960 W/fuel or electronic problems. HELP PLEASE!

[nondem]

I've got a 1996 Volvo 960 w/2.9L engine 182K Miles. This car has one fuel pump(In-tank).
About a month ago it started occasionally having "spells" where it would sputter and/or stall when I hit the

throttle but only occasionally. Between "spells" it ran PERFECTLY at any throttle setting. It had "spells" with increasing regularity. Eventually it became "cronic" to the point that it wouldn't even idle. I was stranded. When I came back to the car it started fine and I was able to limp 10 miles home as long as I didn't give it too much throttle.

I put in a new fuel filter and but the problem returned in a day. The car has progressively gotten worse to the point that it will

start and idle but I can't give it much throttle or it will stall and eventually it sputters and stalls anyway. It "appeared" to be fuel starvation initially.
I've got a OBD-II device(ELM327) and previous to this problem have had a Check Engine fault with the "Air Injection Unit" but that code had been showing months and it stayed cleared a few days each time I cleared it.

Since it started showing long before this problem I don't think it's related, but I'm open to being corrected.

When monitoring live data I'm seeing misfires in the cyclinders when it's sputtering/stalling and The "Closed Loop fuel system" reports an "O2 Sensor fault" but no more detail.
The only hard fault I've ever seen was in the Air Injection System I mention above. It's not come back since I cleared it at the start of my troubleshooting efforts after limping the car home.


I hooked up a fuel pressure guage...Here is what is happening:
The pressure is 42psi constant at idle - Normal.
If I give it throttle the pressure instantly goes up with it but it never makes it to 50 psi.
If I hit the throttle quick the pressure snaps up but it stalls or almost does.
If I give it throttle slowly it'll rev up to near redline and the pressure creeps up with the throttle. In anycase - too much throttle makes it stall.

It seems like anything that makes the fuel pressure flirt w/50psi causes it to stall. I mean it almost looks like there is a

overpressure safety switch at 50psi. Does this make sense?

The only fuel related repairs it's had in the last 10K miles has been the fuel rail and the filter I put on it.

Thanks for any advice you guys might offer,I appreciate it...

[billofdurham]

Welcome to Matthew's Volvo Site.

These are the symptoms of a clogged filter. I would check/replace the sock filter on the fuel pump.

Bill.

[peregrine]

Another thing to check: When installing a new fuel filter, it's very easy to kink the short fuel line between the tank and the filter. That leads to weird and unpredictable fluctuations in fuel delivery, and mimics a fuel pump or fuel pump regulator going bad. Maybe that happened when your fuel filter was last changed; the symptoms don't always show up immediately.

I always check the simple stuff first! By the way, if it's kinked and it's been that way for a while (with or without symptoms), it may be best to replace it because it probably has settled into that (incorrect) shape. It's not an expensive fix.

Hope this helps,
Jeff

[nondem]

Another thing to check: When installing a new fuel filter, it's very easy to kink the short fuel line between the tank and the filter. That leads to weird and unpredictable fluctuations in fuel delivery, and mimics a fuel pump or fuel pump regulator going bad. Maybe that happened when your fuel filter was last changed; the symptoms don't always show up immediately.

I always check the simple stuff first! By the way, if it's kinked and it's been that way for a while (with or without symptoms), it may be best to replace it because it probably has settled into that (incorrect) shape. It's not an expensive fix.

Hope this helps,
Jeff

I'll give that another look. The MAF should be in tomorrow so I'll at least be able to rule that out.