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Old 11-30-2008, 08:18 PM
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Ford 7.3 hard to start

I am working on a friends 02 F350 with a 7.3 engine.It acts like the glow plugs are not coming on.All I have is this P.O.S. Haynes manual.It has no schematic.No info on testing the glow plug relay or the sensors for oil temp or coolant temp.The thing is useless.Any body out there got a Chilton Shop Manual for the 7.3 in a 2002 F350?I need to know how to test if the glow plug solenoid is good or what the resistance on the thermistors for oil temp and coolant temp should be since they control how long the glow plugs stay on.I changed the fuel filter and that cured the stall at idle.It shows no code so I suspect the glow plug solenoid.I live in cold country and the block warmer for this truck can't always get plugged in.I did not find anything on this when I searched this thread.I have no diesel engines of my own so I need all the help I can get

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Old 11-30-2008, 08:51 PM
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Its almost always the glow plug relay even if you hear it clicking. You'll have to test it. The dirty way to test is to turn the key on, listen for the click, then watch the volt gauge on the dash. When it clicks, the volt meter should change a little. If its not, its a bad relay. You really should verify it though with a volt meter at the relay itself. They're cheap and easy to replace.

Don't use the "glow plug" light on the dash, use your ear. The light on the dash goes on and off based on the computer's idea of when you should start. The actual relay cycles differently from the light. Listen for the click when you're watching the gauge.

If the relay is good, you'll have to test the individual glow plugs. There is a flat connector that goes through the valve cover gaskets. Pull that connector off and test the resistance to ground of each peg on the connector. I'm not sure what the resistance should be, but the manual should tell you. If it doesn't, look for consistent readings that are within about 10% of each other. If you have some that are way off, they are bad and need to be replaced.

Relays should be purchased at NAPA. They have good relays for the cheapest prices. IIRC, the correct part number is either GPR109 or GPR104 but its been a while
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Old 11-30-2008, 08:59 PM
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Thanks Curtis.It does not come on at all.I guess I better just replace the solenoid
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Old 11-30-2008, 09:11 PM
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Verify that the solenoid is getting juice from the computer. If its not, then I would suspect the sensors you talked about.

But... 9 times out of 10 its the relay. Even if you buy one and find out that its not the problem, it will be someday and you'll have a spare

Glow plug relays are like Kryptonite to the powerstroke. The engine will run forever... as long as you have a spare GPR.
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Old 10-07-2012, 10:26 PM
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I know this post it's old but found some good info researching the same issue here. Glow plug testing... - Diesel Bombers
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Old 10-07-2012, 10:42 PM
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cold country ?

I worked at Mercury one winter , compared to the Northern rockies, I thought it was summer. I have an 01 7.3 and it is hard to start 9 months of the year, I have a friend that is a tractor mechanic and he said he would put some gasoline on a rag and put it in the air intake to start his, I haven't tried that. I have been told NOT to the use ether, some of the tractors have an ether squirter built in, and My case 580 needs ether 6 months of the year to start if not plugged in.
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Old 10-08-2012, 04:53 PM
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The best fix I have found for the GPR is the one made by KEM. The biggest issue with the Fors relays, and others built to that exact design, is they have a "limiting" pin inside that limits full engagement of the relay contacts. This causes arcing, and burns out the contact areas; thus a clicking relay, that doesn't work. The KEM part eliminated that pin, allowing full contact, and longer relay life.

Normally, I am not a KEM fan, as they are one of the lower end parts brands. But, in this case, they have come up with a superior product. Maybe not by design, but that how it worked out.
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