Need to change Glow Plugs

garyjh2 · #1 12-20-2005, 07:48 PM

I am thinking of changing glow plugs myself this weekend, saving the $600.00 the local shops charge. 1999 250sd. I am hoping someone will offer tips and advice, do's and don't do's. From what I have been able to find out, its not that big of a job, or am I wrong?
When cold, it will start, not real happy about it though. Sometimes I have to hit it 2 or 3 times, smokes and idles very rough for a few seconds. If plugged in, it will start just fine. I am thinking the GPR is working, just a few bad glow plugs. Have not put the test meter to anything yet. Is there something else I am overlooking?
Thanks in advance....

ModMech · #2 12-20-2005, 07:58 PM

I would DEFINATELY test them before replacing them. I worked for a fleet with dozens of these (50+) and only changed 2 glow plugs in 8+ years.

Almost always the relay is bad, either it is not closing at all, or the contacts are burned and the voltage to the plugs is reduced due to high resistance of the contacts inside the relay.

The realy is under $30, and can be changed in 30 minutes or less including disconnecting and reconnecting the batteries.

289mustang · #3 12-23-2005, 09:49 AM

Wow, where are you getting your relays at ModMech? They sell here at Ford for around $100! I'm sure aftermarket is cheaper, but thats some serious savings!

ModMech · #4 12-23-2005, 11:28 AM

International dealer.

garyjh2 · #5 12-30-2005, 08:17 PM

Thanks ModMech,
Here's what happened. I tested the GPR, several times, it tested at 11 volts, but seemed inconsistant. Sometimes 11 volts, sometimes less. I unplugged the connecton to glow plugs. Tested front 2 pins and rear 2 pins on each side. I found one bad glow plug on drivers side bank (front pin) and one on the passenger bank (2nd pin from front). I went ahead and changed all 4 glow plugs on drivers side bank (ran out of time to change passenger side bank). It seemed to me that having just one bad glow plug would still allow an easier start. The truck still has a tough time starting on cold mornings. Have I done something wrong, or, is having just 1 bad glow plug going to cause hard starting?
After removal of old glow plugs, just out of curiousity,I applied the tester to each, and every one tested bad. Why good test when in the block and bad when not? Do I go ahead and change out GPR?
I hope yopu can make sense of my ramblings....
Thanks again.

ModMech · #6 12-30-2005, 09:42 PM

This is how I have ALWAYS tested GPs, and I have never been fooled:

Using a standard 12V test light, attach the "alligator clip" end to the POSITIVE battery post (or GP relay "+12V" post), and touch each GP tip in turn while they are installed. Alternatively, with the 7.3L PowerStroke enignes, touch the OUTERMOST terminals in the valve-cover harness. If the light lights, the plug (and connection) is GOOD.

Once the GP is removed, you can only really test it with a good ohm-meter. The resistance between the terminal tip (where the harness plugs onto) and the THREADS should be close to 1-3 ohms. Over 5 ohms, and it is bad (the spec is really 10 ohms, but over 5 you have reduced it's heating to the point of being useless).

To test the GP relay, you need to check voltage on BOTH larger terminals with they relay energized. You can do this by grounding the ECM wire at the (-) terminal of the relay (one is + w/jey on - DO NOT ground this one!). Simply place the key in the RUN position, gound the (-) small terminal, and measure voltage at BOTH LARGE terminals. If they are DIFFERENT by more than 0.5Vdc, the GP relay is bad due to high internal resistance).

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