300 TE cold start problem - PeachParts Mercedes-Benz Forum

whutchens · #1 02-24-2004, 05:31 PM

have a 1992, 300 TE with the 103 engine. When the temperature is cold it has the following behavior:
-starts instantly
-then loses rpm to the point it quits.
after you start it a couple times it catches and cold idles up to about 1000 or 1050 rpm's and runs fine

So what I do when the temp is cold is:
-goose the throttle just barely about two times and run it up to about 1100 rpm's for an instant on each "goosing"
-then it sets in at the right cold warmup rpm and operates correctly

This behavior is not changing or getting any worse. When the weather is up into about the mid 40s or higher, there is no problem. So the car seems to be a little lean on cold startup.

Anybody had this one with their 103 engine?

thanks,

sbourg · #2 02-24-2004, 06:29 PM

I'd guess either the cold-start valve isn't opening, or the idle air valve is sticking.

Steve

whutchens · #3 02-25-2004, 09:38 AM

thanks Steve,

Any suggestions as to how I might diagnose those two valves? I have thought of the cold start valve but was wondering if there is a simple way to test it.

WH

ctaylor738 · #4 02-25-2004, 11:50 AM

It gets power from the fuel pump relay. You can:

Check for power to the valve
Check that the ground is good
Check that there is some resistance across the contacts on the valve

You could also unplug it and see if the starting problem stays the same.

sbourg · #5 02-25-2004, 11:57 AM

First step would be to connect a voltmeter across the cold-start valve. See if you can relate what you see there with the symptoms. Normally expect the ECU to turn it on for a few seconds on cold start. If no voltage, likely not a valve problem, but check to see if it is shorted. If you do get a voltage, it will have to be removed to check for mechanical operation. If no voltage, a temp sensor may have failed - I don't know offhand which to have you test next. Check your service literature.

If both the valve and the control signals are normal, then you might just want to remove the ICV and clean it with throttle body cleaner, rubbing alcohol, or whatever does the trick. I used the latter on mine, but the other may be more effective.

Steve

ctaylor738 · #6 02-25-2004, 02:33 PM

My bad.

After finding the right diagram (P 120/2 on the 124 ETM). It looks like it gets power from the OVP relay, and that the control unit provides ground. The coolant temp sensor is shown in the loop inside the control unit. So another scenario is a bad coolant sensor - the control unit is thinks the coolant is warm and is shutting the cold start valve down.

hookedon210s · #7 02-25-2004, 09:11 PM

I had a similar problem with my 91 300TE. It would idle at very low rpm's when first started cold and almost stall. When warm , it would miss a bit like the fuel pressure had leaked down after hot shut down. I threw an idle control valve, OVP relay, fuel pump check valves and injectors at the problem. Turns out the basic fuel mixture had just drifted too lean for the lambda system to compensate. A simple readustment of the duty cycle to 50% cured all of the problems. I feel like such a bonehead. Sometimes it's the simple, overlooked problems that are the culprit.

whutchens · #8 02-26-2004, 09:15 AM

You are obviously not a bonehead. "A simple readjustment of the duty cycle to 50%." How do you accomplish that?

by the way, thanks for the reply .

hookedon210s · #9 02-26-2004, 09:50 AM

Do a search on this site for "fuel mixture adjustment". There are many posts for this. Be sure that your engine is up to operating temperature before making adjustments. Simple to do if you have the 3mm alllen wrench adjustment tool and a digital multimeter that will read frequency/%. Mercedes specs say for checkup purposes a 40% to 60% duty cycle is acceptable but if you are adjusting the mixture you adjust to 50%. Good luck.

Sundman · #10 03-04-2004, 06:43 AM

Quote:

Originally posted by hookedon210s
I had a similar problem with my 91 300TE. It would idle at very low rpm's when first started cold and almost stall. When warm , it would miss a bit like the fuel pressure had leaked down after hot shut down. I threw an idle control valve, OVP relay, fuel pump check valves and injectors at the problem. Turns out the basic fuel mixture had just drifted too lean for the lambda system to compensate. A simple readustment of the duty cycle to 50% cured all of the problems. I feel like such a bonehead. Sometimes it's the simple, overlooked problems that are the culprit.

Seems like my problem ideed hmmm... se url here... But how do you measure the "duty cycle"... Obvoisly really, i don't understand - sry ;-)

hookedon210s · #11 03-04-2004, 08:20 PM

There are many threads on this site that explain all aspects of troubleshooting and checking your system. However, the short answer is to obtain a good digital volt ohmmeter that will display %. With your car fully warmed up insert the positive lead into pin 3 in the diagnostics plug located on the driver's side fender close to the firewall and the negative lead in pin 2 or to ground. If all is well your meter should display a fluctuating reading between 40 and 60. If the reading is a constant whole number, your system is not operating in closed loop and the problem needs to be diagnosed and remedied. If the reading is not between 40 and 60 at idle, it can be adjusted with a 3mm long allen wrench at the air mixture controller to obtain a setting of 50. If your meter reads properly, making the mixture richer (turning the screw clockwise) will result in a lower number (I think). My meter reads the opposite but this doesn't really matter since you are aiming for a 50. Once this is set, rev the engine to 2500 rpm and check the reading. It should not vary by more than 10. All that my car needed was to be richened up to correct the hard cold starts, stalling and rough idle. The lambda system can only correct the mixture so much and doesn't correct the mixture when the engine is cold (operating on open loop).

Sundman · #12 03-05-2004, 04:03 AM

Quote:

Originally posted by hookedon210s
There are many threads on this site that explain all aspects of troubleshooting and checking your system. However, the short answer is to obtain a good digital volt ohmmeter that will display %. With your car fully warmed up insert the positive lead into pin 3 in the diagnostics plug located on the driver's side fender close to the firewall and the negative lead in pin 2 or to ground. If all is well your meter should display a fluctuating reading between 40 and 60. If the reading is a constant whole number, your system is not operating in closed loop and the problem needs to be diagnosed and remedied. If the reading is not between 40 and 60 at idle, it can be adjusted with a 3mm long allen wrench at the air mixture controller to obtain a setting of 50. If your meter reads properly, making the mixture richer (turning the screw clockwise) will result in a lower number (I think). My meter reads the opposite but this doesn't really matter since you are aiming for a 50. Once this is set, rev the engine to 2500 rpm and check the reading. It should not vary by more than 10. All that my car needed was to be richened up to correct the hard cold starts, stalling and rough idle. The lambda system can only correct the mixture so much and doesn't correct the mixture when the engine is cold (operating on open loop).

Thanx, I will be able to check this now... :-)