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Discussion Starter #1
I have a 99 with a 5 speed manual. Motor is entirely rebuilt with the exception of the vp44 at this time.

No engine codes. I have a scan tool and have recorded live data while driving with nothing abnormal.

I had a new vp44 in there, it started having hot start issues conveniantly 6 months after warranty. I have a FASS 165 feeding it with an Isspro reading out my pressures. I happen to have a working spare VP laying around and swap it in. Did seem to be a bit more powerful with the swapped in VP. Haven't driven it enough to confirm my hot start issue is gone, however, I had a second issue as well. When coasting in neutral, my RPM's like to bounce around, anywhere from almost stalling itself at 500 to a bit over 1000. As soon as I come down to just under 5mph the idle fixes itself to normal and the truck behaves. Stumped as to what could be next. No codes, no other problems with the truck. Ran great for the past 8000 miles or so. Any suggestions?
 

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Maybe a a couple of things. But first some questions. Have either the apps or Crankshaft position sensor been replaced, recently or with rebuild. If so what brand or price level? Timbo maybe? Original injectors? Are you sure it droops to 500 rpm when bouncing? Why the rebuild, how many miles before hand?
 

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Discussion Starter #3
Maybe a a couple of things. But first some questions. Have either the apps or Crankshaft position sensor been replaced, recently or with rebuild. If so what brand or price level? Timbo maybe? Original injectors? Are you sure it droops to 500 rpm when bouncing? Why the rebuild, how many miles before hand?
No on APPS and Crank sensor

Injectors are brand new RV275’s

Positive it bounces down that low, nearly stalls itself. I recorded live data while driving and can see it dipping that low and then shooting back up.

Rebuild was due to previous owner tuning it heavily. Entire block was swapped as the bad one was a 53. I have a Mexico block now.

Most my components except the block are just past 200k.

I did have a faulty Map sensor code. Replaced sensor, code is gone. Problem still persists.
 

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Replace the Crankshaft sensor with a good quality brand one. No cheap junk. At least 90%+ of the time if a tach is irregljular droping below 700 idle the sensor is going bad or gone.
 

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Discussion Starter #5
Ill get one ordered.

Any particular brand suggestion besides OEM?
 

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Name brands like Standard, AC Delco, Duralast(autozone), Echlin(Napa), Any of the old brands or top of line store brands, just not cheap online or cheapest store brands. Most of those "second line" brands will fail soon, same for transmission electronics. My current Duralast is about 10 years old.
NO Chinese junk.
 

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Discussion Starter #7
Installed a new one with no change.
Note that the fluctuations are amplified by braking, however it will still bounce around on its own coasting.

Edit: never mentioned, my fuel pressures do not change to the slightest ever except at WOT which then it dips down about 1.5-2 psi. Problem definitely seems computer/electrical related as once my speed drops below 5ish mph the idle goes to normal. A lot of you with stick shifts will know it has a slightly elevated RPM when coasting at high speeds. That is what I need back, however cannot find any issues.
 

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Discussion Starter #8 (Edited)
Here’s what I’m reading:

Intake air temp: 116
Water in fuel volts: 5
APP sensor volts 160.8 (that’s what the scanner says, I don’t believe it, how is this checked manually?)
APP Sensor Percent: 0 (I’m just idling)
Calc engine load : 25%
boost volts: .5
Boost pressure: .12 psi
ECM engine rpm: ~800
Engine speed ckd sensor: ~800
Engine speed cmp sensor: ~800
Inj pump rpm: ~800 (this sensor seems to bounce higher and lower than the others
Key on counter: 25,461
ECT: 186*
ECT: .74 volts
Ambient temp: 78*
Intake air temp: 2.03 volts
ECM volts: 13.75
Inj pump volts: 12.26
Inj fuel temp: 94.6*
oil press: 1.46 volts
Oil press: ~25psi (at idle again, gauge doesn’t read that low on the dash, it lies, but the computer is reading this)
Vehicle speed: 0
Transfer pump: on
Fuel shutoff driver: on
Inj pump fuel shutoff: off
Idle select: off
Idle validation sw1: on
Idle validation sw2: off
Final fuel state: not set

rest of the parameters are 100% useless like idiot light signals or all the cruise control settings. Let me know if this helps or anything else I should check. I can read all the on board computers (abs, ecm, bcm, etc).
 

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To check apps insert a probe into the apps plug or wire for pin #3 and turn key on. Read with any voltmeter. Red to that and ground to ground. Should read about .05 at idle and smoothly graduate to about 3.8v at WOT. You can and should run that with a set of junjper wires to the cab and monitor it while driving
There is nothing in the truck that is 160v anything. NO truck, manual or auto, should drop below 700 idle in neutral parked. Lugging in gear is the only way for that to happen for a second
 

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Discussion Starter #10
I checked and even replaced the APPS. All symptoms are still present, no codes present at this time. Everything still seems and behaves normal except for that coasting in neutral, and just the other day it actually did stall itself while coasting. Could this be a VP or ECM issue? Anything else possible? Been doing a lot of reading on Mopar Mans site but have yet to find a culprit, just a learning a lot about the wiring of this truck. Any more suggestions would be helpful. Thanks
 

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Discussion Starter #11
What’s the potential that it could be bad fuel causing such an issue? I understand that it seems too electronic of an issue considering it fixes itself under 5mph, but just a thought.
 

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I doubt it is bad fuel. Possibly sucking air between lift pump(if external) and the tank. How does the tach act when it dies. Does tach go to zero first or after it stops running? I did not think it would be APPS. Does it miss or surge at idle in park or drive at a stop? What did dash voltmeter do when it was dying?
 

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I forgot, have you replaced the crank shaft sensor? If not try it, it is only 40 bucks. But only buy a good one one. Autozone Duralast , Standard Motor products o Borg Warner etc.
 

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Discussion Starter #14
Tach goes to zero with the motor. The tach does not seem to be off at all. And nope, it behaves perfectly normal otherwise. Maybe a touch underpowered but nothing else. No abnormal smoke, no missing, nothing is bad except it stalls itself.
 

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Discussion Starter #15
And yes on the crank sensor
 

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Unless sucking air, it almost has to be a short somewhere. Maybe internal in the ECM or VP, or maybe just a wire. Do you have a fuel pressure gauge, is it consistent.
 

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Discussion Starter #17
I do have an Isspro FP gauge and its a very consistent 17-18 at idle and 16-17 when moving. Doesnt go much under 16 if at all at WOT.
 

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Good unless it looses presrue when/before dying.
 

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Discussion Starter #19
It maintains pressure throughout. Watched it as it stalled before. Gauge doesn’t move then just bounces back to 17-18 when it’s off just like I bumped the key to prime.
 

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Discussion Starter #20
Thinking more about my diesel question. Just felt I should note that I only recently took care of an algae problem in this truck. But since have been good since clearing it out and a few filters later.
 
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