In the interest of saving a few pennies on gas, I filled up with regular instead of my normal hitest. The car seemed to run fine with no audible knocking or seat of the pants power degradation. It has however started to diesel. Do I need to, or is there a way to retune, setting idle/timing, for the lower octane or is the only solution to go back to hitest?

timing.

retard timing a few degrees and/or lower idle speed. If it is an automatic just shut it off in drive and then shift to park. You will get better milage with the lower octane fuel. My son drives a 94 Firebird with an LT-1. With 91 octane on the interstate at 80 mph it gets 24-25 mpg with 87 he has gotten as high as 31 but 28 is the norm.

I ran mine on 91 octane with no hints of knocking or pinging but it would not shut off either I backed my timing down but that did not help either so I did what 64elcamino said and shut it off in drive and every thing was fine, until about 300 miles it started to knock so I checked every thing out and could not find any thing wrong. I pulled the motor this week too find three pistons that had chunks missing out of them graemlins/angry.gif Im sure your setup is different than mine but just be careful. Check my post in performance "Pistons came apart" from a couple of days ago. One of the guys said you may not hear much of a knock, but still cause damage with low octane.

I was always told that to stop dieseling, you should reduce the idle speed. From my experience, it doesn't take much - 50 to 100 RPM's has always done it for me.
It's the easiest fix to try anyway..
Wes
Edit - like the other guys have said though, with a lower octane fuel, you've got to watch for pinging. If you were running as much advance as you could get away with on hi-test, it'll likely ping with the lower octane fuel. If it's rattling, knock your timing back.

I'll take the timing down a couple of degrees, recheck everything and then write back.

Thanks

Even with a Holley 4 barrel and an Edelbrock 2101 performer manifold on my 350, I use an anti-diesling solenoid. The one I have was available from Holley. It enabled me to set my idle properly with no diesling when I shut the engine down, no matter how hot.

I backed off the timing by 3 degrees and it seems to be ok. It is hard to tell because it is much cooler today. Why would timing affect it so much ? Is it the additional heat?

I'm still of the opinion that idle speed is the cure for dieseling. From my experience, backing the timing off will usually slow your idle down a bit - so the dieseling might stop if you don't set the idle back up.

As for the recommendations for backing off the timing here, I think the main reason is prevent knocking or pinging with your use of lower grade fuel. Higher octane fuels burn more slowly and are harder to ignite than low octane fuels. If your timing was set at say 10 Degress BTDC on hi-test, the higher octane fuel doesn't get a good burn until after the piston reaches TDC and teh intake valve has closed. With lower octane fuel, the burn is faster, so you are more likely to get a good burn in the cylinder before TDC - or before your intake valve has closed. That's what causes the pinging on low grade fuels.
Low grade fuels (because they ignite easier/burn faster) are more prone to dieseling.
MalibuJerry's solution of adding an idle solenoid is a good one in my opinion. When the ignition is energized, the solenoid is activated and it increases the idle speed. When you shut off the key, the solenoid is deactivated and the idle is backed off - preventing dieseling. When you're idling at the lights, idle speed is maintained at a high enough level to keep the oil pressure up and prevent stalling.
So, the way I see it you've had answers to two questions - one you asked (dieseling) - reduce idle speed, add an idle solenoid if necessary. The other one you didn't ask but people were trying to help you prevent the most common (and more damaging) problem with lower octane fuels which is pinging - back your timing off.

Sorry for being long winded - didn't have time to draft a short answer.... Hope it helps.
Wes

Thanks again. Well, it's been 2 days since I backed off the timing 3 degrees and it is not dieseling. It still puzzles me why this would cure it since the ignition dies when you turn the key. Anyway, it seems to work. Thanks for your input.

can someone please tell me what "dieseling" is? is it like detonation? in just curiouse to find out what it is.


aaron

Dieseling is when the engine continues to run - sometimes backwards - after you've turned the ignition switch off. They call it dieseling because the heat & compression are what is igniting the fuel - just like a diesel engine.
Wes

oh ok thanks...sometimes my velle does that too, ever since i adjusted the idle, but it only last a sec or 2.


aaron

Well the temperature has risen and the dieseling has returned. I like the idle stop solenoid idea. I can purchase a solenoid at Advance Auto but there is no listing for the bracket it attaches to. Does anyone know where I can get one?

Originally posted by Don C:
Well the temperature has risen and the dieseling has returned. I like the idle stop solenoid idea. I can purchase a solenoid at Advance Auto but there is no listing for the bracket it attaches to. Does anyone know where I can get one? I got the solenoid and bracket from a speed shop that sells Holley parts. The solenoid had to be ordered. The car came with one, on the old Rochester 2 barrel, and the idle is easier to adjust with one installed. The reason for the solenoid in the first place is to close the throttle completely to "starve" the engine when you shut it down. The engines were running leaner, hotter, and had a faster idle due to emission changes. Hence, the tendency to diesel when shut off.