Engine/Transmission (1998.5 - 2002) down shifting

[new2dodge]

i heard that if the rpm's are over idle speed and your foot is off the throtle, the inj. pump starves for fuel ( cooling ). my question is when i down shift at a stoplight am i wrecking my pump?

99 qc 2500 nv400 power puck

 

[rkressg]

Don't worry about it. Your electric lift pump, when working properly, constantly supplies 7 psi when your foot is on the accelerator, and around 14 when it is not. I. E. when you downshift and take your foot off it. If you are worried about it, get a mech fuel press gage.

P. S. welcome aboard the TDR

 

[bighammer]

So a long downhill mountain grade would kill every truck that comes down? Yea right. Whoever told you that one is misinformed.



When your foot is off the go pedal, there is no fuel injected in the engine. (unlike a carburated engine) The pump's internal guts are still bathing in fuel the lift pump supplies. As long as the lift pump is OK downshift all you want.

 

[CFAR]

bighammer



Not intended as a flame, I'm just curious, I always want to know more.



So what you are saying is that when I'm sitting at a stop light at idle I'm not injecting fuel?



Doesn't make sense.



pls explain

 

[duaneparks]

I don't want to step on bighammer's toes here...



I think what he means is, when your engine speed is above idle and your foot isn't on the go pedal the injection pump does not inject fuel. At idle it does inject fuel. If you downshift and coast down in gear, there is no fuel being injected until the engine reaches idle rpm unless you hit the go pedal again.



I too was worried about this when I first heard about it, but figured that if there was a problem with this setup, there'd be a TON of VP44 failures out there no matter what.



I don't usually downshift if it will bring my rpm's above 2000 though...



HTH.



Duane

 

[new2dodge]

duaneparks hit it on the head. it all sounded a little fishy to me to begin with. as they say, an ounce of prevention... ... .....

thanks to everyone for the responses.

 

[MCummings]

Originally posted by duaneparks

I don't usually downshift if it will bring my rpm's above 2000 though...

I don't usually downshift if it will bring my RPMs above 3,700RPM



The deal is like this. What you probably heard was that early '98. 5 trucks would end up with a Dead VP44 after installing a "Performance Box" (aka PE Comp). The way the '98. 5 pumps die (Even with enough fuel pressure) is When you are WOT the pump is building heat because the solenoid inside the pump is working harder than before. The pump gets nice and toasty inside, then when you lift off the fuel pedal, you get no more fuel to cool off the pump. The pump siezes up (motor is still turning over since you are slowing down) but when you get back on it,, there is no power (like if somebody turned the key-off) becuase the VP44 is broken. The '98. 5 trucks were very prone to killing VP44's because the tolerances were not good enough, the problem has since been fixed. There aren't many '98. 5 trucks over 100K running on their original injection pumps.



As an FYI (not saying you don't know) a diesel motor has no throttle body or an Air to Fuel ratio to worry about. RPM and power is regulated by how much fuel is introduced into the cylinders. Like has been said, if you are above idle and not on the fuel, you have no fuel going into the cylinders.

Want to make more power? Just inject more fuel with more air. Simple enough ? Your puck is just adjusting timing and adding a little bit more fuel. A PE COMP or TST COMP box pierce the injection pump wire which directly controls the Injection pump solenoid. The longer the solenoid is open, themore fuel that gets injected (The solenoid is a natural closed, power open) as you hold the solenoid open longer you make more power, but also heat up the solenoid more, this is why sometimes you may hear the Highest setting on a box referred to as "Kill". It doesn't mean you are going to kill the competition, it means you are killing your injection pump



Hope this is informative enough.



Merrick Cummings Jr



EDIT: To Answer your question, yes, you are slightly killing your inj. pump, and your clutch by downshifting. Peter @ South Bend clutch will tell you about the dangers of "reverse torqueing" a clutch.



Have you checked your Fuel Pressure lately? Don't worry about downshifting killing your VP44. Downshifting will probably kill your clutch first.

 

[CFAR]

MCummings

As an FYI (not saying you don't know) a diesel motor has no throttle body or an Air to Fuel ratio to worry about. RPM and power is regulated by how much fuel is introduced into the cylinders. Like has been said, if you are above idle and not on the fuel, you have no fuel going into the cylinders.

duaneparks

I think what he means is, when your engine speed is above idle and your foot isn't on the go pedal the injection pump does not inject fuel. At idle it does inject fuel.

So basically you both agree that the injection pump does not inject fuel into the cylinders coasting down a hill in gear?



I would think that you would be injecting the same amount of fuel as at idle.



Isn't fuel controlled by the apps and some type of fuel map?



If I'm going down hill and push in my clutch fast, the truck doesn't die.



If you mean that coasting down the hill, the injection pump injects no more fuel than at idle, that makes sense.



More simpler explanation pls.

 

[RustyJC]

Originally posted by MCummings

To Answer your question, yes, you are slightly killing your inj. pump, and your clutch by downshifting. Peter @ South Bend clutch will tell you about the dangers of "reverse torqueing" a clutch.

The exhaust brake manufacturers certainly wouldn't be happy to hear this since downshifting to keep the RPM's up is critical to achieving maximum exhaust braking.



Rusty

 

[MCummings]

Originally posted by RustyJC

The exhaust brake manufacturers certainly wouldn't be happy to hear this since downshifting to keep the RPM's up is critical to achieving maximum exhaust braking.



Rusty

Rusty, You are 100% correct. Ask Peter if he likes E-Brakes.



CFAR Easiest explanation. Do you know anybody with an '02 Ford PowerStroke? Get them to rev it up in Neutral. Rev it way on up to about 2,500 and let off the fuel,, You'll hear the truck get real quiet while the RPM comes close to idle, then the injection starts.



When your motor returns to idle, injection of fuel into the cylinders starts again. A Cummins with BIG injectors will show you this. My truck will catch itself hard when returing to idle. Does a quick Romp-Romp.



If fuel was injected above idle and you are not on the fuel pedal, then on a long down hill EGT would stay up, atleast SOME, but instead, on a LONG downhill with the motor above idle and you are off the fuel EGT will fall down to only 100*.



Hope this helps.



Merrick Cummings Jr

 

[Amianthus]

I agree with you Merrick with one exception. Using my FMS as a guide, when I get off the throttle as in going downhill, the FMS is reporting a "9" signal back to the ECM. Idle is around "10". SO I disagree that no fuel is going into the engine. Just less than would be needed for idle. I also see this in my pyrometer readings. The pyro doesn't drop out to ambient or just slightly above. Mine stays around 250 on a good downhill run. So I believe that there is some fuel being injected. Just not enough to worry about.



To answer the original question, don't worry about it. Downshifting won't hurt it (VP-44) and coasting won't cause it to sieze up and die. That's what blown lift pumps do.