Why does dieseling happen?

[Mustvid]

Can someone explain the reason dieseling happens? I have a 92 5.0 with box stock AFR 165's heads & Ed Curtis cam plus all the bolt on's you can do. it's got an AOD tranny with 3:73 gears. Timing is set at 16 degrees. I'm running the AFR recommended Autolite 3924 plugs. The heads and cam were installed in the last 45 days and I have been trying to get the tune right. It does not deisel when the engine is cold. Once it's at 190 is when it will deisel. I usually have the tranny in drive and it happens when it shifts into third around 1500 RPM 25-30 mph. It only last for a few seconds. I have taken it to the track and had the timing at 18 degrees and it was real bad at high RPM and WOT. Once I backed the time down to 16 it ran fine at WOT and high RPM. It only does it now at part throttle and low RPM. I have searched many threads on different web sites and I can't find an explanation for while dieseling happens. Someone told me that advancing the timing only effects the spa!
rk in relation to top dead center. Is this happening because of to much timing? Is it because it is to hot a spark or to cold? Should I try a colder plug? I did try running mid grade gas but it made it worse. Once I put premium in it has become fainter but is still there. It is driving me nuts because I don't understand why it happen. Once I can understand why then I will know what I can do to fix it.

[Brcuda]

What is happening is that the throttle is remaining partially open when the engine is shut down, which gives the hot engine sufficient fuel to run without a spark from the spark plugs. Most hot engines have sufficient carbon build-up that remains glowing red hot and acts as an igniter for the fuel. The solution is to make sure that the throttle closes completely when you turn off the ignition switch.

[grey88gt]

I may be way off here but I think hes refering to pinging, or predetnation. Slow your timeing down, your igniting the fuel way to soon and your hearing the uncontroled combustion in the clyinder, thats why it got worse with mid grade fuel becuase it burns at a faster rate. I think 18deg timeing is insane. Iam suprized you havent cuased other problems in yout engine because of to much timeing. Buy the way dieseling is when an engine trys to keep running after you trun it off, also caused buy low grade fuel, to much timeing, ideling to fast, hot engine with very high compresion. and what Brcuda says.

[Ieatcamaros]

Good posts above. You're on the right track. Back your timing back to 14 and you should be fine. Are you getting any pinging with it at 16? But like grey88gt said, it is too much timing or too much compression. Either of these combined with low grade fuel will diesel as you already know. But I bet on the timing being too high.

[tmoss]

I run 18* intitial (is only 32* total with DA1) and get no hint of ping with 89 octane gas. Deck height, quench area and chamber condition have a lot to do with ping.

[Ieatcamaros]

Originally Posted by tmoss

I run 18* intitial (is only 32* total with DA1) and get no hint of ping with 89 octane gas. Deck height, quench area and chamber condition have a lot to do with ping.

I should have clarified that 16-18 was probably too high for the combo he is running.

[Capri306]

Correct, tmoss & Ieatcamaros. The geometry of the combustion chamber has a lot to do with the proper spark timing for optimal combustion. However, I completely agree 16 degrees BTDC is way too much. He might even make more torque with ~12, but a chassis dynamometer is the only way to know for sure...which I highly recommend once major components like cylinder heads are changed.

Keep running 92-93 octane gasoline. The octane rating (in simple terms) is the ability of the fuel to resist detonation, which is NOT the same as combustion. You are not "dieseling," you are getting spark knock, plain and simple. So-called dieseling occurs at shutdown only on carbureted engines (or fuel injected ones if the circumstances are right), just to clarify. Anywho, just back the timing off, be sure your injectors, fuel pump, and fuel pressure are up to snuff, and you should be good to go.

BTW, have you done a spark plug reading yet? They're probably covered in aluminum bits by now, anyway, so you'd better replace them before making too many more high-speed passes.

[89fast5oh]

Whenever I set my timing I just get the engine to operating temp and go out on the hi-way, and put it in 5th gear and just keep turning the distributor till I get a bit of ping, then turn it down so it stops.

[grey88gt]

One other thing to consider, is that the more timing you run the more rod load you increase on the lower end.