Tuning with CIS

[pentoman]

All,
I'd like to state that I've found some success in changing the running of a 1990 190 16v by fooling the management system to control the EHA as per my wishes.

If you're interested in this then read on.

At its launch the 16v was criticised in the British media of the period of 'lifeless low rev performance' and in the strongest examples I even read something akin to 'the worst throttle response I have ever experienced'. I can only agree. People who have fitted EFi (electronic injection) and done a custom fuel map have reported big improvements.

I wanted therefore to have a rudimentary attempt at seeing what effects the fuel system's componenets have.

Air mass sensor position signal (or the throttle potentiometer)


Well, so K(E) jet CIS Mercedes have real no throttle position sensor. Instead there's a signal for the position of the air flap. When the air flap moves down the ECU knows it should supply more fuel using the EHA. Why, I am not sure because the movement of the air flap itself injects more fuel anyway. I guess this is what they call 'acceleration enrichment'.

I tried disconnecting mine to see what effect it has. This made the throttle response really bad. There was an even bigger delay before pushing the throttle and getting the engine to do work. However if you get jerky acceleration then disconnecting this seems to fix it.

Wide open/closed throttle switch

All it does actually is signal WOT and Idle positions. It makes no difference to the throttle response in all the in-between scenarios (i.e. 99% of driving) so I have ignored it apart from testing that it is working correctly.




Coolant temperature sensor


This important sensor signals engine temperature to the ECU, and the engine will run richer if the engine is cold (and raise its idle rpm). But you can abuse this by faking a signal at this sensor to fool the ECU to run the car richer. This is what I tried. I got the engine nice and warm but fitted a resistor to make it think it's only 60 degrees C. At low revs the car responds really well. At high revs... well... it was really jerky. Really horrible. I am not sure why. All over the place whenever you moved the throttle position. But then I had an idea, I disconnected the air flow sensor position potentiometer. Wow, the jerkiness went away and I was left with a great performing car that had staggeringly much better throttle response low down.

So I am convenced there is some basic tuning that could be done by changing the fuelling to improve the low-down response. However it would be important to understand what other effects doing this would have, just for example of fuel consumption. It's possibly the factory set up sacrifices response for fuel economy (all the Mercedes brochures boast about the economy of the 16v engine). If true then this would mean that there is room to retain the stock Jetronic CIS system but also to change its behaviour to the benefit of response and performance. My vision is of finding an effective way to force the ECU to richen or lean the mixture. Then it is necessary to apply some tuning work so as to translate that ability into the desired improvements. The ideal solution would be a mappable system such as Megasquirt used in conjunction with the stock ECU system to adjust the fuel map as desired. This would allow mapping on a rolling road.

The EHA:


I fitted a spare EHA and changed the hex screw adjustment on it - nothing good could come of it . Therefore I ruled out EHA adjustment as a sensible tuning method for performance gains.

My car has been returned to standard after this experiment but it has given me food for thought.