Vacuum in fuel tank causing lumpy running/power loss

[AllHailKingVolvo]

Hi all,
I've noticed recently that my 940 tends to lose a bit of power when warm, or ten mins into a journey, and gets a bit hesitatey (for want of a better word!) up hills and under power, as it did before I did an ignition system overhaul.

Yesterday, I fuelled up and noticed that there was a significant vacuum buildup in the tank when I took the cap off...more than I have experienced with other cars, and not something any of my previous Redblocks have done.

Oddly enough, the car ran like a peach for the next ten mins, then the running issue reoccurred. On my return journey, the same thing happened...ten mins of smooth, then the lumpiness was back.

So, I pulled over, unscrewed the fuel cap by 2.5 turns and there was the vacuum again. Immediately the idle smoothed out, I hopped back in and took off, and the car was instantly much better. LOADS more power and a lot smoother.

Any idea what is causing this? A duff cap or a blocked evap system?

[Laird Scooby]

Originally Posted by AllHailKingVolvo

Any idea what is causing this? A duff cap or a blocked evap system?

Yes, one of those!

You should have some charcoal canisters under the bonnet Phil, they store the fuel vapour and release it into the engine under ECU control. Worth testing the solenoid valves that do the control and also the hoses to/from the canisters.

However, the most likely cause is a duff fuel cap. I assume it's the original cap? If so, have a look on it and see if the words "Ohne Luftung" are printed on it anywhere (usually on the inside face) and if so, the cap isn't your problem. Those words are German for "without ventilation".

If that is the case, you need to check the routing of all the pipework for the charcoal vapour system to ensure the one that allows air back into the tank after the vapour has been purged isn't kinked, blocked or whatever.

I'm not too familiar with the system used on the 940s, just the basics of how they work so it'll be a bit of a suck it and see exercise but i'm sure you'll find it if you're methodical.

Volvo still sell fuel caps, only in black nowand if memory serves, they're about £23 and you can get aftermarket ones for about half that. However the aftermarket ones are carp.

Good luck with it, let us know what you find!

[griston64]

I'll have a spare on my breaker 940 if you need one Phil. I'm going to be at car tomorrow

[AllHailKingVolvo]

Originally Posted by griston64

I'll have a spare on my breaker 940 if you need one Phil. I'm going to be at car tomorrow

Thanks Mark! I'll take it, please let me know how much you need for cap and postage and where to send!

[griston64]

Originally Posted by AllHailKingVolvo

Thanks Mark! I'll take it, please let me know how much you need for cap and postage and where to send!

Ok Phil

I'll pm you

Cheers