My 850 GLT 2.5L 10V Manual has developed a fault when applying power or decelerating. It is especially noticable at low speeds. When depressing the accelerator to go faster or taking your foot off to decelerate the car jerks (single jerk)as it takes up speed or slows down. The fault also makes gear change unpleasant and difficult to do smoothly. It's as if the engine revs are incorrect for the speed being travelled. The car sometimes struggles to maintain constant revs at slow speeds (20-30mph) The engine also sometimes over-runs after turning it off .

I've had the distributor cap, rotor arm, HT leads and coil changed and have had the throttle control unit cleaned out, all to no avail.

I was begining to put it down to wear and tear and old age but strangely, for two days recently, the car suddenly drove perfectly and was back to its old self and as smooth as silk but the fault returned on the third day. Is it a ECU problem? No fault codes show up.

Was your battery disconnected during any of those mech. ops? If so, this causes a relearn in the computer, a sort of reset. It has to build memory of driving conditions etc. after the battery is disconnected. That's probably why it started to drive fine again after a little while.

Before all this started the car broke down. It wouldn't start and had to be jump started using another vehicles battery. Fault codes showed up when restarted but they made no sense. Since then I've had the problems as reported. I must admit the car has been running better recently but still over-runs now and again.

My guess is it's a fuel problem - check your fuel pressure at the rail - maybe failing fuel pump, relay, fuel pressure regulator, or maybe dirty injectors too (especially if you are experiencing dieseling - overrun).

Yeah, I meant to say in the original message that the injectors have been cleaned out too. Thanks I'll look at the other things you've suggested.

have you changed the air filter and cleaned the air meter from that?
(Do you use the standard air filter or oiled ones?)

I'vre taken out what I think is the air meter, little plug with holes in it and a small circuit? How best do I clean it?

The air filter has been changed but I'll check the meter.

Please excuse my ignorance but please explain the difference between oiled filters and standard ones. Thanks

Harvey850

The standard Volvo air filter is dry, though you can buy 'performance' filters which have a light coating of oil. If over-oiled the oil can coat the air sensors (in the tubing just off the filter) and cause it to either give false reading or fail.