1999 V70T5M56 Poor cold idle troubleshooting Topic is solved

[abscate]

The only rationale I can come up with for the warm fix is that when the car goes closed loop on ECU, the extra air leaking in is compensated for by the trim. This makes smooth performance and idle. Im really surprised I wasnt tripping P0171 trim codes from this bad hose.

[wizechatmgr]

Better luck next leak? =p

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And, how did this vacuum leak suddenly cure itself? Did they hose get sucked onto itself and close off?

Sounds crazy

[abscate]

Couple of clean up items

1 we bitch about these cars but the engine management system is robust. I had a 5mm intake manifold leak directly Into the manifold at cylinder 3 but got part time cold running roughness, good Power, good fuel economy, and no codes.

2 on reflection , there was a slight flutter on running to closed throttle (think townstop sign) that I see now gone. Just on a whim, I pulled the hose on the warmed up car and it’s was back. This is probably a good marker of a small leak, a slight flutter of revs between 850-1050 on run down in gear. You slush box guys might not get this as your overrun characteristics are different, that’s why manuals rule.

I may have seen a bump in highway mpg, too early to tell. At 70mph I am getting 27 reported vs 25.5 before

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Upthread a drawn diagram of the vacuum hose system on the T5 with hose sizes

...and my intuition was correct.

I did a VAT inspection of the hoses, found nothing, so I removed the ones I could not see buried under the intake manifold and...obvious check valve fault.....

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[abscate]

My brain solved the mystery of the sudden click and clearing of the rough idle problem while I was watching first half of LIV TOT today

This is a VVT car, exhaust

On cold start, Volvo plays with the exhaust valve timing to push flame into the exhaust to clean up the cold start fuel. This also kills the manifold vacuum.

After a bit, click, exhaust VVT changes to warm mode and Presto, vacuum back to normal.