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I have read that replacing the glow plug harness on the 02 cars fixes the trouble light issues. I have a 2004 PD Jetta with intermittent glow plug trouble codes. After testing and tightening all the usual suspects the engine still takes about 1.5 seconds to start (turning over after glow plug light is out) instead of the normal instant start. Should I replace the harness.
Where does the harness terminate on the PD cars. The 4 prong plug is not in the square harness holder behind the air box like the 02 cars.

toolgg
 

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Do you have any other codes? There are a bunch of things that can throw a GP light.

Test the glow plugs between the tip and ground, it should be between 0.5-1.5 ohm. Too big of a difference will throw a code. Corrosion on the plugs, tips, or connector will throw a code. If you find corrosion on the harness or bad wiring then I would replace the glow plug harness.

A possibility is a bad coolant temp sensor or plug. The GP are only supposed to go off below a certain temp. If the coolant temp sensor doesn't sense this, the GP won't go off. Look behind the flange coming off the driver's side cyl head, same place as the 2002's sensor. Unplug the sensor (don't remove unless you are prepared to a hot coolant shower) it to trick the ECU into thinking it's cold and it may help.

There is also a fuse on top of the battery plate.

Lastly, your hard starting issue could be a fuel leaking out preventing pressure, low battery voltage, clogged fuel filter, a shorted wire somewhere along the line., etc. And glow plugs do age and get weaker over time. The pumpe duse glow plugs are ceramic unlike the pre pumpe duse and are also more fragile so handle them with care.
 

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I think you have to splice into the harness to replace. Could be a timing issue, you can adjust the cam sprocket slightly (assuming that you have a vag com) to make for easier starting. If you have a rough idle it could be a sign that timing has slowly degraded and is giving harder starts and rough idle.
 

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The harness can get old and frayed, compression could be low, timing could be off? The glow plugs stay on after the glow plug light goes off too. Perhaps the battery voltage is a little low and is making the engine crank slower?
 

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Timing is a big cause of delayed starting. Air in the fuel? I think the tandem pump had a recall for leaks that would let air in.
 
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