PSA: My glow plugs work fine - or *do* they???

[tonydca]

Executive summary: Glow plugs can begin to wear out, rather than just stop working all together. When this happens, they'll still get hot and look hot, just not hot enough, fast enough. And if you don't have a $10 multimeter/voltmeter from Princess Auto or Canadian Tire, consider getting one.

Backstory: Rough cold starting. Needed a couple of cranks and then lumpy low idle with lots of white smoke.

Just had a new cylinder head and fuel filter installed; air filter fine. So problem is likely either fuel pump seal (crap), poor piston compression (extra-double crap), or bad glow plug operation. Let's check the cheap stuff first.

Test #1: Check that power is getting to the glow plugs. With the engine cold, set your multimeter to measure DC Volts. Clip the (-)/Black lead to the battery common (-) terminal, then reach under the intercooler and touch the (+)/Red lead to the metal bar connecting the four glow plugs together. Should read 0.0 Volts. Have your capable assistant turn the engine key to the "On" position. When you hear the loud "Clunk", the measurement on your multimeter should jump up to around 12 Volts, give or take. If this happens, the glow plug activation circuitry is working fine, now you need to check your plugs. This was my case.

Test #2: When you pull the plug out, you *can* test them electrically as shown below (*but*....):

http://www.ngk-dpower.com/en/technik/te ... ik-tipp-1/

... because of what happened to me when I physically tested them as described below, I don't 100% agree with the details of that post, despite it being a manufacturer's website. Here's why:

- I got out my set of *COPPER* jumper cables. Copper is soft and will not damage the threads on a plug. Connect one side of the Black/(-) cable to the battery (-) terminal. You will not need the Red/(+) jumper cable.

- I popped out my first glow plug and connected the other side of the Black/(-) jumper cable to the hex nut/threads on the body of the plug, being careful not to touch the terminal/top threads of the plug.

- Holding the plug by the cable clamp, I reached under the hood and carefully touched the terminal top of the plug to the (+) battery terminal. Start counting: "1-Mississippi, 2-Mississippi," etc.

- By 9-Mississippi, the end of the plug was a nice cherry red. "Great" I thought.

- Got out my second plug and did the same thing. By 5-Mississippi, the tip was glowing orange-yellow hot. Hmm....

- Plugs 3 and 4 tested the same as plug 2.

- Measured the electrical resistance of the plugs - set the multimeter reading to the Greek "Omega" symbol (for "Ohms - resistance). Plug #1 was 3.5 Ohms, plugs 2-4 were all 1 to 1.3 Ohms.

- Replaced Plug 1; now she starts beautifully.

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** DANGER!! SCIENCE ALERT!! **

The higher the resistance of an element in an electrical circuit is, the harder it is for electrical current to pass through it. In the case of my worn glow plug, there would only be 1/3 of the current flowing through it as was flowing through the others (since the resistance was 3x higher).

The kicker is: When it comes to producing heat, if you drop the current by 1/3 and the resistance stays the same, you actually drop the amount of heat generated by 1/3*1/3=1/9!!

So instead of my glow plug acting approximately like the filament in a 100W incandescent bulb, it was closer to a 15W bulb.

***************************

So what I took away from this is that even though the bad plug was getting red-hot, it was but a shadow of its former self, as evidenced by the way the others tested out. And if I hadn't physically tested the plugs myself, but just went with the manufacturer's web-site info, I'd have thought they were all OK, since they were all within specs.

So I believe that it is important when you are checking your glow plugs to make sure that they all either have the same electrical resistance, or behave the same (similar hot in similar time) when you physically test them.

[robbieg]

Science: it works, bitches.

http://xkcd.com/54/

[Mr. Flibble]

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