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The Hummer Knowledge Base

Starting the Diesel in Cold Weather


When your engine starts cold, is it rough running? Do you spew alot of white smoke? If so you may have a bad Cold Advance Temperature switch. Once I replaced mine (about $20.00) Lucy fires off immediately, with no hesitation (minimal battery suckage)...


My cold advance switch went bad also. This is really easy to test. The switch should set when the engine temp is below 75deg F. Unfortunately you really can't access the switch from the top but you can check the voltage on the injector advance or the cold start advance solenoids. The cold start solenoid is on top of the injector pump and very accessible. Just turn the keyswitch to on and you should see 12V at the plug on the solenoid.


I have started mine also in 10 degree weather w/o being plugged in without a problem. For me, I turn the key to on then depress the pedal once and release. This sets the cold advance. Then I crank and it fires right up into the fast idle. If I don't depress the pedal and release it, it struggles to start. The cold advance solenoid is not strong enough to move the linkage, it can only hold it open. Mine is a 95 so I am not sure if this changed with the DB4 pump in the 96+ models. Anyway, if the beast does not fast idle, then your temp switch is bad (like others have mentioned) or the solenoid. You can also depress the throttle slightly while cranking to see if that does anything (manual fast idle/cold advance). If fast idle is working, then it might be the glow plugs (I am assuming that the wait light is staying on like it should). When everything is warm, make sure there is no water in the tank or lines. Drain the water separator.

Hope this helps.


Keri and others.......

On mine ( 95 NA diesel ) the cold start solenoid doesn't 'automatically' kick in. I've tested it by applying 12 volts to doesn't extend the plunger down by itself, but, if I manually extend the plunger (by hand, if I'm testing under the hood, or, by foot, if I'm in the cab starting it..) the solenoid holds the plunger out as intended.

My starting procedure: Turn key to 'on' position, have right foot ready, when 'Wait' light goes out I then manually depress the pedal and release, then crank her over. Most of the time this works just fine. Occasionally it doesn't fire up in fast idle mode ....but still fires up.

I haven't experienced any really cold weather since I've owned the beast (since '96), plus it stays in a garage here at the house.

Something I've noticed though, and it bothers me a bit......I seem to have a small, slow drain on the batteries. The batteries are fairly new....about a year old now. I have a digital voltmeter in line in the system. Each night they drain down to about 11 to 11.5 volts. Sometimes, during the first start of the day, while the glow plugs are doing their thing, the voltage will pull down to as low as 8 volts but the beast still cranks, although not easily. (I can imagine the current draw at that low voltage!)

I believe that if I were in a cold area, or the beast were parked outside, that it would take even longer for the glow plugs to work, thus, drawing the available voltage down even lower and I'd just be SOL.

While running though, the alternator seems to be doing it's job.....the digital voltmeter always shows at least 13.X volts.....sometimes as much as 14+ volts. As long as the truck is started at least once daily, there is never any problem starting for the remainder of the day.

I've yet to find the reason for the slow drain at night.......I've got several misc. electrical items installed but I can disconnect them entirely and still have the same problem.

Rick Crider
Monroe NC
95 diesel wagon

The electrical cold start advance and fast idle solenoids are on the same circuit. That is to say the switch supplies voltage to both solenoids in parallel. The cold start switch supplies voltage when the block temp is below 75deg F.

You do have to "tap" the accelerator pedal to allow the fast idle solenoid to set. The cold start advance will just work.

You can easily test either solenoid by applying voltage. The fast idle solenoid when set should hold approx. 800rpm. When the cold start advance is set there is an audible clatter in the engine sound.


Rick explained it well. The fast idle solenoid does not have enough force to actuate the linkage itself, but will prevent it from returning all the way down. As soon as you turn the key, the solenoid will energize (if the temp switch turns on). So by doing the Stomp, you allow the solenoid to extend and it holds the linkage up.

Now as far as cold advance and fast idle, I don't have the book in front of me. What does happen when you depress the pedal is that the fuel injection does advance. So by just having the "throttle" open a little, you are advancing the timing. That's why I was asking Kevin if he tried this. I don't remember a separate cold advance solenoid. I thought power went through the temp switch to the (multipurpose) solenoid. I'll have to double check.

