Revs dropping at idle whilst moving.

[Squired]

Hi guys, bit of a strange problem with this here's the back story.

My car (2008 S Max 2.0 duratec) started making a loud hissing noise intermittently a couple of weeks ago, some journeys it would make a noise and some it wouldn't.

Idle was high and it would often surge to 1500 - 2500 rpm when started then settle down.

Had a look and couldn't pinpoint where it was coming from but then it got worse and was happening near enough every time I drove it so investigated further and thanks to you helpful lot I narrowed it down to the PCV hose behind the intake manifold.

Took the plunge and put a new one on myself (£50 from ford's, for a small bit of rubber tube. TRB)

Anywho, yep the pipe was split, and after 4 hours of fiddling I finally got the job done.

Fired up straight away and idle nicely + no hissing, Great.

Went for a test run and whenever I slowed down at a junction, roundabout etc with the clutch in the revs would drop to 500 or so, pick up to 750 then drop again to 500 repeatedly until the car stops moving at which point it idles as it should at about 780?

It's only actually cut out once, but it's still rather concerning. Any ideas what could be causing this? I've checked all air hoses, electrical connections several times and all looks good. Only happens when the cars moving above about 2mph?

Thanks in advance for any help.

[Squired]

Anyone have any ideas?

[unofix]

I think what may of happened is that when you had the air leak your idle speed was high. The ECU has learnt this and has compensated and reduced the idle speed. Now you have fixed the problem the idle speed is too low. You could use Forscan and go in and delete the learnt values, but I kind of think that over the next few days/week it will relearn the correct idle without the need for you to do anything.

I would just like to add this is only a theory and someone with more know how might have a better idea

[Epaulnet]

Hi Squired. Have u try to disconnect the battery for at least one hour? Or sure will help but it's worth to try. Keep in mind that u will need a radio code. Good luck with that. Let us know.

[SiW0020]

Did you ever fix this? My 2012 2.0 Duratec randomly starts erratically idling after driving for a bit. No reason, no way of repeating it, just happens randomly. Garage took a few looks but we’re stumped. I.e. just stop at traffic lights and starts hunting around and then can clear up during journey.

 

regards,

 

Si

[Tdci-Peter]

3 hours ago, SiW0020 said:

My 2012 2.0 Duratec randomly starts erratically idling after driving for a bit. No reason, no way of repeating it, just happens randomly.

Air leaks as the OP had cause consistent raised RPM. Most common cause of random RPM variations is a sticking throttle valve. If you are lucky, a really good internal clean with alcohol or carb cleaner will cure it. Electrical problems in the motor or position sensor in this valve also cause the same symptoms.

[SiW0020]

Okay thanks. I've brought some particulate/diesel/fuel cleaner to add to the tank, I'll try that this weekend.