Hard starting when warm

Hayden simpson

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Here is the situation with my 1985 6.9idi,

When I first start the truck for the day it starts great, 2 or 3 cranks. The issue is when the truck is warm If i go to start it once its warm and it has only been off for up to like 10 minuets, say I run into a store, it starts great. The trouble is once it is warm and sits. If I drive it go fish or workout for like an hour or so I go to start it and I have to crank for close to 30-45 seconds. I dont crank that long but crank in like 8-10 second intervals. Eventually i hear it try to fire up, like a few coughs then it runs. I am wondering if it is a fuel vaporization problem, I had that on my 68 bronco, but that was gas. Any help is appreciated. Thanks
 

sjwelds

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Next time it does it, try pouring some cool water on the back of the injection pump. If that works, your injection pump is on its way out...
 

chillman88

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Ill try that thanks, just curious why does the cold water help

Simple version is as these wear out the tolerances change, and when they get hot certain parts swell. The cool (not super cold) water will cool things down enough for it to pump again.

If that works, it means it's time for a new injection pump.
 

IDIBRONCO

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Unless you know for sure that the injectors are pretty new, it's usually a good idea to replace the pump and injectors at the same time. They wear out at practically the same rate.
 

david85

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When fuel stops flowing, the engine shuts off, but that fuel also doubles as coolant for the injection pump. The aluminum outer housing expands at different rates than the steel parts inside. So as the engine rests in a hot state, the injection pump temperature will actually go up for the first few minutes after shutdown.

As it cools down over longer periods of time, the components begin to return to a more neutral condition that allows tight enough tolerance to build high enough pressure fire the injectors again. This is why the cool water may get you started, but it can be very risky, especially if you're trying to do this while cranking. For the same reason, it's a very bad idea to power wash one of these engines while it's running.

This is why the symptoms resemble vapor lock that old carburetor gasoline engines suffered from before EFI became standard (EFI fuel pumps have enough pressure to defeat the vapor-pressure of gasoline even at high underhood temperatures). Diesel can't vapor-lock because it's nowhere near as volatile as gasoline.
 

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