When cold, my truck makes a puff. It is not a cloud, just a puff. Keri, you have a 95 if I remember right. I never saw the puff in my truck b/c the tailpipe is way in the corner facing down. Once it was moved to a side exit behind the driver rear wheel, I can clearly watch it when starting via the mirror. Nothing changed in the way it started so it must have done it all the time. I have seen others do it also so I too feel a small puff is quite normal. When it is really cold (20 deg) it will make a larger puff unless I double cycle the plugs.


Gerald and others, some clarification and observations.

When cold, two things happen. Both controlled by the temp switch (which is in the back of the passenger side head and accessible easily by removing the engine cover or not so easily from under the truck). One is the fast idle solenoid which is on top of the injection pump. This solenoid just gives a little more fuel and you can do the same thing by stepping on the pedal a little. Opening the "throttle" with the pedal does NOT advance the timing, it just gives more fuel. More fuel is a good idea when cold because there is more fuel droplet area available to ignite. Gerald's stomp is a good idea. The other thing that happens is timing advance. This is low on the passenger side of the pump. Advanced timing produces the louder rattle. Advancing the timing helps combustion with a cold combustion chamber because the fuel gets more opportunity (time) to ignite. It is in the combustion chamber longer. The timing is advanced by allowing the case pressure in the pump to bleed off, this allows the rotating group to spin on the axis and the result is the injection happens a little bit earlier in the cylinder. If your engine has significant miles on it, you may not hear the increased combustion rattle since the timing chain does wear and the result is that the injection timing is retarded.

As others have mentioned, the first troubleshooting is to determine that you have 12 volts on both wires (green I think) when cold. If not, look at the wiring or replace the temp switch. The fast idle solenoid is on top, you can measure the stoke to see if it's working or listen for a click when you apply 12 volts with a jumper wire. BTW, the usual failure for the fast idle solenoid is the very little wire that connects to the spade terminal breaks off and can be soldered back on. The advance mechanism won't necessarily work if there is 12 volts. They stick inside the pump when they fail. To check the cold advance, when it's cold, start it up, then quickly and carefully remove the lower green wire on the passenger side of the injection pump and see if the engine sounds different and quieter and maybe rougher. If yes, then it's working. If not, when the engine is cold again, start the engine and use a jumper wire to apply 12 volts and see if the engine sounds different and louder. It should, and if it doesn't, either your timing chain is stretched to the point that the timing needs to be adjusted, or the advance mechanism is sticking. I think this requires pump replacement. Have the timing checked before replacing the pump. Also make sure the lift pump is functioning properly and the filter is clean before replacing the pump. Always start with the lift pump and filter function first. Many injection pumps have been replaced unnecessarily. All of this is correct for NA engines (DB2 injection pump) for sure. I don't know if the details are the same for the electronic pumps (DB4 pump) on the turbodiesels.

Some other observations:

  1. The prechamber GM 6.2/6.5 will start better than any direct injection diesel in cold weather because the compression is higher which results in more compression heat which ignites the fuel. This statement assumes all other variables are equal. The biggest variable is charge air heating. If the charge air is heated better in a direct injection engine, it can start just as good or better than a prechamber engine like our GM's. The Cummins for sure and I think the Navistar now use a heater grid in the intake plenum to preheat the air instead of glow plugs. My dad is a Dodge guy (poor him) and his Cummins starts fine down to 10 below, but my Hummers start easier and smoother.

  2. I live in Wisconsin. I have started my 93 6.2 na in 35 below zero weather without it being plugged in. It didn't like it (oil pressure was slow to come up) and neither did I, but I had no choice. It fired right up. I mention this so you all know that if everything is working right, the engine will work very well in cold weather. It is much easier on the engine to plug it in, so I do whenever I can if the temp is below 20.

  3. I use a non-alcohol fuel additive with lubricity additive all year.

  4. Normally, I get a little white smoke (unburnt fuel) when I start mine. More white smoke when cold and a little more as the Hummer gets older and the compression is dropping off. Right now, I am troubleshooting a rough start, with lots of white smoke problem. It is intermittent, doesn't happen all the time. My lift pump is on it's last legs and may be the problem. I will replace it and the fuel filter first. The other possibility is the tempswitch is going bad or the advance mechanism in the pump is sticking occasionally.
Hope this helps.


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