My 1999 S40 (automatic) has just started stalling and misfiring. I might be driving at 100km/h and feel the car suddenly slow, very briefly. Or I might be at a traffic light, or parking, and the engine stalls. I've had the car for several years, and it's never done this before. There is also an engine light on the dashboard, but I'm fairly sure that's been coming on intermittently for years.

I've experienced something very similar before in a Ford, and I'm pretty sure that was a problem with the fuel filter. Today, I took the S40 to a mechanic who said the computer might be sensing a fault and reducing the fuel supply.

My feeling is that replacing the computer would be an expensive job, and might not make a difference. But equally, the fuel filter might not be the problem.

Any advice?

 

You're getting way ahead of your actionable info. There are about 1,000 things that could possibly cause that problem.

Thing is, your car is screaming at you, trying to tell you where the problem is. Read the OBD codes and it's going to point you in at least a general direction (often a pretty specific direction). Better yet, if you can find a Volvo-specific two-way scanner, it's probably going to really help you focus in on where the problem is. If you're a total nerd (like many of us here) you can get VIDA loaded on a laptop and really drill down.

But asking a hopelessly generic question on a forum is a great way to throw lots of money, time and parts at the problem without fixing anything (and very likely, causing new and exciting problems). ;-)

 

digging a bit into what your mechanic said - they are on the path of thinking of fuel delivery issues. First step as Habby noted it to scan for codes. You probably wont get a direct diagnosis from the codes since your model doesn't have a variable pump unless some specific sensor is wonky. Most sensors will get flagged by an OBD code - excepting perhaps the ECT, and you did note a check engine light but didn't mention what codes were found - not much to go on so far. Most likely you may see a misfire issue or a fuel trim issue. Here's a few things I'd check out before you start randomly buying parts:

1) check air boots, vacuum lines etc for anything that may show cracks or dry rot
2) pop the head cover off - inspect the wiring harness and connectors to the coil packs, and look for a date code on the spark plug wires (anything over 5 years is old). These cars are known to develop cracks in harness connectors to the coils.
3) pop out the spark plugs - inspect/clean/check gaps. Anything oily or damaged?
4) find the fuel pressure regulator on the side of fuel rail (small device with a fuel line going in and a vacuum line into the bottom). pull the vacuum line off - if fuel drips out, you have a torn diaphram which can lead to poorly managed fuel pressure
5) If you can find/rent a fuel pressure gauge, check the pressure at the rail. There are vids on Youtube to do this - pretty direct tool - there's a schrader valve (same type as on a tire) that the gauge screws into. Google around for specs
6) pay attention for any other symptoms - ie longer cold start cranking? fluctuating idle? surging at steady throttle?
7) scan for OBD codes, write them down and post up - plenty of info on Google to detail what the code is really telling you. You don't need a big Volvo specific scanner to get started. I use a $30 US tool to read codes on several cars including my son's 00 S40.