flat spot at low revs 850 T5

Hi all I have owned my bright red 850T51996 for 3 years and its done 60000 trouble free miles in that time now totalling 125000. Unfortunately the engine is now misbehaving. At idle the engine is lumpy and pickup is poor-once 4000 revs are reached power is normal I hasten to add I tend not exceed 3500. Engine is properly maintained by me never thrashed new plugs points and dizzy cap a few weeks ago. As far as can be seen there are no obvious splits in the air supply tube from the air filter box to the turbo.The turbo reluctantly kicks in. Am I correct in suspecting a throttle body or air control unit problem. Is there any D-I-Y stuff I can do I have the usual circuit testing gear and a voltmeter Many thanks in advance ancell

Just an update. Very little power and lumpy till tubo kicks in then smooth as silk. Now if only I could move from stopped at 3500 revs. Going for diagnostic test this afternoon. Will post progress. Ancell

I would look at your cam and crank position sensors. As you have done the dizzy cap/rotor and plugs.

First thing I'd try is to measure the output of the throttle potentiometer. Look for poor conductivity at the lower throttle openings.

Many thanks you guys. What a great resource this forum is. My son and I will get under the bonnet tomorrow. In the meantime I will read up on sensor and throttle threads. On the diagnostic front- garage diagnostic software could not read fault codes. The garage was more than fair ran me home and picked me up did not charge a penny I will start using them for my MOTs. I have booked the car in to a tuning/diagnostic centre on Monday. I am reluctant to use Volvo as it means driving through Edinburgh during the festival with almost no low power pick up. Ancell

Hi all Under the bonnet today.No volt/ohm tester leant out not returned. Removed air cleaner hsg and throttle cable cover inspected all the rubber/plastic turbo pipes except the lower pipes. Removed and inspected every small rubber tube and bend.Cleaned every connection accessible. Noticed a ticking sound from coil-is this normal-or could faulty cause this. Noticed "misfire/fuel starvation" is less serious with cold engine whereas engine rough when hot in the 1000 to 1500 rev range. The temp gauge is its usual rock steady self and no warning lights are coming on. Hope to get my voltmeter back tomorrow.Bought all the components for D-I-Y fault code reader. ancell

Thank goodness the fault has developed into a full blown failure. Car wont start. As far as I can determine coil/amp has packed up-but-as we know there are many reasons for no spark at the coil on this engine. The diagnostic guys took it away this morning fingers crossed its the coil. ancell

Hi all the story goes on and on. Tuner/dignostics very quickly determined faulty coil-replaced. Read fault codes- no faults showing Fault returned on road test. Replaced MAF sensor no improvement[no charge to me]. Vacuum gauge shows 10 PSI? inches mercury? this indicates a valve/mechanical problem-previously checked all rubber elbows/pipes. Checked cam belt-fitted by Volvo- looks like problem here the timing marks are so far out the car would not run-but-it does. A major problem is that the faint scribe line on the back face of the splined crankshaft pulley cannot be found and there are timing marks on the main grooved crankshaft pulley but no mark on the block to line them up with. At present we are waiting enlightenment from Volvo-----------.