Page 1 of 2 12 LastLast
Results 1 to 20 of 36

Thread: 6.5 Diesel Stutter and Defueling

  1. #1
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default 6.5 Diesel Stutter and Defueling

    So I am an active member on Duramax forum and have posted this issue on that site with no suggestions. The truck in question is a 94 6.5 TD 2500 4x4 5 speed (4:10 gears) with an F VIN engine. The truck has 145,000 miles on it and I have installed a Heath Diesel PMD relocate about 6 years ago, a cold air intake, 4" straight pipe exhaust, new injectors at 108,000 and new glow plugs. I also installed a new lift pump about 6 years ago as well.

    I have 2 issues with the truck that I have been trying to find an answer for:

    1. When I am driving down the road at cruising speed it will occasionally stutter, almost like a quick miss in the motor. this is a split second event just enough to feel the stutter but no dash lights and the gauges don't move when it happens. It doesn't do it all the time only occasionally. I took a 350 mile trip with it not too long ago and I would say it did this about 6 times on the whole trip. Another thing to note is that when the truck is cold started (50F or below), it will smoke and misses like crazy for about 10 seconds. After that, it runs idles flawless and it starts easily every time after.

    2. Issue number 2 is especially noticeable when pulling a trailer. What happens is when I hit a hill and I give it more pedal my boost will pick up but then it acts like it defuels then boost goes to almost zero then back up to 10 then back to zero and on and on. I have wired the wastegate shut thinking it was a vacuum issue, but it still does it which makes me believe it is defueling. I also found out that at 2500 rpm the pump will naturally defuel for some odd reason, but this boost surge happens at rpms below 2500 as well. I have read online that it could be a multitude of different things from AI temp sensor, boost sensor, PMD issue, and so on, but I was hoping someone could give me a really good starting point.

    Any starting points would be greatly appreciated.

  2. #2
    Join Date
    Oct 2001
    Location
    Granby, Missouri, USA
    Posts
    3,081

    Default

    Welcome!

    1. Some folks call this "fishbite" and is a common occurrence. I have not found a cause or solution. Mine used to do it, but now that I think about it, it hasn't happened for quite some time now. It may have disappeared when I replaced my IP a couple of years ago. Your smoke and miss at cold start is probably not related, but is most likely due to a faulty glow plug or two, or perhaps slightly low compression in one or two cylinders.

    2. Your ECM is most likely defueling due to a lack of boost control. The ECM is very sensitive to boost, and tries to control it through the vacuum-operated wastegate control system. Wiring the wastegate shut will make the problem worse, since the ECM can't control the boost, and will then go into Limp mode. There are a few ways around this issue. The first way is to make sure the entire boost control system is functioning properly: good vacuum from the vacuum pump, good wastegate solenoid valve, and no leaks in any of the vacuum lines. What I did with mine was ditch the vacuum pump, installed a Turbomaster manual wastegate control, and installed a chip into the ECM from Kennedy Diesel to keep it from going into Limp mode. Works awesome now.

    Casey
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

  3. #3
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    Quote Originally Posted by arveetek View Post
    Welcome!

    1. Some folks call this "fishbite" and is a common occurrence. I have not found a cause or solution. Mine used to do it, but now that I think about it, it hasn't happened for quite some time now. It may have disappeared when I replaced my IP a couple of years ago. Your smoke and miss at cold start is probably not related, but is most likely due to a faulty glow plug or two, or perhaps slightly low compression in one or two cylinders.

    2. Your ECM is most likely defueling due to a lack of boost control. The ECM is very sensitive to boost, and tries to control it through the vacuum-operated wastegate control system. Wiring the wastegate shut will make the problem worse, since the ECM can't control the boost, and will then go into Limp mode. There are a few ways around this issue. The first way is to make sure the entire boost control system is functioning properly: good vacuum from the vacuum pump, good wastegate solenoid valve, and no leaks in any of the vacuum lines. What I did with mine was ditch the vacuum pump, installed a Turbomaster manual wastegate control, and installed a chip into the ECM from Kennedy Diesel to keep it from going into Limp mode. Works awesome now.

    Casey
    Thanks for the reply.

