190E cutting out.

[Robert_2253]

Can anyone help me out ? My treasured 1988 190E 2.6 straight six petrol motor keeps momentarily cutting out just as though there is a broken wire in the low tension ignition circuit. The problem started 2 yrs ago, very occassional at first but has progressively worsened. The strange thing is that the problem gets worse the warmer the weather. When the outside air temperature is below freezing there is no problem at all and she goes like the wind. A warm spring afternoon, especially if the car has been parked in the sun, is an entirely different matter, and the car is virtually undriveable.
I've done the usual things including a full service at a Merc main agent garage but to no avail. The ignition switch has been changed, as has the fuel filter, valve guide oil seals, plugs, plug leads etc. Both the distributor and the ECU have been substituted without any improvement. During repeated visits to my local garage the wiring and fuse box connections have all been checked. The car recently passed an emissions control test without problem.
When the car does decide to run ok (on cooler days) it now feels a bit down on power at times with the economy meter further towards the red sector than usual, and at other times it gives a feel that it is only 'just' running.
The garage has now had the car for a month without success.
Any suggestions that anyone can give would be very much appreciated.
Thanks in advance. Robert.

 

[television]

After all the work you have done there is the fuel pump and relays.
Some more info would be useful, does it start and idle OK, does it missfire. you are also saying that it runs OK in the cold weather, and the engine is hot.

Malcolm

 

[Robert_2253]

The car starts first time every time without problem. The majority of the time it idles fine, but sometimes the revs rise to about 1200 / 1400 all by itself and then either drops back to 500 before reverting to a steady 800, or sometimes it drops back to zero and stops.
There is no 'misfire' as such - it is more like a complete ignition failure for a split second - sudden enough to throw you forward against the seat belt. When travelling at speed it causes the car to slow, but usually it sorts itself out before the car comes to a halt. At low speeds, such as when negotiating junctions, the engine stalls but will immediately restart without problem. The indicated engine coolant temperature is fine. At Xmas I drove it 400 kilometers to Paris in sub zero outside temperatures without any occurrence of the problem untill entering a tunnel on the Peripherique - the outside air temp rose to 7 deg C and the problem commenced, and continued for the time I was in the warm city. On returning home the next day the problem went away as soon as I got out of the city into the colder countryside and I drove the 400 kilometers home without any further recurrence.
Hope this helps. Thanks for your time.

 

[Stevie A]

There's few things I think could cause this.

The OVP relay next to the fuel pump relay can cause cut outs. If ABS light has been coming on occasionally, with erratic idle now and then, I would replace OVP.

A faulty fuel pump relay can cause cut outs too.

The ambient temperature effect though is the puzzle. Go and check all your breather hoses and vacuum pipes thoroughly and clean them if they are blocked and reconnect anything that might be loose Also, lift off the air filter cover and clean in the inlet area if it is covered in gunk. You can push the round thing in the air filter housing down a bit and clean the walls of the inlet too. Be careful and gentle when doing so though to avoid damage to components in there.

The only other thing I can think of is a crank or TDC sensor needing cleaned or replaced. I'd try the OVP and fuel pump relays first though.

 

[Robert_2253]

Thanks Stevie.

I will give the breather hoses and vacuum pipes a good going over.

I presume the relays you refer to are the two identical ones that sit side by side in the fuse box. They have been swapped with each other to no effect. The ABS light has not been coming on, although a snowy day some 7 weeks ago proved that the ABS works just fine - thankfully ! The ignition timing sensor, and its lead to the ECU has been changed without any change resulting. The garage have tried directly feeding the fuel pump with power from the battery through a temporary additional cable, but that made no difference either.
I wonder if anyone in Siberia would like to buy a Merc ?

 

[Stevie A]

The OVP relay and fuel pump relays are in beside the left hand side of the battery in by the firewall behind a plastic flap and not in the fuse box. Try searching for "cut out" or something on the forums and you may get more pointers as to cause. I've read of a dodgy fuel pump or fuel pump relay causing your type of trouble.