4.3L Dieseling

Joined
Aug 30, 2010
Messages
8
4.3L/LH 2-BBL (2004) was running hot (up to 190 deg) with hot riser on one side so changed the impeller and motor is now running 160-175 but now experiencing some dieseling unless I let motor idle for several minutes before turning off.

Used some seafoam during this same run because had trouble getting to 4500 for WOT and that seemed to help quite a bit. Wondering if the seafoam contributed to the dieseling because didn't have this problem earlier.

Haven't checked timing and getting ready to order Merc service manual, does anyone know the setting/rpm for adjusting the timing. Also is this a simple point and shoot procedure.
 

Fishermark

Vice Admiral
Joined
Oct 19, 2003
Messages
5,617
Re: 4.3L Dieseling

now experiencing some dieseling unless I let motor idle for several minutes before turning off.

I believe Mercruiser put out a service bulletin that recommended exactly that. Let it idle for a minute or two after a hard run to let it cool down. Sounds normal to me.
 

ziggy

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Jun 30, 2004
Messages
7,473
Re: 4.3L Dieseling

ya might wanna read mercruiser service bullitein 97-17.
while this isn't the real s/b. it appears to be copied.. http://www.marinemechanic.com/site/page154.html

under suggestions to minimize 'run on' it does appear to be missing item #7 which is...

7. v6 and v8 only: install a 140* F (60*C) thermostat.
 

scooper77515

Senior Chief Petty Officer
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Sep 3, 2010
Messages
747
Re: 4.3L Dieseling

Mine did this the first time I took it out, just the week before last. Kinda upset us, having problems on the first trip.

I did a full tuneup, new plugs, wires, dist cap, rotor. Went out two other trips after that and it didn't do it again.

Not sure if the tune up fixed it or if I just didn't run it the same way and let it idle a bit before killing it. :confused:

I assume it has the stock thermostat so a swap will happen if it occurs again.
 
Joined
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Messages
8
Re: 4.3L Dieseling

All good suggestions. I'll try the plugs first because I just changed them and noticed my new plugs weren't exactly the same as I was replacing, but didn't think anything of it at the time.

Also good opportunity to take thermostat housing.
 

scooper77515

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Re: 4.3L Dieseling

Dieseling is usually a carb-related issue, not electronics.

I think it may just be chance that mine quit after the tuneup.

Dieseling is usually from the carb continuing to supply fuel after you kill the engine, or burning what is left in it (hence, idle for a few minutes before killing it to burn it off).

But, since mine stopped after a tuneup, which it SERIOUSLY needed, as it had stock plugs, cap, rotor, and crunchy wires, I didn't hurt anything by swapping it all out. And it seems like the dieseling is gone, so go for it, and I will have my fingers crossed for you! :D
 

NHGuy

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3,631
Re: 4.3L Dieseling

Not true
Dieseling is usually a carb-related issue, not electronics.

I think it may just be chance that mine quit after the tuneup.

Dieseling is usually from the carb continuing to supply fuel after you kill the engine, or burning what is left in it (hence, idle for a few minutes before killing it to burn it off).
Nope, dieseling is what occurs when the engine runs without spark, hence the term diesel...It occurs with very hot combustion chambers and can be caused by excessive timing advance.
BTW carburetors don't care if the ignition is on, they are supplied fuel by the pump if the engine is turning, and they provide atomized fuel when the engine turns which pulls the fuel in.
 

fossill

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Jun 20, 2009
Messages
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Re: 4.3L Dieseling

Also seen it caused by carbon buildup in combustion chamber and/or by running a too hot spark plug. Both provide a hot heat source for igniting the fuel-air mixture with the key off. Running a can of carb/combustion cleaner through it and giving a a good hard run will help if thats the cause and or replacing spark plugs to a cooler heat range.
 

scooper77515

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Re: 4.3L Dieseling

My plugs had 7 years and 185 hours on them and looked pretty bad. Would that start a point of combustion to assist with dieseling?

