850 2.5 20v - Very occasional stalling

[AndyPants]

Ok so my lovely 850 has an annoying problem, very occasionally it exhibits the following behaviour, at all other times it is perfect:

• Starts perfectly then misfires and stalls.
• Misfiring, hesitation particuarly on slight load.

The following definately increase the likelihood of it happening:

• < 1/4 tank of fuel
• Re-fuelling the car with a full tank and then restarting (done this twice).
• Re-starting within about 10 minutes after a cold start.
• Slight inclines.

The car has had the following new parts in the past 3 or 4 years:

• New Bougicord HT leads
• Recent Volvo spark plugs
• Recent new Bosch MAF
• New genuine fuel pump relay
• new Bosch distributor cap & rotor arm.
• New genuine engine coolant sensor.

I did once run out of fuel 2 or 3 years ago (the computer lied). I recently drove it 310 miles with no issues, then 100 miles in all weathers inc. heavy rain with no bother. Then drove about 5 minutes in it on way back and stopped/restarted. And it stalled 3 times, before eventually going back to normal.

When I was changing the ABS sensor I noticed that the fuel pump was leaking a small amount of fuel out the top with a full tank - possibly a slightly perished seal?

I'm thinking the fuel pump might be on its way out, but I wondered if anyone else had any ideas? Since fuel seems to be a distinct factor I suspect that is where the issue lies. Car has done 120k miles and there are no fault codes.

Thanks

[740T]

You seem to have replaced the common failure points. Have you replaced the coil? Might be worth doing that for all it costs.

[AndyPants]

Originally Posted by 740T

You seem to have replaced the common failure points. Have you replaced the coil? Might be worth doing that for all it costs.

Thanks, yeah I realised last night that I'd forgotten to mention that I recently replaced that too! With a new Bosch one. The ignition system is pretty much good as new now.

[The Slug of Doom]

I have to say I would replace the fuel pump as the symptoms you describe do point to that since you have replaced the relay already.

A known good one off a breaker would be my course of action to avoid significant expense.

The filter should also be changed as a matter of course with the pump.

[AndyPants]

Ordered a, hopefully, perfectly working one from a Volvo breaker on eBay, so figures crossed it works! What I think I'll do is swap them over and see how the car is, if the issue is still there I might buy a new pump anyway but at least I'll be able to rebuild it at my leisure

[byootox]

You could check your fuel pressure before and after you replace your pump to see if there is any difference.

[Cypher007]

could be the camshaft crankshaft sensors. or I'm wondering what a bad fuel pressure regulator would do?

any codes?

[rogerb]

It may be worth trying a crank position sensor. That is the one above the flywheel.
Also try cleaning the throttle and idle valve, with some carb cleaner.

Roger.

[Cypher007]

yes the idle air bypass valve can stick. mine went through a phase of doing it recently. it didn't stall though, just rough idle.

after the second time it stuck (after using a screwdriver, carb cleaner, and GT85 to work it loose) I soaked it for a few mins in GT85, seems to have fixed it.

the other thing is vacuum lines especially the elbow hidden behind the manifold near the PAS pump.

I had a P1 850 and I knackered the fuel pump once by running the tank low. but the car wouldn't start at all.

[AndyPants]

Thanks for the excellent replies!

Originally Posted by byootox

You could check your fuel pressure before and after you replace your pump to see if there is any difference.

That's a good idea, I've got a tyre pressure gauge but assume that's probably not suitable!

Originally Posted by Cypher007

could be the camshaft crankshaft sensors. or I'm wondering what a bad fuel pressure regulator would do?

any codes?

They were my 2nd best bets I thought after the fuel pump, the car has had a long time very occasional jerkiness problem (hence all the bits I've changed!) and I guess maybe it has finally started showing itself properly.

There are no fault codes using Volvo FCR, although I did once get one for an out of range fuel signal - or something along those lines. The petrol gauge never quite reaches maximum so I figured it was probably that.

Originally Posted by rogerb

It may be worth trying a crank position sensor. That is the one above the flywheel.
Also try cleaning the throttle and idle valve, with some carb cleaner.

Roger.

I did clean the throttle body 5 years - 20k miles - ago, so in theory it should be fine still. It had a new genuine idle valve too about 4 years ago, so that should be ok.


It's one of those really annoying intermittent issues. I once had a hot start problem but I cured that a couple of years ago by either the new fuel pump relay/new coolant temp sensor/or unsquishing a hose I'd caught in the throttle cable cover.... My confidence in the car has been somewhat dented now until I get to the bottom of this! Still I should give it credit for doing 900 miles in a week even with this v. occassional issue! It's parked at my mums so I hope it behaves on the journey back to mine!