3.0 mercruiser dieseling

[Pdevito]

I have a brand new mercarb 2 bbl tks. Ive set the idle at approx 725. Why else can cause deiseling. Have not touched factory fuel mixture

 

[JonBoat55]

What kind of engine do you have? Raw water cooled? What temp is it running at?

 

[Grub54891]

Check the timing.

 

[Scott Danforth]

725 RPM is too high

have you checked timing?

if all is good with timing after you get the idle within spec, let it idle a bit prior to shutting it off. if you have to pick up a skiier, key off while in gear

 

[Scott06]

All above are good suggestions. Would also maybe fatten the idle mix. I decarbonized it by using GM top engine cleaner( others will work as well), seemed to help

need to be careful with the dieseling eng8ne can run backwards and suck water up into it

 

[Pdevito]

What kind of engine do you have? Raw water cooled? What temp is it running at?

3.0 mercruiser at 175 degrees warmed up

 

[Pdevito]

The tech at the shop said to set idle in gear between 725 and 750. Ive had it as low as 650 until it starts starving while in gear

 

[cyclops222]

You know more than the tech does.

 

[Scott06]

You know more than the tech does.

Agreed 650 or so, tach may be off. If it stalls in gear could be timing or idle mix issue.

3.0 mercruiser at 175 degrees warmed up

If your engine runs a 160 thermostat this is ok, if it’s a 140 it’s getting a tad hot. What year is engine.

 

[Scott Danforth]

If the 3.0 is fuel injected, it will have a 160F thermostat. Carbureted, should be 140 degree thermostat

 

[Scott06]

If the 3.0 is fuel injected, it will have a 160F thermostat. Carbureted, should be 140 degree thermostat

Around 2003 they went to 160 for emissions reasons. My carbed engine if that vintage has 160

 

[charliedaubitz1]

be sure and check for a vac leak around the carb base. there are several style base gaskets, some don't seal good. A spray bottle filled with water is good to test with.

 

[Pdevito]

What kind of engine do you have? Raw water cooled? What temp is it running at?

Water cooled

 

[Pdevito]

I have a brand new mercarb 2 bbl tks. Ive set the idle at approx 725. Why else can cause deiseling. Have not touched factory fuel mixture

 

[Pdevito]

650 in gear

What kind of engine do you have? Raw water cooled? What temp is it running at?

 

[Pdevito]

Water cooled running at 170. Water temp is 72

 

[paulswagelock]

Common issue on these motors. When I had mine, letting it idle for a few minutes helped, and shutting it off in gear helped.

 

[JonBoat55]

My 1999 carbed Merc 3.0 came to me with a 160 thermostat installed. Had slight dieseling issues and warm start issues for a couple seasons. Eventually swapped it out for a 140 and havnt had any issues since.

 

[Scott06]

Certainly anything to get it running cooler, like 140 thermostat or cool down idling will help.

Two other things that helped on mine was running GM top engine cleaner down the carb to remove carbon deposits from the combustion chambers. The top engine cleaner works great, as does sea foam or berrymans. Pour down carb at fast idle eventually stalling the engine let it sit 20 min, then start it. Just dont pour it too fast as you can hydrolock the engine (speaking form experince).

Also try fattening up the idle mix especially if the plugs look lean.

 

[charliedaubitz1]

Another thing to consider besides an air leak on the base is, that engine should be run on fuel with no alcohol, it will run lean if using Gasohol.

V6 and All Other Older 4, 6 Cyl & V8 Models, Except Supercharged Models.

87 (R+M)/2 Minimum Octane Unleaded Regular or Premium Without Alcohol DO NOT USE Leaded Gasoline