Truck is lurching forwards when stopped

[Ben Geller]

Weird problem just popped up.



When sitting at a light stopped in gear, the the truck idles at 550RPM (which it always has), but now is lurching forwards as if someone is rear ending me. It's very noticeable, when vehicle is in neutral at a light, it doesn't do this.



Any ideas?

 

[Bob McDonald]

That is the infamous tranny downshift 'bump'. The very first time that happened to me, I looked in the mirror to see who had rear ended me.



It will get to the point where when you are ready to go again, it 'still' will have not downshifted into first and you will try to start in a higher gear. I had to manually pull the shift lever down to first to get the darn thing to start off in first gear. A real pain.



Also, you will begin seeing the flashing 'O/D Off' light symptom.



I noticed that mine did that when the weather started to get warm. I ended up adding a second transmission cooler to keep the fluid temp down and that has help a lot. I didn't have any of the symptoms last summer.



Eventually, you are looking at potential major transmission work being needed.



Good luck.

 

[Ben Geller]

Thanks for the reply.



I am being optimistic that it's not the "bump". The reason I say this is because it seems to do the "lurch" when the truck is idling in gear and stopped and it feels like the truck is almost stalling out because of the low RPM's. Could this have anything to do with it?

 

[EddieS'04]

Not the bump. Bump only happens on gen2 6 speed.

 

[Bob McDonald]

Sorry Eddie. We Gen1 folks had our 'bump' before there was a Gen2. lol

 

[Yardsale]

Thermostat?? In my '01 Job 1, when the t'stat went, the idle is raised (to pull air through the radiator) and it messes with the shifting. Just a thought.

 

[Doug Watts]

Clean the IAC. There is a project on it. It's probably getting a litlle sticky.

 

[Ben Geller]

Thanks for all the tips.



Have been looking more into this hoping to give you guys some new info.



Truck threw a CEL a couple days ago and it was the P0401, so I cleaned the IAC, cleaned the throttle body, and changed the DPFE again (did this last year made the mistake of not buying OEM to save a couple bucks), also ran a coat hanger down the exhaust side to clear anything.



When I was in ford getting the DPFE, I left the truck running, and my daughter said the truck was "bucking", the exact same thing it's doing when i'm stopped at a light. It seems that the RPM's are dropping to 400ish, and the truck bucks (I'm assuming to avoid a stall) and the RPM's raise back to 600.



So it seems that the lurching is caused by the engine trying to stall out. I've changed the DPFE and reset the CEL but as of yet, this hasn't corrected the stalling/bucking.



What the heck can I do to fix this? :banghead:

 

[EddieS'04]

Another thought. Are you using the SD performance coil. Some members had theirs go bad.

Also if your IAC is good. To me it could point to a fuel or spark problem.

 

[Ben Geller]

Everything is bare bones stock on mine.



Time to go after the wires and plugs I think. Only next logical step.

 

[Gavin Allan]

Sounds like a bad torque converter.

 

[Mike Gennello]

Had this problem, found dealer cracked anti-lock brake ring on front when they did a brake job.

 

[Ben Geller]

I really think it's something to do with spark (plugs or wires), and i'm going to try changing out my plugs and wires and see if this helps. Might even try the project on raising the idle. The truck will only idle at 700ish when it's cold.

 

[Jeff Harwanko]

Have you tried adjusting the idle speed? I had a problem 2 winters ago, when stopped at a light with the heat running the truck would surge and then try and die. It ended up being that when you have the heat on or (AC in this weather), the AC compresser kicks on and if your idle speed is just a little too slow, it puts a noticable strain on idle speed and the truck wants to die. I followed another members project on adjusting the idle speed and haven't had a problem since. Just a thougt....

 

[Ben Geller]

DE- That sounds EXACTLY like what I have. Even though i'm not running A/C I can hear the compressor kicking in, and the truck almost stalls out, then lunges forwards. Good thing the brakes are good or i'd be eating someones bumper in front of me. I'll try the idle project and see how that goes.

 

[Yardsale]

You might be money ahead taking it to the dealer. I know they're kind of spendy, but usually a one-stop fix (instead of the hunt and peck you are doing now - your time is worth something).