190E running problems

Hello!

I have had a problem with cold running for some time now. The car is a 85 190E Euro (no cat.). Idle when started cold is about 1000 rpm (in Park), but if I leave the car idling, the idle speed will, after a minute or two, start dropping until it's about 700, and at this time its running quite rough. The idle will slowly rise as the engine warms up, and when reached operating temperature it idles smooth at about 1000 rpm. After previous post I was advised to replace the overvoltage relay, but it made no change.

This is also a problem when the car has been parked just so long that the engine temp has fallen to about 30-40°C. Then the idle will be very low (especially when in D or R) until driven a while. If the engine temp is lower than this, it will idle fine at about 1000 rmp (just as when started cold). I guess its at this temperature or lower the idle air valve sets in.

I am thinking about a faulty temp sensor, so I have tried to test it. First, to check if the circut was functioning, I removed the connectors with the engine at idle, and it almost stalled. Then I checked the resistanse at various temps by removing the two connectors and measuring the resistance between them. With cold engine (ambiente temperature about 15°C) it read about 8000 ohms, at 50°C about 3500 ohms, and with warm engine (80+°C) about 450 ohms. Anybody know if these readings are good or bad?

In addition, I always has to use the accelerator to keep it running the first seconds when starting, even when the engine is fully warm. It will start quickly, rev up, but then the revs will immediately falls down to about 500 rpm, and the car often stalls at this point.

Any help will be greatly appreciated!

[stevebfl]

Sure is a lot to cover. Most of which has recently been discussed.

The idle valve controls rpm all the time not just at certain temps. It has different set values due to temp. All of your readings indicate an uncontrolled idle. The proper tool for monitoring this is a multimeter in the amps position mounted in series with the circuit. Proper warm values should be about 600ma. One good test is to turn on A/C, it should increase the current by 50ma from where ever it was.

If your current readings seem to be unrelated to engine speed you must verify the throttle switch, air flow potentiometer, and possibly a decel switch (not on early models).

The idle of course is a reflection of all the other fuel control functions. On your euro model as with US models fuel control is effected by changing the differential pressure (between upper and lower chambers in the fuel distributor). This is done by opening a controled leak with more or less current flowing through the EHA (electro hydraulic actuator). This signal varies on the different KE systems. Pre 1988 190s use only positive current. After 88 and all 6 and V8 motors use plus and minus current for greater control and better limp home characteristics.

The reason for the better limp home comes from the default position (no forms of enrichment engaged). The plus/minus systems default to zero with the ability to go plus or minus for control. The early 190s use 8ma as the default possition, with the ability to drop to zero or add to whatever. Naturally if your default is zero and you loose control the same result is in effect. If your default is 8ma and you loose control you in effect get a -8ma lean correction.

From the default position there are a number of enrichment corrections. Most systems go to around 20ma with KOEO (key on engine off). Most get 40-60ma starting enrichment (when starter is engaged only). All get cold engine enrichment 0-30+ma depending on temp. Acceleration enrichment (determined by rate of change of air flowmeter potentiometer) 0-10ma. Barometer and feedback lambda control through O2 sensor exhaust monitoring round out the control for US cars. The lambda correction for zero default cars is from -10+ma to +10+ma. On 8ma default systems control is from 0 to around 16ma.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-26-2000).]

Thanks Steve! You sure have helped me to better understand the EHA-system.

I think i get the cold-start enrichment thtough the EHA since its always reving up when started. But I am not so sure about the cold-enrichment. Since this is controlled by the temp-sensor, the sensor is what I first would suspect, and therefore I'm interested in knowing what the resistance is supposed to be. Anyone?

I belive it's a difference in the idle air valve system between Euro and US-models, or maybe by different engine types or model years. According to my Haynes Guide, my engine has an idle-air valve with a internal bi-metal strip and spring, which gradually closes the valve as it warms up. According to the book, its only working during cold starts and cold operation. Another solution to the problem could therefore be that the valve is closing to soon.

BTW, my car has the 2.0 l 4-cyl fuel-injected engine, which was the standard engine in Europe (in most ways similar to the 2.3 entry-level in the US).

Regards
Meb

[stevebfl]

The values you give for temp sensor resistance sound proper although I never use those readings.

The appropriate way to read temp sensor values is by reading the pin voltage at the ECU (or at the sensor). The ECU sends out a 5v reference signal and the resistor allows this signal to be pulled toward ground. By measuring this value the whole circuit is measured. After all the ECU doesn't care about the resistance it cares about the voltage it sees.

The voltages start around 3v depending how cold and wind up less than a volt. (around .6-.8v if I remember correctly).

If your car has the bi metallic auxiliary air device then you do not have an idle management system. You have a manual adjustment and a thermostatically controlled auxillary air valve. Similar in theory to everything back to the manual choke.

One thing to remember. Engine speed is determined by the amount of air not the amount of fuel. Remember when you step on the "gas" you actually step on the air. The best the fuel can do is get it right. Too lean you run slower, too rich you run slower.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-26-2000).]

How exactly do you test the temp-sensor circut? Can I just hook up the multimeter (in voltage-mode) to the two connectors at the temp-sensor? I guess the ignition must be on, but do the engine have to run? I am a bit afraid to do some damage to the ECU...

I guess my car has the somewhat "older" system for controlling the idle (Probably due to less restrict pollution laws in Europe compared to US). At least you can adjust both the idle speed and mixture (CO) by turning some screws at the airfilter housing. The auxiliary air device controls the idle speed while the ECU controls the micture, am I right? Is there any way to test the air device? My problem could of course be a faulty auxiliary air device, and not be mixture-related.

Any info appreciated