    1. I am leaning toward the missing issue on start is due to low compression in a cylinder but have not verified this. I installed new glow plugs because of this issue and a new glowplug controller and tested each glowplug before installing them and they all worked but did not solve my issue on starts. I do have the original IP from 94 still in the truck, so maybe that could be some of the issue with the miss. It also is a bugger to start once it is below 15F which when I bought it with 72k on the clock it would start without being plugged in at -10F.

    2. I am wondering if my whole vacuum system isn't starting to show signs of aging from what you are saying. I haven't checked the vacuum at the wastegate actuator, but I was always thinking of going to a turbomaster. The weird thing is that when I am empty and I get on it climbing a hill it will overboost initially (causing the truck to defuel) then it will build back up to 7PSI and hold steady until I hit 2500 RPM and then I hit the defueling point (this is with the wastegate wired shut). But when I get a trailer on the back it overboosts and then enters a vicious cycle of defueling and refueling to the point where I have to downshift because I cannot maintain power.

  4. #4
    Join Date
    Oct 2001
    Location
    Granby, Missouri, USA
    Posts
    3,081

    Default

    Quote Originally Posted by 6.5_Diesel View Post
    Thanks for the reply.

    But when I get a trailer on the back it overboosts and then enters a vicious cycle of defueling and refueling to the point where I have to downshift because I cannot maintain power.
    When my '95 would defuel, it would stay in that mode until you turned the key off and restarted. I believe the 1994 models are a bit unique and have a little different programming compared to later models. If you have the original IP, that is definitely a big suspect.

    Just be sure to realize that a Turbomaster by itself won't solve anything without changing the programming in the ECM to accept the new boost levels that it can't control. That's why I had to install the chip from Kennedy Diesel into my ECM.

    Casey
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

  5. #5
    Join Date
    May 2014
    Location
    us, ga
    Posts
    166

    Default

    6.5_Diesel
    I had a similar problem where every once in a while my rig would stumble as if lost fire then immediately lit again. Over about a 6 month period, it happened more and more. Ended up being the PMD. The devil lives in all PMD'S by the way.

  6. #6
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    Quote Originally Posted by arveetek View Post
    When my '95 would defuel, it would stay in that mode until you turned the key off and restarted. I believe the 1994 models are a bit unique and have a little different programming compared to later models. If you have the original IP, that is definitely a big suspect.

    Just be sure to realize that a Turbomaster by itself won't solve anything without changing the programming in the ECM to accept the new boost levels that it can't control. That's why I had to install the chip from Kennedy Diesel into my ECM.

    Casey
    If I only set the TurboMaster to allow 7-8 psi max, would I still have an issue with boost control? I could understand having an issue if I say set it at 10psi but if I stay within the computers limit of 7-8 psi would I be ok or would I still need a reprogram?

    I think I may order a PMD and see if that helps the "fishbite" beings they are $130. Would you recommend getting the No.9 resistor? If that doesn't fix the issue I am guessing it may be a pump problem beings it is the original yet.

  7. #7
    Join Date
    Oct 2001
    Location
    Granby, Missouri, USA
    Posts
    3,081

    Default

    Quote Originally Posted by 6.5_Diesel View Post
    If I only set the TurboMaster to allow 7-8 psi max, would I still have an issue with boost control? I could understand having an issue if I say set it at 10psi but if I stay within the computers limit of 7-8 psi would I be ok or would I still need a reprogram?

    I think I may order a PMD and see if that helps the "fishbite" beings they are $130. Would you recommend getting the No.9 resistor? If that doesn't fix the issue I am guessing it may be a pump problem beings it is the original yet.
    It will still probably defuel. It's not really the amount of boost, but the fact that the ECM expects to control the boost over a certain RPM, throttle input, speed, etc. Once those parameters are out of spec, and the ECM realizes it cannot control boost, it goes into limp mode. I tried setting my Turbomaster to low PSI, but every time I towed or got on the throttle, she would go right to limp mode.

    Yes, I've got the # 9 resistor. I prefer and recommend the Dtech PMD:

    http://www.dipacodtech.com/All-Produ...New/DT19209057

    I've had real good luck with the Dtech.

