Just started happening today. '97 850NT stalled when slowly moving through a parking lot. Finally restarted but it surged between 500 - 2000 rpm. Could be driven fairly normally but stopping at intersections was a guess whether or not it would stall.

When surging, exhaust showed real rich mixture & smelled bad. A couple times when I just let it surge with my foot totally off accelerator, it seemed to rectify itself after a minute or so, other times it just died.

Obviously some sort of engine control sensor but which one? I'm thinking it's not related to ignition.

 

Possible faulty coolent temp sencor. When the computer sees the engine as cold but it's really is warm, it will send way to much fuel through the injectors. It will start up just fine because the engine is cold. It will go down the hi way just fine because of high air flow and high RPMs. But as soon as the pull off of the hi way and try to idle the engine, everything goes haywire. The computer can not handle the over rich condition.

 

Thanks Trektech. Your explanation makes perfect sense. Where is the temp sensor located?

 

It's threaded into the front of the thermostat housing below where the top radiator hose leaves the engine. It's a big brass bolt looking thing with a wire coming out of it - see picture.

 

Auto part's store only showing a sensor for a turbo. Is there a difference between NT & Turbo sensors? I'm guessing there would be no difference.

 

No difference

 

Well, that didn't solve the problem. installed new sensor but as soon as engine temp started to rise the symptoms returned. It all made so much sense. Any other ideas?

 

there's also a temp sensor on the intake air path

 

Where's that temp sensor located on the intake air path?

 

I'm guessing by intake temp sensor you're referring to the MAF sensor on the outlet of the air cleaner box? I inspected and cleaned that with CRC MAF sensor cleaner to no benefit. The Haynes manual suggested the EVAP valve as a possibility so I changed that also to no benefit. The car is running worse yet today, it won’t even idle. Next I’m considering the crank and/or cam position sensors? Idle speed motor? ICM –Coil assembly? Anyone want to weigh in on any of these ideas? I have checked the codes several times and have found none which seems odd for such a situation.

The way this occurred, out of the blue when the car was running so perfectly seems to indicate some sensor or module just quit doing what it’s supposed to do. Any brainstorming is welcomed! TD

 

Try searching google on "volvo 850 intake air temp sensor" for some related posts that may help

 

The happy ending to this saga occurred late today. To date I had replaced the coolant temp sensor, the EVAP valve and the cam position sensor. I checked the fuel pressure & did a hard reset on the computer module, but all this was to no avail.

Yesterday I ordered my next three options from B&R Wrecking and they showed up today at a bargain price; a MAF sensor, a coil/ICM unit, and a CPU. It never hurts to have spare parts around for the next malfunction, right?

I decided I'd install them in order of "ease of installation" which meant that the MAF was first. I cleaned it and installed it and ... "Shazam" no more problem!

These cars are so great when the work, and such a vexation when they don't, but most times the answer is so simple .... just have to find it!! TD

 

Good deal. I probably should have noted in my comment that the most likely source of idle problems are vacuum leaks, MAF sensors and the idle air control valve. On the newer models the electronic throttle can act up as well.