Violent Intermittent Buck or Bang (Like being hit from the back)

[SRubenstein]

Hey all. I'll preceed this by telling you this is going to be a tough one to diagnose, due to the frequency of the problem. In the past 2 weeks the problem has happened 3 times. Dodge just replace my turbo under warranty a few weeks back. I don't think the prob is related but wanted to mention that anyways. First time it happened I was sitting at idle, in drive, foot on brake, maybe 90 seconds. Second time was driving home medium throttle, approx. 80mph. Third time was just like second. The truck all of a sudden gets a violent "buck" or "bang" that you feel through the whole truck. It's like you just got rearended by a semi. I can safely eliminate the differential due to the fact it happened while not moving the first time. If anyones ever felt a torque converter lock up when not moving, it's kind of like that. The fact that it happened at 80 mph kind of takes away a TCC lock up problem out of the picture because it should already be locked at that speed. Has anybody ever experienced this type of problem. I'm an ASE Master Tech, and pride myself on difficult diag, especially electrical. If the prob happened all the time this wouldn't be that bad. Any insight is much appreciated.

 

[HOBrian]

If it is hitting that hard and no codes are thrown, the best I can say is if you can put the transmission in neutral as much as you can to see if the problem only happens in gear.

 

[SRubenstein]

If it is hitting that hard and no codes are thrown, the best I can say is if you can put the transmission in neutral as much as you can to see if the problem only happens in gear.

What makes this difficult is how unpredictable it is and how infrequent it happens. It has never happened when idling in park, however it's not often that I do that this time of year.

 

[HOBrian]

Does the engine bog down any? Drop RPMs? I am thinking now it's the transmission Because if it were the engine you would fell it slow down and bog/struggle. (when sitting at that light) and if it comes on that hard I would believe it would throw a code.

 

[SRubenstein]

Does the engine bog down any? Drop RPMs? I am thinking now it's the transmission Because if it were the engine you would fell it slow down and bog/struggle. (when sitting at that light) and if it comes on that hard I would believe it would throw a code.

The one time it happened while sitting and idling in gear I did catch the RPM's drop. I was talking with a buddy and I thought I got rearended by a truck. It's a one time shot and then fine again. Have you ever done a TCC lockup test in gear? That's what it reminded me of. As far as the two times it happened on the highway I was just cruising along 70-80 mph and bang, it happens so quick I don't know if my RPM's dropped. I am also leaning towards transmission but there's no trans slip or power before it happens, just a sudden bang.

 

[HOBrian]

I think the drop in RPMs is the result of the bang. I have the 6spd so I am not at all familiar with the auto.



If you had to guess do you think it is engine or transmission?

 

[steved]

Sounds like a miss to me...

 

[Wingate]

Do you have a boost pressure gauge installed on the truck?... IF so, instead of watching the RPM, watch the boost pressure gauge, see if the engine is taking on more load when this occurs. It could be a good indication of a transmission/torque convertor malfunction. I would guess that it would be in the transmission or transfer case due to it happening at highway speeds.

One other thing that comes to mind, does the steering feel funny for an instant when this happens?. . Could be a transfer case/ front end issue. Almost like the front end is trying to engage...

 

[SRubenstein]

I think the drop in RPMs is the result of the bang. I have the 6spd so I am not at all familiar with the auto.



If you had to guess do you think it is engine or transmission?

Yes I would completely agree. Initial thought is transmission right now.

Sounds like a miss to me...

I don't think so because it does not feel like a skip. It's like a 6 cylinder miss if anything.

Do you have a boost pressure gauge installed on the truck?... IF so, instead of watching the RPM, watch the boost pressure gauge, see if the engine is taking on more load when this occurs. It could be a good indication of a transmission/torque convertor malfunction. I would guess that it would be in the transmission or transfer case due to it happening at highway speeds.

One other thing that comes to mind, does the steering feel funny for an instant when this happens?. . Could be a transfer case/ front end issue. Almost like the front end is trying to engage...

Boost pressure gauge is on the way from diesel manor. should be here by weeks end. The transfer case issue is an interesting thought. During plow season I had a couple of instances where the 4WD light would not turn on or shut off when shifted. The salt used in the northeast this year was ridiculous and this is a plow truck. I have had electrical issues in the harness before from salt contamination. I am going to mess around with the transfer case shifter and wiring harness to see if I can make anything happy. This is a manual shift transfer case btw.

 

[cerberusiam]

As far as the two times it happened on the highway I was just cruising along 70-80 mph and bang, it happens so quick I don't know if my RPM's dropped. I am also leaning towards transmission but there's no trans slip or power before it happens, just a sudden bang.

If its happening at that speed its not the converter as it is locked.



It could still be in the trans but highly unlikely as the things that would cause that at speed or idle would break something with just a one time event.



They will bang like a backfire when the grid heaters activate some times so am wondering if you don't have an ECU problem that is momentarily shutting everything down then coming back abd the heaters cycle.



You have on 05 so check the wire that that activates AC clutch for abrasions or chaffing back thru the harness ont he passenger side of the engine. A temporary ground on that wire will throw all sorts of quirks into the fuel system control.

 

[vherner]

Had the same problem with a low mileage 92 chrysler newyorker. stop for a red light and swore we had been rear ended. this problem wouldnt go away, had the trans changed and that took care of it.

 

[SRubenstein]

If its happening at that speed its not the converter as it is locked.



It could still be in the trans but highly unlikely as the things that would cause that at speed or idle would break something with just a one time event.



They will bang like a backfire when the grid heaters activate some times so am wondering if you don't have an ECU problem that is momentarily shutting everything down then coming back abd the heaters cycle.



You have on 05 so check the wire that that activates AC clutch for abrasions or chaffing back thru the harness ont he passenger side of the engine. A temporary ground on that wire will throw all sorts of quirks into the fuel system control.

The ECU/ electrical problem is more likely where the problem stems from. As I stated before the salt this year was ridiculous up here. I had an incident during the winter where the salt contaminated my wiring harness so bad that the parking lights were getting power from the trailer harness B+ lead in the harness to the rear, causing the parking lights to stay on constant. The thought process of putting an ECU on the underside of this engine exposed to the elements is mind boggling. I'll look a little closer at the wiring harness, ecu, and ac area tomorrow. I was hoping that I'd find someonelse who had experienced this before, my gut tells me I'm gonna have to dig to find this one. Thanks to all who have given me suggestions so far. Please keep em coming, sometimes we forget to look for the obvious.

 

[HOBrian]

Please, if you find the problem let us know!