Idle speed takes time to settle

[guest5061]

I have an '02 plate base model manual Jazz with about 75000 miles which I'm delighted with.
The original exhaust recently started blowing which made the idle speed run high (approx1200prm) as opposed to the usual 700 ish prm.
I had the mid and rear exhaust sections replaced at a tyre fitter place.
The idle speed is still high and takes approx 10 seconds or so to settle back to how it used to be.
I can't feel any air leaks in either of the exhaust joins with the engine running and hand underneath the car & the joins are also watertight (indicating they're also airtight). My mechanic cousin suggested that a dash warning light would appear if serious but no light is illuminated. He also suggested that the idle speed valve may need cleaning. I have disconnected the battery so that the ecu supposedly relearns the idle speed but to no avail.
I'd be very grateful if anyone could advise what we should try next and/or if you've experienced anything similar.
Thanks very much.

[culzean]

High idle speed could be caused by a leak in the inlet manifold or maybe faulty PCV (positive crankcase ventilation valve )  which allows crankcase fumes to enter inlet manifold under certain conditions.  Any  extra oxygen to enter the system can fool the oxygen sensor in exhaust into sensing that air / fuel mixture is wrong. Dirty air filter,  or dirt on the MAF (mass air flow sensor) or MAF sensor faulty.

Dirt can get on the MAF from the gunge from crankcase coming into intake manifold from PCV valve.

[madasafish]

I'd vote for the MAF first.. It will have encrusted dirt particle on it. Cleaning is possible with carb cleaner and a cotton bub. BUT take great care: the wire is fragile...

[guest5061]

Thanks very much for the replies. I'll look up how to clean the MAF sensor as suggested.
It just seems too coincidental for the issues to only start occurring as soon as the old exhaust started blowing.
I was wondering whether there might still be a small air leak at the cat & mid exhaust section join which would effectively replicate the leak when the old exhaust started to leak??? - it seems reasonably logical to me.

[guest5061]

Ok, so I've now thoroughly cleaned & flushed out the MAF sensor with aerosol carb cleaner as suggested. Reconnected the battery and driven for several miles and the issue is still present.
What would you kind folks suggest to try next?

[guest5061]

My mechanic has now double checked everything I've done, run a diagnostics check and found no fault codes.
We're going to try a new idle air control valve first to see if that fixes it.
I've bought a refurbished one online for £45 rather than a used one from a breaker. Will update with progress when it's fitted.

[guest5117]

Have you tried cleaning the EGR valve as well? Also try cleaning the MAP sensor. There are videos guides for these on youtube. Easy to do and then you have two less unknowns.

[madasafish]

Try cleaning teh throttle body ... it may be encrusted with dirt preventing teh throttle flap from closing quickly.

Carb cleaner and cotton buds again...

Also check the air filter whilst you are at it...

[guest5061]

OK, the problem has finally been solved!
It was the PCV (Positive Crankcase Ventilation) valve after all. Interestingly, nothing showed up on either independent garage or Honda main dealer diagnostic equipment. Honda suggested the PCV and, bingo, it's fixed immediately.
I'd like to add how helpful my local Honda main dealer (Johnson's Oxford) were, especially as they didn't charge me for the diagnostic work and only for the PCV (£61 inc VAT & fitting) which they did while I waited. I also got a replacement drivers door lock barrel fitted whilst there at £104 inc & another spare key which, again, I thought was reasonable.
Thank you very much for your comments and suggestions.