Jumping rpms

[99GreenCummins]

To start: I was hauling a piece of equipment home Saturday and my truck started to get a surge(jumping RPMS) driving down the road. Pulled off, checked all my apps connections(TIMBO APPS), connections to VP. Went to get back on Interstate and the RPMs started jumping on their own any where from 1500 to 2500 back and fourth.

Had a scan tool put on it, said truck was reading 19.5lbs of boost at idle from the MAP sensor. Today I changed the MAP sensor and the Comp box harness since I had it laying around.

Started it today, didn't do the jumping when I first started it up. Came back about 2 hours later, idled around to hook up a trailer and it started again.

Anyone have any ideas now? Comp box having issues? VP dying?

Really needing my truck at this point in the year, any help is very much appreciated.

 

[99GreenCummins]

Been working on it here in the past few mins, has CEL, I'm assuming from changing map sensor and harness........don't have scan tool here.

Now it has a dead pedal for probably the first 3-5 seconds of idle, then pedal works.

 

[diesel_importer]

Remove comp first. Clear codes and drive. Is the map reading correct now?

 

[99GreenCummins]

Remove comp first. Clear codes and drive. Is the map reading correct now?

I unplugged the comp and it was the same, maybe worse. Guy is coming with a scanner tomorrow to clear codes and monitor the sensor.


I have a very bad feeling the VP is dying

 

[mathews8pt]

Check the harness where the stock lift pump is/was. The comp plugs into something right there and I have had a truck that rubbed through the harness in that area. It killed 2 vps before I pulled it all apart and found the issue.

 

[99GreenCummins]

Well had it scanned again and cleared all the codes. MAP sensor voltage was 3.2-3.3V........should be 1.5. RPMs will still flutter or jump around when ran up above 1800 rpms.

I am beginning to doubt my assumption of a dead VP now. The RPMs were just floating back and fourth, not cutting out and jumping around like I originally thought they were.

The guy that is helping me (D_L performance on here) is going to look to see if he has an ECM to swap out and try. If that doesn't work, going to start tracing wiring harness to see if its rubbed through and back feeding the MAP sensor

 

[99GreenCummins]

Check the harness where the stock lift pump is/was. The comp plugs into something right there and I have had a truck that rubbed through the harness in that area. It killed 2 vps before I pulled it all apart and found the issue.

Was it in the truck harness or the comp harness?

Anyone else have any ideas to try? I will try to get a good video this evening showing the jumping.

 

[zfaylor]

ECM? The map sends the voltage back to that to determine fueling am I correct?

 

[99GreenCummins]

ECM? The map sends the voltage back to that to determine fueling am I correct?

If I can find one locally for a 99 manual I am going to try one......everyone so far has a spare for an auto truck, not a manual:bang

 

[mathews8pt]

Was it in the truck harness or the comp harness?

Anyone else have any ideas to try? I will try to get a good video this evening showing the jumping.

Truck harness. Saw the vid on FB, very odd to say the least.

 

[dzlsmokin]

My truck displayed the same behavior yours is. My ecm is bad...

 

[99GreenCummins]

We decided it was the pump. Put a known good apps on it....still went crazy. Just unhooked apps still went crazy. With apps unplugged and reading the crank sensor(saying normal volts) it still went crazy.

Ordered a bluechip VP, the rebuilt on with a new computer on it. Hoping its here monday-tuesday so I can get my truck back.

 

[1stack]

My 99 done that real erratic finally found a loose butt connector around the vp. Not sure which wire though.