Okay, we replaced Idle control valve, mas air sensor, hoses, etc. Car runs great going down the road, but it surges up and down on the idle when sitting.

Still sounds like air is escaping a bit from under intake, but all hoses look good. Would a bad intake manifold gasket cause the air sound and surging idle?

Any ideas?

Thanks

Kelly

 

We had this problem a couple times ... turned out the Coolant Temp Sensor (CTS) was bad. It can be tested with multimeter for proper resistance. Don't make the same mistake we did tho: we bought another CTS but it was the wrong one. In other words, the 1990 760 (and perhaps other Volvos too) uses a dual-sensor in a single unit. One sensor goes to ECU (engine control unit found inside cab on pass side kick wall) and the other goes to ICU (ignition control unit). We initially installed the wrong CTS (with a blue cap, $15) and it started hard and ran poorly. Then we found the right CTS (with a black cap, $45) and everything cleared up. Oh, also the Check Engine Light (CEL) was lit and we had a bad CTS code at the blinker box ... but we thought something else was causing it because the blue cap CTS was new.

Oh well. Bottom line seems to me that the CTS is pretty important to getting things running right. And they are kind of a pain to get to and replace.

I have posted a thread on this forum on this CTS saga ... search a bit and you'll find it.