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Engine RPMs revving while driving

mhixson

Member
Messages
13
Reaction score
28
Location
Mount Vernon, WA
Noticed recently that my truck (1996 AM General Hummer with GM 6.5 TD) would sporadically rev its engine just a bit while idling. Took it for a spin and once while slowing down from 50MPH or so to take a turn the engine revved while I was off the throttle. I've had a failing crank position sensor before and this doesn't feel the same. In that failure it was like the truck was abruptly downshifting and didn't know what gear it wanted to be in. This just seems like I'm tapping the accelerator when I'm not.
I've only noticed it while decelerating, coasting downhill, or idling at stop. I haven't noticed it when on the throttle while accelerating or climbing my steep driveway.
I just replaced my PMD with a new Standadyne and used the same resistor that was in my old one. Same exact symptom persisted. My air filter looks reasonably clean and I also replaced the fuel filter. The old one was pretty dirty.
Two codes I have now are

P1214
P1218

Any help or diagnostic suggestions would be greatly appreciated.
 
Welcome @mhixson what type of scanner do you have? others will chime in soon with help, but are you able to read live data and check the pump calibration numbers in the PCM?

also check all of your grounds. on pickups they are normally located on the passenger side of the engine there is a stud on one of the rear intake bolts. usually is three small wires on eyelets. take them off and clean well. Not sure where they are on a Hummer.

P1214- injection pump timing offset
P1218- injection pump timing calibration circuit

putting a call out to @Will L. and @Hummer H1 y'all are hummer guys.
 
Welcome @mhixson what type of scanner do you have? others will chime in soon with help, but are you able to read live data and check the pump calibration numbers in the PCM?
I have an OBDLink MX+ which is a Bluetooth OBD-II reader. I can read live data with it, but I don't know if I can read pump calibration numbers. I'll try to figure that out.

also check all of your grounds. on pickups they are normally located on the passenger side of the engine there is a stud on one of the rear intake bolts. usually is three small wires on eyelets. take them off and clean well. Not sure where they are on a Hummer.
I'll take a look at that tomorrow.

Thanks for the help!
 
see if you can use the android app called torque pro on your bluetooth scanner. iirc Quadstar has the PID's for the app for these engines on his site for free. in I'm not mistaken this app can read those values among other things specific to 6.5's where other "generic" obd2 scanners cant.
 
you should be able to test the app before purchasing the paid "pro" version. I would be using it myself but one truck is fully mechanical no computer and the other is OBD1.
 
Looking for the pump calibration id number it should read somewhere in the neighborhood of "114 or 117" something like that depending on the resistor in your PMD. one of those codes indicates the PCM isn't reading the resistor properly. the other one is what I think is the PCM has lost the TDCO offset setting or the IP timing is out of spec, maybe from timing chain wear or something in the IP. this one is why we need to verify good grounds to the PCM.
 
96 hummer is turbo with ds4 obd2. Same engine as vans. Vacuum system controls hvac controls- otherwise same as the pickups.

Ds4 not my strongest suit so stepping back for the better guys on this one.
HummerH1 is a db2 guy like me.

I always start with clear return line from ip- bubbles and contaminants.
Clear line from fuel-line.com its 1/4” diameter and about 5” long.
Fuel pressure tapped at ip inlet - choose your electric gauge and mount permanently in dash, get tap adapter from modmafia.com
 
Oh, my limited response besides needing to know of air intrusion and incoming fuel pressure at the IP (injection pump)
Is what fuel additive for lube are you using and how much or do you live somewhere that still uses full sulfur diesel instead of ulsd?
 
I'm still researching where to find my ground locations.

Just wanted to mention that I just did a short test drive through the neighborhood after letting the truck warm up for 10+ minutes. I was in/around it this entire time and did not notice any engine revving at idle. Also nothing on my test drive.

To recap: problem persisted after replacing my PMD. It (so far) has not persisted after replacing my very dirty fuel filter. Could that fuel filter actually have been the problem? Here it is.
 

