I resistance checked the crankshaft position sensor, it checked out fine with 845 ohms.
I then checked the air flow pot as you recommended saumil, at idle in drive I get almost bang on 0.7V, fluctuating around 0.71 as the idle is slightly unsteady. As the revs are raised the voltage slowly raises with them, no jumps at all. I went upto 3500rpm, way beyond my problem zone.
I will check the throttle valve switch next for completeness. Where do I go from here?! Is the TDC sensor important?

I think the air sensor is ruled out also. Did you verify the EHA function when the car is cold ? Can you check intake vaccum ? At idle, it should be between 16-22 inch Hg. If it is too low, there could be an exhaust clog OR a EGR valve (if your car has one) malfunction. Also what happens to intake vaccum when you press the gas pedal ? Can you check the vaccum that controls the EGR valve ? That valve should be closed at idle, i.e. the vaccum should be lower than 2 inch Hg. Then as you accelerate, at 2000 - 2500 rpm, the vaccum at EGR port should be 5 to 6 inch Hg, enough to open the valve. If this valve does not work like this, idle would be rough and you would also have hesitation during acceleration. These are some other possibilities I can think of. My 87, 260E does not have an EGR valve so would not know its location but should be somewhere around the exhaust manifold since its job is to ricirculate the exhaust.

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Saumil S. Patel

I also do not have an EGR valve on my car or the means to check the vacuum. I'm considering seeing a 'reputable' mechanic later this week though so he can do such a check. I replaced the boot underneath the airflow sensor, the hoses to the idle control valves and the thin vacuum hoses for the injection system though. Is there anything else I can check without a vacuum gauge?

I thought the air intake temperature sensor was pretty unimportant to be honest. Anyhow, running out of ideas I looked on alldatadiy.com where I read "The intake air temperature sensor which is located in the induction system measures incoming ambient air temperature. Information concerning the incoming air temperature is sent to the CIS-E and EZL control units and is used to calculate necessary adjustments to the air/fuel ratio and ignition timing. Low ambient air temperatures will tend to enrichen air/fuel mixtures and high ambient temperatures will tend to retard ignition timing."
So I resistance checked it, it should have the same resistance profile as the CTS. At room temperature I got 16kohm, telling the KE-ecu that the air is -30C and so enriching the mixture. This would explain my sooty black spark plugs too. I'll have to wait until wednesday for the part to come from the big merc warehouse in MK to find out though but I'm hopeful. Also would explain why without the electronic side of things the car performs better, as the base mixture is good.

I think it should contribute as you read, but I am not sure how much it is weighted in the air/fuel ratio calculations. The reason I am saying this is because my 87, 260E does not have this sensor. But this is an inexpensive part and if you done see a decrease in resistance with increase in air temp (you can use a hair dryer), it is bad.

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Saumil S. Patel

The air intake sensor was definitely bad, an ohmeter across the new one gave ~2kohms (old one was 16kohms) but still no joy. I'm running out of ideas, but I think I'm going to go though the entire KE-Jetronic test procedures before bringing it to a mechanic.

Throttle position sensor checked out okay, only just below 1ohm, but below nonetheless, cleaned up the pins too. Looked inside my ecu connector and all was well, no corrosion. I also had the wrong fuel pump installed, with one pin (87H) missing, put the correct one back, no change. It's off to a Merc shop tomorrow.

Yes, check the KE system one last time (though you have done most of the tests already), and if it is not the KE system the only thing I can think of now is vaccum. Perhaps a clog in the exhaust system.

You should confirm that the sensor signals are indeed reaching the ECU (i.e. if the harness is o.k.) Specially the coolant temp sensor cables.

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Saumil S. Patel

Okay I visted Lymington motors where they told me they had fixed another tuned car earlier on in the day. They were meant to check and adjust my EHA valve. When they eventually got the pressure tester onto the fuel distributor (it measures the upper or lower chamber pressure) the tool seemed like a compass, changing reading as you moved it. Anyway they seemed to establish that the pressure differential was only 0.3 bar when it should be 0.4.
They removed the valve and were put off by the screw which they though was the adjustment. The prats were trying to turn it clockwise almost rounding off the threads! I informed them that you undo the screw and an allen key is adjusted underneath. Bottom line was no change and really not much of an idea either except that they initially did a leakage test, all cylinders over 50% so I'm going to strip the engine down and do the rings. Also going to check for a vacuum leak by blacking off vacuum lines one-by-one.

I dont think press diff of 0.3 compared to 0.4 would cause the hesitation at 2000 rpm, for that rpm, the diff would be quite larger, unless ofcourse for some reason the pressure difference becomes erratic around 2000 rpm. The difference should increase smoothly as your rpm smoothly goes up, this happens by reduction in lower chamber pressure (upper chanber pressure is constant between 5.3 and 5.5 bar) as you depress the gas pedal.

The 0.3 vs. 0.4V may cause a difference during startup. Do you have a problem starting the car or idling ?

The hesitation, does it occur around 2000 rpm also when you gradually depress the pedal past it ?

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Saumil S. Patel

Forgot to ask, is the fuel still leaking in the air sensor ?

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Saumil S. Patel

I know I posted the solution elsewhere but in case anyone stumbles upon this thread after a search...

The black sooty spark plugs were due to the base CO adjustment being VERY rich (I'm talking 5 rotations rich, over 10% CO). How it got there I don't know but that's what it was.

The hesitation was due to the ignition control unit which mounts on the inner fender and uses vacuum and an R16 resistor value to calculate the spark signal should be sent. See my later post for more details on this.

Thanks to everyone to helped me solve this, especially Saumil who sent me lots of KE test literature and zakh who spotted the solution!