LB7: Hard/Rough Starts Cold

[Bdsankey]

I've noticed my LB7 doesn't like to start smoothly when below around 20F. I have put in a brand new GM glow plug controller, brand new GM glow plugs, new GM glow plug "tie bars" (don't know what else to call em) and verified that each glow plug does receive power when the "wait to start" light is on. I have not used an amp clamp to determine what each bank and the intake heater is pulling (intake heater is still in the bridge). It'll start in just about any weather, but it runs like crap for a minute or two until it builds a little heat in the cylinder which leads me to believe its got a weak hole (guessing ring issue). The truck doesn't have a ton of blow by BUT I have only ever checked it warm, not cold (this is the next diagnostic step). It does have 270k miles on it and will be coming out this winter for new bearings in the bottom end (low oil pressure) as well as some oil system upgrades.

My thought is the previous owner (or who knows, before he got it) filled the crankcase with fuel and washed down a cylinder causing ring damage as well as causing bearing damage. The truck does have brand new bosch injectors, new bosch reg on the cp3, shows now fuel or coolant in the oil via an oil analysis.



Anyone ever experienced this issue?

 

[ikeG]

Haven't done a compression check yet?

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[Bdsankey]

Haven't done a compression check yet?

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I have not, I will be once the motor is out to determine if I need to pull the heads off. No matter what it needs some love in the bottom end.

 

[ikeG]

I have not, I will be once the motor is out to determine if I need to pull the heads off. No matter what it needs some love in the bottom end.

The starter sure comes in handy when doing a compression check

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[Bdsankey]

The starter sure comes in handy when doing a compression check

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Those are optional, I like bluetooth starters. I'll check the compression before the motor comes out. It's been a long morning, rear main on my 2013 GMC Terrain decided to blow out this morning pulling into the parking lot due to a frozen PCV valve. GM's TSB only covers them for 10 yrs/120k on this issue so its on me to repair.

 

[baggedLB7]

How about the intake heater?

 

[Bdsankey]

How about the intake heater?

I don’t think it would explain the issue since 04.5+ trucks don’t have them and start great in the cold but I’ll definitely check it to make sure it’s getting power.

 

[DylOrr]

I don’t think it would explain the issue since 04.5+ trucks don’t have them and start great in the cold but I’ll definitely check it to make sure it’s getting power.

Just LLYs don't have them factory, LBZ+ absolutely do, but from experience I've noticed LB7s don't cold start well, and the more tired the engine the worse it is.

 

[Bdsankey]

Just LLYs don't have them factory, LBZ+ absolutely do, but from experience I've noticed LB7s don't cold start well, and the more tired the engine the worse it is.

I realized that after I typed it, been up since 3:30am and ~650mi ago so thanks for catching that. My LLY has been the best starting Duramax out of the 5 we’ve had between my father and I.

I’m wondering if it’s tired or if it’s potentially a glow plug issue where one bank may be getting proper voltage but not enough amperage. I’ve got some diagnostics to do and I’ll report back. I need to voltage check and grab an amp draw rearing from each glow plug bank as well as the intake heater.

It’s higher mileage, don’t get me wrong, but at 271k it doesn’t have a ton of blow-by. I don’t know what compression is so that is another thing I need to check.

 

[baggedLB7]

I had mine turned off in the tune one summer but as soon as it got super cold it would start just fine but it would run a little rough for a bit. Got it turned back on and the roughness went away.

 

[Bdsankey]

I had mine turned off in the tune one summer but as soon as it got super cold it would start just fine but it would run a little rough for a bit. Got it turned back on and the roughness went away.

I’ll reach out to my tuner and see if it’s turned off. Didn’t even think of that. Betting most guys turn them off by default?

 

[baggedLB7]

I dont remember how long it stays on but mine stays on for probably 10 sec after the motor fires.

 

[Bdsankey]

I dont remember how long it stays on but mine stays on for probably 10 sec after the motor fires.

