I pinned out all the inputs to lh2.4 ecu on my 94 940t today and found a couple faults I wanted to get some feedback on:

• I don't think my car has a cold start injector as I got nothing from the pin-out test and I cannot identify under the intake manifold between #2 and 3 runners. is that feature not included on every car? i think this car was originally imported to California where I doubt a cold start injector was ever necessary.
• there is no wire to pin #3 on my harness??? which is for full-throttle contact switch test. i also couldn't get any reading off of pin #2 for the throttle idle contacts. does this suggest a TPS failure?
• engine RPM signal flashed ~6V while cranking. Bentley recommends a minimum of 8V. car cranks/turns on normally so I'm not worried about this but thought I'd flag it since it is out of spec.
• finally, my O2 sensor tests both were out of spec: no continuity with sensor connected, but continuity with O2 sensor disconnected and green wire grounded. does that suggest a failing sensor or merely faulty wiring? I suspect wiring since the green wire grounded should not have continuity. but my car has been running rich with a lumpy idle and poor mpg so I suspect a failing o2 sensor. i have not yet tested the sensor directly.

thanks for y'all's help as always.

 

Unplug the O2 sensor and drive the car - does the lumpy idle go away?

You can easily send a false rich or lean signal to the fuel injection control unit.

With the O2 unplugged - touch the green wire (from the control unit) to the body - this will send 0 volts to the control unit telling it - hey i'm lean, give me so more gas !

Then, lick a finger on each hand grab the green spade connector with one and touch your other wet finger to the positive battery terminal - that will send a small voltage through your body to the control unit saying - I'm rich, lean me out !

Then note any differences, the o2 sensor is not capable of changing the mixture very much.

Crazy but it works - and used to do it often (years ago) to check what - I don't remember. Maybe O2 function with an exhaust gas analyzer.

 

they got rid of cold start injectors some time ago when they realized the main injectors can just do an extra squirt for the same effect. I think our 87 had one and our 92 didn't.

I don't think turbos have the full throttle encrichment switch, just an idle switch..

I test o2 sensors by unplugging the single pin round connector, and wrapping a single strand of thin copper wire around the pin, then plugging it back in, so that thin strand sticks out... set my volt meter for a single digit DC Volts range, connect it to that thin wire and ground, and run the engine long enough for it to warm up, you should see a voltage that fluctuates every few seconds between 0.1-0.3V and 0.7-0.9V and back. if you gently rev the motor, the fluctuations should be faster, like every second or something, while at idle it might be every 5 ro 10 seconds. if its TOO slow, or it doesn't change at all, your O2 sensor is probably dead. they need to be changed every 60K miles or so for best results.

 

thanks for the info guys.

re o2 sensor: mine is a three wire into a single terminal for the main harness, and I can't seem to backprobe it either. I figure I have to disconnect it to test it.

if the o2 proves decent, i plan to look at the Cat and fuel pressure for lumpy idle, which is worst in D at a stop.

 

My bad, you have the three wire flat connector for the O2 - never the less - the o2 sensor cannot change the mixture enough to make a "lumpy idle". You can still disconnect it to see if the "lumpy idle" goes away. The two black wires are for the o2 heater.

The cat has nothing to do with the way the engine runs, (unless it has blocked the flow of exhaust) It's a ceramic honeycomb coated with platinum and rhodium that converts hc and co to co2 and h2o. and it's after any sensors or engine controls.

The fuel pressure regulators rarely go bad, and when they do the fuel pressure is too high when first started and floods the engine. They work fine after that.

For a lumpy idle I would look for an air leak to one cylinder, making that one lean (and missfire at idle) - like a intake manifold gasket - grab some carb cleaner, spray around the intake manifold to head area - if there is a leak the idle will change. Look for the simple/common stuff first -

 

thanks again yall. sounds like the ECU pin outs are basically fine overall.

i unplugged o2 sensor and drove around the block - and it didn't make a big difference. so I assume it's most likely working.

i may have been inaccurate in my description - the car doesn't so much idle lumpy as it just shakes a good bit in D at stop. I don't know if that might be a transmission mount issue or something, but upon further reflection, that is a more accurate description and it may not be a result of the idle at all. still, i'll check more stuff out today and see.

 

Your engine mounts are probably flat. When new they have a compressed gas and liquid in them - and they leak over a period of time until they are flat and transfer the vibration of the engine to the sub-frame/body. I had the most complaints in cooler weather and worse when car was first started. On some cars the oil pan will rub against the sub-frame when the mounts are flat. Most cars have a rubber bumper to prevent the oil pan from touching the sub-frame directly, so check the clearance between the rubber and the oil pan. You could check the clearance of the fan in the fan shroud - but you should have an electric fan on a 94.

With new mounts you can grab the oil cap and move the engine, they are quite flexible - with flat mounts you can't move the engine very much.

The trans mount on your car fails in a similar way - it will split/sag and can transfer vibration - but is usually heard first over bumps or dips in the road.