What controls or holds the idle speed on a '99 e300 td?
When engine is started cold it idles at approx. 900 rpm. it will stay at that speed until the accelerator pedal is tapped a few times, then it will go down to 600 rpm. it also does this when engine is warm. sometimes when idleing it will creep up from 600 to 800 rpm.

Hook it up to an obd2 device that shows live data and watch the TPS and load as this is happening. I suspect your TPS is acting up.

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I'd wager on the throttle position sensor as well. I have a Bluetooth scanner from Amazon and an app called torque. It's great for checking the basic diagnostic codes for these cars and monitoring live data while driving. Intake temp, coolant etc, live fuel economy.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Jake, if it is the TPS, is that something that can be adjusted or should it be replaced?
What is the brand and model of your scanner, I will get one from Amazon!

It can't be adjusted, only replaced. It's electronic, sometimes they go bad.. I've seen a few on w210s that were bad. All gas cars, but the same concept applies. Mine is made by Foseal for android and iPhone.. I found it on Amazon for $20 and that's more than I paid. I'd try eBay, last time I looked that one that I have is cheaper on there!

Before ordering any parts, make sure to scan it first for pending codes.. It'd be a waste to order it and not be the TPS. But we will go from there once you get the scanner.

BTW, my 99 E300 started doing this since the glow plugs went bad.. It's really nothing to be too worried about, I've been living with this for quite some time.

If the motors hot, and you go to start it again it will go back up to 7-800RPM right? That's what mine does.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

I do have an OBDMATE OM123 scanner, I will see if it works on this car. I do have a check engine light on.
Yes you are correct, restart hot engine and RPM will go back up to 7-800.

Yeah I'd see what the code is and check back

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

The code is P0380
glow plus/heater circuit A.
Also noticed a faint buzzing sound coming from the black box behind windshield washer fluid container (glow plug relay?) while key was switched on to second position while scanning. when I turned the key off that buzzing sound continued for about a minute or longer and slowly tapered off and became silent.

I would still check the live data from the TPS.

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CENSORED due to not family friendly words

Thank you for your input.
Please keep in mind I am very "green" at this stuff, I just now figured out how to plug the scanner in!

that sound is normal.. nothing to worry about all my e300 cars do it, i haven't figured out what it is myself. clear the code and see what happens

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Its the vacuum transducer that operates the egr valve, there is also one for the turbo the other side of car under the air box.
The turbo one is tucked down lower on the car so the buzzing noise is less noticable.

My car was a 96E300d, and this is what I did when I got the TPS code. I know that your vehicle is different than mine, so the information may not be of any use to your situation, but its worth taking a look at. Good luck

The TPS code can be thrown if any sensor in that circuit is faulty or not functioning as designed. The circuit consists of the Throttle Position sensor, then the small sensor that is connected by a small black hose coming off the intake manifold, and two other sensors (same part number) located on the side of the air filter box. One of those sensors operates the EGR valve, the other the resonance flaps. In my case, I inspected all the vacuum lines leading to each sensor and resonance control valve to insure there were no leaks. Then I removed each electrical connection plug and sprayed it with electrical contact cleaner, and reconnected them. Once having done that, I cleared the code, and it has not returned in over a year. Hope yours is as easy to fix as mine turned out to be.

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96 E300d

Thanks for your help, I will do as you did and see what happens!

Thank you, I could not be certain where it was coming from, difficult to pin point. Makes sense to me now.