The vehicle belongs to a friend. He's long been complaining that the vehicle won't tick over correctly. It surges between about 400rpm and 1500 rpm intermittently.

I scanned it using Solus-Pro, came up with the following code: 17961/P1553 (Barometric/MAP Signals - Implausible Correlation).

The timing belts were changed by Seat 10k ago. I removed the timing cover and checked the condition of the belts and tensioners anyway, all perfect.

I carried out a smoke test, which didn't show any leaks in the induction system.

After doing a search on various forums, I removed and cleaned the MAP sensor (which was very dirty), removed and cleaned the throttle valve actuator, and re-set the adaptations. No difference.

Observing the actual value of the MAP sensor with the engine off, ignition on, the value was noted to be very erratic, jumping randomly By about 30mb either side of atmospheric pressure. I ordered a new sensor, fitted it this morning, and guess what, still the same.

It's a genuine sensor from Seat, and displays the same erratic tendencies with the engine off.

Delving a little deeper, I checked the feed and earth to the sensor, 5volts, plug and terminals in good condition. I checked the signal wire to the ECU, disconnecting it at both ends, putting a 12volt feed on the wire from the battery, and checking the condition of the wire using my Teslite leads, no voltage drop at all.

All the plugs and pins at the ECU are found to be in good condition. There was a little corossion at the earth terminals near the ECU, cleaned them all up.

Started vehicle up again, erased all fault codes, re-set throttle valve adaptations again, left it ticking over whilst we refitted the scuttle panel, wiper blades etc. Vehicle ticked over perfectly, until we tried to drive it, the fault reoccurred straightaway.

Since resetting the throttle valve adaptations though, intermittently when the fault occurs, the ASR light illuminates, and throttle pedal is lost too.

I'm a Mercedes van specialist, and have very little to do with cars beyond routine servicing, and despite my best efforts, I appear to have made things worse here. I'd be grateful if anyone has any ideas on this one.