Volvo Repair: Volvo s70 turbo revolutions problem, torque convertor, volvo s70

Question
Dear Sir,
I have 1997 Volvo S70 Turbo. In the last few days I start experiencing very unusual problems with the revolutions. The car is working fine but when I am trying to stop my car, revolutions are significantly dropping down from usual 900 to anything between 400-500. This happens on and off. Sometimes it is just working fine but sometimes it is such a nightmare. I don't know if I can relate this problem as electrical or mechanical. The problem occurred a few days ago when I installed a new stereo. It was done by the professionals. Apart from a new stereo I got lots of other things installed before such as turbo timer, boost gauge, neon lights etc. Also I was advised that my battery is old and soon it needs a replacement. When the car is not moving the revs go back to usual 900 and when it is going everything is fine too. This problem really worries me and I don't know what to do. Can you please advise me what to do.
Sincerely,
George


Answer
Could be one of two things:

1) The lock up clutch in the torque convertor isn't releasing.  When the car is cruising the torque convertor locks for efficiency but it has to disengage when the cars stops otherwise the car stalls.  Since its only at decelleration this is a good possibility.  There are no service parts for Volvo transmissions though so if that turns out to be the problem the whole transmission would have to be replaced.  The only way to tell for sure that this was the problem is to do a transmission line pressure test with the volvo diagnostic computer hooked up to activate the different solenoids and note the proper changes.  Only a dealer can do this test though, unless the shop is volvo specific with the proper tools.

2) There is a fuel related issue that isn't adapting properly for deceleration.  Normally you get almost 0 fuel or total cutoff with hard deceleration, so if that isn't adapting properly then it could cause a stalling type issue.  It would most likely be momentary though and not constantly during deceleration.  That would be very difficult to diagnose and there is no way to do that at home.  A reputable shop would need to trouble shoot that one.  

Thats all I can think of right now that could cause the symptoms you're describing.  If you find any more details later you'd like to relay to me I'd be happy to update and diagnosis I could give you.  If i find anything new or think of other possible causes I'll let you know.  Hope that helps.

James