6.5 Diesel Stutter and Defueling

[6.5_Diesel]

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.

[arveetek]

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

[6.5_Diesel]

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.

[arveetek]

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

[Aftermath]

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.

[6.5_Diesel]

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.

[arveetek]

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