And not to be picky, in my previous post, I reported it wasn't an electrical problem, but a carb issue. The carb keeps supplying air and gas after you turn off the electronics.

I was corrected, but still sorta right. Right? Carb still delivers fuel and air to engine after you kill it, and continues to run without spark? :confused:
 

jkust

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Messages
4,942
Re: 4.3L Dieseling

Run on is pretty common for the 4.3's. I have a 2003 4.3 which I bought at around 40ish hours three years ago. It dieseled then and would diesel now if I didn't do the two things to stop the symptom. I simply let it run for 30 or so seconds especially after a hard run and what really helps is higher octane gas. I only use 92 octane and that works well. I once forgot and filled about a half tank with 87 octane and there was a tiny bit of run on. I finally broke 100 hours on the engine so will get the full 100 hour tune up with the winterization and will see if that does anything. EFI engines won't have this same issue.
 

Alpheus

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Messages
1,757
Re: 4.3L Dieseling

I was corrected, but still sorta right. Right? Carb still delivers fuel and air to engine after you kill it, and continues to run without spark?



Ya but electronics or power to the engine has nothing to do with the carb atomizing air/fuel mixture and supplying it to the engine. You can turn the engine over by hand and as long as the carb has fuel in its bowls it will still function.

Changing plugs,making sure you have a good clean fuel supply or use a step up in octane usually works.

Better / higher octane fuel will burn slower and needs more heat to burn. This is why it helps stop dieseling. Higher octane has a higher flash point this is why it is used in higher compression engines. Combine heat and high compression and the fuel tries to combust before the plug fires.(pre-ignition)
The higher flash point allows the fuel to keep from burning before the plug fires...
 

jkust

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Re: 4.3L Dieseling

My plugs had 7 years and 185 hours on them and looked pretty bad. Would that start a point of combustion to assist with dieseling?

And not to be picky, in my previous post, I reported it wasn't an electrical problem, but a carb issue. The carb keeps supplying air and gas after you turn off the electronics.

I was corrected, but still sorta right. Right? Carb still delivers fuel and air to engine after you kill it, and continues to run without spark? :confused:

Scooper, with 185 hours on the plugs did you notice any difference after replacement. I can't discern any difference between when I bought at 40 hours and now at 107 hours. I am wondering what if anything my full 100 hour service (at least the tune-up parts) will do when I run it next season.
 

scooper77515

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Re: 4.3L Dieseling

Other than it isn't dieseling anymore, a little quicker response at the throttle, hard to say.

I only drove it twice. At test drive, and at maiden voyage. Decided to pop a plug to take a look, and it was worn down a bit and gapped out. Then noticed they had engine paint overspray on them and figured they were put on at the factory. So decided to swap out all of the above. Even plug wires...they were a little stiff and starting to get crunchy at the boots.

Only thing I haven't done is the impeller. Have it in the boat, just haven't swapped it out yet. May wait til when I pull it out of winter storage. I hear that they don't do well being stored a long time without moving.

Boat planes pretty much instantly, and gets to top speed/rpms quite quickly, I would guess 6-8 seconds. So no complaints, so far.

I may start using premium if dieseling re-occurs. Just since it IS dieseling, suggests to me that it is asking for a bit more octane.
 

jkust

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Re: 4.3L Dieseling

Other than it isn't dieseling anymore, a little quicker response at the throttle, hard to say.

I only drove it twice. At test drive, and at maiden voyage. Decided to pop a plug to take a look, and it was worn down a bit and gapped out. Then noticed they had engine paint overspray on them and figured they were put on at the factory. So decided to swap out all of the above. Even plug wires...they were a little stiff and starting to get crunchy at the boots.

Only thing I haven't done is the impeller. Have it in the boat, just haven't swapped it out yet. May wait til when I pull it out of winter storage. I hear that they don't do well being stored a long time without moving.

Boat planes pretty much instantly, and gets to top speed/rpms quite quickly, I would guess 6-8 seconds. So no complaints, so far.

I may start using premium if dieseling re-occurs. Just since it IS dieseling, suggests to me that it is asking for a bit more octane.


