Cold Start Stumbling, Etc... Fuel Pump?

[mattsk8]

Got another question. I have a 99 V70 R, and (not all the time, but often) when I start it cold first thing in the morning it seems to stumble fairly badly when I accelerate. Also, the other day I was driving it mainly highway for about a 30 mile trip. After it warmed up, the car ran great for a bit, but then started stumbling terribly and almost wouldn't even go 60mph, it also stalled at a stop sign. It did this for about 10 miles, then just cleared up and took off like nothing ever happened. This was about a week ago and the car's been running great ever since, even the cold starts have been good. The car runs great otherwise.

About a week prior to this, I got a code P0137 which is for the rear O2 sensor (no engine light prior to the O2 sensor code showing up). I got the codes read again today hoping for some light to get shed on why it was stumbling, but still just the code P0137.

I also noticed my mileage went from an average of about 23 down to 20 mpg on this last tank of gas (the tank that had the stumbling episode). That's w/ me doing the math on my mileage, not just using the the thing in the instrument cluster that shows mileage.

Any suggestions? Could this be indicative of a failing fuel pump? If it is an indication of a fuel pump going, what pump do you guys recommend for it?? The car has about 160k on it, so I know a new fuel pump is probably a good idea anyhow, just want to be sure I'm not overlooking something else.

[Ihatespeedbumps]

Checking fuel pressure at the rail is the only way to verify the fuel pump is doing it's job. These engines can run until (albeit poorly) down to about 1 bar before they stall. Because of the difficulty installing a fuel pump in the AWD models you want to be certain it's the culprit. Normal operating fuel pressure should be about 4 bar.

That said, I would first check the age of the fuel pump relay. If the date stamp is original it would be a great idea to replace it as more of a general maintenance item. Also, cracked ignition coils will cause a stumble on cold starts. The reason it doesn't store any misfire codes is because the ECM is still operating in open loop and knock is not detected. This is a pretty common problem with these cars.

The code for the rear o2 sensor would be unrelated to your issue.

[mattsk8]

Checking fuel pressure at the rail is the only way to verify the fuel pump is doing it's job. These engines can run until (albeit poorly) down to about 1 bar before they stall. Because of the difficulty installing a fuel pump in the AWD models you want to be certain it's the culprit. Normal operating fuel pressure should be about 4 bar.

That said, I would first check the age of the fuel pump relay. If the date stamp is original it would be a great idea to replace it as more of a general maintenance item. Also, cracked ignition coils will cause a stumble on cold starts. The reason it doesn't store any misfire codes is because the ECM is still operating in open loop and knock is not detected. This is a pretty common problem with these cars.

The code for the rear o2 sensor would be unrelated to your issue.

Thanks for all the info!!

Just out of curiosity, when I check the fuel pressure, what's one bar? How many psi should I be seeing for me to determine it's good, and what psi would be the cutoff to determine I should replace it?

[Ihatespeedbumps]

1 bar = 14.7psi. Fuel pressure should always be hovering around 4 bar. If you're getting dramatically lower or higher the fuel pump should be called into question.