Help - feels like fuel starvation

[gimbals]

Hi all, I have a 1999 C180 - a problem has developed recently - if I am accelerating for any length of time or travelling up a long hill at 70mph, the engine starts to hesitate - it feels like fuel starvation. As soon as I ease off or drive gently, everything is fine.

The car has 69000 on the clock with the original fuel filter. Can anyone shed any light on what might be wrong?

All help gratefully received....

 

[teky]

new fuel filter, then come back :wink:

 

[gimbals]

new misfire...

Ok, I changed the fuel pump and it has cured the problem of hesitation at speed, however, a second problem has been uncovered. When on the motorway travelling at say 60mph, if I accelerate hard by depressing the kick down switch the engine starts to misfire and then will not exceed 60mph. When I stop the car, it idles like it is running on three cylinders.

As soon as I switch off the engine and immediately restart it, the problem is cured and I can drive normally as long as I do not use kickdown.

The misfire feels like an engine management thing.

Can anyone help?

 

[gimbals]

sorry, I meant to say i changed the fuel filter not the pump......

 

[Ade]

If you have a catalytic converter I would check it is not blocked. I had similiar symptons on my 1991 230TE. Does the engine idle freely when you first start up? If so I would go for the Cat being u/s.

 

[gimbals]

when I start up from cold it revs at around 1500rpm then settles to around 800rpm. Is that what you mean by idle freely?

 

[Ade]

What I mean is does the engine cough and splutter before you pull away i.e. misfire etc.

Without the benefit of actually test driving your car I still think it's your CAT as your engine sounds fine. On mine, the car feels ok until it's being running for a bit and then the symptoms you describe start (as CAT starts to warm up and clog). The 60 mph top end is the giveaway (restricted exhaust/backpressure). How you can test the CAT I have no idea.



My CAT is being completely removed this week (it's a pre '92) so I'm lucky but unfortunately you'll more than likely have to replace yours. However, I have heard you can get CAT cleaners although from where or how effective they are I have no idea .

I went through all the same suspects you are looking at and none were the cause but it could be your kickdown relay but again no idea how to test it.

 

[gimbals]

thanks for clarifying Ade. I think mine is different. It only has a 60 top speed when it is misfiring badly. As soon as I switch off then switch on the ignition, the misfiring clears and (as long as I do not use kick down and rev the engine hard) I can creep up to any speed I choose.

If I floor the pedal from a standing start (and depress the kick down microswitch) It accelerates well but is very jerky when changing gear. It does not change up until it hits 6500 revs which worries me a little. Is this normal? After accelerating like this it always develops the misfire which, as I mentioned earlier, can be cleared by switching the ignition off and on.

Any ideas?

 

[Ade]

Hi gimbals

The only thing I can suggest on the information you have given is the kick down relay could be playing up. I'm no expert but this would seem to be a logical place to start as my understanding is this controls fuel delivery as you kick down.

You may be right but dont dimiss the CAT altogether though. I never thought in all my investigations that the CAT was the problem until an unfortunate episode with a dodgy mechanic (recorded in previous topics) revealed the true cause.

 

[justin1820]

air flow meter every time

 

[gimbals]

I have cleaned the air flow unit but it still seems the same. I also noticed that I cannot get the engine to rev any higher than 4000rpm when the vehicle is stationary. Is this a design thing with MB and auto boxes or is it part of the problerm?

 

[jberks]

hi,
Im not a mechanic and the its a different engine, but vaguely similar issue and I guess the injection system is a similar design. My E240 is very slightly hesitant at speed (slight variations in acceleration on a static throttle, and on kick down it has occasionally missed, not wanting to go beyond 4000rpm in 3rd). Apparently, according to the diagnostic done last week, this is down to the oxygen sensor being very slightly out of spec (new one on order). Amazingly the air flow meter was ok - to the surprise of the mechanic. Perhaps if yours is way off, the engine management is going into a 'get you home' mode and by restarting the engine, you reset it ?
By the way,
The 4000rpm at idle rev limit is deliberate, my C220 did it.

Just a guess but you never know.

good luck
Julian

 

[mjtray]

I 2nd the opinion that it may be your O2 meter...and yes your car will not rev past 4000 whilst in park as it is designed to saveguard transmission etc.

Good luck

 

[gimbals]

update

thanks for the good advice - I cleaned the O2 meter with brake cleaner and although i initially thought there was no improvement, i was wrong.

Acceleration and performance is much better - but - the engine has a habit of developing a serious misfire, as though running on three cylinders and refusing to travel above 60mph. It usually happens if I accelerate hard and use the kick down switch. If i drive gently the problem does not arise.
I can clear the misfire by stopping the car, switching off the engine and restarting. It must be electrical and it must be in the central control system but does anyone know where and how to fix it? :cry:

 

[jberks]

Hi,
the only thing I can suggest is that given the fault appears to be engine management related and that the computers on modern mercs seem to log everything, it may save you a lot of aggrevation to have a diagnostic done. Chances are that the computer has logged the fault and will tell you exacly what it is. It may be preferable to potentially replacing parts on the off-chance. Plus the misfire may well kill your catalytic converter if left and they can be pricey. There are some good non franchised specialists around, I think the diag on mine was only about £75 and it took the guesswork out at least.
cheers,
Julian