Idle Control Valve or Crank Position Sensor?

[lepton]

Hi,
I have a 1990 300SE Auto which has hot stalling problems similar to those reported elsewhere on the forum. The car runs very smoothly when cold or at speed when the temperature is about 78 C, but when standing at lights etc in Drive, the idle revs which are usually very steady at about 700 swing between 450 and 1200. The revs drop back to a steady 700 if the car is slipped into neutral. I have replaced the OVP, fuel filter, the thermostat and the coolant to no effect. Previous threads suggest the problem could be related to a sticking Idle Control Valve or a faulty Crank Position Sensor
(£104). Which is more likely to be the culprit?
Any help appreciated,

Alan

 

[television]

Hi Alan, I do not think it will be the cranshaft sensor, it has an easier life at low revs and I feel sure it would show up at the higher revs.

Malcolm

 

[lepton]

Thanks Malcolm, thats useful to know. The problem only occurs during slight overheating (85 -90 C). Some threads have suggested that the Crank Sensor can fail at high temperatures, though I would have expected the ignition to die completely. Why does the idling speed recover completely when the gearbox is moved from Drive to Neutral?

Alan

 

[television]

Thanks Malcolm, thats useful to know. The problem only occurs during slight overheating (85 -90 C). Some threads have suggested that the Crank Sensor can fail at high temperatures, though I would have expected the ignition to die completely. Why does the idling speed recover completely when the gearbox is moved from Drive to Neutral?

Alan

when your car is at idle it runs on the fuel pump relay, the main ECU is out of action, as soon as you press the throtle it switches over to the main ECU.

Re the crank sensor, strange thing can and do happen, we can all find and learn something new every day. though I said that I doubt that this is the fault, they are not expensive. the fuel pump relays vary on the cars. those with air con and autos speed the engine up to prevent stalling when turning the air con on, or going to D.

malcolm

malcolm

 

[MR DAMAS]

Hi Malcolm can you tell me which ECU is "out of action" at idle whould that be the injection or the ignition?

 

[television]

Hi Malcolm can you tell me which ECU is "out of action" at idle whould that be the injection or the ignition?

you need ignition at idle. The ECU that is out of action is the main ECU.
The switching depends on the injection system that is fitted to the car, on some cars there is a switch on the throtle cable,on others the switch points are the lower ones in the distributer, it is then different on fly by wire systems. you really need to know what system is fitted to your car.

malcolm

 

[lepton]

Malcolm, what you said about the fuel pump relay sounds promising. Although the fault is intermittent, it does only occur when the car is in D or shifted into D from N or P. Where would I find it on a 126?

Alan

 

[television]

Malcolm, what you said about the fuel pump relay sounds promising. Although the fault is intermittent, it does only occur when the car is in D or shifted into D from N or P. Where would I find it on a 126?

Alan

The last very big thread on a 126 was your problem and it was the fuel pump relay. I may have one ,will come back later

Malcolm

 

[lepton]

I have ordered a new fuel pump relay. Do I need to disconnect the battery before replacing the old one?

Alan

 

[television]

No you do not need to disconnect the battery,just make sur that the ign; is off.
I hope it cures it, I cant just tell someone to go and buy a relay, they are complex things.

malcolm

 

[lepton]

Picked up the relay from the local dealer on Monday (no packaging - £106). Plugged it in and attempted to start the engine. The engine fired straight away then died. The relay would only work if plugged in when the engine was warm and stopped working when attempting cold starting. The dealer reluctantly replaced the relay which has worked fine since. The car now starts on the button (it used to take 3-4 seconds of turning over) and the stalling problem seems to have been cured. Thanks for the help Malcolm.

Alan