1998 GCL w/ 4.0 & automatic - 250K showing

The engine cranking speed is the same as always (fast) but the amount of cranking time before the engine starts is gradually getting longer and longer. It always starts but it used to start almost instantly and now takes maybe 6-8 seconds of cranking.

The problem is worse starting from cold - it still starts right up when it's been run recently.

Is there a check valve in the fuel system which could be allowing fuel to drain back into the tank? That's all I can picture so far.

How about you? Any ideas? <g>

PHM
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How old is your battery? Your battery may be starting to lose its ability to hold a charge. When the truck has recently been run, the alternator has been juicing the battery, so it fires right up. But when its been sitting for a while, its starting to lose mojo and it struggles to kick the starter out.

 

Battery is new; a month old. Some expensive NAPA unit. As was the previous battery. <g> And the cranking speed is always tip-top - never even slightly fades. I can check the voltage when cranking and see if it plunges.

PHM
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I guess you have a well maintained vehicle, air filter, fuel filter, wires, plugs, cap, rotar, coil, ect. When you turn the key to the run position without starting the engine listen for the fuel pump to engage. It is located in fuel tank, you should hear a buzz start when the key is turned to the run position. The buzzing sound will last unitil the fuel system is pressurized, that takes normally 2 to 3 seconds. So the question is how long is your pump buzzing/running after turning the key to the run position?

 

Sorry, I misread your OP. Then I agree with Tony below, your fuel pump has just become a suspect. I cant remember if the 4.0 fuel rail has a schrader valve, but if it does, hook up a pressure gauge and see what the fuel pressure is.

 

Yup. #1 suspect is failing fuel pump. Same symptoms on my '97 4.0. Replaced fuel pump and it now starts instantly hot or cold.
If you determine you need a new pump, don't cheap out. Only buy the Bosch assembly. Rockauto has good price.

 

First of all, your fuel pump is fine, but you're loosing the residual pressure in the fuel rail because the pressure regulator is leaking pressure back to tank when it's shut off. I think on your 98 the pressure regulator is in the fuel filter if it's outside the tank or part of the fuel pump module if not. A simple solution is to install a remote check valve in the fuel line if it's in the pump module. As close to the tank as possible.

Also, the fuel pump only runs for a few seconds when you turn on the key. It's a timed cycle and has nothing to do with pressure buildup.

Here's something to try. It's called the "poor man's prime." Turn the key on, then off a couple of times before you crank it. It should fire right up.

If you were experiencing a ragged start, I would think you have a leaking injector, but you don't, right? Besides, that would happen every time you started it.

Check to see if the filter is mounted on the frame. If it is, replace the filter.

 

I just looked at one for your jeep, and it seems it's on top of the tank mounted on the pump module, but is external. It's got a pair of o-rings that fit into the pump module.