RPM drops while driving?

[ACerf]

I posted something similar to this not long ago, thinking it was a transmission problem. But it appears that is not the case. It happens when I am going about 60+. The RPM drops to idle for a second and then goes back to normal. It does not slow the speed down because it happens so fast. I was wondering if the FCA might be the cause. I am sure that it is some sort of sensor, but which one? I am not getting any codes other then P0633, which I knew that one was already there, and has been for a long time. Anyone have any experience with this problem or any ideas what could be causing this RPM drop? Thanks for any help I can get!

 

[LNaugle]

on 03 trucks you would want to check the apps. may have a bad spot in it. intermitently happens fast enough that computer doesnt recognize you are not pullin your foot off the throttle.

 

[ACerf]

on 03 trucks you would want to check the apps. may have a bad spot in it. intermitently happens fast enough that computer doesnt recognize you are not pullin your foot off the throttle.

Ok, not sure I understand what apps. you are referring to? Is this something that can be fixed by reflashing the computer?

 

[JJPage]

Could be the APPS, It's a sensor under the black plastic shroud on the left front of engine, but first take the EZ off...

APPS:
http://www.dodgecumminsapps.com/

In this thread it will show how to adjust the TV cable after changing the APPS.
https://www.turbodieselregister.com...ions-2003-2007/224368-jerky-transmission.html

Jess

 

[ACerf]

I visited Timbo's website and now I know what APPS is all about. Thanks! I called and spoke to him. He said that it could be the start of a failing APPS. He suggested that I get a OBD II scanner and drive with it and see if I get a Rail Pressure change (FCA) or a Throttle position change (APPS). But if anyone else has any ideas or experience with this problem please let me know, Thanks!

 

[ACerf]

Could be the APPS, It's a sensor under the black plastic shroud on the left front of engine, but first take the EZ off...



APPS:

Timbo APPS



In this thread it will show how to adjust the TV cable after changing the APPS.

https://www.turbodieselregister.com...ions-2003-2007/224368-jerky-transmission.html



Jess

Ok, do you think that this could cause a problem? I have had it on the truck since 2004 and never had a problem. In fact a couple of years ago I took it to Edge and had it checked out and some software updates done. But I guess I could disconnect it and see if the truck still has the problem.

 

[JJPage]

Ok, do you think that this could cause a problem? I have had it on the truck since 2004 and never had a problem. In fact a couple of years ago I took it to Edge and had it checked out and some software updates done. But I guess I could disconnect it and see if the truck still has the problem.

Yes, I believe this(EZ) could be the culprit, takes 2 seconds to disconnect. And it may be in just one of the EZ's levels, might just do it on 2, and not on 3...

 

[HBarlow]

Is engine rpm actually dropping momentarily or is the tachometer dropping with a false reading?

Driving down the highway engine speed cannot instantaneously drop then return. Engine speed could slow back to idle and slowly return to cruise rpm. Its not clear from your description which is the case.

 

[cerberusiam]

Is engine rpm actually dropping momentarily or is the tachometer dropping with a false reading?

What he said. If you aren't loosing speed the rpms are not dropping. If the rpms really dropped that far the truck would nose over noticeably. More likely an electrical glitch in the cluster or possibly CPS.



Since you have an 03 I would be looking at the ECU connectors for possible corrosion or the big connector on the firewall having connection issues.

 

[ACerf]

Is engine rpm actually dropping momentarily or is the tachometer dropping with a false reading?



Driving down the highway engine speed cannot instantaneously drop then return. Engine speed could slow back to idle and slowly return to cruise rpm. Its not clear from your description which is the case.

The drop and return is so fast that it is hard to tell, but it does feel like the truck speed is dropping off for that split second. I can feel the truck buck momentarily. There is no slow back to idle in this case, this is drop to idle then right back up to speed. Checking the connection on the ECM might be a good place to start. I live in Arizona in the winter months and I would not think I could have much chance of corrosion, but anything is possible.

