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Matt, Mechanical Engineer
Category: Ford
Satisfied Customers: 22932
Experience:  BEng hons Mech engineering, in auto industry 22 years
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mondeo 1.8 zetec T.D.C i 59 reg. engine self revving between

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mondeo 1.8 zetec T.D.C i 59 reg. engine self revving between 1250 and 1500 rpm.making car surge forwards even when slowing down in traffic, or on a bend. sometimes when not in gear it revs up and down constantlyand once it has occurred revs dont drop below 1250 rpm. Ford dealer has had it on a diagnostic machine but no faults were found, all values were reset by computer but problem persists.They blamed supermarket diesel but it has been filled up 3 times with BP ultimate but it makes no difference.They now want it back in for 3 full days for tests, but they dont fill me with any confidence.I think they will just start replacing sensors and trying things willy nilly until something works.Any suggestions

Diesels will happily run off engine oil so I'd 1st check the state of the crankcase breather (PCV)
these are sprung loaded and are only meant to 'burp' excess pressure every now and then. The exiting gas is piped back to the inlet , however should the valve stick open then the engine tends to suck the air out of the crankcase all the time and this carrys oil over with it.

If you remove the air intake pipe and look inside where the crankcase breather is plumbed in you may find evidence of oil.

If you do find oil replace the valve

if this is OK then this could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.
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