    Casey
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

  8. #8
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    Quote Originally Posted by arveetek View Post
    It will still probably defuel. It's not really the amount of boost, but the fact that the ECM expects to control the boost over a certain RPM, throttle input, speed, etc. Once those parameters are out of spec, and the ECM realizes it cannot control boost, it goes into limp mode. I tried setting my Turbomaster to low PSI, but every time I towed or got on the throttle, she would go right to limp mode.

    Yes, I've got the # 9 resistor. I prefer and recommend the Dtech PMD:

    http://www.dipacodtech.com/All-Produ...New/DT19209057

    I've had real good luck with the Dtech.

    Casey
    Any good or bad reviews on the PMDs that SS Diesel sells?

  9. #9
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    So a little update on the truck. I tested the AIT sensor and it ohms out correct. I also realized that when I plumbed in my boost gauge I put a tee coming out the top of the manifold (one side is the AIT sensor and the other is boost) and I am thinking this may be causing heat build up due to no air circulation over the resistor. Another thought is that my CAI is pulling in too hot of air under the hood, especially when towing, causing to high of intake temps. Anyone have insight as to if these items could cause defueling?

  10. #10
    Join Date
    Jan 2001
    Location
    Knoxville,Tennessee
    Posts
    2,639

    Default

    Is your CAI open to under hood heat? Or is it enclosed and getting out side air via the snorkel in the fender?
    "The Constitution is not an instrument for the government to restrain the people, it is an instrument for the people to restrain the government."
    -Patrick Henry


    A5150nut
    2006 K3500 D/A
    94 6.5 4x4 5spd Sold

  11. #11
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    Quote Originally Posted by a5150nut View Post
    Is your CAI open to under hood heat? Or is it enclosed and getting out side air via the snorkel in the fender?
    My CAI is not a box type and pulls all its air from under the hood. I am getting to the point where I am really digging into this because it is such a pain when pulling a trailer. I am getting my hands on a vacuum tester today to test the vacuum pump. I was told 15+ inches HG at the actuator is what I should have. I also ordered a new boost sensor and boost solenoid that should be here this week ($60 so I figured what the hell the ones on the truck are the originals).

    If the pump tests good and I put a new boost sensor and boost solenoid in and the problem is still there, I am guessing it either has to be my CAI or how I moved the AIT sensor when I installed my boost gauge. When I installed my boost gauge I pulled out the AIT sensor and put a Tee fitting in its place and stuck the AIT sensor in one side and the boost gauge in the other. This may be causing a hot spot due to the brass fittings and the sensor not being directly in the intake air stream.

  12. #12
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    First, pull any DTC's that may be present. A paper clip and the key is all you need for that. If it's defueling, code(s) are likely present and the SES lamp should be on, if only during the event.

    I see a couple issues. Your "cold" air intake is not cold. It's pulling hot engine compartment air. Your IAT sensor is not sampling intake air. It is sampling engine compartment air temperature and radiant heat. The sensor must be IN the intake air to sample it. It's likely the sensor is being heat-soaked from the Tee, which is being heat-soaked by the engine compartment. Either of the issues could cause the problem you are having. Both together are nearly a certainty.

    Restore your IAT sensor to original and relocate your boost gage fitting. Fix your "cold" air intake plumbing so it is actually pulling air not from the engine compartment, or restore it to original.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  13. #13
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    Quote Originally Posted by DmaxMaverick View Post
    First, pull any DTC's that may be present. A paper clip and the key is all you need for that. If it's defueling, code(s) are likely present and the SES lamp should be on, if only during the event.

    I see a couple issues. Your "cold" air intake is not cold. It's pulling hot engine compartment air. Your IAT sensor is not sampling intake air. It is sampling engine compartment air temperature and radiant heat. The sensor must be IN the intake air to sample it. It's likely the sensor is being heat-soaked from the Tee, which is being heat-soaked by the engine compartment. Either of the issues could cause the problem you are having. Both together are nearly a certainty.

    Restore your IAT sensor to original and relocate your boost gage fitting. Fix your "cold" air intake plumbing so it is actually pulling air not from the engine compartment, or restore it to original.
    Thanks for the suggestions. I was looking at ordering a boost bolt to relocate my boost gauge and returning the AIT sensor back to original. I still do have the original air box for the truck, so I may just go back to original, but I was wanting to see if I could remedy the issue without it.

