run on

Have a look at this Merc service bulletin on dieseling.

Service Bulletin 97-19

On other question, when you checked the timing, did you put it in base timing mode per the manual?

 

[b18apwr]

Re: 4.3L Mercruiser dieseling / run on

Have a look at this Merc service bulletin on dieseling.

Service Bulletin 97-19

On other question, when you checked the timing, did you put it in base timing mode per the manual?

Thanks Don. I know that service bulletin well. I did jumper my purple/white to battery negative when I checked the timing. The timing mark was exactly on when I checked it this way. I also had the purple/white jumpered when I set my idle. As an aside, after running it for any length of time, I let my boat idle for about 4 minutes each time before shutting off and it would still diesel. Anything else I should try?


One odd thing about the service manuals. I downloaded #25 for my engine. In section 4B that I'm using for adjusting my timing, it says it is for a Thunderbolt V system - and I have a Thunderbolt IV system. Everything in the manual looks correct for my Thunderbolt IV. Just wondered in this was a typo. I thought the Thunderbolt V had knock and a few other goodies that my IV system doesn't.

 

[Bondo]

Re: 4.3L Mercruiser dieseling / run on

and I have a Thunderbolt IV system.

Ayuh,... There's No jumpering Nothin' with a T-Bolt IV.....
It's just Hook up, Point,+ Shoot....

The T-Bolt V comes in a few flavors, including the Goodies you mentioned, or Your stripped down version...

 

[Don S]

Re: 4.3L Mercruiser dieseling / run on

You have a Thunderbolt V, they stopped using the IV in 95 or 96, forget which.

 

[b18apwr]

Re: 4.3L Mercruiser dieseling / run on

Wow. Ok. My flame arrestor cover with all my engine, transom, and outdrive serial numbers says "Thunderbolt IV" across it in big letters. Interesting information to say the least. Ok. Well, then I have a Thunderbolt V and I have been following the service manual to a T.

 

[Don S]

Re: 4.3L Mercruiser dieseling / run on

What's your engines Mercruiser serial number?

 

[b18apwr]

Re: 4.3L Mercruiser dieseling / run on

What's your engines Mercruiser serial number?

Engine Serial # 0M083552
Drive Serial # 0L978175

 

[Don S]

Re: 4.3L Mercruiser dieseling / run on

Engine Serial # 0M083552

Either you have the serial number wrong, or you have a 5.0 or 5.7 EFI engine.

OR !!!!!!!!!!

you have the wrong plastic cover over the top of your 4.3L carbed engine.
There should be a metal tag just above where the starter mounts on the engine. That number will mean something.

 

[b18apwr]

Re: 4.3L Mercruiser dieseling / run on

Yep. I typed that incorrectly. Engine is 0M063552

 

[Silvertip]

Re: 4.3L Mercruiser dieseling / run on

Anti diesel solenoids were common equipment on cars in the 70's and 80's for this very problem. It should be standard equipment on boats as well. It is energized and adjusted to push the throttle linkage to normal idle speed. Key off - the solenoid drops out and the throttle plates now close fully. No more diesel.

 

[b18apwr]

Re: 4.3L Mercruiser dieseling / run on - SOLVED!

Re: 4.3L Mercruiser dieseling / run on - SOLVED!

Just wanted to updated everyone. I have resolved my problem. Turns out the throttle linkage was the culprit. In an earlier post, I mentioned that it didn't seem like my throttle was always returning against the idle cam. Well, I turned the barrell on the throttle cable about two more turns away to add a little more tension to my throttle cable. That's all it took. I ran hard all day today and could drop back to idle for about 30 seconds to a minute after running and turn the boat off. No dieseling. Turned right off. Thanks to everyone for their help. I love this forum. It's a wealth of information and I'm just wanting to give back. Happy boating everybody!

 

[teamneon]

Re: 4.3L Mercruiser dieseling / run on

lol seems like im following you, im not i swear, we just have the same boat and apparently the same problems.
Where did you find the manual online?
I really need to trace the tach jumping problem and also the speedo not working problem and need the manual for this.