Worked on 300E, now it stalls with idle valve connected! - PeachParts Mercedes-Benz Forum

pawoSD · #1 12-03-2008, 11:52 PM

I did a bunch of work on our new 300E over the past couple days attempting to solve a cold-start problem.

Symptoms: Car is very hard to start when cold, but starts ok when warm, once started and let run a minute or two, it will never stall out. Idle is very low (500) at all times.

Those were the original issues.

I replaced the idle control hoses (NOT FUN), cleaned the idle control valve, checked all sensor connections, installed a brand new OVP relay (old one had blown fuse but they wrapped wire around the base!

)

I get it all back together, begin cranking....nothing, nothing....battery weakening....so I hit the gas during cranking, it fires Right up and holds like 2,000 rpm with the pedal, I hold it around 1200 or so for about a minute, let off the pedal, it dies. The engine coasts to a stop, then the fuel pump shuts off about 2 seconds later.

I restart, it will fire, rev, die. I start using pedal, starts right up, I can hold the idle as low as 700 using pedal....let off, dies. I'm ready to set it on fire and get a diesel....ok maybe not yet. In desperation I unplug the idle control valve, turn key, fires right up! Idles at 750ish, smoother and more steady than ever before. In drive it lowers to 600. Check engine light comes on. I take it for a drive. It runs great, idles smooth, acceleration is strong.....0 issues. Come back, turn off for 10-15 mins......start it back up, fires right up and idles around 700.....I put it away for now, and we'll see tomorrow evening if it will cold start properly....(doubt it).

What did I do??

The engine wiring harness is Ultra-brittle....but everything seems to be plugged in ok (aside from idle valve). Why would unplugging the idle valve stop it from stalling?

Could my starting/idle issue be a weak coil? I don't want to throw parts at the problem.....that'd only dig the hole deeper.

duxthe1 · #2 12-04-2008, 12:14 AM

With the valve disconnected your idle should be much higher...1500+ When not powered up the valve defaults to full open which should give much more than 700 RPM. I'd suspect a general running problem that is exacerbated at idle. Hard to diagnose what without being under the hood but I'd start by getting a steady idle by whatever means and listening closely for a vacuum leak.

pawoSD · #3 12-04-2008, 12:41 AM

So if it is only idling at 800ish with the valve disconnected then their is another issue at hand? What could that be? The car litterally drives perfectly with the valve disconnected....better than it used to WITH it connected.

Not sure on the cold starts as I haven't tried yet....obviously its not operating as intended.

whipplem104 · #4 12-04-2008, 01:53 AM

This is really odd. You may try with the air cleaner off pushing down on the fuel door and seeing if the idle goes up. This would indicate that the fuel mixture is too lean.

Ron in SC · #5 12-04-2008, 07:03 AM

I had same problem which was cured when I changed the the fuel computer.

babymog · #6 12-04-2008, 09:00 AM

The fuel pump is kept running by a signal from the ignition, if the engine dies the fuel pump shuts off. That is normal.

Quote:

Originally Posted by whipplem104

This is really odd. You may try with the air cleaner off pushing down on the fuel door and seeing if the idle goes up. This would indicate that the fuel mixture is too lean.

I believe that this would indicate the engine was running too rich, an easier way to check would be to unplug a manifold vacuum line to introduce more air and see if it runs better = too rich. Running worse with extra air indicates too lean. Gasoline engines are sensitive to air leaks unlike your diesel, and to temperature-dependent mixture settings.

It is difficult to diagnose completely from symptoms, often a sensor out-of-spec will cause mixture issues, would need to be diagnosed with an ohmmeter/DMM vs temperature using your FSM. Can be the fuel computer also, they do go bad, I've had two replaced under warranty from driveability issues.

It is unlikely IMO that it is the coil based on your symptoms.

If you can retreive the fuel computer PN, I will look to see if my spare is the same, which would be a 5minute swap, and one way to eliminate that from the mix.

I don't know what years the wire-harness was bad, my '91 was only 5y/o when I sold it, wiring was fine, others here very likely can tell you if yours is a suspect year.

Hot-wired fuses/relays are never a good thing, did the fuse remain good after replacing it? Only the fuse was bypassed, not the relay?

Check also the wiring to the O2 sensor, an unplugged O2 sensor will cause the engine to default to mechanical-injection settings. I don't think it'll affect idle though.

How do the engine grounds look? When it doubt, clean it. Weak grounds can cause you to chase symptoms forever, a couple of millivolt leak to a sensor ground can signal a whole different parameter. Ditto with the fuel computer's ground.

pawoSD · #7 12-04-2008, 09:37 AM

The grounds and wiring connectors all look ok, the car is a desert car, so there's no corosion on anything...

Only the fuse was bypassed....I haven't checked to see if the new relay-fuse has blown. The relay itself looked old/original.

