Howdy everyone,

I have a 98 Ram 1500 with the 5.9L V8 and a manual transmission. The issue is when the vehicle is in motion (above ~5mph) and the engine is at or above 1500RPM if you press the clutch, the engine stays at or around 1500RPM. It will remain this way until the vehicle comes to a stop or a higher gear is selected that pulls the engine rpm down. If it is below 1500 rpm when the clutch is pressed it does drop but its slow to return to idle.
So far, I have checked for vacuum leaks (haven't found any. plugged all the vacuum ports on the intake with a gauge hooked up, no difference), cleaned the throttle body, changed the intake manifold to solve a plenum leak (now has the Hughes FI Airgap), replaced the IAC motor and have verified that it opens and closes, replaced the throttle position sensor, replaced O2 sensors (for unrelated reasons), and replaced the MAP sensor. One thing I have noticed is that if I unplug the TPS the issue goes away, but the engine runs more rich and it throws a check engine light.

I am not sure what to do next for troubleshooting this issue, any help would be greatly appreciated!

 

Sounds like the clutch is dragging.

 

Thanks for the lead but that doesn't seem to be it. Starting the vehicle in gear with the clutch pressed in doesn't cause the vehicle to creep forward. Even with the rear end jacked up the rear wheels don't spin with the clutch pressed. Also wouldn't clutch drag be more dynamic? I.e as you slow down it allows the motor to spin slower? I'm thinking it's something more on the electronics side as unplugging the throttle position sensor seems to temporarily eliminate the behavior.

 

Did you thoroughly clean the IAC well when you did the throttle body? Did you reset the computer after you changed the IAC? What brand IAC did you install? I have had very little success with the aftermarket fellers....

 

Yep you're on the right track although I don't know exactly how to help you. Was the TPS Mopar? Have you checked voltage when closed?

 

The TPS that currently is on it is not a Mopar TPS, I do still have the Mopar TPS. Interchanging them does not make a difference, and the voltage at close is about 0.71v

I did clean the IAC valve when I cleaned the throttle body. I do not know if the IAC that was on it is a Mopar part or not. I did end up replacing that IAC valve with an aftermarket IAC from standard motor products. I am looking to replace it with Mopar IAC just because i do not trust it. The computer was reset after the install of the new TPS and valve IAC.

I did some more experimenting with the truck. I plugged the old Mopar TPS in with the other TPS still in the throttle body. Took it for a drive and it returns to idle as it should in this configuration. I hooked up my scan tool to it and compared TPS values, the one on the throttle body reads 13.4% at idle and the one not connected to the throttle body reads around 6.4%. This leads me to think the problem lies with the TPS is informing the PCM that the throttle is still open and the PCM is not commanding the IAC valve to close. Is there any way to adjust the indexing for the throttle position sensor on the throttle body?

 

This should help with your TPS adjustment - https://www.hughesengines.com/Upload...nSheet2011.pdf

However, you shouldn't have to modify/alter the sensor for a stock motor if the sensor is working properly.

 

I had the same generic symptoms… for me it was bad aftermarket TPS… after swapping out several, I put in a new OEM sensor in it’s been fine ever since.

 

I spent the afternoon trying to adjust the TPS. I went from about 0.74v to 0.65v with no change. Just to see what would happen, I adjusted it back so there was no tension on the sensor with no change.

I am ordering both an OEM TPS and OEM IAC valve. For now, plugging in a TPS that is not hooked up to the throttle body seems to "fix" the issue so the truck is drivable. Will update with the results of changing those parts when they get here.

 

When you cleaned the throttle body, did you also clean out the well where the IAC piston extends into the back of the throttle body? I ask, as this area commonly builds up with carbon and must be cleaned good for the IAC to function properly.