Any Idea?.....3406E Cat Problems

oh ya. these new trucks when they have electrical issues is usually something smple and stupid. the expensive problems can be diagnosed by just pluging into the diagnostic port. it's the little electrical issues that can rack up tens of thousands is shop bills. It doesn't take long at $60per hour. There are so many connections its really just a crapshoot as to if you find the right one or not

 

Nothing like an old mechanical Cummins with a PT pump. Even if the whole electrical system was ripped out of it, you just turn the thumbscrew in, Have somone push you with a forklift or something, and pop the clutch.

Same goes for almost any old mechanical engine.

 

Speedo is bouncing at idle? An incorrect VSS (Vehicle Speed Sensor) signal could cause surging. When the truck is hooked to a diagnostic scanner, does the VSS signal stay steady at an idle? It should.

The last time I turned wrenches on a truck, they didn't have computers in them. I was, however, a Specialized Electronics Tech and worked in GM dealerships for about 10 years. Computer controls are computer controls are computer controls...

Some thoughts I have are... Does it have a cable actuating the throttle? I'm visualizing the cab moving up and down while it goes down the road... and a damaged cable causing a surge that wouldn't happen on the cruise. Or a speed sensor sending an erratic signal while moving. If the computer thinks you're suddenly going from 35mph to 70mph, it will vary the pulse width of the injectors and cause a surge.

If it's a fly-by-wire system, the cab moving up and down could still be causing a poor connection on a wire.

Do the big trucks have OBD-II now? If so, plug in a scanner and do a data capture. Watch for things like fluctuating TPS and VSS while holding the pedal steady. Then watch it while on the cruise. Compare the pulse width on the injectors while on the cruise or the pedal.

A mechanic that knows their stuff should be able to plug in a scanner and see right off, when it surges, what is going on. If they work on it, and still have no clue, go somewhere else. I've worked with more than a few supposed Electronics Techs that knew how to plug a scanner in, but had not a clue what they were looking at on the screen. Out of my whole career as an auto/truck mechanic, working in a multitude of different shops, I worked with maybe a handful of guys who really knew their stuff. The rest were parts changers.

And MedicineMan is 100% correct... new parts do not equal good parts. I had a customer bring back a car that I'd changed the plugs in. It had an intermittent misfire which I thought was maybe a loose plug wire. After I hooked it up to the oscilloscope, I saw what appeared to be an intermittently fouled plug. I pulled the plug in the offending cylinder and nothing appeared to be wrong with it... til I turned it upside down and saw the center porcelain was broken. It was dropping down over the gap and causing the misfire.

 

did the mechanic clear the ecm? even though the repair was made if he didn't clear the ecm it's probley still picking up the code,i'm not sure all obd-2s,clear the codes,there are hard ones and soft codes,hard codes are like,crank sensor,cam sensor,tps sensor,temp sensor,and the like,I hope this helps,maybe the loose wires were the problem,hope so,seeya.

 

please tell me about the wreck as far as where it got hit and what damage was done and how fast truck was going. when throutle sencer was changed did some cheet and not change the wires? You can get just the sencer or the complete one with wire harness.

 

I believe that you have a broken wire between the throttle position sensor & the ECM. These can be temperature sensatice as the insulation contracts & expands, position sensative as the cab & chassis move on the road. The least expansive "cure" might just be a fresh harness, be it a dash harness or a cab harness. Shy of inspecting each & every wire associated with the throttle position sensor about the only way to eliminate this posability.

 

I would be looking at the fuel system, preasure, flow, and for air in the return fuel. could be sucking air or combustion preasure in the fuel system.
Also clean the battery connections, and Grounds.
Testing the CAC is not a big thing and is always a good thing to do if it is a low power complaint.
I take it they did a valve set?
The ECM even in a cat reacts quick enough to compensate for any power fluctuations. Just a thought

 

Just a longshot but because it's only when it's on the throttle and not the cruise I would be looking at the potentiometer ( not sure of the spelling) and it's wiring, could have been damaged, it could cause this.