Hey guys, I've noticed lately my truck will diesel after I turn it off occasionally. Its a fairly fresh engine, (under 10k miles), so I always blamed the electronic box or too rich of a carb letting fuel pool up. I changed my plugs last months and it would still do it every once in a while. What I finally figured out is this. Whenever I can I fill up with E10 ethanol gas, at most stations around here its cheaper and higher octane (89). Whenever I fill up with 87 regular is when I get the dieseling.
Its not a real problem to me other than the embarassment of it happening. I didn't think I built the engine that high performance, but it does have the Edelbrock Performer intake and cam, with probably slightly higher than stock compression just because of having the heads milled flat.
Any thoughts on the subject?

 

i think changing fuel and octane levels is your culprit. the dieseling is related to the timing.
i'm sure someone else here can expand more on effects of running higher octanes and relationship to engine timing.

 

Check the timing, probably setting the timing to 10 to 12 degrees BTC should be fine.
Check the vacuum on vacuum advance on distributor, check if it has continual vacuuum or no vacuum at idle. Their should be no vacuum during idling of the engine.

 

I believe my timing is about 12. Kind of where it likes to run. Like I said, I always try to filll up with the higher octane ethanol, but some stations have it 10 cents more than regular, while others have it 10 cents less. I know the lower octane is the reason. I just want to open up some discussion on the subject.

 

Set the timing back a couple of degrees and it should go away. So 10 would seem to be the number.

Or just run 89 octane.

You could also try something like Seafoam to clear out any deposits that may have built up in the combustion chambers.

Or, you could try the next colder heat range spark plug. For instance, if you're currently running a Champion RF10YC, you'd run a RF9YC. If I remember those numbers correctly. It could be that the grounding strap is heating enough to light it off. A colder spark plug would prevent this, if it is what is happening.

 

ethanal gas dosen't burn as hot as gas. burn two tanks of 93 octane gas will help or try an octane adative. if timming is off engin would miss.

 

I have seen engine dieseling at 14 degrees BTC. Recheck timing with timing light and vacuum on vacuum advanced removed.

Lower the engine base idle.

 

try easy thing frist like running good gas (high octane) and work your way to the hard and costly thing .

 

What's your idle speed?

 

The 390 I built for my highboy would diesel something fierce... constantly...

Even on 93 octane (11:1 compression).

I switched to an MSD box from the old Duraspark, changed the timing around (a LOT!) and got rid of the dieseling.

However, since I took it apart, I now know what was going on. There was so much carbon built up on top of the pistons it was amazing - I had an intake vacuum leak, sucking oil in, and it really clogged everything up. Still ran great though...

What the MSD and re-timing had to do with it, I don't know, but ... definitely get everything setup properly first.

 

I'm coRnfused. :/
Why all the talk about ignition timing?
The ignition is off, right? :/
Dieseling is about carb-settings and carbon build-up ain't it?
Straighten me out please. :)

Alvin in AZ

 

Here's the thing: Over-advanced ignition timing will heat up those carbon deposits before you turn the motor off. When you turn it off, they're still heated, and you get dieseling.

So like I said. Either clean 'em out, or set the timing back.

 

Ok, I'm pretty sure theres no carbon deposits because its a fresh engine. I do have my timing somewhere around 12-14, I haven't checked it lately. My idle is set at 800 in park with the choke off and warm engine. I can't guarantee it, but I don't think there is a vacuum leak, again, because its a fresh engine. I would like to back the timing off because in the cold I occasionally get a backfire throught the carb till it warms up.

 

Check the spark plug gap, 0.042-.046" for electronic ignition, and check the firing order. Recheck the fuel mixture on carburator.

 

I had a similar problem on a SBF I built once. Turns out I forgot to recheck the torque on the intake manifold..... That can cause one he'll of a vacuum leak.