Peugeot 406 "P" reg idling speed problem.

Agile · 22 May 2010

This is a 1996cc injection with a rather unusual looking "carb".

The accellerater cable twists a puley on the carb which is spring loaded and returns sharply.

The basic idle speed is about 850 rpm but often returns to 1300 and stays there for several seconds.

What sets the minimum speed?

Whats likely to be causing the reluctance to return to the real idle speed.

There is a kind of diaphragm valve on the two tubes which allow the carb to suck air from the crankcase and rocker cover. Is this likely to be sticking and causing the problem?

Tony

Burnerman · 22 May 2010

The 'carb' is a single point fuel injection unit, which sprays fuel down into the inlet manifold.
The idle speed is usually goverened by the ECU, and the idle speed is often courtesy of its own idle valve, which allows a small amount of fuel mixture to by pass the main throttle body.
Often these rotate in their housings, and are prone to seizure with general engine grot.
Its worth removing it and cleaning it up with WD40.
John

Agile · 22 May 2010

Can you clarify exactly what you suggest I remove and how to identify it?

Thanks

Tony

Burnerman · 22 May 2010

I don't have any pics of this or a manual, Tony but perhaps if you could google Motronic MP5.1.1 this is the type of injection system your car has.
The description of it is MFI-i.
The idle speed is 800 RPM + or - 500 RPM and it is ECU controlled.
John

Stivino · 22 May 2010

I think you mean + or - 50 RPM

Burnerman · 22 May 2010

Sure do!

John

Agile · 23 May 2010

I had read it that way without even reasing it did not actually say it !

This is difficult because I dont understand modern cars so well but just how does the ECU control the engine RPM when that accellerator turns a spindle on the carb?

Now if it was just a pair of 1½" SUs then I would be able to balance them and set the mixture without any problems!

But these modern engines seem to be just a bit more complicated!

Tony

Burnerman · 23 May 2010

Rotating valves are controlled by stepper motors in the injection system, and the fuel flow is regulated by the time that the injector is actually held open.
For other such pleasantries such as a cold start, an extra injector may be brought into play.
All of these sequences are governed by sensors measuring air and fuel pressure and air and engine temperatures (together with a variable resistance from the throttle body position) - all of which feed back to the ECU. Ecu failures are pretty rare really - and when you consider how much info is relayed back to them its a miracle that the engine runs at all....
Its only in this way that engines can happily burn unleaded fuel (due to automatic ignition retardation) and give acceptable fuel economy.
Give me diesels any day though!
John

Peugeot 406 "P" reg idling speed problem.

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