91 7.3L Diesel

My boss has a 91 Ford Tow Truck with a 7.3L diesel. Every morning when he starts it it makes a terrible noise (rattling, like a bunch of metal parts inside the engine are just rattling around) and a rough idle. this lasts for about 10-15 minutes until it is warm. Now when he leaves it plugged in overnight it is fine but once it sits for 12 hours or so and not plugged in then it sytarts like crap. It has been like this for about 6 months, i am no diesel mechanic so don't know too much about them. I have tested the glow plugs they were all good (tested around .4 ohms) except for 1, replaced it and still no luck. I mean this shouldn't be running like this I understand if it was very cold outside but it is in the 70 here now and even in the winter it is in the shop all night (where it is at least 60 degrees).

any ideas or suggestions. Appreciated.

It is a Ford F-450 Super Duty Lariat XLT 7.3L diesel, if that helps.

Have you checked to be sure that the glow plugs are getting power, is the controller cycling on and off like it should? I have had glow plugs check good with an ohmmeter, and still wouldnt heat. They are usually swollen and burst open, and can be a real booger to get out . Also check the temperature switch that controls the cold/fast idle solenoid, and the cold/timing advance solenoid. If the timing advance isnt working properly, it can cause hard starting and excessive rattle on cold start. It is located behind the thermostat housing. It is a two pin heat sensitive switch that looks like a temp sensor, and its not real easy to get to. lol A crowfoot wrench will get it, but its a pain.

Have you checked to be sure that the glow plugs are getting power, is the controller cycling on and off like it should? I have had glow plugs check good with an ohmmeter, and still wouldnt heat. They are usually swollen and burst open, and can be a real booger to get out . Also check the temperature switch that controls the cold/fast idle solenoid, and the cold/timing advance solenoid. If the timing advance isnt working properly, it can cause hard starting and excessive rattle on cold start. It is located behind the thermostat housing. It is a two pin heat sensitive switch that looks like a temp sensor, and its not real easy to get to. lol A crowfoot wrench will get it, but its a pain.


Thanks for the reply.

How do I test the Tempature Switch?

With the engine cold, an ohmmeter connected across the two terminals of the switch(harness disconnected) should show continuity, or closed switch. When the engine reaches operating temp, the switch should open, killing power to the fast idle/timing advance solenoids. Or you can test for power at the two solenoids. A cold engine with key on should have battery voltage at both, warm engine no voltage. Power for this circuit comes from the glow plug controller to the fuel shut off relay on the pump, then to the temp switch, then to the fast idle/timing solenoid.

I checked voltage to the glow plugs with the key in the ON position and the engine off. the voltage went up to about 9.2-9.5 volts on each plug then the glow plug 'timer' kicked (clicked on and off) on and the voltage jumped up and down. I read somewhere that the voltage should be above 10 volts and if it is below 10 v to check the glow plug relay. I have no idea where the glow plug relay is at. the owners manual doesn't say where it is located.

The glow plug relay should be on the rear of the intake.

where exactly is the relay, I know that the timer is on the back of the intake. I really haven't had a chance to look at it, we have been pretty busy lately.