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Old 09-25-2017, 09:03 PM   #1
240sleeper
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Default Misfiring on startup

I have a 93 naturally aspirated 940 that has been having some issues when cold. The engine is pretty hard to start in the morning. It chugs for a few minutes and feels like its misfiring, and then will clean up and run fine once running for a minute or two. There is a strong smell of fuel and a decent amount of smoke coming out the exhaust. Once the engine is warm it will run and start perfectly fine throughout the day. Once the engine is cold it starts chugging on startup again. So far I have replaced the cap, rotor, plugs, wires, fuel injectors, and probably a few other things. I work at a junkyard so I get most of these parts for free although I'm getting sick of replacing things. Any idea what this issue could be caused by?
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Old 09-25-2017, 10:38 PM   #2
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Check ECU's temp senosr
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Old 09-26-2017, 01:15 AM   #3
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Yup, sounds like ECT is out of spec. Mileage in the tank a bit as well?

Could also be a leaking injector, but ECT is pretty likely.
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Old 09-26-2017, 12:01 PM   #4
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I'm going with leaky injector. What you can do at home is a cylinder drop test. Unplug each injector one at a time and see if you can detect which hole is misfiring.

I just diagnosed a leaky direct injection injector in 10 minutes. Misfires heavy on cyls 4 on the scan tool (modern vehicle, duh) and swapped coils and it still missed on 4. Smoking, fuel stank. Compression ok, so that leaves the injector.

On these old cars, no scan tool data, so gotta do the cyls drop test. Or maybe use a labscope. Or after you find a hole which is missing, swap the injector over to another hole and see if it follows. And of course a visual of the injector. Pull the plugs too and check for fouling or excessive gap.
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Old 09-26-2017, 07:57 PM   #5
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I already changed the fuel injectors, the coolant temperature sensor that is on the back of the engine I changed a few months ago. I noticed my gauge cluster temperature gauge bounces around so I'm going to change the front temperature sensor. I also have a spare ECU

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Old 09-26-2017, 08:42 PM   #6
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Have you checked the ECT resistance on both pins? I've heard a few sources lately that have had some bad experiences with new sensors failing in short order.
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Old 10-24-2017, 02:47 PM   #7
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Thanks for the tips everybody. It turned out that the coolant temperature sensor did go bad after a few months. I had bought an after-market one and apparently it was of low quality. I remedied the situation by buying an oem Bosch sensor and it starts perfectly fine now
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