W126 500SEL advice

Page may contain affiliate links. Please see terms for details.
Year is important. Earlier cars used CIS to 1985 which is purely mechanical and later cars from 86 on used CIS-E which is mostly mechanical but had 02 sensors.

When the engine is cold, a cold start injector may be assisting along with the auxillary air valve to give more fuel and air that may cause it to run better. Those may be stuck which can impact hot running. As will the fuel mixture , vacuum leaks , so on.

The testing process on these cars is very manual , requiring fuel pressure guages, smoke maxhine , timing light and exhaust gas sniffer. Give us the year and we will be able to offer better suggestions.
 
I’m no expert on 126s but I’m not sure all 1986 onwards car had O2 sensors, most old Mercedes cars of this era didn’t have catalysts fitted.

I think Grober could be on the money with the ignition module :cool:
 
If the engine was removed and re-installed. I suspect that the issue is likely more related to something that was not put back correctly. A disconnect vacuum line (or incorrectly connected), unplugged temp sensor etc would cause those symptoms.
 
Mid to late 1980's versions of KE Jetronic do not necessarily have an O2 sensor. KE was initially a modified version of the purely mechanical K Jetronic, adding electronic control of some functions like cold start enrichment and fuel cut off on the overrun. My 1989 190e was of this type. When cats came along in I think 1990 or 1991 then of course they did have O2 sensors. A relatively simple and reliable injection system in my experience having run one for 21 years but I did have problems with cold start injector not turning off when it should. You could try just pulling the electrical connector off it to see if it makes any difference to running.
 
In the U.S maybe. Not here in the U.K. None of my KE Jet cars have 02 sensors, as they don't have cats.

Interesting, I was always under the impression that the EHA regulates pressure from duty cycle readings. I'm not sure how it would function without readings from an 02 sensor.

Oxygen sensors are not exclusive to cars with cats as any fuel injection system that requires air fuel mixture as an input will use one in order to adjust fuel trim.
 
Last edited:
Again, I’m no expert - but I thought without the lambda/O2 feedback it ran in open loop, no feedback, just relying on airflow meter potentiometer readings and temperature etc?
 
Evidently not all KE Jetronic ECU/systems are the same . The ECU's changed from Analogue to Digital processing along the way and extra input/output features were added to over the years such as idle control [ motorised?], lambda closed loop control , and laterly knock sensor inputs KE3 (CIS-E III) When the new v8's were introduced in 85 they were initially available in RUF form = no catalytic convertor or KAT form= catalytic convertor /unleaded fuel in anticipation of new clean exhaust regulations [ 1989 Germany] I am guessing that the early adoption of Catalytic Convertors in the American market meant that they were all equipped with lambda sensors post 85 whereas the UK market may have seen a later introduction of the technology hence the different experience on different sides of the pond?
see Wiki
KE-Jetronic (1985–1993)
Electronically controlled mechanical fuel injection. The engine control unit (ECU) may be either analog or digital, and the system may or may not have closed-loop lambda control. The system is based on the K-Jetronic mechanical system, with the addition of an electro-hydraulic actuator, essentially a fuel injector inline with the fuel return. Instead of injecting fuel into the intake, this injector allows fuel to bypass the fuel distributor, which varies the fuel pressure supplied to the mechanical injection components based on several inputs (engine speed, air pressure, coolant temperature, throttle position, lambda etc.) via the ECU. With the electronics disconnected, this system will operate as a K-Jetronic system.
 
Just to update this thread... The problem has now been found and solved. It turns out that it was one of the pick-ups between the engine and gearbox on the flywheel (the ring with the teeth on it) that was causing the issue.

Thank you for all for your help!
 
Just to update this thread... The problem has now been found and solved. It turns out that it was one of the pick-ups between the engine and gearbox on the flywheel (the ring with the teeth on it) that was causing the issue.

Thank you for all for your help!

If that's the pickup that goes from the back of the block to the EZL, it's nice that you found one. They were NLA for some time.
 
If that's the pickup that goes from the back of the block to the EZL, it's nice that you found one. They were NLA for some time.

Well if it is that one then I think the specialist had one sitting around or has sourced one! So I'm pretty lucky I guess. Atleast its solved it and she'd sprung back to life! I should have her back in a few weeks... I'm a tad excited!!!
 

Users who are viewing this thread

Back
Top Bottom