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Duramax 6600 Discussion Forum for the Duramax 6600 Diesel Engine, including the LB7, LLY, LBZ & LMM engine specific topics.

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  #1  
Old 12-09-2008, 05:06 AM
LoClass LoClass is offline
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Default LB7 hard/rough starts in cold weather

Recently I've been having harder starts with my '03 LB7 in colder weather (20* and under). A little background on my truck: '03 Silverado LB7, 110,000 miles, Federal emissions. Injectors were replaced at 74,000 miles when the turbo went out (failed internal oil seal).

In the past a normal cold start (no block heater) down to about 0*, after key on, the glow plug light would go off after about 6-8 seconds and the truck would start with no issues. Indicated voltage would be in the 11-12V range for 30 seconds or less and then jump to 14-15V, with a noticable increase in interior light brightness when the voltage increased. I believe that jump is due to the glow plugs/intake air heater being turned off.

Anyway, in the past few weeks winter has set in here in Wisconsin and I've had a few cold starts in the 10* to 20* range, no block heater, where the truck would start, but run rough for a few seconds and then smooth out. Over the weekend I intentionally parked it outside overnight when it got down to 0* with no block heater. When I tried to start it the next morning, it cranked for a few seconds longer than normal and when it fired up, it bucked hard for a few seconds before the engine smoothed out, like it wasn't hitting on all 8. The other thing I'm noticing now is that when I perform a cold start I don't get the step up in voltage that I used to get. As soon as the engine starts, the volt meter steadily ramps up to 14-15v within a couple seconds.

The truck has not given me an SES light yet and the wait to start light functions as it always has. Based on the research I've done on the forum, this sounds glow plug related to me. I haven't done any testing yet, but I plan to check to see if I'm getting any voltage to the glow plugs at a cold start. I'm suspecting the glow plug controller may be bad, but that's only a guess. Based on what others have experienced, does this sound like the place to start?

Thanks
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Old 12-09-2008, 09:41 AM
DmaxMaverick DmaxMaverick is offline
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In any case, check for stored DTC's first. It may be very simple.

Check your fuel condition. Are you using winter fuel? Or straight #2 with an additive. If none of the above, suspect fuel waxing/gelling. Be sure to use an additive for winter conditions, at the correct concentration for the temperature you are seeing (according to the label), and/or winterized fuel rated for the temperature. If fuel is not an issue, read on.....

Sounds like some failed glow plugs and/or failed glow plug controller. Could also be some air intrusion. If the glow plug cycle seems normal, but the voltage draw seems less than previous, it's likely some failed plugs (or failed intake heater). If the voltage draw seems normal, but the WTS lamp is shorter than previous, or flaky, it could be the relay. Either way, it is simple to diagnose, and not an expensive repair. Go right to the horse's mouth and check the glow plugs. Just remove the bus bar (connects to the GP lugs) and check them for continuity with a VOM or test light. Any GP lugs not short to ground indicate a bad plug. You don't have to remove any plugs that indicate good, as it's likely they are heating well.

Option B is to just replace the GP controller and the plugs. All can be had for less than $150, including necessary cold beverages. Should take less than an hour or two, the first time, including periods of refreshment. After replacement, test and keep good plugs for future backup. Also, keep the controller, as the relays are replaceable, and should be compatible with the new controller. Do not buy an individual relay. They are more expensive than a complete controller.
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Old 12-09-2008, 11:51 AM
LoClass LoClass is offline
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I don't believe it is a fuel related issue as I'm using winter blend fuel along with an additive. I've had instances in the last 2 winters of gelling in extreme cold even with additives, so I've had first hand experience with cold weather fuel issues.

I plan to get at it this weekend when I'll have some time to drop the wheel liners and test the plugs. I'll have to stop by the local Autozone and see if they'll scan for stored codes. My only concern with testing and/or replacing the plugs is corrosion and possibly breaking one off. I've owned the truck since new and it has 6 Wisconsin winters, so it has seen its fair share of road salt. If I do have to remove plugs, should it be done on a warm or a cold engine and would soaking them with a penetrating oil (like PB Blaster) be of any value?

