Though I post this as a separate thread.

One of the things I have done to improve cold starting on my 93 F250 Turbo 7.3l was to disconnect the electrical lead on my IP to my timing advance. I noted during trouble shooting that the electric advance comes in when the engine is cold and turns off when it warms up. By disconnecting the timing advance the idle during engine warm-up is about 150 rpm higher (900 rpm) than normal which does not really bother me. There is also a brief "puff" of white smoke (unburned fuel) during starting on the really cold days (< 32DEG F) that may or may not have been there before. I have been running like this for over a year now.

My question is: DOES THE ELECTRIC TIMING ADVANCE ON THE IP COME IN AT TIMES OTHER THAN DURING COLD STARTING?

(For example it would make sense to me to advance the timing under turbo load, however I don't see how that gets activated as there are no turbo-boost sensors (other than the wastegate) on the 93' Factory turbo set-up.

 

High idle and timing advance are both controlled by the same coolant temp sensor.
The only time it works is when the coolant is below 115 degrees.

 

Thanks Dave,

Got any theories why the factory does the change in timing for cold-starting?

All I can tell you is ever since I got rid of the timing advance electrical connection my cold starting has gotten a lot easier. The engine seem to fire on the first or second cylinder rather that having to crank thru several cylinders.

 

The advanced injection timing gets the fuel into the cylinder a little bit sooner. This gives it more time to ignite and burn completely in a cold cylinder. As cylinder walls and piston combustion surfaces warm to operating temps, there's no need to get the fuel in 'sooner'. So, the advance drops back to regular time. The main benefit to advanced injection timing for cold start is the elimination of white smoke. Your engine seems to start easier because the fuel is igniting a bit late in the cold cylinder, and providing more of a push after TDC. White smoke, but effective.

 

Thanks Dave,

That is kind of what I figured was happening. Given the price of a starter rebuild, I think I will do whatever is easier for cold starting starting (i.e. keep the timing connection off during cold starting)

I guess you could apply the opposite thinking in that retarding the injection for cold starting allows the cyclinder to heat up more due to compression and hence have a hotter air in the cylinder when the diesel gets injected.

 

Well, not really. Keep in mind that those cold cylinder walls really suck up thermal energy. When the fuel gets injected, the compressed air isn't as hot as it should be; that's why the white smoke (incomplete combustion). The stuff still ignites, though, and the cylinders quickly warm up to operating temps. Bottom line: mechanically, you're not hurting anything. Ecologically, you're causing several members of the Sequoiah Club to experience blood pressure spikes. *grin*

All seriousness aside, if you want to see what happens when gear heads have WAY too much time on their hands, go here:

http://www.dieselinjection.net/frames.html

Intended for Cummins 855 motors, there's a LOT of good general knowledge in these articles. If you decide to print them out, have a lot of paper on hand. I printed this stuff out over a year ago to use in a class, and it came to almost 70 pages of info. Bruce has added to it since then. Great reading!

 

Lots of good reading, thanks.