95 850 2.5 20v Unable to stop it surging !

[se13allmylife]

Hi guys , desperately seeking advice on this one , My '95 855 2.5 20v auto (app. 180k) has had this problem for nearly a year . On medium throttle between 2000/3000rpm the car surges i.e. accelerates - brief pause - accelerates - brief pause e.t.c. On hard acceleration it's fine and same if gentle which means I have to drive like a learner or like my trousers are on fire !! I've tried replacing all the vacuum pipes , the car has had new Volvo plugs , air filter , oil and fuel filter , oxygen sensor (Bosch) , MAF (Bosch) , coil (Bosch) plug leads , rotor arm , distributor cap and injector cleaner through the system , all to no avail ! Not getting any fault codes on the OBD1 . When it first started fuel system cleaner seemed to make it better but only for a few weeks . The car only gets light usage now because of my company van but it's still driving me mad that I can't enjoy driving the old girl the way I used too . I was wondering if it could be a failing fuel pump or injectors , although they sound ok and I would think hard acceleration would suffer more if it were them ! Also fuel consumption has become a bit crappy since this started . Any suggestions gratefully received , you may save my sanity !!!

[jreed]

Unusual symptoms! I have no real idea what the problem is, but can offer up only ideas for things to inspect and measure in the hopes that you might find a clue that will help. Perhaps some of the experts here can weigh in with suggestions...
The first thing I thought of was to hook up a gauge on the manifold so you can observe the vacuum level while the problem is happening. This might or might not be helpful but it's relatively easy to do.
Does this car have OBD2? If so, you might want to connect a scan gauge to it and see if you can see what the computer thinks is going on with the coolant temperature and other sensors.
Are there any strange symptoms like backfiring, oil in the coolant, leaks, sounds or smells that might help point to a problem?

[se13allmylife]

The car is only OBD1 and the problem only shows whilst driving , at idle it's smooth as silk and revs freely . The engine shows good compression , there's no leaks apparent and no contamination of the oil or water . If you accelerate hard from a standstill it does 'cough' once but then takes off like a scalded cat , gentle acceleration is faultless . There is a slight buzz from the auto box on initial pull away and I have wondered if that may have some bearing on the issue but the noise and problem don't occur at the same time . Like I say it only happens on medium acceleration between 2000/3000rpm when the car is moving , this has really got me scratching my head !!

[se13allmylife]

SORTED !!! It turns out it was the fuel pump , replaced it this morning , road tested it and she drove like a dream for the first time in over a year ! Must say this was a bit of a last resort because as I said before I thought a failing pump would be worse under hard acceleration , still it worked and I'm a very happy fella !!!!

[jreed]

Glad to hear it's fixed and working well. Out of curiosity, did you notice anything broken, plugged up or worn out with the old fuel pump?

[se13allmylife]

All the hose clips were tight , the hose appeared good and the electrical connections were secure . The pump itself didn't rattle or feel like anything was loose internally . The only thing that appeared really cruddy was the gauze type filter on the bottom of the pump , it had turned a dark grey in colour ( I'm guessing it was the same as the housing originally ) and the gauze looked very clogged. I don't know how long the fuel line should hold pressure but when I disconnected the fuel lines the return feed was bone dry and maybe a thimble full dribbled out of the feed but the car had been standing for 18 hours . I was also surprised by how little came out of the pump itself , maybe an eggcup full , still it was a rewarding hours work .