I've had my truck about 75k miles now, and I've noticed this since I got it. When heading down a steep slope or hard braking, the idle speed goes up enough to hear. The opposite is also true - when facing up a steep hill or braking while traveling backwards, the idle speed drops enough that the truck vibrates more. Anyone else noticed this?

 

The idle drop sounds like fuel.....the pump itself can be having trouble with fuel supply, fuel quality or even idle set too low to begin with....
Have you done any work to the engine, pump, changed fuel source or just happening on the latest tank of fuel????


bob...

 

>>>”Have you done any work to the engine, pump, changed fuel source or just happening on the latest tank of fuel????”

No, no, no, and no. It’s happened since I got her. I’m not too worried about it, because everything else is so great…starts within one revolution, even below zero without being plugged in, and gets over 20 mpg in the summer. I’m just wondering if anyone else has noticed this. Automatic trucks would probably never notice. When I say a steep hill, I mean steeeeeep. Like steep enough that 4WD has trouble making it. I’m thinking one of two things: either the flyweights in the governor are affected by the G’s, or the fuel head between the engine and the pump is, or both. So if gravity pulled my governor weights aft (towards the tailgate) would this restrict fuel or add fuel?

 

I've never noticed this on my truck. Of course I've never operated the truck at extreme angles... When the roads dry out, I'll do a panic stop or 3 and see if I notice anything.

 

It sounds to me like your lift pump may be delivering less than required fuel. This would explain how it gets better/worse depending on your angle. You would expect a faulty lift pump to have a complete meltdown alot before 75k though.

I wonder if there isn't some sort of restriction in your fuel lines somewhere, which fuel can get by easier when greater pressure is applied going downward. The lift pump not having to work so hard etc. This is a bit of a tricky one isn't it?

I can tell you though that I have put my truck through alot of real nasty angles, lots of bumps and wet/hot and drycold conditions, without ever experiencing a slight variation in engine performance / idle.

It sounds like you're in for a real time tracking this one down.

I'd start with checking the lift pump output, and move along the fuel system/line to test flow from there. There's been anumber of threads where guys have tested this, and they will surely chime in and give you the output numbers if you ask. I don't know them myself.

Good luck with her

 

You're probably right, it could be the lift pump I guess. I haven't ever measured the fuel output while cranking like the FSM says, but I know the lift pump is at at least putting out a little positive head. If I crack the low pressure bleeder while idleing fuel comes flying out. Has anyone tapped a fuel pressure gage in the bleeder hole? The bleeder is a straight thread isn't it? That might not be so easy to do.

 

I don't know if I'd tap a pressure guage in there. It's asking for an air leak.

I'd just borrow or rent a pressure guage and check the output at the pump for the best results.

 

I've got a very similar thing with mine at idle, coming to a stop facing downhill it idles great, but loses a little bit of rpm when facing a steep uphill...it needs the idle sped up a little on it anyway, but when on a good uphill, it idles pretty darn slow. Tach is on its way, I'm curious to see...haven't bumped up idle yet cause POD's are sitting upstairs, just came today Kinda wierd tho...

Brian

 

DDD, There was a recent thread in TDR where one of the guys with similar symptoms found a restriction in his fuel lines/pickup comming from the tank.
I would recommend checking this out before going crazy with the lift pump.
Jay

ps..if you need help drive to Torrington and I'll give you a hand.

 

I've got the same symptom only during hard braking - my idle speed rises just enough to hear but not enough to see any difference on the tach. I also thought the LP was the culprit.