dieseling

[mcvette]

I have a 1997 Chaparral 1930 sport with a 4.3 lt gen+ engine. While out on the lake yesterday I noticed when I would shut the engine down several times it would diesel. I turned the key back to the on position and it would continue to run.

Any thoughts or ideas what causes this, will it hurt anything and what should I do to correct it. The engine has fuel conditioner and was winterized properly.

Thanks.

 

[Bondo]

Re: dieseling

will it hurt anything and what should I do to correct it.

Ayuh,... There's always the potential for Hydro-lock, which will Destroy your motor..
When was the last time the motor was Tuned Up,..??

 

[Don S]

Re: dieseling

The number 1 reason for dieseling is to high an idle rpm, it needs to be down to 700 rpm or lower.
Have a look at the service bulletin on diesiling.

Service Bulletin 97-17

 

[scipper77]

Re: dieseling

I had an Oldsmobile that did this a number of years ago. My mechanic told me that the dieseling was caused by carbon deposits in the combustion chamber that were hot enough to ignite the fuel. With a mechanical fuel pump the engine just continues to run. The "fix" he suggested was to try shutting down the car in gear and the extra load on the engine would keep the engine from dieseling. It worked but obviously wasn't a desirable solution.

 

[Chris1956]

Re: dieseling

McVette, Let the motor idle 30 seconds to allow cool down, after a hard run, before shutting it down. That should cure it. If not, leave it in forward, and turn off the key.

 

[familyguy]

Re: dieseling

I have a 1998 Glastron GS 185 w/ a 4.3L volvo penta GL engine and it would do this same thing on occasion when I tried to turn the engine off after a hard run across the lake w/ engine rpms up in the 4,000 range. Try what Chris1956 wrote. Let the engine remain at idle for 30 sec. to a minute especially after a hard run before trying to shut down. After doing this very thing I haven't experienced any more dieseling.

Your engine has the potential to reverse rotate when it diesels which could draw water up your exhaust, past your exhaust valves and into your cylinders causing hydrolock-way bad. This isn't a concern on a vehicle that deisels because the open end of the exhaust pipe is only exposed to air, unlike a boat which has the open end of the exhaust "pipe" below the waterline.

When you turn your key back to the on position when it's dieseling you're reapplying power to your fuel pump and ignition (spark supplying) system which will allow your engine to keep turning. Dieseling is exactly what scipper77 wrote- carbon deposits begin to glow from high heat in the combustion chamber much like embers in a fire will, and this glow supplies the "ignition" or spark needed to keep an engine turning when it's otherwise turned off. It will eventually stop because there won't be a constant supply of fuel with the fuel pump shut off, but it may not stop before it reverse rotates.

Higher octane fuel helps prevent the excess buildup of these deposits.

 

[IVAZ]

Re: dieseling

Higher octane fuel helps prevent the excess buildup of these deposits.

I thought the opposite was true. I was under the impression the higher the octane the more deposits that are left behind (the owner?s manual to a ford Focus lists that premium fuel is not recommended due to carbon build up).
I thought the benefit of higher octane was a less violent combustion.

 

[familyguy]

Re: dieseling

You're right in saying that higher octane results in less violent combustion if the combustion you're refering to is the engine knock, aka detonation that occurs from using too low an octane recommended for your engine. At best, the argument that less deposits will occur from a using a higher octane rating is debateable. I certainly haven't ever heard using a lower octane rating than recommended for your engine will result in less deposits.

 

[IVAZ]

Re: dieseling

I agree you should always use the octane rating your engine was designed for (with the Focus it was 87 octane).

 

[Mark_VTfisherman]

Re: dieseling

A properly tuned engine shouldn't do this- I think that is a carburetor engine and may have an issue with the float if a decent tuning doesn't stop it.

 

[familyguy]

Re: dieseling

I think this is common with a carburerated marine engine like mine because this engine is just a marinized car engine and most car engines don't run at 4,000 rpms for a long period of time like these boat engines sometimes do. Because of these hard runs carbon deposits that all engines build up stay "glowing" for a short time when the engine is brought back to idle. If you let the engine idle for a short bit before turning it off you allow these deposits to turn back to just deposits instead of an ignition source. Remember, as long as you have adequate compression, proper fuel/air mixture, and a somewhat properly timed ignition source you are going to have combustion. A 2-barrel Holly carb will deliver fuel as long as there is fuel in the float bowl and vacum to draw it into the cylinder.

Everything else written put aside, you should always let your engine cool down at a more steady rate that idling can provide, unlike the shock that will likely occur from just shutting it down after taching it out around 4,600 rpms (my max)

 

[TomV1]

Re: dieseling

I would start first by bringing the idle down to spec 650 or 700rpm, make sure you put it service mode if it requires, while in service mode you may as well check your timing and idle mixture, make sure you take it out of service mode when your done. and you can also change your thermostat from 160 to 140 degree this will let the engine run a little cooler, this is what we did to a friends 4.3 and has not had any problem since.

Tom