671 "Hesitating"

[gus]

Just south of Seattle my 671 with Spicer 4sp began "missing" about every four or five minutes yesterday after 3000 mi of faultless operation the past week.

It acts just like it has passed a small bit of water in the fuel which has happened a couple of times over the years but always stopped after once or twice.

I added fresh fuel thinking that would help but, if, anything it got worse.

My next move is to change fuel filters but fuel filter clogs usually just reduce power and don't cause "missing".

One thought is that there is an electrical problem in the fuel shut off circuit, maybe a wire grounding out when moving? I doesn't happen at rest and idle.

Doesn't seem to be any worse when pulling hard.

Injector??

Please feel free to diagnose!!

All ideas accepted!!

[Don4107]

Air leak?  Might want to pull the return line and run it into a container to see if you get air bubbles.

Good luck
Don 4107

[zubzub]

Hi Gus, if you want to simplify things remove the control wire from the skinner (or whatever it's called) that way you can bypass all the wiring from front to back, especially the rear engine bay switch panel.  WOuld be a weird short to "+" but could be.  Let's hope it's that simple.

[gus]

Zub,

Good idea. Power keeps the air valve closed, any momentary interruption of power could cause the air valve to open and shut off the fuel.

I hope it is just a loose wire somewhere in the engine compt, maybe at the rear engine control switch panel or on the shut off valve itself.

I can't do anything now because of weather but was hoping for a bunch of ideas to get my brain going!

[Kevin]

Had similar problem a few years ago with my '04. Turns out the Racor water seperator drain plug threads were a bit buggered-up and allowing air to suck in, which caused the motor to "hiccup".

We were just leaving for the coast for our customary anniversary get-away when I decided to drain any water off the Racor. My wife said, "why'd ya have to screw with it ten seconds before we were ready to leave?!"

It was the weekend and the local truck supply house was closed. I couldn't find anything to thread into the Racor plastic bowl. I was getting desperate! I phoned a friend and asked if he could think of anything. He cleverly suggested an "expandable plug", the sort of things plumbers use to temporarily plug a pipe to check for air/liquid tightness.

I picked one up at the local hardware store, plugged-up the Racor and away we went. I actually ran that thing for a few months before finally purchasing a replacement Racor bowl.

Maybe, as Don 4107 suggested, it is an air leak somewhere.

Good Luck!

Bus  Boy

[zubzub]

Hi Gus.  I hesitate to ask, but did you find the problem?

[harpold700 3]

zubzub could have the answer, I had the same thing happen on one of my Kenworths.  Bad electrical to the air valve, caused much excitement pulling up out of Leavenworth one night.  Hope it's simple, Gord

[PCC]

I had a similar situation with my 8V92, and I finally found that it was a 12 volt accessory connected at a point where the device's start-up draw affected the input voltage to the ECM, causing what I called a hiccup.

If you know an electronic geek as a friend, have them monitor the 12 volt input to the ECM and see if there is a short voltage drop.

Another idea is to turn off every accessory, or perhaps you can remember what was turned on just before the problem began.

My problem I found purely by chance, because I turned the small inverter on while I was in slow idle, which are conditions that I have very seldom.

Hope this does not gives you something else to be frustrated by.

[johns4104]

Your ecm is probably ok.

John 4104 as simple as it gets!

[zubzub]

If by ECM we are talking about Gus's brain, I'm guessing  it's fine.  It's one of the older models, hardwired and time tested.

[PCC]

I was not saying the ECM was bad, only a momentary flicker in voltage (12VDC) to the ECM could, as it did in my case, cause a hiccup in the running of the engine.

The voltage flicker could be being caused by something totally unrelated.

[zubzub]

I'm just joking around PCC, Gus has a 4104 with no ECM.

[PCC]

My ignorance is showing - And I thought I was being helpful !!!

[gus]

Well, actually you have all been helpful. Please keep it up because the more ideas the better.

I haven't gotten into the problem yet because of steady rain and my granddaughter's graduation festivities. Since we will be here a while I will wait until the whirlwind calms down a bit before getting into it.

Supply fuel air leaks are a real possibility because it is a mechanical suction system but I'm going to attack the electrical part first.

I'm now even suspecting the engine run toggle switch on the panel because I've had three of those original switches fail in the past two years.

I don't really understand how the fuel return line could cause this problem but I'm willing to learn??

My personal ECM is probably the weakest part of the whole system!!

Keep the ideas coming and I'll be sure to post the outcome once I get into it.

[Just Dallas]

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