I just called to set up my 100 hour service and without any tune up items it is $1000 (winterization, impeller, 100 hour drive maint). I see why people skip it now. I've gotta think I could put plugs and wires in myself.
 

Jeepster04

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Messages
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Re: 4.3L Dieseling

I just called to set up my 100 hour service and without any tune up items it is $1000 (winterization, impeller, 100 hour drive maint). I see why people skip it now. I've gotta think I could put plugs and wires in myself.

$1000 sure seems high to me if they are not doing any tune-up what so ever. If they are just doing winterization, impeller, drive maintenance(probably just change the lube, MAYBE check alignment), you could do that yourself if you have any experience with turning a wrench and have a good manual for your boat.
 

jkust

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Re: 4.3L Dieseling

$1000 sure seems high to me if they are not doing any tune-up what so ever. If they are just doing winterization, impeller, drive maintenance(probably just change the lube, MAYBE check alignment), you could do that yourself if you have any experience with turning a wrench and have a good manual for your boat.

I think I have resigned myself to professional maintenance and biting the bullet. I can just see me screwing it up or taking more time than I have to spend. The only rational I can come up with for the $1000 price tag is the dealership is located on the water in an uber wealthy part of town where I bet most don't bat an eyelash at the price. I half expected him to chuckle when he said the number. Also I see they don't package the services together such as the Platinum package or Gold package but charge separately for each group of services like other marinas do.
 

John_S

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4,269
Re: 4.3L Dieseling

4.3L/LH 2-BBL (2004) was running hot (up to 190 deg) with hot riser on one side so changed the impeller and motor is now running 160-175 but now experiencing some dieseling unless I let motor idle for several minutes before turning off.

Used some seafoam during this same run because had trouble getting to 4500 for WOT and that seemed to help quite a bit. Wondering if the seafoam contributed to the dieseling because didn't have this problem earlier.

Spray down carb version or fuel additive? I only use the former on my strokes (once a year). It does remove carbon, and that generally helps prevent dieseling, but 4-strokes do not typically need it, unless you have been running very rich and had specific carbon build up.

Haven't checked timing and getting ready to order Merc service manual, does anyone know the setting/rpm for adjusting the timing. Also is this a simple point and shoot procedure.

You probably have a Thunderbolt V ignition, and that requires to be put in base mode to check and adjust timing. You should also set idle mixture and idle speed. You probably have a TKS type carb, and that may have some additional steps/procedures to do so. Follow the service manual. I expect it will say 10 degrees BTDC @ 650rpms (base mode), but have not referanced a 2000+ model, like yours.
 

scooper77515

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Sep 3, 2010
Messages
747
Re: 4.3L Dieseling

My tuneup cost around $200 (plugs were upgraded to $8 plugs, didn't need them, though). Took me less than an hour to do the work.

I expect 30 minutes to swap the impeller.

I don't winterize, down here in TX.
 

sethjon

Senior Chief Petty Officer
Joined
Jun 8, 2010
Messages
692
Re: 4.3L Dieseling

4.3L/LH 2-BBL (2004) was running hot (up to 190 deg) with hot riser on one side so changed the impeller and motor is now running 160-175 but now experiencing some dieseling unless I let motor idle for several minutes before turning off.

Used some seafoam during this same run because had trouble getting to 4500 for WOT and that seemed to help quite a bit. Wondering if the seafoam contributed to the dieseling because didn't have this problem earlier.

Haven't checked timing and getting ready to order Merc service manual, does anyone know the setting/rpm for adjusting the timing. Also is this a simple point and shoot procedure.


Before you do timing you better find out why its running so hot.175 is not acceptable. You should max out at 160. That problem is more serious than dieseling.
 

John_S

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Jun 21, 2004
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4,269
Re: 4.3L Dieseling

Before you do timing you better find out why its running so hot.175 is not acceptable. You should max out at 160. That problem is more serious than dieseling.

That is normal with most latter model mercs. 175 degrees at stat housing with 160 thermostat.
 
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