  14. #14
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    I suggest addressing only one thing at a time, starting with the sensor. I suspect that will cure most of the problem. Once the issue is resolved, then do something to get less hot air into the intake.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  15. #15
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    Quote Originally Posted by DmaxMaverick View Post
    I suggest addressing only one thing at a time, starting with the sensor. I suspect that will cure most of the problem. Once the issue is resolved, then do something to get less hot air into the intake.
    I am going to vacuum test tonight just to make sure my pump and solenoid are up to snuff. I just ordered a boost bolt for my boost gauge so that I don't have to drill and tap into the manifold (just more convenient), so when that gets here I will install it. I will hook the trailer up and see if moving the AIT sensor helps. If not I will move onto the air intake then onto the boost sensor and wastegate actuator.

  16. #16
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    So I tested the vacuum pump last night and I got about 23" Hg from the line off of the pump and about 17" Hg at the wastegate actuator. One thing I did notice is as I was unplugging the hoses from the boost solenoid I was holding the solenoid in my had and it was vibrating like crazy and no it was not due to the engine because I removed the solenoid from the mount. Does anyone know if it is supposed to do that? It is almost like a valve internally is opening and closing very rapidly.

    I also reinstalled the AIT sensor back into its original location and ditched the Tee fitting I had to add my boost gauge. I should have my boost bolt Friday, so I will put that in and take it for a drive.

  17. #17
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    The unplugged, vibrating solenoid is normal. By disconnecting the vacuum lines, the ECM went into hyper-mode trying to modulate the vacuum, which it obviously couldn't. It likely set an according DTC. The SES lamp may have come on as well.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  18. #18
    Join Date
    Mar 2017
    Location
    ND
    Posts
    20

    Default

    Quote Originally Posted by DmaxMaverick View Post
    The unplugged, vibrating solenoid is normal. By disconnecting the vacuum lines, the ECM went into hyper-mode trying to modulate the vacuum, which it obviously couldn't. It likely set an according DTC. The SES lamp may have come on as well.
    The solenoid was doing the vibrating while plugged in idling too, but when I pulled vacuum at the actuator it was steady at about 17" Hg. Is it supposed to vibrate when hooked up and at idle? I would think if there was an issue I would have seen it on the vacuum gauge at the actuator.

    My boost bolt and new boost solenoid and boost sensor will be here today, so I am going to get my boost gauge plumbed in and take it for a spin.

  19. #19
    Join Date
    Jan 2001
    Location
    New Hampshire - Live Free or Die
    Posts
    6,057

    Default

    Don't know if this has anything to do with your problem, but, the two ports on the solenoid are not interchangeable. If you connect it up backwards it will not work correctly.

    One port has a restriction, visible when looking in to it. Unfortunately, I forget which way it needs to be connected. I recall, (I think), that the idle vacuum on the wastegate actuator should be about 11 inches. (I'm thinking the port with the restriction goes to the pump, but could be wrong; sold my '95 12 years ago...)
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC, John Deere X758
    New ride: MB GLS450 - most stately
    Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510

  20. #20
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    Quote Originally Posted by 6.5_Diesel View Post
    The solenoid was doing the vibrating while plugged in idling too, but when I pulled vacuum at the actuator it was steady at about 17" Hg. Is it supposed to vibrate when hooked up and at idle? I would think if there was an issue I would have seen it on the vacuum gauge at the actuator.

    My boost bolt and new boost solenoid and boost sensor will be here today, so I am going to get my boost gauge plumbed in and take it for a spin.
    The vibration is normal. ECM boost control is not on/off, but is modulated and will allow wastegate opening at a gradual rate. 17" is a little high, but within the margin of error. 15" is ideal at sea level at any "full closed" RPM, with 11" the minimum. Less than ideal variations could be a matter of a well seasoned solenoid, or aftermarket programming adjustments. If you have an aftermarket power program, it wouldn't be unusual to have increased wastegate actuator vacuum, as a means to compensate for increased exhaust back pressure against the wastegate.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •