truck dies if I let off the pedal at higher RPM's

[Talvez]

So forgive me if this has already been discussed but I went back 20 pages and didn't see anything to help.  2001 HO 6-speed. FASS DRP, 75hp DAP injectors, Quad XZT+.  New PCM a couple of months ago because I had been running an external voltage reg but hated the flickering lights and engine lope at idle.  

I replaced the vp44 with one from DAP 2yrs ago after intermittent stalling at highway speeds.  Ran fine with 2-3 instances of a 1 sec miss at highway speeds last summer. thought this was possible injectors as it has 215K on originals so replaced them 7 months ago.  this July when it hit 95+degrees I stepped on clutch to turn at an intersection and the truck died.  would not start. Just crank and crank, finally started after 20-30 min. Idled fine, took it for a drive and about 5 miles later same thing.  No start. towed home and as I got to the house tried to start and it fired up.  So now I can recreate it everytime.  If the engine is cold (hasn't been running) it will start fine if I had previously shut it off on purpose. Will run good till engine warms up then i get a slight miss at idle and if I tap the pedal and let off quickly it will drop below 700 rpm and hiccup before resuming idle. If I take it for a drive, its runs like a champ but as soon as it warms up and I let off throttle it will die like I turned off the key. When it does start after a die, it starts up like it may have air in lines.  sputters and a few catches then evens out after a bit to the slight miss at idle. Sent vp44 back under warranty, got it back, installed  and same issue.  fuel pressure 7psi WOT, 10-14 psi at idle to normal load. countless new fuel filters, new CAM sensor. Hot wired vp44 when warm, same symptoms.  One thing I noticed with fuel pressure was that with cool engine my pressure guage needle had a slight buzz to it (1psi variation) but once it warms up I get a large vibration in the needle (5psi variation). tested APPS voltage and it is ok. by the way, I removed the XZT+ and made sure all wires were back to original locations. Have talked to several places including local shops, DAP diesel, Bluechip Diesel, GenosGarage.  Everything seems to point back to vp44. Please help as this has everyone baffled and the shops I talk to tell me that after everything I have done they would start with a new vp44.

[Dieselfuture]

It sounds like psg on vp, pour cold water over vp when it acts up see if it helps. Have you checked ac noise on  alternator, you mentioned some external regulator. Sounds kinda like vac may be doing its thing. 

Your lift pressure is still lower then we all like to see.

Any codes

[Mopar1973Man]

I also agree that you need to have the alternator tested for excessive AC noise. 

[Jeff Edwards]

 Not sure if this is your problem but had a similar problem going on with my truck use the scan tool truck never said any codes but found that the PCM would not read the ECM  truck  gave me the same symptoms that you're having when the motor was cold it would run once it warmed up to temperature shut off  so I removed the ECM put it in my refrigerator to cool all the circuits  once cool down was able to get the scan tool so that the PCM would read the ECM the result was a failing ECM  once ECM start warming up with the electrical voltage flowing through it it would kick out. Reading it replace the ECM was what problem

[SilverMoose]

10 hours ago, Jeff Edwards said:

 Not sure if this is your problem but had a similar problem going on with my truck use the scan tool truck never said any codes but found that the PCM would not read the ECM  truck  gave me the same symptoms that you're having when the motor was cold it would run once it warmed up to temperature shut off  so I removed the ECM put it in my refrigerator to cool all the circuits  once cool down was able to get the scan tool so that the PCM would read the ECM the result was a failing ECM  once ECM start warming up with the electrical voltage flowing through it it would kick out. Reading it replace the ECM was what problem

Nice detective work

[kzimmer]

On 8/31/2017 at 3:11 PM, Talvez said:

So forgive me if this has already been discussed but I went back 20 pages and didn't see anything to help.  2001 HO 6-speed. FASS DRP, 75hp DAP injectors, Quad XZT+.  New PCM a couple of months ago because I had been running an external voltage reg but hated the flickering lights and engine lope at idle.  

I replaced the vp44 with one from DAP 2yrs ago after intermittent stalling at highway speeds.  Ran fine with 2-3 instances of a 1 sec miss at highway speeds last summer. thought this was possible injectors as it has 215K on originals so replaced them 7 months ago.  this July when it hit 95+degrees I stepped on clutch to turn at an intersection and the truck died.  would not start. Just crank and crank, finally started after 20-30 min. Idled fine, took it for a drive and about 5 miles later same thing.  No start. towed home and as I got to the house tried to start and it fired up.  So now I can recreate it everytime.  If the engine is cold (hasn't been running) it will start fine if I had previously shut it off on purpose. Will run good till engine warms up then i get a slight miss at idle and if I tap the pedal and let off quickly it will drop below 700 rpm and hiccup before resuming idle. If I take it for a drive, its runs like a champ but as soon as it warms up and I let off throttle it will die like I turned off the key. When it does start after a die, it starts up like it may have air in lines.  sputters and a few catches then evens out after a bit to the slight miss at idle. Sent vp44 back under warranty, got it back, installed  and same issue.  fuel pressure 7psi WOT, 10-14 psi at idle to normal load. countless new fuel filters, new CAM sensor. Hot wired vp44 when warm, same symptoms.  One thing I noticed with fuel pressure was that with cool engine my pressure guage needle had a slight buzz to it (1psi variation) but once it warms up I get a large vibration in the needle (5psi variation). tested APPS voltage and it is ok. by the way, I removed the XZT+ and made sure all wires were back to original locations. Have talked to several places including local shops, DAP diesel, Bluechip Diesel, GenosGarage.  Everything seems to point back to vp44. Please help as this has everyone baffled and the shops I talk to tell me that after everything I have done they would start with a new vp44.

 

I think you're sucking air in the supply line to the lift pump. I had a similar problem last November, turned a corner and stepped on it, RPM's way up, as soon as I let off the go pedal it died. No start. Towed it to my garage and scratched my head for a couple hours. Cracked injector lines 3, 4 and 5, bumped the starter to force lift pump to run for 30 seconds, cranked for 30 seconds, snugged up the injector lines, and all was well. This however does not explain the relation to engine temperature. Hopefully that's a coincidence.

 

I now keep a 3/4 wrench in my truck at all times... haha.

Edited September 1, 2017 by kzimmer