Also, where is the air intake heater located ? I'll test that as well.

Thanks.
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Old 12-09-2008, 12:56 PM
DmaxMaverick DmaxMaverick is offline
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The intake heater is located in the pipe just before the intake plenum (before it enters the heads), under the plastic engine cover with the Duramax label. It'll be about a 1" dia. plug with a large wire attached to it's lug (looks like a huge glow plug). It can be tested the same as the GP's.

Verify you have batt voltage at the GP's and heater during the WTS cycle. If not, the controller needs to be replaced. This is more likely your problem (not too uncommon). Several GP's have to fail before starting is noticeably harder. A failed controller means no GP's at all. A couple years ago, I tested a few no-glow/heat starts. Temp was about -10F and I disconnected the GP's and heater. It started about like what you describe. It did start, however, which is impressive. The heater didn't seam to make a difference at all, enabled or not. Anymore, I hardly WTS. If it's above freezing, I turn the key from off to start, and it starts the same with or without waiting. Below freezing, it becomes noticeable, depending on the temp (I wait, if the temp is below about 35). The intake heater is not necessary for a start except during extreme cold. It's more of something to make the EPA happy, IMO. If you are starting at 20 below, it's probably helpful.

Don't sweat the GP corrosion just yet. You don't need to remove them for testing. Just remove the connectors and test the GP lug for continuity to ground. If using a test light, clamp the wire to batt+ and probe the lug. Light on = likely good plug. No light = definitely bad plug. Corrosion can be an issue, but should only apply to bad plugs (you don't have to remove good plugs). If you remove a couple and corrosion is present, I suggest removal of all of them, and reinstall with some anti-seize. They may not be corroded, yet, and now is as good a time as any to prevent it. This should be an early maintenance procedure, especially for trucks used in road salt and coastal areas. Take your time and don't force any of them, and it should be OK. If they are stuck, soaking them with PB Blaster may help.
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Old 12-12-2008, 12:20 PM
LoClass LoClass is offline
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I was able to spend some time on the truck this morning and found the root of my problem. The buss bar on the left (driver's) side of the engine was rusted through between the #1 and #2 glow plugs (the two closest to the firewall). That's why it acted like none of the glow plugs were working. Only 1 of 8 was getting power.

I tested all 8 glow plugs and #1 (left side, closest to firewall) was bad. I'm not sure if it's coincidental that the only one getting voltage was also the only dead plug. I find it strange that the computer didn't set off the SES or set a DTC. I also checked to see that I was getting voltage from the controller at key on and it is, so I believe that a new glow plug and new buss bars (I'll replace both while I'm at it) and I'll be back in business.

I was able to pick up a glow plug locally and managed to replace it without snapping the original off. I soaked it for quite a while with PB Blaster and it came out pretty easy. I have to wait on buss bars since the local dealer is out and my wife is picking up a set from the dealer in the city where she works.
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Old 12-12-2008, 12:52 PM
DmaxMaverick DmaxMaverick is offline
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Glad to hear you figured it out. Simple enough.

The Federal Emission engines have a "dumb" GP system. CA/NE Emissions PCM's monitor GP voltage, and have a cow if only one fails (or isn't connected, like in your case). CA engines have each GP wired individually from the controller (no buss bar).

I'm not surprised the #8 plug failed (cylinders are 2-4-6-8 on the driver side, starting at the front). It was getting a full cycle with more juice than it should have, with other plugs unable to share the load. You got out of this one pretty easy.
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Old 12-22-2008, 12:14 PM
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NeilLB7 NeilLB7 is offline
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Wouldn't they normally all just get +12.5V each from the buss bar? Each one draws some amperage "x"...and thats about it. The fact that only one was hooked up shouldn't mean that its going to get more amperage or voltage. It draws what it needs. Nothings being forced upon it.

Unless the Dmax glowplugs are different than what I'm used to on other diesels.
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