wandering idle

Pcar · Post by **Pcar** » 14 Feb 2011, 10:21

Hi all , I have a 1999 xc70 with 150k that has just developed a strange problem . when the engine warms up the idle starts to pulse up and down between 200 to 500 rpm every second. It only does this in neutral or park or when coasting at a slow speed (10-25mph). This doesn't happen when the engine is cold or when driving at speed. No dash codes and the cars seems to run very well the than that. Just changed the plugs and filters and no joy.
Thanks for any advice, Mike

Post by **JRL** » 14 Feb 2011, 11:29

Either a vacuum leak or the ETM is starting to "go"

Pcar · Post by **Pcar** » 15 Feb 2011, 09:31

Thanks for the info . One thing puzzles me (mostly cause I don't want to replace a $600 part) , if it's the ETM and that problem is cause by actual physical contact by the potentiometer inside the throttle control unit , wouldn't this also happen when the engine is cold ,since the same worn out parts would be rubbing together ? When the engine is first started in the morning it runs and idles fine ,until it warms up. And if it's a vacuum leak, any thoughts on where to check first ?

thanks, Mike

Post by **jblackburn** » 15 Feb 2011, 09:40

A small vacuum leak often won't rear its head until after the car starts to warm up - and the O2 sensor begins to take readings. That is where I would start.

Pcar · Post by **Pcar** » 02 Mar 2011, 13:31

Success !!! I have a '99 xc70 that had an erratic idle for the past 3 months. Sometimes as much as 1000 rpm variation in park or neutral once the car was warmed up . Did a tune up ,cleaned the MAF to no avail . Took it to Volvo and they said a new throttle body ( $1200 ) was needed. So I took it to an independent to have my throttle cleaned as suggested by this site. The car now purrs like a kitten and the idle is as stable as a rock. Thank you all who made suggestions . You saved me a $1000. And now a warning to all those like me who are new to this site. Do your research , ask questions and try all the simple inexpensive suggestions first. Members of this forum have been very gracious with there advice and I really appreciate it. The irony is if I had taken Volvos word for it and changed my throttle body ,it would have fixed the problem and they would have said " We were right, you did need a new throttle body" , when all it needed was a cleaning.

Thanks again , Mike

Post by **JRL** » 02 Mar 2011, 15:00

I had the same thing in my 99
(see post, "I'm stumped")
Today I installed a new MAF and the idle is as smooth as silk ...so far!