I've had my 500SL R129 (UK 1991 model with 119 engine) for 12 years now. Recently advised by MB dealer to get a replacement EZL (also known as iginition control unit) - trouble is they are pretty expensive, and when you're talking £1200 a go, you can't really "throw parts" at the car at this level of spend per part.

I have read quite a few posts on the various forums on this site. There seem to be a raft of problems with rough idling, and the parts that seem to come up the most are:

O2 sensor
Throttle valve switch
Decel switch
Idle control valve and hose
Fuel pump relay
Over voltage protection relay
Voltage regulator
MAF
EGR
Vacuum hose under inlet manifold
Crank position sensor
Air flow position sensor
Electronic Throttle Actuator
Coils

MB technician told me that with my fault, none of these are an issue - indeed, some of the listed parts where not even fitted to my car.

My fault is that on a very hot day (32C or 90F plus), having driven my car for around 15 minutes on a motorway at 60 mph, if you slow down to a stand still my car surges at idle. If you gently accelerate there is an initial drop in power then the engine picks up. This and the surging will clear after a couple of minutes moderate driving, and it will never occur with temperatures under 30C or round town driving - only ever occurs as exactly described above.

So, some advice needed please.

Could anyone offer an opinion to confirm if what described is probably the EZL, or maybe offer up other ideas.

As to replacement EZL, the part number on my car is 012 545 5732, but I have seen threads saying that various ones can be used (on different cars and engines to mine), so can anyone state if this is a possibility on my car and if so which ones.
Finally, does anyone know of an electronics expert in the UK or Cape Town South Africa that can rebuild these things?
(Not asking much, am I - but it's got to be worth a try!)

thanks

Graham:

It's my opinion the MB dealer in your locale is looking to rid your wallet of cash.

The surging could be caused by a number of items in the list you presented. I would suspect any of the idle control mechanisms or an introduction of additional air.

Someone here more knowledgeable about your model will narrow it down, but don't buy the high $$$ EZL. At least not yet.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Send Phil at Partshop an email....I'm thinking he can get you one for the same amount quoted to you, except in cheap $US

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

Since the problem is temp related...have they tried a new temp sensor?

Some times adding 100-200 ohms resistance to that sensor will allow for better summer weather operation..

Does the car have ASR? code 471.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
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44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

you have hit the nail on the head this is temp related.now is there any diff in the engine temp gauge when this occurs?does the temp usually stay at 85 and now it's going to say 95 or 100?this is one of the early 119 engines that suffered from the "green crud" remove both of the distrib caps and look to see if you have the green crud disease both on cap and rotor buttons.pull a couple of plugs at random and look for anything unusual.do you run the climate control all the time?does the a/c begin to load up at the temps you describe?because if true then it may be nothing more than vac leak.send some answers and maybe i can help. david poole european performance dallas tx.

Many thanks for your replies so far.

I have had the following replaced by new Mercedes Benz parts in the past two years:
Both dist caps and rotors, all ignition leads and caps at both ends of all leads, new injectors and their holders and the seals, vacuum hoses across the entire centre of the engine (the ones that sit between each injector), idle control valve, coolant temp sensor, outside coil, new spark plugs (by way of normal MB service), two new fuel pumps and fuel filter, new fuel pressure regulator.

The fuel system was purged by a Bosch technician who also evaluated the KE-Jetronic fuel injection system. The fuel distributor is running good.

The car runs perfectly at idle or during acceleration, outside of the condition I have raised.

Since I wrote the first post, I have to say the issue I have described has become worse in that the symptoms now start in ambient tempratures of around 19C - quite a signifcant and swift detioration in only a few days.

I appreciate what some say about Mercedes attempting to get money out of me, but I have to say I am not convinced in this particular case, as they are going to fit the EZL for a few days to see if it solves the problem. If it doensn't, they will return the part to Mercedes and it won't cost me a penny.

Grateful for further thoughts.
thanks.