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About a week ago my car began being hard to start. The glow plugs would burn for the "normal" (like they have in the past ~5 sec) duration, but the car would turn over for a while before starting. The hard starting began shortly after i filled up at a hole in the wall station (was in a hurry). I attributed it to bad fuel. Filled back up at my local high-volume BP, and had great starts for the past 2 days. Temperature has been low 40's, high 30's.

Today, 28*F outside, the glowplug light would only burn for less than a second as opposed to the normal 4-5 seconds. Even after several cycles, the car killed its already weak battery trying to start.

My father is convinced that the short duration glow is because of the low battery, even though it cranked for 15 seconds afterward. It seems to me that rather than not glowing because of a low battery, the plugs would simply kill the battery with a longer glow, with no regard to battery power.

I read on here to check the coolant temperature sensor. I used an OBD scanner with live data, and at first start, the reading was 10* below ambient. 45 seconds after starting the reading was 30* above ambient, and rose steadily at idle corresponding with the "feel the coolant line for warmth" test. I'll test it again in the morning with respect to ambient.

What else can I check?
 

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First I would focus on the simple things: change the fuel filter or at least drain the water separator, test the battery and alternator. Did you scan for codes? Did the malfunction light come on in the dash mine came on when the glows were bad. Glow plug fuses are located in the fuse holder on the battery cover. Simple test:disconnect glow plug harness, then check each one with a test light hooked to positive on battery it should light up if good. I had to do this all the time on my 7.3 ford
 

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If it's cranks too slow it won't inject fuel so if the battery is really weak it could cause that no start problem. The coolant temperature sensor should show ambient if it's working correctly on a stone cold engine. Here's a cheap test: unplug the sensor and this will tell the ECU to give maximum glow plug duration. If the car starts fine the sensor is bad. Other common problems are in the wire harness and plugs.
 

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Discussion Starter #4
If it's cranks too slow it won't inject fuel so if the battery is really weak it could cause that no start problem. The coolant temperature sensor should show ambient if it's working correctly on a stone cold engine. Here's a cheap test: unplug the sensor and this will tell the ECU to give maximum glow plug duration. If the car starts fine the sensor is bad. Other common problems are in the wire harness and plugs.
First I would focus on the simple things: change the fuel filter or at least drain the water separator, test the battery and alternator. Did you scan for codes? Did the malfunction light come on in the dash mine came on when the glows were bad. Glow plug fuses are located in the fuse holder on the battery cover. Simple test:disconnect glow plug harness, then check each one with a test light hooked to positive on battery it should light up if good. I had to do this all the time on my 7.3 ford
The fuel filter has less than 3k miles on it, but I'll drain after church today. No codes, and no MIL/CEL.

When I get back home, I will:
Compare the coolant temperature to ambient temperature with the scanner
Drain the fuel filter
Check the fuses
Unplug the coolant temperature sensor and see how it does
Test the glowplugs with a test light

I'll report back, thanks for your help.
 
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