The Diesel Stop banner

Strange problem w/4R100 transmission stumps my mechanics

2K views 5 replies 3 participants last post by  Ryanr79 
#1 ·
Hello! I have a 2000 f-550 super duty bus with the 7.3, 2wd, it has a strange problem at freeway speeds, in that the rpms never settle down after reaching cruising speed. Going up a hill at 70, rpms sit slightly over 2700, once I crest the hill and go down, rpms are still at 2700. Normally it should idle down while cruising to about 2200, but this never occurs, happens this way 100% of the time. If I depressed the brake, even with a feather touch, the rpms drop by about 500, and if I release the brake they go right back up. It almost feels like overdrive is not engaging, but when I push the od off button it downshifts and screams to 3300 rpms. I had a lot of work done on the exhaust and turbo pipes, they were very leaky, replaced the hysroboost, and replaced the fuel water separator valve as it was leaking diesel. I noticed that there was two small air lines (red and green) going into a detached port with a sensor connected to it, my mechanics ic says it's for the waste gate, but one of the lines was disconnected, reconnecting and making sure it wasn't leaking did nothing to fix this. Any help or ideas to try would be much appreciated.
 
#2 ·
You've likely got a 4.88 ratio rear axle, so your rpm at 70 MPH should be between 2600 and 2700 rpm.
My truck with a 3.73 rear axle runs right at 2000 rpm at 70 MPH

How you're getting below that at 70 MPH is the mystery.


You may still have a transmission issue however.
 
#3 ·
I didn't know it could be geared to run at such a high rpm. Everything "feels" fine, I'm just not used to a vehicle never having the rpms drop down. I just noticed this change since everything was repaired and now it produces proper power and actually powers and sounds like a turbo diesel again. Will running at 2700-3000 rpms for long periods of time damage or wear out the motor?
 
#4 ·
Probably won't hurt the motor, but will suck money out of your wallet in the form of diesel fuel.
 
#5 · (Edited)
When you crest the hill, your rpm should never change, unless you remove your foot from the pedal. In fact it should start to accelerate, and the rpm should increase. So I see no issue there. As well as your speed is bang on. The red and green tubes are for the waste gate, red, and the cpu intake pressure, green. The red one can be totally removed and plugged at both ends to prevent dirt getting in. The red line opens the waste gate, and has been known to open way too early, raping you of power. I assume that's why it was originally removed. I left the sensor module intact with the green tube still on for the cpu to use, and plugged the red to get max boost all the time. The way it works is the green line picks up vacuum from the intake tube, reading the air coming in. When it builds up, it pulls the plunger in the sensor thingy, which opens the red line, allowing the waste gate to move. Red line not needed, and really the green too, but the cpu uses the green line for a signal, so I left mine intact, although not critical.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top