D5 (2005 on) Help please- rough XC70 D5 idle with AC off?!

[Decky62]

Hi, I need some help please! I’ve had a rough idle on my XC70 D5 2009 for a couple of years now and I can’t pin point the problem. The car runs fine with the air con on, as soon as the air cons turned off it starts shaking, rough idle and almost stalls. The revs also drop when coasting coming to a stop, pressing the clutch will stop it from stalling. It seems to idle rougher when warm/hot.

I do a full service every year
Replaced MAF
New battery
New glow plugs
New alternator
Cleaned throttle body
Replaced camshaft and crankshaft sensors
Checked all sensors and connections
Cleaned most of the earths
Showing no errors on diagnostics

Any help much appreciated, car is on 198k and is a manual.

[SnineT]

I can't say what it is but the aircon when engaged brings the revs up 100 rpm or so which then masks the problem.

Has it only just started doing it or has it always done it, it's possible it's been owned by a fuel miser who had the idle turned down at some point in the past.

Sorry to say it but it's one of those can of worms problems without proper diagnostic readings to work with, one of the other chaps might have seen this before and be able to narrow it down for you.

At a guess I'd be looking at the fuel shut off system if it's trying to stall when coasting.

[Decky62]

Thanks for your reply. The aircon seems to push the revs up 20-50rpm which it seems to like. But after a while the idle goes rough even with the aircon on.

I’ve owned the car for 6years, it’s only been rough for the last 2/3years but it’s getting far more frequent now.

Il look into the fuel shut off, cheers for that

[ca2n]

I would try to find someone with VIDA to do a scan on your car for any fault codes. I'd also be particularly interested in the live fuel rail pressure readings.

Hope this helps.

[Decky62]

Thanks for that. Il have to try and find someone that’s got one. When I had it plugged in a couple of days ago there were two readings for the mass air flow in mg/s if I remember correctly. I can’t remember the terminology but the top reading in mg/s was fluctuating with the idle. When the air con was on it would settle down. This is why I tried a new MAF but it made no difference.

The car still has loads of power and no warning lights on the dash

[hi tower]

Possible air leak somewhere after the maf?

[dingov70]

Hello there , start simply . With engine running shine a light on crank bottom pulley and watch to make sure it is running true . An increased loading can level the pulley up and the issue is less noticeable , but over time the "wobble " returns and the rough running returns . Increasing RPM can make the duff part run more accurately for a short time

Also try and look for cracks in the rubber sandwich indicating failure is on its way

Another question is , do you have a rumbling sound once it runs rough ? as a similar issue affects dual mass flywheel assemblies . I encounter these on almost a daily basis when performing MOT emission tests .

[brickman]

Dual mass flywheel failing or as above, the harmonic balancer/bottom pulley is failing. The pulley failure isn't obvious without getting your face in there to see it and check for it falling apart, quite common at this age and can lunch your engine when it goes quite easily.
both items would be masked by increasing aux load, AC/alt/PAS pump putting load of the system.

[Decky62]

Thanks for your replies. I’m still stuck on this one. DMF is fine, I’ve had a full timing kit on it and water pump. The rough idle is still there! It’s runs fine with the AC on, once it’s hot and the AC is turned off it still idles rough. The interior lights also flicker slightly when it idles rough. Any help appreciated

[Simmy]

try a leak off test on the injectors also check that they are matching /check coding.