850 GLE, 1995, (very) high idle at startup, then up and down

unxetas · Post by **unxetas** » 30 Dec 2009, 18:57

Hi guys,

Been trying to fix this on my 850 for a while, thought it would be the engine coolant temp sensor, but alas, just replaced it and didn't change a thing

The car idles at around 2000rpm after a cold start, slowly climbing up to 2100 or 2200rpm for 10-20 seconds, then starts alternating between 1000 and 1500rpm, at 0.5-1 second intervals. It doesn't go up and down "smoothly", like a sine wave kinda thing, it's more like a switch, up, down, up, down. A square wave if you will

Could be related or not, but a few months back, the car started dying randomly while idling after a long-ish drive. Didn't happen very often, but every now and then, I'd be idling waiting for someone, and it would stall. It would then have trouble restarting, spluttering and throwing a little fit for a few seconds until it would eventually start like nothing had ever happened.

Any thoughts would be really greatly appreciated! I've replaced the ECT sensor and the thermostat (old parts seemed fine, and as I said, the problem hasn't changed in the slightest!). I've got spark plugs, dizzy cap and rotor ready to swap in, but they aren't that old and I'd rather not replace them unless there's a slight chance that they could be part of the problem (which doesn't seem very likely?!)

Oh, strangely enough, no codes at all..

Cheers!

Post by **JDS60R** » 30 Dec 2009, 19:16

I would like to see the fuel trim data first but hunch says its a vacuum leak, I understand the elbows tend to go bad over time.

unxetas · Post by **unxetas** » 30 Dec 2009, 19:23

As I understand, I'd need an OBD reader to check the fuel trim, right? Will ANY OBD reader do that (like el cheapo from the local parts store) or..?

Actually, come to think of it - will any OBD reader be able to reset the service light as well (VDO instrument panel..)?

Post by **misha** » 30 Dec 2009, 19:44

I just saw that you have '95 model.You allready have diagnostic unit under the hood.Here is how to read & erase the codes:

http://www.volvospeed.com/CheckEngine.php

unxetas · Post by **unxetas** » 30 Dec 2009, 20:03

thanks, but I HAVE checked the codes, there were none

I was talking about what they have at the dealerships, that allows them to check different values in real time (like fuel trim, as JDS60R mentioned)..

Post by **FlyingVolvo** » 30 Dec 2009, 20:07

Mass airflow sensor?

unxetas · Post by **unxetas** » 30 Dec 2009, 20:25

hm.. just ran down to the garage and tried unplugging the maf connector.. Oddly, plugging it in and out had no effect whatsoever on the car..?

The car started at the 2000rpm idle for a while - unplugged the MAF, no change.

After a while, it started cycling between high and low (well, this "low" is still higher than normal, at 1000rpm). Again, unplugging the MAF at this stage made no difference.

I left it running for a while and after a couple minutes, the idle stabilized a bit more at approximately 1400rpm.. Still cycled about 200rpm or so, but not 500-700 as before. Unplugging the MAF had, you guessed it, zero effect again.

Does this help at all?

rcannon · Post by **rcannon** » 30 Dec 2009, 20:54

Just curious what IS the correct idle speed for 97 850 T? Mine idles at 1000 which seems a bit fast.

unxetas · Post by **unxetas** » 30 Dec 2009, 21:01

correct idle speed should be 850rpm..

unxetas · Post by **unxetas** » 30 Dec 2009, 21:37

tried unplugging the IAC valve.. No real reason, just thought I'd try it. The idle went smooth right away, but very low.. seemed lower than what it should be anyway. It kept slooowly creeping down until it eventually stalled, but it took maybe a couple minutes and it was, funnily enough, pretty smooth.

That doesn't tell me much, just hoping it'll help someone here give me a little hand