Low RPM misfire

[volvy740]

Is any of these the fuel pump relay? I'm assuming the pumps are relayed, but Haynes was no help at all in answering this.

[stephend]

I think it's the big green rectangular one at the left of your photo - but tbh I'm not sure. They're well known for failing (mine went at 150k miles, but the garage changed it, which is why I'm not 100% sure on identification). Beware that later models have a white relay, and they're not interchangeable, so if you buy a new one, make sure you're getting the right one.

[czech240]

Take it out.....open up the cover and reflow the solder..... should be all it needs....

Have a close look beforehand and you'll see a few joints that have cracked....

[Will M]

I have a similar problem in my 940, is the relay the same in that, thanks.

[stephend]

Dunno, I'm afraid, Will. Best post what engine yours has, which may help someone knowledgeable about 940s to advise.

[Will M]

engine is B234F

[volvy740]

Took the relay out and it does look in bad shape. My camera couldn't pick up enough detail to post pictures, but there were hairline circular cracks around some of the solder joints. On the flip slide of the circuit board, there's a patch of brown crap at the base of some of the components that doesn't look like it should be there (vented capacitor, maybe?). Smells kinda funky, too. I'm gunna order a new relay rather than try and save this one. I'll post back with the results.

[volvy740]

So, changed the relay and guess what...yep, it made NO difference. I'm gunna have to take a baseball bat to this car soon - it's causing me a lot of stress! But before I do that, I would like to get to the bottom of this mystery issue, even if it's only to satisfy my curiosity. Below are the remaining rational thoughts that I have left...

1). In light of the discovery that this car has two fuel pumps and I was listening to the wrong one for a priming buzz, I took the fuel filler cap off and put my ear down the fuel filler whilst someone else cycled the ignition key. Nothing. complete silence. I don't really understand the role of having two fuel pumps, so can only speculate how this ties in the symptoms. My thinking is that a bad pump in the tank would give lower fuel pressure than the car is accustom to. Before condemning the pump, I would like to test it. Maybe fish it out the tank and give it supply voltage to see if it pumps. If not, change the pump, if it does, move to the wiring. I'd need to know what voltage it needs though. Before I even do any of that, does the relay I replaced serve both pumps? Or is it two pumps, two relays?

2). In my first post (I think) I suggested the ignition amp. My thinking on this is that the issue seems specific to rev range and is intermittent. My experience of bad fuel pressure is that I gets worse the faster you go, too (which isn't the case here). Is this still a possibility or is it just a dumb idea?

Waiting whilst trying to breath calmly - Rory.

[baggy798]

I thought that the in tank pump only really helps when the tank is low or near empty, but when it's over about 3/4 full the gravity feed is sufficient enough for it not to be noticeable.

Edit: here's a quote from turbobricks:

Originally Posted by smokeyfan1000

If intank pump has failed..... car runs OK when tank is full- 3/4 full. Once tank's level reaches 1/2 tank or less, the engine will quit(due to vapor lock). Let sit @ 5 minutes, and engine runs again for @ 15 miles, then scenario starts again.

Everyone I've had fail did this.

[volvy740]

"Low or near empty" pretty much describes how my Volvo spends much of its life. I don't do much mileage, so I only put about £25 in per week (unless I'm making a long journey).