Thanks for the heads up. I didn’t think of them turning it off by default. I’ll report back what they say and if it’s turned off I’ll get it tuned back on and see if it improves.

 

[DylOrr]

I’ll reach out to my tuner and see if it’s turned off. Didn’t even think of that. Betting most guys turn them off by default?

Take the IC tube off and look. It should get red hot I believe.

 

[Bdsankey]

Take the IC tube off and look. It should get red hot I believe.

Once it warms up I will. It’s been pretty chilly before/after work. It’s easier to plug it in vs freeze my fingers off. It looks like it’s supposed to warm up tomorrow through Wednesday and get us out of the single digits/low teens.

 

[Bdsankey]

Update, confirmed with tuner that the tune calls for it to turn on. I will check with a multi-meter tonight after work as well as confirming it does get hot by either pulling CAC tube or by using my IR temp gun on the housing.

 

[Bdsankey]

Checked resistances and voltages tonight once the truck cooled down enough to turn the intake air heater on, below is what my Fluke told me. Based on these resistances and voltage drops to the glow plugs I’m thinking it’s time to rewire those circuits? ~2.7v drop seems quite excessive. My other thought is maybe the controller cannot handle the current demand but that wouldn't necessarily be true since the stud that feeds the glow plugs is higher than glow plug voltage but significantly less than supply voltage.


Voltages:

Intake heater circuit at glow plug controller: 10.6v
Intake heater circuit at heater: 10.56v
At glow plugs (each bank): 8.26v
At stud/lug for glow plugs on the controller 9.6v
Supply: 12.26v key off, 10.9v key on


Resistances:

Driver side (#6) to lug on gp controller: 6.6-7.6ohm
Passenger side (#5 & #7) to lug on gp controller: 7-15ohm
Intake heater circuit to lug on gp controller: 1.5ohm

 

[Bdsankey]

More info, unless I remember incorrectly I have both wires with barrel clips going to the rear terminal. Looking at replacement controllers they have one on each terminal. When I did injectors I just undid the wires at the controller instead of risking breaking the aged plastic. Looks like I have another data point to check out tonight. This is how it was wired from the PO but I had never had it in winter until March 2019 (parked it for quite awhile until I made putting injectors in it a priority then parked it from August to January since I try not to drive it in the salt but it became a necessity).

I do have the intake heater on the front terminal. I'll move the smaller barrel connector wire to the front terminal tonight (based on the images from RockAuto). Truck is a 2002 Federal emissions.

 

[dndj]

Those 2 blue wires are feedback to the ECM. One is used to monitor glow plug voltage, the other is for the IAH voltage, both downstream from the relay. If you had both connected to the same terminal, it seems like the ECM may throw a code?

P0380 for the glow plugs, P0540/P0543 for the IAH.

 

[PureHybrid]

Checked resistances and voltages tonight once the truck cooled down enough to turn the intake air heater on, below is what my Fluke told me. Based on these resistances and voltage drops to the glow plugs I’m thinking it’s time to rewire those circuits? ~2.7v drop seems quite excessive. My other thought is maybe the controller cannot handle the current demand but that wouldn't necessarily be true since the stud that feeds the glow plugs is higher than glow plug voltage but significantly less than supply voltage.


Voltages:

Intake heater circuit at glow plug controller: 10.6v
Intake heater circuit at heater: 10.56v
At glow plugs (each bank): 8.26v
At stud/lug for glow plugs on the controller 9.6v
Supply: 12.26v key off, 10.9v key on


Resistances:

Driver side (#6) to lug on gp controller: 6.6-7.6ohm
Passenger side (#5 & #7) to lug on gp controller: 7-15ohm
Intake heater circuit to lug on gp controller: 1.5ohm

Voltage seems low for just having the key on, as well as voltage to the glow plugs. I agree that the voltage drop across the glow plug wiring seems excessive too