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Old 01-11-2008, 07:59 PM
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Bad glow plugs

I'm changing my glow plugs out right now, and I'm wondering if you can tell by looking at the old glow plugs if they are bad or not. My truck has been starting really rough if at all (when it's really cold I get a few cranks and that's it-break out the jumper cables!) and I'm hoping the new glow plugs will do the trick. I bought the truck in November and the GPR was done in March, I had the batteries tested and they're good, and it has a new starter. So far the glow plugs don't look too different, I was just curious if they're anything like spark plugs that get fouled and you can tell by looking at them.
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Old 01-12-2008, 02:09 AM
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I am not a fan of replacing parts unless I know they are bad, or worn out. You needed to Ohm test the GP's from the UVC harness, this would have told you the condition of each plug. I will post how to troubleshoot GP system below. There is no way just by looking at them if they are bad. Just because a place like AZ tells you your batteries are good doesn't mean a thing. They may be strong enough to start a Yugo for another 5 years, but our trucks take a lot of juice to kick over, especially when cold. Get a volt meter, if the batteries drop below 11 volts while cranking they are bad and need replacing, no matter what their machine says. I have seen GPR's go bad in two days, so just because it was changed in March means nothing. If you really want a good relay go with the Stancor model. Also if you live up North you should be running a thinner oil in the winter. This will make a huge difference in starting.
Good Luck

How to check Glow Plug System

To check the Glow Plug Relay (GPR)
· Be sure the engine is cold, so that the PCM will tell the GPR to turn on. If the engine is hot, you won’t have as much time to check.
· Locate the GPR – Its behind the fuel filter on top of the engine, a little bit toward the passenger side of the valley. There may be two relays there. If so, the rear one is the GPR. It will have two fairly large wires (yellow and brown) connected to one of the large posts.
· With your multitmeter set to DC volts, and 15 V range (if not autoranging), clip the positive (red) lead to the output terminal (with yellow and brown wires connected), and the negative (black) lead to a good ground point (like the battery ground terminal or someplace metal directly on the engine block.)
· Turn the key to ON (do not start)
· If your GPR is good, it should click, and you’ll see 11 volts or so on your meter, then, depending on temperature, it will click off up to 2 minutes later. You should do this a couple of times to make sure it consistently makes the connection.
· If you don’t get voltage with this test, confirm by retesting as follows.
· Remove the two small wires from the smaller two of the four GPR terminals.
· With jumper wires, apply voltage from the battery across the two small terminals. If your voltmeter now reads voltage on the output terminal, your GPR is OK, and your problem is in the PCM circuit that tells the GPR to activate.

If your GPR is bad you can use the factory replacement for around $75, Napa's GP110 is close to this price maybe $10 cheaper. But you can get a GPR 109 from Napa for around $22.00 This is the same exact relay as the GP110 except the mounting holes are rotated 180 degrees, which is no big deal as the wires stretch just fine.
Now if you are tired of replacing your GPR and want a H.D. alternative may I suggest the Stancor 586-902. This is a large relay and it can truly handle the large AMP draw our trucks call for at start up. Gopher Electronics has these for under $40. I know several folks that live way up North (Alaska, Canada) where they know about serious cold starts and they all swear by the Stancor. I am very happy with mine, I believe I have pics of mine installed in my webshots.

To check Glow Plugs.
· Remove the electrical connector on the inboard side of valve cover at the gasket. Press down on the top of the connector latch and pry gently with a screwdriver. Photo of disconnecting one and another Photo of it loose.
· There will be 9 pins on the valve cover gasket where you removed the connector. The two pins furthest forward and the two pins furthest back are for your glow plugs.
· With your multimeter set to resistance (ohms) and low range (single digits) if not autoranging, clip the negative (black) lead to a good ground point.
· Probe each of the 4 outer pins individually with the positive (red) lead, noting the resistance. Good glow plugs will have a resistance between 0.6 and 2 ohms. If you get infinite resistance on any glow plug, that one is either bad or the connector under the valve cover has come loose.
Take Care

Also let me add, that if your GPR does test pad you can use a Napa model #109, their book will call for the #110 but the 110 is around $60+ where the 109 is low $20's. They are the same exact sensor spec wise, the only difference is the mounting tabs are rotated 180 degrees, don't worry the wires make it fine. However if you really want a HD relay and one you won't have to change yearly, you should replace it with the HD Stancor unit, the model # is 586-902 and it is made to handle the 200 amp draw, whereas the others are made very cheaply. You can see photos of this relay in my webshots.
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Old 01-12-2008, 09:46 PM
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Thanks-that all makes logical sense. I haven't used a multi tester yet, so I think that will give me a definite on what is working and what is not. I'm done with the glow plugs- 7 hours later-we'll see in the morning if it did the trick! At least one of the glow plugs looked pretty scorched, so I hope it wasn't all in vain. Once again-thanks for your help.
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Old 01-13-2008, 08:12 PM
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If you want to check your old plugs just use a battery and a couple a pieces of wire. When I changed mine out I laid them out like they came out of the engine so i could see which cylinders were bad. They get very hot and it only take just a second on the battery to get them hot. Just be sure to hold onto the top as the tip will leave a mark HA!!! As far a the lighter oil I put in 5w40 synt rotella a few months ago and it really made a big difference. Starts well and runs smooth when warming up. When I checked my plugs I wanted to see how many wouldn't heat at all. I guess just because they heat don't mean they are working like they should. Maybe one of the techs will shed some light. Good luck and I hope the G plugs done the trick.
99 Red Stroker
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Old 01-14-2008, 10:31 AM
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Thanks-so far so good-started no problem this morning at around 15 degrees. I'm leaving it outside all day today not plugged in to see how she does.
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