Rough cold start idle - Mercedes-Benz Forum
 
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post #1 of 9 (permalink) Old 05-04-2019, 10:26 PM Thread Starter
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Rough cold start idle

I've been reading posts and haven't found one yet that matches my exact situation. Had the car serviced recently and the car runs good when warm with an idle in the 750 rpm range. Plugs all changed and spark plug wires checked. But when doing a cold start in the AM, it starts up quick but the idle drops to 500 +/- and you can smell gas and the engine shakes bad. Warm up regulator? Car doesn't stall when I step on accelerator while it is still warming up.
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post #2 of 9 (permalink) Old 05-04-2019, 10:28 PM Thread Starter
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Forgot to mention I have a '79 450SLC. Previously the rpm was always high at 1000 rpm when in idle and it did not have the cold start issue.
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post #3 of 9 (permalink) Old 05-05-2019, 06:11 AM
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I've been reading posts and haven't found one yet that matches my exact situation. Had the car serviced recently and the car runs good when warm with an idle in the 750 rpm range. Plugs all changed and spark plug wires checked. But when doing a cold start in the AM, it starts up quick but the idle drops to 500 +/- and you can smell gas and the engine shakes bad. Warm up regulator? Car doesn't stall when I step on accelerator while it is still warming up.
Check to see if you have 12V at the WUR harness.

Check and see if your Aux. Air Valve is stuck closed.

Sounds like your cold start valve is working.

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post #4 of 9 (permalink) Old 05-05-2019, 08:49 AM Thread Starter
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Thank Nobby.

For others, found this thread to assist.

https://www.benzworld.org/forums/r-c...topics/1600137
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post #5 of 9 (permalink) Old 05-10-2019, 05:09 AM
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Thank Nobby.

For others, found this thread to assist.

https://www.benzworld.org/forums/r-c...topics/1600137
Don't go down that rabbit hole. He is explaining a D-Jet fuel delivery system.

YOU have a K-Jet.

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post #6 of 9 (permalink) Old 05-10-2019, 05:42 AM Thread Starter
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Thanks again.
Found this but still looking for one better describing mine. But this should.be closer.


https://www.benzworld.org/#/topics/2182425
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post #7 of 9 (permalink) Old 05-10-2019, 06:54 AM
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Thanks again.
Found this but still looking for one better describing mine. But this should.be closer.


https://www.benzworld.org/#/topics/2182425
Myndex is explaining how the frequency valve operates and the nightmare it takes to configure the kjet lambda system in spec.

The WUR is essentially an automatic choke that decreases the fuel pressure.

In the fuel Distributor, there's a spring loaded system pressure regulator. When the fuel pressure is higher, more fuel enters the system making it run rich.

As the WUR warms either by 12v or residual engine heat, the fuel pressure drops and the system regulator lets less fuel in. This leans the mixture out.

A failure of either the WUR, system pressure spring, and/or fuel Distributor diaphragm will make for a bad day.

Given the symptoms, I would suspect your WUR. However, these cars like to fool you and the only way to tell is with a fuel pressure test kit. I recommend you purchase one and start by checking the fuel pressure. Make sure it has kjet fittings!

This is done by hooking the test kit up between the WUR, the fuel Distributor fuel fees line, and taking readings at room temp, warm up, and full operating temp.

There are a few guides floating around here on how to do all of that.

Once you do all of that, we can move onto your lambda system next 😉

Good luck!
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post #8 of 9 (permalink) Old 05-10-2019, 11:40 AM Thread Starter
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Thanks for the explanation. That helps.
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post #9 of 9 (permalink) Old 05-18-2019, 11:58 AM Thread Starter
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As anticipated, it was the WUR. Replaced it and now every thing is running smooth.
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