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Discussion Starter · #1 ·
Let me first start out by saying thanks for all of you that take the time out and help total strangers. I bought this Truck last year and didn't have any real problems, aside from ones that could've been avoided until Probably 4 months ago. I've had three Mechanics look at it with the third and most recent one being a fleet diesel manager also part of this forum and is a great Mechanic. All have been stumped and all are having trouble even finding a term that best fits what it's doing. Basically when I start it up it will idol fine for awhile and then will do small cut outs or hiccups randomly. When you dig into the pedal over 2,000 RPMS the hiccups become violent as if there is an ignition interruption for a second and keeps doing it until you let off. Not fuel surge symptoms they said. It does it at at steady speed without shifting and even when I manually switch gears. I don't think its transmission related because it does it seconds before a shift, steady speed and at idol, just not as violent. I'm no expert so if it does sound like transmission, please say. I also have no codes. The most recent Mechanic hooked it up to a JPRO and revved it up to watch Rail Pressure and many other things. I filmed it in case anyone wants to watch it and see if it helps better explain the graph and what he was testing. He said Fuel Rail Pressure was fine, voltage was ok and no huge red flags, but we didn't take it out for a spin to graph when it does the violent hiccups, just revved. No fuel or vacuum leaks. Their best bets are ECM related or a wire somewhere that is barely working. This might not be related but I can hear my BCM near my fuse box clicking for the first 5 minutes of starting it. below I'll try and list everything we've done to it and information that might be needed. I'm so frustrated and really need this fixed so it's safe for my wife 4 year old and new one on the way. Again I have a few videos if need be. Thank you.

-Things done to the truck and Important info-
-Truck is a 2010 3500 LMM 6.6 4x4 automatic Duramax long bed 4 door with 230k miles with an engine swap that is around 100k on engine.
  • 5 stage switch with EFI LIVE, DPF EGR Delete and block plate installed. (No Monitor screen).
  • Installed Fuel Rail Plug and PCV reroute.
  • Maf Sensor, Air Filter cleaned.
I'm sure there is a few other things missing but that's pretty much everything I can think of.
- New crankshaft Position Sensor, fuel filter and head assembly.
 

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2017, 2500HD, LTZ, front guard, Duraflap mudguards, Leer bed topper
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I am sure you and the folks who have checked the truck have done due diligence and are trying to help… so, bare with me..

So.. on “fly by wire” systems, the accelerator pedal input is “read” by the ECM and the actual “throttle” control device opens as much as the ECM tells it to. The FICM must be in lock step as well. Well, if there is a “glitch” in wiring between the accelerator position sensor, ECM or throttle actuator or FICM, you might see the problem you describe.

I have seen electronics like the ECM cause erratic behavior… and because the problem you describe is so “consistent”, it could be many different things as alluded to above.

A possible “clue” is the BCM you mention… which BCM do you describe makes the noise? Does the noise correlate with the hiccup problem?

it seems there have been a rash of battery issues lately… how old are your batteries? All terminals clean? Good charge? If you connect jumper cables to another running truck, does problem continue? (This would help isolate an electrical power issue)

On vehicles with “cable” throttle, there are sensors on the accelerator and throttle plates… which the ECM reads and puts fuel in to try to achieve stoichometric combustion… based upon air pressure, volume, temp, etc.

There could be a “flutter” in the turbo control vanes..
that is also controlled by ECM and related sensors.

But they all must have proper power from batteries… checking all body grounds and reseating them, especially since there was an engine “swap”…

Wires can corrode internally and you may never notice until it breaks completely.. shake them while checking continuity…
 

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Discussion Starter · #3 ·
I am sure you and the folks who have checked the truck have done due diligence and are trying to help… so, bare with me..

So.. on “fly by wire” systems, the accelerator pedal input is “read” by the ECM and the actual “throttle” control device opens as much as the ECM tells it to. The FICM must be in lock step as well. Well, if there is a “glitch” in wiring between the accelerator position sensor, ECM or throttle actuator or FICM, you might see the problem you describe.

I have seen electronics like the ECM cause erratic behavior… and because the problem you describe is so “consistent”, it could be many different things as alluded to above.

A possible “clue” is the BCM you mention… which BCM do you describe makes the noise? Does the noise correlate with the hiccup problem?

it seems there have been a rash of battery issues lately… how old are your batteries? All terminals clean? Good charge? If you connect jumper cables to another running truck, does problem continue? (This would help isolate an electrical power issue)

On vehicles with “cable” throttle, there are sensors on the accelerator and throttle plates… which the ECM reads and puts fuel in to try to achieve stoichometric combustion… based upon air pressure, volume, temp, etc.

