300e 86 Stalling at junctions no one knows why

[Peterr]

Hi all,
I have a 1986 300e Petrol 4 door saloon
I have not been about for a while doing charity stuff abroad.
I came back in March and took the old girl out. She was fine for a week or so, then backfired several times and then wouldn't start. RAC didn't have a clue after two hours of fiddling. We then took it to my garage. The owner found the float value blown out of place and pushed it back. All seemed ok then, when she got warm she would stall at junctions.
I had everything checked and tuned by the best guy around here. The rubber boot changed as it had perished.
She runs perfectly on both tick over and revs when she is stationary, both when cold and hot.
However, when one starts to drive her when you take your foot off the accelerator she stalls

I have had the man and his dog look at it; they say change the injector system, or the whole carb...
BUT she runs perfectly well when stationary, its just when she is rolling or coming to a junction when I take my foot off the accelerator.
She has now languished for over a month at the garage. Chris the owner is fantastic but for the first time is stumped.

Please help, I miss her terribly

 

[Alex Crow]

hello there, does this ALWAYS happen, or just occasionally, or just when hot/cold? does it struggle a bit before it stalls? the grey microswitch on the throttle linkage needs checking, also the idle position switch on the throttle valve needs checking. both these switches should be closed circuit when the throttle is closed btw. also the idle control valve can get gummed up with carbon. failing all that, and if the symptom is constant, try driving in position 3 instead of D, if this stops the stalling then it will be the inhibitor switch, assuming you have an auto.

 

[Peterr]

Thanks for your rapid reply Alex,
Yes she stalls all the time, no she doesn't struggle, seems OK to drive off then at the end of the road stalls.
When it first started happenning it was only when she was warmed up.
When warmed up she also stalls when trying to reverse.

I hope this helps.

I will try driving in "3" and Yes she is an auto
Peter

 

[Alex Crow]

from the sounds of it the inhibitor switch problem is unlikely, but do try it in 3. those 2x switches will be the chief suspects. the idle position switch on the throttle valve has a connector on the manifold on the end of a cable. there are three pins in the connector, you need to test the continuity between the central pin (common) and the two outside pins. one of the outside pins is for closed throttle as i said, the other is for full throttle and not a concern to us.

 

[Ivanerrol]

Fuel Pump Relay.

 

[Peterr]

Thanks a million folks, so from the little I have said you don't think I need to change the carb system?
I will take these details down to the garage tomorrow and update you.
PeterR

 

[Alex Crow]

hello peter, your car does not have a carburettor. to change the entire fuel injection system is simply not practicable, you will need to pinpoint the fault and repair/replace as needed. go with what you have so far and get back to us with more questions or the good news!

 

[television]

The car uses the fuel pump relay to speed the car up when in D or the air con is turned on, does it speed up when you put it in gear

 

[Ivanerrol]

Here is an image of where your electronic controls are. This is a Japanese spercced car - the arrangements of the items are a little different but basically follow the same pattern. The array is behind the battery. Remove the plastci cover and you will find these.
The OVP is the Over voltage protection relay.
The FPR is the Fuel Pump relay
The AC relay is for the Air conditioning.

The FPR supplies voltage to the fuel pumps which are under the rear axles area below the fuel filler. When you first turn the key to on you should hear a whirring sound from the rear for a second or so - that's the fuel pumps priming the system. The FPR restricts fuel flow from the pumps in the case of over revving. Due to age, heat and humidity the FPR's and OVP's electrical circuit boards break down. The OVP and FPR's were known trouble units from the start and MB revised the units many times.
You can remove the FPR and then reseat it. This sometimes fixes your problem - for a short while.
You can also remove the cover off the unit and if you are handy with a soldering iron, touch up the solder joints on the circuit board.
90% of stalling problems are the FPR.

These are an expensive replacement unit. If you can find another from a wreckers yard or if you have a friend with the similar model you can swap them over to diagnose.

If this turns out to be the problem then buying a new one is the Best remedy. You can get them from the great auctions site or from the US at much cheaper prices

 

[Alex Crow]

although the fpr soldered joints are common for failure, especially on the relays, the o/p has clearly identified a stalling problem that ONLY happens at closed throttle. it just so happens i resoldered a w124 102 one today, a stisfying and certain cure, this car was cutting out at any throttle position. when i consult a customer with cutting out on any of the ke-jetronic engines the first thing i ask is if it is only when slowing/coasting/at standstill - ie closed throttle, or can it happen when cruising/accelerating.

 

[television]

Alex,,could the fact the the trouble started after the car blew back,,could this have damaged a diaphragm somewhere, or the rotary idle valve

 

[Alex Crow]

the usual is just to blow a pipe off or push the plate upwards and jam it as happened here. i think the o/p said the rubber half of the metering housing has been changed? i think also the idling is fine until it has been driven. hard for us to guess here though.

 

[television]

I wonder if the plate should be re set, I have the instructions


The fast idle should be checked by putting a volt meter between the + terminal and terminal 16 of the ECU multi plug. switch on ign, and select P or N if correct there should be the full battery voltage. check that when in D a drop occurs