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Oh, my limited response besides needing to know of air intrusion and incoming fuel pressure at the IP (injection pump)
Is what fuel additive for lube are you using and how much or do you live somewhere that still uses full sulfur diesel instead of ulsd?
I live in western Washington State so I think we're ULSD. I occasionally use some Lucas or Amsoil injector cleaner.
 
Yes- that filter does not look good at all. Guessing no fuel pressure gauge yet. Low pressure would 100% indicate that was the restriction.
Mine is torn apart for a higher power build along with truck modifications I am trying to afford. But when mine goes together this time it will have not just a pressure sensor at the ip inlet but one at the fuel filter inlet so I can compare pressure before and after filter. There will be a difference all the time obviously but when I see an increase in the differential it tells when the filter is getting plugged.

I can’t really tell in the picture, but you should learn about fuel algae aka fuel bugs.
It is possible that is what you have. I can’t think how many miles it should take to get a filter that dirty in normal use.
 
Will beat me to it!! I was thinking the same thing... fuel bugs. you might want to continually watch this. if you can run the lift pump and drain a sample of fuel into a clear soda bottle just to see if there looks like there is anything there. post a pic too.

Get a pressure gauge on there and see where your at at idle and acceleration. even if you need to run a hose and strap the gauge in front of the windshield.
 
Pull a fuel sample into a clear glass container that can seal air tight.
Mason jar is ideal. Draw the fuel from before the filter, Siphon from tank is ideal.
Take picture as soon as in the jar, then 24 hour intervals afterwards
 
Pull a fuel sample into a clear glass container that can seal air tight.
Mason jar is ideal. Draw the fuel from before the filter, Siphon from tank is ideal.
Take picture as soon as in the jar, then 24 hour intervals afterwards

Just siphoned this sample from my main tank. Looks pretty clean to me. I was going to do my aux tank also, but am having trouble getting the tube in far enough to grab any fuel.

Presuming that diesel bugs thrive in the dark I've sealed this jar and put it in a dark cupboard in my shop.
 

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Get a pressure gauge on there and see where your at at idle and acceleration. even if you need to run a hose and strap the gauge in front of the windshield.

Picked up a fuel pressure test kit today. I'm guessing this brass piece in the line between my fuel filter and IP is where I attach this kit. Is that correct?
 

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You can test there- the problem is it doesn’t verify the fuel line from the filter to the ip. So that’s ok for testing 3/4 of the system. That will test pressure going to the fuel filter, not after it. So imagine your filter you just changed was 100% plugged- you could test there and think fuel supply to the engine is perfect when it is getting none.
 
You can test there- the problem is it doesn’t verify the fuel line from the filter to the ip. So that’s ok for testing 3/4 of the system. That will test pressure going to the fuel filter, not after it. So imagine your filter you just changed was 100% plugged- you could test there and think fuel supply to the engine is perfect when it is getting none.
This is after the fuel filter and before the IP.
 
Someone relocated it then. That originally went between the ffm and the drain.
Is so, fine. Just replace tje fuel line from there to the ip inlet with new sae30r9 hose because the original hose throttle the rig fails under the ethanol fuel we have now. If the hose after that schrader valve fails internally- you won’t know and again would think pressure is good when it is restricted.

Search the picture from warwagon- his was a classic example. Seen so many people buy new ip replacing good ones because the fuel line fails inside.

Doing that will tell you the pressure now. Rig the hose so you can read the gauge while driving. Jammed under the windshield wiper is a common move for that. Idle and revving the engine while sitting still will barely cause a draw. Truck under heavy acceleration going up a hill is a completely different level.


Long term
Buy this:
Then choose an electronic fuel pressure gauge with compact sending unit.
Tad on HML had one done at speedhut that replaced the clock in the dash that always fails. His turned out looking very stock.
 
Keyed up with lift pump humming, but before starting, the gauge reads about 5PSI. At idle it falls to what looks like 0 to me. Is that indicating a blockage at the fuel filter? I'm thinking I need to take the whole housing off and clean it out with how dirty my filter was.

Also, here's a photo of the line running from the test point to the IP. Should I replace it?

Thanks again.
 

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