M103 hesitation + other problems / fuel pressure testing

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May 29, 2013
190e 2.6 / W123 230 / W123 230E
Hi everyone, what follows is a slightly long and rambling post, but any help would be appreciated.

Ever since I've had my 190E 2.6 the throttle response hasn't been quite what you'd expect for a car of its power. The car is perfectly driveable (indeed, I've driven it over 20,000 miles in this condition) but it's not as enjoyable to drive as it should be.

The main issue is poor and/or uncertain throttle response at lower RPM, particularly if the throttle is opened suddenly. The car can be hesitant to a varying degree and lacks urge, sometimes feeling like it's on the verge of bogging down. When this happens, there's also often a strange surge of power when lifting off, almost as if the fuel delivery is taking a while to catch up with the air. Things are better if the throttle is 'led' down gently. Oddly, if a wide throttle opening has been used recently, the response seems to be better if you then stick your foot down again. Above about 3000-3500rpm it seems to 'take off' and revs cleanly to 6250rpm.

Bizarrely, the engine seems to be more responsive when you turn corners somewhat enthusiatically (?!?!). Response drops off slightly just after you straighten up.

Another problem is 'dirty' starting - the car usually starts 1st time, but sometimes it takes two goes - firing once and dying, but always then starting 2nd time. In many instances the car doesn't seem to start very cleanly, but coughs into life, taking a second or two for the idle to settle down. Strangely on the hottest days of last summer (ambient air temp ~30C) the car needed the throttle to be opened slightly to start.

The ignition system is, I think, in good condition. The distributor cap and rotor arm are fairly new Bosch items (no difference noticed when I changed them), the (non-resistor) plugs are OK and the HT leads are undamaged.

Other things I've checked are the coolant and air temperature sensors (both OK), and the air flow meter potentiometer (replaced not long ago, voltage OK). I tried disconnecting the O2 sensor, but it made no difference.

The car has always passed its (cat) MOT emissions test with no problem. Every year I've had it tested the CO level is well below limits and lambda is bang on 1.

I've carried out the basic checks using the on-board fault diagnosis. All the static tests are fine - with the ignition on and engine off duty cycle is 70%, 10% with air flow sensor plate deflected, and 20% with the throttle wide open. Interestingly the duty cycles with the engine running are slightly out of spec - at idle (warm and with the purge line blocked) it jumps around 49-52%, at 2500rpm it's 63-66%. The spec is no more than 10% deviation between the two. Does anyone know how bad this deviation is, and does this indicate if my mixture is rich or lean?

I don't think it's an OVP issue as my ABS works fine, and there's never any problem with cold starting, performance when cold, etc.

Has anyone got any idea of what might be going on here? I suspect that the starting issue is the injectors leaking (the car is now not far off 170k and there's no record of them ever having been changed). As for the other issue, every possible explaination seems to have reasons for and against. I'm keen not to just throw parts at this problem, or make any over-hasty adjustments of things like the basic mixture strength (which may just be masking the real problem) but I don't really know where to start.


Possibly related, possibly not, is a rather abrupt 2nd-3rd gear shift when cold, particularly at small throttle openings. This gets a lot better when warm. There are no other problems evident with the gearbox - 3rd-4th change is always fine, the fluid and filter are just over 3 1/2 years and just under 20,000 miles old with no discolouration or nasty smells. Might this be a vacuum issue?

Obviously having the correct fuel pressure is critical, so I've been looking at fuel pressure testers. Either they seem to be unsuitable for the KE-Jetronic system, of uncertain quality, or very expensive for something I'll hopefully only have to use once. This got me thinking... has anyone tried making their own? You can get pressure gauges, hoses, T-pieces and the like fairly cheaply.

Many thanks.
Bogging when accelerating can be caused by bad ignition as the fuel/air mixture "blows out" the spark. It could also be caused by lack of fuel pressure and/or delivery and the starting problem does suggest this may be an issue

My K-Jetronic Mk I Golf GTi was difficult to start when hot because of fuel vaporisation - similar problem of lack of fuel pressure

Some W124s have dual fuel pumps, an accumulator and a filter at the tank end. The 190E may be similar. I'd check that both pumps are working and that the filter isn't blocked with crud. Maybe remove & backflush the pumps if they are accessible

There will probably be a stated minimum fuel rate from the supply with the pumps on and it'd be worth checking you're getting that minimum rate. Also check you have the recommended pressure and that the fuel pressure regulator is working OK

Maybe check the injector flow rate & spray pattern as well

Nick Froome
I had similar problems with my w126 with an m103 3 litre engine, this is an 1989 and has no cat though.
After doing the easier stuff I got a pressure tester and the first thing I learned was that to check mine properly you needed 2 gauges so you can compare the pressure differences in the 2 parts of the metering head! I didn't know it at the time but this was where my problem lay, after gawd know how long on Google and *****ing about I took the EHA which controls the difference off and set it by trial and error.
Previously mine was spot on from cold but as it reached 75c the throttle response and gearchanges abruptly became much poorer, after my guesswork it warms seamlessly with a smooth change and 21-25mpg locally after 197k.
The kit I got and used just the once, as you point out, is something like this this one.

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Check the micro switch on the quadrant of the bowden cable .It sould be contacting the cam of the micro switch. Check and see if the switch gives out that little click as you move the roller in /
Thanks for the replies.

Checking the fuel delivery volume is a good shout. The 190E 2.6 has 2 pumps. The fuel filter should be OK - I changed it at a similar time to the ATF and it made no difference to the performance or starting issue. I do have a new MB fuel filter that I'm planning to fit at some point.

By the way - how much noise are the fuel pumps supposed to make? When you turn the key, the noise of them priming the system is quite audible even inside the car with the doors and windows shut. With the engine running, if you stand at the rear of the car you can hear them running (making a high-pitched whining noise).

The only thing that makes me think it might not be a pump problem is that the car seems OK at high revs.

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