Im a new 7.3l powerstroke owner and want to make sure this is normal and im not having issues. First of I own an early 99 F250 7.3l. Original motor and automatic tranny.

First issue: When im driving down the road (at any speed) and I let off the throttle the truck jerks/thumps once and then when I push on the throttle it jerks/thumps once again. (seems to be worse when letting off throttle) No issues between shifts. I have replaced the pedal assembly, and I dont think its motor mounts because if the truck is in park and you push on the throttle you dont get the jerk/thump. I live down a pretty bumpy road and when you're at the speed where it doesnt need much throttle to keep driving its pretty jerky because its on/off throttle constantly. This also cant be a carrier bearing issue as I do not have one on my truck. I was under the truck and was seeing if there was any play in in the drive shaft and I found some play in the slip yoke (when truck is in neutral with parking brake on). It was definitely at the yoke and not the rear diff. Is this normal?

Second issue: this may just be a diesel thing but when im driving down the road above 45 mph and i let off the throttle, it sounds as if the motor isnt idling (very quite) until it hits 40-35 mph and then you can here the great 7.3l idle. Maybe this is just a 7.3 or diesel thing but ive never had that happen on any other truck ive owned.

Any guidance would be greatly appreciated!

 

Greater minds will be along shortly, im certain. Welcome to the forum, congrats on the truck! Nice to see another E model in here.

First thing im curious about is milage, and specifically, milage on that transmission. Have you checked fluid? Is it the right level, and is it clean?

 

Thank you! 149k miles, transmission is stock. Just had the fluid changed 3k miles ago. They said there was some flakes on the magnet. I know ill need a new transmission down the line (no slipping just a harder shift here and there, nothing major at all) Im moving down south this year and hoping itll last till i can get a BTS transmission. I didnt think transmission for the throttle issues because it doesnt do it when shifting.

 

Awesome the fluid is good. My E99 is a zf6, so i cannot compare apples to apples, but my 2016 f150 eco.... has a new engine with original trans... and its doing what you are descibing. Acceleration is fine, but clunks are happening on throttle off. Im about to have mine flushed/cleaned.

@SkySkiJason @Sous @cleatus12r can shed light and/or tag others who will.

 

I had a 2015 ford f150 eco before this and I had a clunk. I greased the slip yoke and the problem went away. Give that a shot if you havent.

 

Let's see, this jerk/thump do you mean it feels like drive line slop or the motor stalls or lopes? If it feels like the motor you may check that the tin nut on the IPR valve is snug. Just snug not torqued, about a 1/4 turn past handy is all it needs. The 42 pin harness rub the drivers valve cover is another cause for quirky engine behavior.

The 2nd issue isn't an issue at all. Completely normal for the engine to go quiet while coasting until the 35-40 range which is when the torque converter is programmed to lock and unlock..

 

Feels like driveline slop

 

The slip yokes on these trucks can be a trouble area. It's not surprising really considering they are 20ish years old and many many miles of running in the same 2 or so inches of stroke on the shaft. You can try greasing it, some have luck with it. If not may start checking with drive line shops to price a replacement/rebuild.

 

Okay, I had a feeling that might be what it is. Ill give this a shot and update when ive done so.

 

@Theplumber1995 , as kd5zll said, welcome to the FTE!

As for the issues you are seeing/observing, our friend udsuth78 has you on the right track, as he usually does. I do not have a carrier bearing on my driveline either and I had a heck of a time convincing some shops that I didn't have the bearing and did have 3 U-joints. Speaking of that, how are your U-joints on the driveline?

You mentioned replaced the pedal assembly already, I am hopeful you used a quality parts supplier. About a year ago we had someone that diagnosed a failed pedal assembly and after 3 attempts with part store/aftermarket brands, a Motorcraft unit fixed his issue. I am not saying that is your issue, just providing some background.

Just in case you are not very familiar with the connector udsuth78 was referring to, you can see it here in another recent thread: click here for pictures!

Do you have a scan tool like a BAFX or OBDLink? If the issue is not mechanical and is electronic related, you will need something to look into the PCM and PID's of the vehicle.

Some light reading material from FTE'r BWST is here: FORScan + FORScan Lite: Class Tutorial Q&A

Use Motorcraft XG-8 lubricant if/when you service the slip yoke. Check the rest of the driveline and come back to us with what has changed, has not changed. Hopefully you will have good news!

 

I had another thought, the lurch when getting on and off the throttle sounds a little excessive for a sloppy slip yoke. I'm with Sous on double checking the u joints especially any double cardigan joints. Those things can pack a lot of slack in a short span. I like to check driveline on flat ground in neutral no brakes with a little slack between chocks to see everything just as movement begins. Also what about the possibility of 1 or more brakes dragging, exaggerating the driveline slop. I know personally how problematic the caliper slide pins can be when they get dry and/or dirty.

 

Jack rear tires off the ground and rotate them and look for slop at u-joints, carrier bearing etc. Also push up on driveshaft and look for play.

Then rotate driveshaft and observe ‘slack’ in rear differential. There should be a little movement before tires turn, but too much play here means rebuild is needed.

Generally, any of these problems have a vibration associated with them.....