I don't have a FSM for the W124....I have been going by what it says for the 1990 300SE W126.....I'd assume the engine/mechanicals are the same, it looks to be. I have a good multimeter/ohm meter. I guess the next step is to verify all sensors to be in spec....

I am pretty sure the cold start injector is working, as there was some fuel residue on its tip when I removed it. After the engine has been running/warming up for about 3-4ish minutes, there is a "click" sound and then it seems to idle a little slower (only slightly) but its noticable. What is occuring when it does this?

I will try to get the part number off the fuel computer, that is the larger unit in the back right? Does the fuel computer control the idle?

I'll also try disconnecting a vacuum hose with it running to see what happens.

Also, when I finally got it started the exhaust was smokey and smelled like unburned gas. Running way too rich I'd assume??

When just sitting there idling with the valve unplugged the exhaust smells normal and clean.....and it runs/accelerates great. If it starts good cold then maybe I will just leave the valve unplugged.

babymog · #8 12-04-2008, 09:42 AM

Sounds like the cold-start injector is working. You can verify by putting it in a jar and watching it when you cold-crank the engine.

I'd also guess that the SE engine controls are identical to yours.

pawoSD · #9 12-04-2008, 09:46 AM

I just hope that by the time I get this all sorted out I am not in need of a new starter!

pifcat2 · #10 12-04-2008, 12:21 PM

Linkage could be stiff. Mine looked ok before I removed it and lubricated the joints. It's worth the 30 minutes work before buying more parts.

What did the CEL code indicate?

pawoSD · #11 12-04-2008, 12:24 PM

Quote:

Originally Posted by pifcat2

Linkage could be stiff. Mine looked ok before I removed it and lubricated the joints. It's worth the 30 minutes work before buying more parts.

What did the CEL code indicate?

How do you read the codes? I know where the diagnostic plug is and the little button + red LED, but I don't know how to use it.

The linkage seems to move ok, I also added lube when I was working on it.

pifcat2 · #12 12-04-2008, 02:19 PM

Quote:

Originally Posted by pawoSD

How do you read the codes? I know where the diagnostic plug is and the little button + red LED, but I don't know how to use it.

The linkage seems to move ok, I also added lube when I was working on it.

Search engine is your friend, got this from a post:

The engine needs to be off, but the key must be in the "on" position.

Hold down the button for 3 - 4 seconds and count the blinks. The number of blinks is the code. Repeat the process again. If you get the same number of blinks, you have just one code. If you get a different number of blinks, then you have more than one code.

When finished, turn the key off, then back on, and hold down the button for 6-8 seconds. When you start the car back up, the CE light will be off.

The only codes that will cause a check engine light to illuminate are codes relating to smog equipment. Your ECU will hold a lot of codes that will not cause that light to come on.

91-93 190E and 300 series (2.6L and 3.0L)
1 No faults in system.
2 Throttle valve switch(full throttle contact).
3 Coolant temp sensor.
4 Airflow sensor potentiometer.
5 Oxygen sensor.
7 TNA (RPM) signal.
8 Altitude pressure signal from EZL Ignition control unit.
9 Current to Electro-hydraulic actuator.
10 Throttle valve switch(idle contact).
11 Air injection system.
12 Absolute pressure valves from ELZ ignition control unit.
13 Intake air temp signal.
14 Road speed signal at CIS-E control unit.
16 EGR.
17 Oxygen sensor signal.
18 Current to idle speed air valve.
22 Oxygen sensor heating current.
23 Short to positive in regeneration switch over valve circuit.
25 Short to positive in start valve circuit.
26 Short to positive in shift point retard circuit.
27 Data exchange fault between CIS-E control unit and EZL ignition control unit.
28 Loose contact in coolant temp sensor circuit.
29 Difference in coolant temps between CIS-Eunit and EZL unit.
31 Loose contact in intake air temp sensor circuit.
34 Faulty coolant temp sensor signal from EZL unit.

The code will immediately or eventually come back if you don't resolve it.

My linkage looked fine too but complete removal, cleaning, and lubing the pins, joints, and moving parts resolved my idle issues. Merely spraying it with WD40 did not.

pawoSD · #13 12-04-2008, 02:39 PM

My car is a 1990, manufactured in Nov 1989....will that make a difference?

I verified that my linkage moves smoothly....and clicks the microswitch properly......so thats probably not the issue.....

Thanks for the code info....I will check that out today....

LUVMBDiesels · #14 12-04-2008, 03:00 PM

If your idle control valve is anything like the one on my BMW this might work. Remove the valve and spray into it with carb cleaner. mine was stuck in a partially opened position due to gunk. I had to spray it about three times before it loosened up.

Also try putting it back on and then disconnecting the O2 sensor. If the engine runs better, the sensor is shot.

pawoSD · #15 12-04-2008, 03:05 PM

I tested the idle valve when I had it off, and it operated properly when I applied 12V to it.....doesn't seem to be stuck, I cleaned it out a few times with carb/throttle cleaner too....