XUD7TE: surging at idle.

[tomsheppard]

Car a little lumpy on startup this morning. Took it out at lunchtime and there is a surge of about 100 RPM about every three seconds. I have a transpparent pipe from the fuel filter to the pump and can't see any air bubbles (Which doesn't rule it out) but has anybody any idea what is wrong please? Injection is Lucas, Prince of Darkness.

[Kowalski]

If it does it warm/hot, then you can rule out general rough running due to glowplugs etc.
The governer in the Lucas fuel pump is hydraulic and doesn't work properly when it gets air inside it. If you've not got an air leak, possibly a restriction on the air intake or fuel filter could cause surging, if its not one of the above things it would have to be something serious mechanical, maybe its compression test time.

[oilyspanner]

Check for air leaks and perhaps a blocked fuel filter.
Stewart

[Dave Burns]

Check the glowplugs aren't coming on and off, temporarily dragging the idle down by increasing the load on the alternator.
Turn the steering, raise the suspension and/or loosen the pressure relief screw, (assuming thie is the BX) but basicaly alter the operating conditions for the hydraulic pump incase its this thats causing the fluctuation.
If the injection pump has any electric gismos on it, unplug them to see if it makes any difference, check the fast idle cable is not dancing about.
I don't think air getting in would cause such a regular rythem.
How long does the increase last for.
One of the most common resons for erattic idle and hunting is injectors.
Dave

[jeremy]

Can a slow idle cause hunting? Theory is engine slows, idle sped governor or anti stall thinks engine is stalling and speeds it up , and then the cycle repeats. Mine used to do this when cold when the cold start device wasn't working properly.
jaremy