RPMs drop to zero, while driving - Can't solve

[Big Dan]

First off, I was unable to find any topics that were of similar nature.  If this has already been discussed, please link it.  I appreciate it.

 

I've been dealing with this for about 2 months.  Truck will start and idle without issue.  However, when driving, in gear or not, the engine will randomly shut off (RPMs drop to zero) for two seconds, perhaps, and then start back up.  Occasionally, this will cause the vehicle to buck, especially in lower gears.  If I come to a stop, after this occurs, the engine will die.  Starting back up is not an issue.

The amount of times this occurs is random.  It will do it 10 times in 10 minutes and then not occur for 30, for example.  Grounds are, presumably good.  I checked them.  However, I'm open to re-looking.

 

I've been unable to figure out what is causing this.  I assumed this was a CPS issue, but, changing it (NOS MOPAR) did not change anything.

 

I'm open to suggestions as I don't want to start firing the parts cannon at this and really don't know where to start.

[Ωhm]

Swap all your cube relays around with each other. See if different problems show up.

[Big Dan]

Will do

[Big Dan]

The plot thickens.  Decided to just use entirely brand new relays and see how that went.  Truck wouldnt start.  Put old ones back in, truck wouldnt start.  At a loss, I pressed the clutch in...truck fired up with no issue.  As soon as the clutch is released, in gear or not, the engine dies.

 

So...perhaps, this is more clutch related and not electrical. 

 

EDIT - with that said, now that I think about it, I believe the BA-10/5 pressure plate is still on the flywheel.  I didnt read that the AX-15 used a different part number until AFTER doing the swap.  As it ran fine, I didnt think anything of it.  Anyone dealt with that before/believe that could cause the engine to stop.

It's been, as is, for a year or two now.

[scaleless]

Doesn't start or doesn't crank over at all with the clutch released?  If something was binding, I would expect no crank or a very noisy crank.

[Big Dan]

It turns over but will not start.  Clutch pressed in, will start.

 

Prior to the non starting there was a rattling sound coming from the transmission, when the clutch was not pressed in.  However, as, at the time, I didnt think it was related to this, I did not discuss it.

 

EDIT - to better answer you question - there doesnt seem to be any difference in noise between clutch pressed or not. 

[jdog]

Check all the grounds, check the one at the dipstivk and if you haven't already done it get a ground wire from the block to the firewall. I suspect your clutch is acting as a ground.

[Big Dan]

I'll check the grounds again.  I do have a ground from the block to the firewall.

[Big Dan]

Grounds look good with a somewhat quick overview.  I'll tear them down, when I can, and give everything a good cleaning.

 

For reference, this is, now, the issue.  As I stated before, there WAS a weird rattling noise coming from the transmission, when the clutch was not pressed in.  However, the vehicle still drove and I did not consider it important, for this discussion, as they didnt seem connected.  Now, as mentioned, that may not be the case.

 

 

[jdog]

Is the clutch pedal hitting any wires? Or moving any harness under the dash?

[Big Dan]

I will check and update. 

However, assuming it was, that wouldnt explain the engine stalling while driving.

[jdog]

Unless the wire is broken and would vibrate while driving

[scaleless]

That is such a clean stall when you let off it has to be electrical.  I think Jdog is right about the clutch pressing or pulling a wire or something along those lines.  It's very odd!

[cruiser54]

Are your engine to trans bolts loose? 

At idle, grab and wiggle electrical harnesses. You might find the issue. Look up near the clutch pedal also. 

[Big Dan]

I'll check the transmission bolts this evening. I checked the travel of the clutch pedal, this morning. Nothing was in its path/it's not hitting any harnesses/wires.

[War_Pony89]

When I changed my transmission from the BA/10 to the AX-15 I remembered I had issues with my Crank Positioning Sensor after installed . Made it home fine but started acting strange. Any major work done say about two months ago?

 

Like others said, that cut off when let'n the clutch down smells of electrical somewhere.

 

Runs smooth, low miles too. Good luck sort'n it out.

[Big Dan]

When I changed my transmission from the BA/10 to the AX-15 I remembered I had issues with my Crank Positioning Sensor after installed . Made it home fine but started acting strange. Any major work done say about two months ago?
 
Like others said, that cut off when let'n the clutch down smells of electrical somewhere.
 
Runs smooth, low miles too. Good luck sort'n it out.

Unfortunately, no, as that would help me narrow it down.

I'll spend some more time, tonight, looking at grounds and looking around the clutch pedal for anything.
But, after looking this morning, there's nothing in the path.

I, however, can't think of what would cause an engine stalling short.

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[Eagle]

CPS wires touching exhaust manifold?

 

[Big Dan]

25 minutes ago, Eagle said:

CPS wires touching exhaust manifold?

 

 

No.  I've had an issue with that previously (which required a replacement). 

[87MJTIM]

21 hours ago, Big Dan said:

...I believe the BA-10/5 pressure plate is still on the flywheel.  I did read that the AX-15 used a different part number until AFTER doing the swap.

I did the swap a couple years ago to an AX15 with an external slave cylinder.  I left the PP from the BA10 in place.  It just didn't feel right when shifting.  I researched part number change for manual trans Jeeps on RockAuto.  The LUK clutch kits changed when the Jeeps switched from internal to external slaves, not just the change from the BA10 to an internal slave AX15.  Searching the individual components, besides the slave cyl/throughout bearing changing to a separate throughout bearing, the clutch disk remained the same for all years MJ/XJs through 2001.  The pressure plate changed when the slave cyl switched from internal to external.  This why I recommend changing the PP when you switch trans with an external slave.

 

That background given, I don't think the PP is your problem.  I think the problem is electrical.  You state it only happens when you release the clutch.  That suggests that the clutch pedal is contacting an open wire under the dash.  Get under the dash with a light and depress the clutch.  See if it contacts or moves some wires.

[cruiser54]

Wiggle test.

[JimB]

Do you have cruise control? If so check the white vacuum break switch. Break not brake.

[Big Dan]

1 hour ago, JimB said:

Do you have cruise control? If so check the white vacuum break switch. Break not brake.

 

I did have cruise control.   As it was not operable, I removed the system.

[cruiser54]

Not an issue Dan. 

[Big Dan]

Alright, I THINK  I got it.  To everyone's point, if this was it, the issue was electrical.

 

One transmission bolt was a little loose and that has now been tightened.  But, I believe, the culprit was the manifold to firewall ground.  The bolt on the manifold side was loose and the ground cable had some movement to it.  That was tightened up and things seem good now.  Engine started without the clutch needing to depressed and no engine shutoff, during testing.

 

I'll drive it around tomorrow and ensure that nothing else pops up.

 

Two things - 1) If that was it, it would make sense that the issue would come and go as driving would cause the cable to bounce around.  However, 2) school me on this.  I didnt believe the manifold to firewall ground was THAT critical (as in complete engine shutoff).  I thought it was more of a secondary ground.