XUD Turbo - lack of grunt | Technical matters | Back Room Forum

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Fri 12 Jan 2007 11:38

XUD Turbo - lack of grunt - lolingram

I carefully eliminated the fuel filter, air filter, intercooler trunking for leaks and general wiring etc,. etc.
I recently acquired a Xantia 1997/Rreg XUD Turbo estate - ECU/Bosch equipped. Shortly afterwards a lack of urge had become apparent after washing the engine. After various discussions here, I disconnected the throttle arm position potentiometer, and checked the tracking of the pot with a multimeter:

1) There is an open circuit between the outer two wires - brown and black in other words infinity.

2) The resistance in ohms between green (centre) and brown starts at 1.3K with the throttle arm at rest.

3) When the throttle arm is opened the resitance progresses up to 1.9K ohms

So.... I ran the engine without the throttle position pot connected. It started jst as normal and idled at 1000rpm - again normal.

{8< SNIP easier to delete than edit. Please see sticky thread on the subject of swearing over in Discussion - DD}

So... what does the throttle arm pot tell the ECU? I am inclined to leave as is. The problem followed a wash down of the engine, and clearly, the throttle arm pot got some water into it which has screwed it up permanently.

Anyone got any answers/thoughts?

Can I leave the pot disconnected?

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Fri 12 Jan 2007 22:15

XUD Turbo - lack of grunt - David G.I.Davies

The throttle pot. you have disconnected is used as an input sensor for EGR control,together with pump (or engine) speed and engine temperature.The engine ECU computes the desired EGR opening from these sensors.The output of the pot.should range from 0.5-0.9v at rest and up to 4.5v at full throttle.If you leave it disconnected the ECU will see no output voltage so application of EGR will be very small.This is why you notice improved running with it disconnected.Rather than testing resistance its better to measure the output voltage with ignition on,but from your resistance readings it seems as if the output voltage would be at mid point most of the time and thus maximium EGR would be applied even on full throttle acceleration when it is normally reduced to zero. In addition the rate of change of throttle pot. voltage is used to regulate EGR,if the throttle is depressed quickly the EGR is shut down in response. Can you leave it disconnected? If it is to be left unplugged you may as well disable EGR with a blanking plate. The emissions conscious answer is to renew the pot. to allow EGR to operate.
David Davies

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Fri 12 Jan 2007 22:54

XUD Turbo - lack of grunt - lolingram

Snip...

Can you leave it disconnected? If

it is to be left unplugged you may as well disable

EGR with a blanking plate. The emissions conscious answer is to

renew the pot. to allow EGR to operate.

David Davies

Thank you for your quick and erudite reply.

1) Can I take it that the throttle position pot. is US, and therefore the source of the problem?

2) Will leaving it disconnected produce any more pollution than running an earlier type of pump with no ECU?

3) Will disabling the EGR with a blanking plate improve matters rather than just leaving the throttle pot. disconnected - performance and/or economy-wise?

Thanks again.

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Sat 13 Jan 2007 20:25

XUD Turbo - lack of grunt - David G.I.Davies

I'll try to answer the three questions:
1 It seems to be,but a voltage output test would prove it.
2 No more pollution ,you are not reducing NOX with EGR as on an earlier system
3 An EGR valve which is not operating or hardly operating will lose the self cleaning feature of a normal rapidly oscillating valve and could jam open under certain conditions,so if you want to run without EGR a blanking plate is best.HTH

David Davies

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