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bleachii's Avatar
 
Join Date: Feb 2008
Location: Huntington Beach, CA
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Initial Startup - High Idle Question

So, I received my Bosch Fuel Injection book by Charles Probst. I've also started re-reading the car won't start sticky to see if I can better understand some of the more technical threads. I have a ton of questions but here are a few if someone wouldn't mind answering for me.

1. Is the 'control pressure regulator discussed in the Continuous Injection - Theory chapter what is referred to as a Warm Up Regulator on our cars?

2. On initial cold start my car idles high and then within 20-30 seconds the idle speed reduces significantly. I think this is normal as it appears the car is receiving increased fuel (or air?) to help with the starting process. Eventually the idle is 'normal'. Does the WUR have anything to do with this process? Seems to me if its the WUR then this process would be more gradual as the engine warms. I read about the electric winding to help with this process but I suspect this does not exist on a '79. My questions are whether this is normal and whether someone could explain why this happens. What parts are involved?

3. The book talks about dieseling defined as the car not immediately stopping when the ignition is shut off. My car will typically do a couple of 'coughs' after I kill the ignition if the car is warmed up (does not happen when cold). The book mentions ignition system issues as a possible culprit. Would this relate to my distributor cap going bad? I have issues with the car once it has been washed and suspect the Distributor cap and now I wonder if this might fix the coughs after shutdown.

Thanks!
Brian

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Old 04-04-2013, 08:31 PM
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This thread, second post answers my first question. Answer is 'yes'.

930 Acronym help
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Old 04-04-2013, 09:21 PM
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My '79 930 does the slight dieseling on shut down too, after its warmed up. I don't think its anything to worry about, I think its just residuals being used up and a delay between turning off the key and the ignition using its last bit of energy thats between the coil, CDI box and plugs.

I always kind of think it's like the car doesnt want to shut down, cause its ready to blast around some more on boost, like a kid that doesnt want to go to bed yet..on a school night. haha

The slight dieseling, doesnt seem like a used up 1970's Detroit iron bigblock dieseling on low octane fuel, that doesn't seem healthy, its more like I mentioned above, that it's residual spark and fuel being used up, after the key is turned off. But just my observation and opinion, someone else may have a more definite answer, if they put the time into figuring out the exact cause.
Old 04-05-2013, 07:21 AM
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Question 2:
I believe the WUR and the AAV both have something to do with how the car behaves on a cold start up. The wur initially adjust pressure (preset) to cold control pressure, CCP, the specs for this, you can find in the manual, it varies with temperature.
I believe your 79 does have the windings in the wur. As soon as the wur receives power it starts heating up, hence changing pressure from ccp to wcp. I beleve the wcp should be around 3.65 bar. This change reduces the amount of fuel, it leans it out. So the engine is indeed given more fuel on start up.

For air, depending on ambient temperature, the aav bypasses the butterfly in the throttle. Hence allowing more air, enabling a higher idle when cold. The aav should then close, as it does also have power to it and closes the little funky moon shaped hole.

So it is a combination of more air an fuel on start up.
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Last edited by jsveb; 04-05-2013 at 07:38 AM..
Old 04-05-2013, 07:36 AM
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Thanks for the responses guys. The windings would make more sense from what I read to explain such a sudden drop in the RPMs.

The author explains how the WUR adjusts pressure based on the temps of the engine (cold vs warmed up). The initial requirements of a cold engine are different than the engine once it warms up. Also, there can be a good 10 min or more before the engine is warm enough for the WUR to make the necessary adjustments (for this reason the WUR is very close or on the engine block per the book). I have no idea where it is located yet on my car. I'm in the Sierra's reading so no access to the car.

The electric winding is therefore engineered to force the WUR to adjust pressure after the cold start but before the engine is completely warm. Basically it tricks the WUR into thinking the engine is warm earlier than it naturally happens.
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Old 04-05-2013, 08:16 AM
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Quote:
Originally Posted by bleachii View Post
Thanks for the responses guys. The windings would make more sense from what I read to explain such a sudden drop in the RPMs.

The author explains how the WUR adjusts pressure based on the temps of the engine (cold vs warmed up). The initial requirements of a cold engine are different than the engine once it warms up. Also, there can be a good 10 min or more before the engine is warm enough for the WUR to make the necessary adjustments (for this reason the WUR is very close or on the engine block per the book). I have no idea where it is located yet on my car. I'm in the Sierra's reading so no access to the car.

The electric winding is therefore engineered to force the WUR to adjust pressure after the cold start but before the engine is completely warm. Basically it tricks the WUR into thinking the engine is warm earlier than it naturally happens.
You've got it!

The AAR (sometimes also referred to as AAV....but it truley is a Regulator and not just an on/off Valve) will wear with age and may cause different warmup characteristics; i.e., may result in a higher, longer sustained high idle, or conversely the opposite of that. The fact that you have high idle when cold and that it drops back down within just a few minutes says everything is probably working as it should.
Also tuck into the back of your mind that both the WUR and the AAR have internal heating elements which receive their power through the rear fuel pump relay.

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Old 04-05-2013, 09:56 AM
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