There could be a “flutter” in the turbo control vanes..
that is also controlled by ECM and related sensors.

But they all must have proper power from batteries… checking all body grounds and reseating them, especially since there was an engine “swap”…

Wires can corrode internally and you may never notice until it breaks completely.. shake them while checking continuity…


Thank you very much for that great information. I will pass this info to my mechanic.
BCM doesn't seem to affect the gauges or change the "cutting out" any. Both batteries are 3 years old and seem to be ok. Terminals are clean. I'll do the shake test again tomorrow. I will also check wiring around accelerator position sensor.
 

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2017, 2500HD, LTZ, front guard, Duraflap mudguards, Leer bed topper
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Thank you very much for that great information. I will pass this info to my mechanic.
BCM doesn't seem to affect the gauges or change the "cutting out" any. Both batteries are 3 years old and seem to be ok. Terminals are clean. I'll do the shake test again tomorrow. I will also check wiring around accelerator position sensor.
so.. 4 years on my batteries.. things “didn’t seem right”.. high charge voltage which remained high instead of settling to “normal”.. little throttle response change… shift not as good…

New batteries and things got better… start speed, throttle.. shift.. then cleaned TMAP sensor..truck runs better than new…

while your truck isn’t an L5P, it must have good power…
 

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Check your grounds! There are numerous places where the wiring is grounded and they have been a problem on this series of trucks. There are many of them to the frame chassis and on the block of the engine too. There is also a big fuse block that sits next to the passenger side. Pull the cover on it and check those connections also. There have also been GM service letters that went out on the LMM about the wiring harness being too short and shorting out as they go around the engine. This could also cause your issue. Dean
 

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Discussion Starter · #6 ·
Check your grounds! There are numerous places where the wiring is grounded and they have been a problem on this series of trucks. There are many of them to the frame chassis and on the block of the engine too. There is also a big fuse block that sits next to the passenger side. Pull the cover on it and check those connections also. There have also been GM service letters that went out on the LMM about the wiring harness being too short and shorting out as they go around the engine. This could also cause your issue. Dean

I will do some more ground hunting and will check the passenger fuse block as well today. I do trust everyones opinions so if everyone thinks I should replace my batteries, I shall. They are almost 4 years old so they're getting up there.
 

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I will do some more ground hunting and will check the passenger fuse block as well today. I do trust everyones opinions so if everyone thinks I should replace my batteries, I shall. They are almost 4 years old so they're getting up there.
Honestly if your batteries are strong enough to start your diesel without dragging the starter down much then they should not be your issue. New batteries will mask a weak wiring connection. The main battery grounds on the block are located at the bottom front corners of the diesel block. There are also some other main wiring grounds there also. Drop your wheel well liners down and you will easily see the grounds I am talking about. Also the ground that goes from the block to the chassi is not easy to get to. I fabricated a new ground that tied into the bolts of the accessories on the drivers side block and ran it to one of the main chassi grounds on the firewall on the same side.

As for the engine wiring harness, I would wiggle the harness with the engine running and see if you feel any difference in the way the engine runs or if it throws a code. Dean
 

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Discussion Starter · #9 ·
Honestly if your batteries are strong enough to start your diesel without dragging the starter down much then they should not be your issue. New batteries will mask a weak wiring connection. The main battery grounds on the block are located at the bottom front corners of the diesel block. There are also some other main wiring grounds there also. Drop your wheel well liners down and you will easily see the grounds I am talking about. Also the ground that goes from the block to the chassi is not easy to get to. I fabricated a new ground that tied into the bolts of the accessories on the drivers side block and ran it to one of the main chassi grounds on the firewall on the same side.

As for the engine wiring harness, I would wiggle the harness with the engine running and see if you feel any difference in the way the engine runs or if it throws a code. Dean
So I did get my batteries tested and also checked voltage and everything on the JPro and no problem there. Checked all harnesses I could find and wiggled them, thought I may have found a few wires coming from the BCM under steering wheel area to make a hiccup but couldn’t get it to repeat. I did notice it’s only a small hiccup when idling or revving but when actually driving and getting past 2,000 rpm to top end that’s when the violent cut outs are. Put it in manual and made sure it wasn’t transmission related kept it in a certain gear but it still does it hard on
top end every time I go above 2,000 and violently. Still not codes, that may also be from EFI LIVE not telling certain codes to go off.
 
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