More power when running out of fuel

[Zaggnutt]

Here's the deal. These pumps have to run with a certain amount of case pressure. The case pressure is part of what maintains the advance mechanism settings. When you gulp air, the air goes into the case... hence lowering the hydraulic fuel pressure inside. Hence... more advance. The "***** ape" running is the result of over advanced ignition. No, you don't want that. It's eating up your glow plugs as that is happening, but thankfully you run out of fuel before it gets serious.

Thanks for explaining this. Based on the youTube video I was under the impression that the injector pump moved air up and out of the pump to the return and in the process the spring regulating the pressure compensated by "increasing timing".

 

[bbjordan]

Here's the deal. These pumps have to run with a certain amount of case pressure. The case pressure is part of what maintains the advance mechanism settings. When you gulp air, the air goes into the case... hence lowering the hydraulic fuel pressure inside. Hence... more advance. The "***** ape" running is the result of over advanced ignition. No, you don't want that. It's eating up your glow plugs as that is happening, but thankfully you run out of fuel before it gets serious.

Mel, thanks for that explanation, but with all due respect, I don't think the difference in timing is enough to explain the total effect. The reason I say that is: consider the effect of the cold advance solenoid. When activated, the case pressure is also lowered to the point where the timing is advanced a minimum of 2.5 degrees on the 6.9 and 1 degree on the 7.3*. But the difference in performance when the cold advance solenoid is engaged is nowhere near the observed effect of a little bit of air in the lines.

I suspect there is more going on. When the air is entrained in the diesel fuel, it must be like a foam going through the injection pump. Think of soda pop or beer...mmm beer ...I'm thinking that the vent wire does not rid the pump of all the air. Some must be getting through the pump into the lines, and eventually the injectors. Here, I think, the air has a two fold effect:
1) It keeps the injection line pressurized.
2) As it goes through the injector it may help the atomization of the diesel fuel. Think perfume sprayer vs squirt bottle.

If this is the case, there may be a potential for increasing the performance of the IDI by introducing a small calibrated amount of air to increase the efficiency of the burn.

That's my 2 cents worth anyway.

* Taken from the Haynes Techbook Diesel Engine Repair Manual Chapter 3 page 34.