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I had a check engine light come on and it was bad glow plug harness .. I replace all the glow plugs and the harness and now when its cold .. I turn my key on and the glow plug light flashes on and then goes out right away .. I have had this car for many years , never done that .. Its a 01 TDI . 270K for miles .. I'm sure its not going to start when it -20 F if its having trouble at 20 F
 

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Is it having problems starting? You don't say that, you just say the glow plug light comes on and goes out shortly after. Which is normal.

If your plugs were on for a very long time previously at 20 F, then that is not normal and is likely due to the failing harness which you got replaced.

This morning it was around 10 F here, I had not driven the bug since Friday night. It was plugged in however.

I got in, turned the key, glow plugs went for about 5 seconds or less (didn't count, it was fast) and turned over the engine right away.

You should only see your glow plug light coming on for a long time if you leave the car out there without the engine heater plugged in and the temperature drops below 0 F.

If it gets down to -30 or -40 F where you live like it does here, then you may see your glow plugs needing to run upwards of 30+ seconds.

If you've got your car in a garage, and it doesn't get super cold, I doubt you'll ever have your glow plugs on for more than a few short seconds.
 

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Yes ... Its starting hard .. sometimes I have to try it 3 times .. Cranks good .. Starts good warm .. but when its 15F the light would stay on for 8 to 10 seconds .. now goes off right away ..
 

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What brand of glow plugs did you put in? Is your battery good? All the connections from plugs to harness good?
 

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Yes ... Its starting hard .. sometimes I have to try it 3 times .. Cranks good .. Starts good warm .. but when its 15F the light would stay on for 8 to 10 seconds .. now goes off right away ..
So this is 8+ hours sitting at ~15F (-10 C) without the block heater plugged in?

If that is the case, then yes that is odd, you should be seeing it glow for 10 seconds or so at that temperature without the heater connected.

It's about that cold here, and with my car plugged in, having both a block heater (might actually be an oil pan heater) and a battery warming blanket my glow plug light is on for only 3 seconds or so at most.
 

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Discussion Starter #6
I havent used my engine heater yet .. When it gets to -20F , I plug it in .. Glow plugs and wire Harness are VW parts .. Glow plugs all are .7 OHMs .. Glow plugs just need to stay on longer when cold .. Thay always used to stay on longer ..
 

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I havent used my engine heater yet .. When it gets to -20F , I plug it in .. Glow plugs and wire Harness are VW parts .. Glow plugs all are .7 OHMs .. Glow plugs just need to stay on longer when cold .. Thay always used to stay on longer ..
Your bad glow plugs on a bad harness used to stay on longer.

Normal operation at 10-20F the glow plugs should only come on for approximately 5 seconds.

Having driven the same bug for almost 14 years now, and having had the glow plug harness fail, I can say with a decent level of certainty that your glow plugs are operating normally now. This isn't a big cummins diesel, the VW glow plugs cycle fast.

Your issue is likely not the glow plugs, however that said I'm not a mechanic and I'm not driving your car. You should take it to a good TDI mechanic to have the battery, starter, fuel, and air all checked. What you're describing could be something simple like not changing your fuel filter at the 20,000 mile mark.
 
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