 

[ACerf]

What he said. If you aren't loosing speed the rpms are not dropping. If the rpms really dropped that far the truck would nose over noticeably. More likely an electrical glitch in the cluster or possibly CPS.



Since you have an 03 I would be looking at the ECU connectors for possible corrosion or the big connector on the firewall having connection issues.

I do think it is electrical, but I am sure it is not the cluster. I did hear the the ECU was having corrosion issues. Good I idea, and I will check that out. Thanks!

 

[cerberusiam]

It really osunds like the ECU is shgutting down for a split second like it lost power. That would make the engine buck and the tach drop like a rock then bounce back up. It is entirely possible the APPS is in a dead spot and on the verge of dying but not bad enough the ECU thinks its is bad also.



Intermittent stuff like this is a pain to track down. Good luck with it.

 

[HBarlow]

ACerf,

I don't think it can happen that way. The tach may drop to idle very quickly and as quickly return to cruise rpm but the engine can't do that.

I think you are seeing an electrical gremlin or a tach failure.

The engine simply cannot instantly drop to idle and instantly return to 2000 rpm or whatever cruise rpm is while the truck is moving at highway speed. The Cummins engine doesn't drop from 2000 rpm to idle in an instant or accelerate back to 2000 rpm from idle in an instant either.

 

[JJPage]

I do think it is electrical, but I am sure it is not the cluster. I did hear the the ECU was having corrosion issues. Good I idea, and I will check that out. Thanks!

Have you disconnected the EZ yet? You can go through all these other processes in due time, do the easy/EZ proceedure 1st.

The circuit board in the EZ's is not high quality. Is it acting like you are turning your key off and on very quickly? If so... unhook the EZ.

 

[AEdelheit]

So does the tach actually go to idle rpm or you just see it go down then bounce back up? If it just falls down then up sounds like a bad connection that momentarily kills power to the ECU as others have stated , of it goes to idle then it has to unlock the torque converter aswell , if that's the case and there is a load in the engine it could go to idle pretty damn fast and bounce back, doubt that's the case but possible.

 

[ACerf]

So does the tach actually go to idle rpm or you just see it go down then bounce back up? If it just falls down then up sounds like a bad connection that momentarily kills power to the ECU as others have stated , of it goes to idle then it has to unlock the torque converter aswell , if that's the case and there is a load in the engine it could go to idle pretty damn fast and bounce back, doubt that's the case but possible.

One of the times that it happened, I happened to see the Transmission Temp warning light, light up. That least I think it was the TTW, it happened so fast that I could tell for your. It was on that side of gauges, which also has the Check Engine Warning light in that area. But that only happened once that I know of.

 

[ACerf]

ACerf,



I don't think it can happen that way. The tach may drop to idle very quickly and as quickly return to cruise rpm but the engine can't do that.



I think you are seeing an electrical gremlin or a tach failure.



The engine simply cannot instantly drop to idle and instantly return to 2000 rpm or whatever cruise rpm is while the truck is moving at highway speed. The Cummins engine doesn't drop from 2000 rpm to idle in an instant or accelerate back to 2000 rpm from idle in an instant either.

Okay, and why is that? I have to say that it sure feels like it is doing that, because the truck jerks with that happens.

 

[ACerf]

Have you disconnected the EZ yet? You can go through all these other processes in due time, do the easy/EZ proceedure 1st.



The circuit board in the EZ's is not high quality. Is it acting like you are turning your key off and on very quickly? If so... unhook the EZ.

I will try this the next time I have it out. I don't drive it every day any more. Anything is worth a try, thanks!

 

[AEdelheit]

Hmm I've seen the edge modules go bad but they usually will miss and run really rough, with the engine running I would start wiggling all the conectors and wiring harness, see if you can get it to stumble. And yes the engine could drop rpm that fast

 

[HBarlow]

Okay, and why is that? I have to say that it sure feels like it is doing that, because the truck jerks with that happens.

The jerking is new information you have not previously offered. That is a new symptom, new information, and of course, it changes the diagnosis.

In order for the truck to jerk power is being temporarily interrupted. I don't know why you are experiencing an interruption.