Newbie here, searched for this answer.

91 300te m103 start low idle problem.
When I disconnect the Idle aircontrol valve connector with engine running at 80C the idle rpm drops.
Is this normal?
If not, can I put battery to 2 connectors and observe operation?
Pulled it and it is clean (sprayed carb cleaner and not much residue).
Thanks

I tried disconecting the icv connector with car running at op temp and my idle went way down! I thought it was going to stall.
I always test mine after cleaning it by hooking it to 12 volts to ck the operation.I like to see it goes full travel I found a bad one doing that test.
Is it normal for the idle to drop?? although mine does I am having alot of problems lately!.......................What symptoms are you having that you are looking at the ICV???

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

I have a 91 m103 with 325K miles.
Car runs ok after starting, dieing, starting, rpms get to normal but have to wait to put in drive,
So,low rpm and die on first start (starts quickly).
Your symptoms>

I dont think its normal for the idle rpm to drop when the IACV is unplugged.

On my M103 (~100k) the idle rpm (IIRC) goes to 1500 rpms.

On my 420SEL (~220k) the idle rpm goes to 1500 rpm when IACV is unplugged.

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(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

I see from your 2007 posts you've been there.
On my 91 m103-
Tested the ovp and everything tests perfectly (several times).
With connector removed, 9 ohms between pins.
Engine on (40c temp) 8.8v between pins, dropping gradually as engine temp warms to 6.5v after a minute or two (just a tic above 40c) when I stopped.
Unplug and idle drops immediately.
Plug back in, no change until I restart the car, then idle ok.
Twice have had someone start the car cold (58f outside).
Starts up but will die unless I slightly hold the air flow sensor plate open, then idles cold fine.
This is driving me bonkers!
Any diagnosis help appreciated!

My car also starts and dies within a second of starting unless I hold the key in the starter cranking position for about six seconds or more to get me past that dead spot! It makes me think there is no fuel in the fuel distributor,maybe just a little residual in the lines by the injectors to give it that initial start and then it dies until more fuel can be pumped up from the fuel pumps.
In other words I think it is leaking back some how. I think there are check valves by the pumps and also a fuel acumulator back there by the pumps ,one of these may be the problem item.

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

I thought about that but those pumps are about 1 qt in 45 seconds.
The accumulator as I understand is only to hold pressure for an hour or so.
Try this-
Remove top of airfilter.
In the morning (cold) have someone start the car.
As soon as the round sensor plate drops,and the car fires, slightly hold it there and see if the car doesn;t idle fine.
Tried that two mornings and it worked.
Now, pulled and tested the Idle air control valve with 12v at the battery and it slams open and shut.
If you are worried about the rubber hoses being brittle, just disconnect the plug and run two 16 ga wires from the pos and neg on the battery and touch the terminals.
Apparently the hose to the intact manifold requires much patience and dexterity to rplace if brittle and cracked.
From the front, pos on the left terminal.
You will hear it open and close as you apply and remove voltage.
Will let you know when I figure the rest out..

I replaced my injector seals and O rings,and it seems to run better and idle a little better but It still idles a little funny and still has a dead spot while driving cold between 1000 and 2000 rpm the car runs great once it gets hot.
I have recently replaced the hoses for the ICV they were split! after that it ran good for a while and now it is acting up again
I still feel it is a vacum leak.

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

For vacuum leak test I use a small hardwarestore propane cylinder with add on valve and tip. To establish baseline, remove an intake manifold vacuum connector and put propane tip at the hole to see what difference it makes in idle speed.
Sometimes, not much, but a change.
Here are the two info testing pieces I used. Accurate.


When you say 1000-2000 rpm what is engine temp?
Might still not be in closed loop.
Different scenarios for acceleration in closed and open loop in my experience.

Still the question- when you unplug the Idle air control valve, your rpm drops hot or cold?

Attached Files

	KE Jetronic Diagnosis eha, pot testing etc..pdf (167.4 KB, 1480 views)
	MB Testing iac eha etc.pdf (139.6 KB, 739 views)

first you have a bad ICV, when you pull the electrical connector the idle should go to to 1900, if not replace it. Second if you hold the air horn plate down just slightly and it runs better! what you did was increase the fuel, so this means that you need a three millimeter alen wrench and turn it c/w to fatten up the fuel. do a search on X11 and setting the air fuel mixture. Replacing the rubber parts is always a great place to start on one of these cars, they get hot under the hood and all the rubber connection parts need changing after 20 years of service, it is cheap but labor intensive, oh yes do not for get that every moving part connected to the heater/AC system is operated by a vacuum module with a rubber diaphragm, they to get worn out. The fuel pumps should turn on with the key and run for about 3 or so seconds this is prime mode, though the fuel accumulator dose not hold max pressure it dose hold some pressure, even over night. Hard start could well be that all the reserve pressure has dropped off and that prime mode just is not long enough to do what we are asking it to do, there fore a long cranking period. When you have lost prime and the first shot is not enough the pumps do not turn on till the crank shaft position sensor reads RPM and thus energizes the pumps again, this is the run mode. I hope I have given you some help. congrats on your newbe status I hope you have a great experience. JNT

Great idea! Plugged climate control line at manifold and got better rpm immediately! Having trouble with that ac anyway- we have mice and packrats and they can eat up wire and tubing.
Also plugged egr line and brake booster line- no change on those.
Where else can I lookfor vacuum leak?

This 91 300te just turned 325K and the adjustment tower has never been touched.
Thought I would rule out everything else first
Temp sensor
ovp
eha
o2 sensor
plugs, wires, dist,rotor
all test good or replaced.
The iac test fine with current, voltage, resistance.
Am going to make a manual with pipe hose and a ball valve to see what happens.
Thank you for the AV/heater direction!
Tom

What color is the plastic on the cold start valve/injector? I was told that the butterscotch color ones had problems on cold starts and were superceded by green ones. On a 90 300TE, changed it to green and it did start alot better; however, many other needed repairs were done at the same time so I can't say that it was definitely the sole problem with the cold start.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

On my M103 the idle speed drops if I pull the plug.
Without power the valve is closed, well almost, there is an adjustment screw to hold it slightly open). If yo put some voltage on the connector it opens, with 12 volts it is fully open.

Rob

Interesting-
James Dean and just-ntime say idle increases when iac unplugged.
Me, Robpruijt, and johnflight1 have idle decreasing at 80c when unplugged.
I have measured current and voltage on mine in running engine and bench tested with 12v. All seems to be good.
Why are three of us (M103) getting the same results and opposed to the other two?

I think the immediate die on cold start is related more to the accumulator or check valves, but the car was just driven 500 miles away (with no problems except the start issue when cold = starts fine while using during the day with short (2 hour) stops) and won't be back till Tuesday.

Me too on the vacuum. Did the injector seals and orings.
Have you checked possibly fuel filter?
I was having problems with climate control so plugged the vacuum line that goes into the climate control vacuum control valve (trace the line from left of the brake master cylinder to the intake manifold ((at least on my 91 300te)).
Got a better and slightly higher idle with that plugged. So ther is a leak in the cabin.
Did not help the die on start issue at all.
Why the low idle with the functioning iac unplugged but working