Replaced fuel pump, fuel pressure regulator, fixed starting issue and higher rpm stutter. But it still has a horrible idle. If I increase the rpms by a few hundred it helps. But when sitting at idle it just strugglings to stay running. Could this be the ignition coils? Or is there anything else that could cause this? It's really frustrating me -_- also plugs and wires were replaced under a year ago.

 

I've actually never sold an S40, only S60 and above but I'm happy to share my thoughts.

First, the only way to service these vehicles is to start with codes in the engine computer and others. That often gives a clue as to what's going on.

The fact that the car runs fine with increased RPMs says that you do not have a fuel delivery issue. Guessing on these cars is not only a waste of time, it gets expensive.

I would first get someone to scan the codes in the computer. I suspect a few things:

- A vacuum leak that causes a lean condition at idle but smooths out as you increase speed.

- Clean the throttle body. A dirty throttle body confuses the computers on these cars since at idle it isn't pulling through the amount of air the profile calls for. To do this properly, I remove the throttle body and clean both sides.

Let us know what codes you find.

 

It had a few codes, a mechanic looked at it but found nothing, I think it might have been a vacuum leak he mentioned. I'll see if I can find the paper. This is my girlfriends car so I'm not constantly driving it and observing it's behavior but I try to keep tabs on it. What would cause a vacuum leak?

 

A vacuum leak can be caused by a broken rubber hose which is most common.

 

Check this S shaped hose between the intercooler and the throttle body. It tends to get brittle and break.

 

I checked it and upon squeezing it some cracks appear but when I let go they kinda retract back into shape, is that enough to mess up the idle?

 

If you have some hose you can try placing a temp piece in place of the old hose to see if anything changes.

Is the check engine light on?

 

 

Yes engine light is on

 

ok time to get a code reader - you can buy one at Harbor Freight for $39 or "rent" (usually free with a deposit) from Autozone or similar. there's a socket under the steering column that you plug into and follow the instructions to read the codes. Once you get those post them up to get some info on what they are telling you.

 

P0128 coolant temp below thernostth regulating temperature
P0420 catalyst system efficiency below threshold bank 1
P0442 evaporative emission system leak detected small leak

 

P0128 Thermostat needs replacing.
P0420 This one usually means you need to replace both top and bottom O2 sensors. The 1st cat convertor may be the problem but usually it's the actual sensors.
P0442 This one is usually either a loose fuel cap or a bad fuel cap. Sometimes it's an actual hose but you have to eliminate the cap as a suspect first.

 

What harm comes from a bad O2 sensor? My car used to foul those left and right but I never noticed any difference in the way it ran

 

Fouling an upstream (pre-catalytic converter) O2 sensor can cause all sorts of gas mileage and power issues since that's the one that the ECU uses to set the fuel / air mixture.

Fouling a downstream sensor will just set an error code (like the P0420) that suggests that your catalytic converter isn't operating properly. Keep in mind that sometimes that's exactly what that error code means, or it can be a bad sensor (they can be suspect at over 100,000 miles). If the cat is "bad" but not "clogged up" and causing exhaust back-pressure, the only problem will be passing emissions if your state requires that test. Otherwise, you could drive it forever with no ill effect, other than the CEL being illuminated.

 

I'd start with the P0442 code by inspecting vacuum lines including the ones by the cannister - a vacuum leak can show up as an O2 sensor issue. Regarding the cat efficiency code, could be sensors or could be an exhaust leak (or could be the intake air leak). I'd work through those before throwing $400 into new sensors on a guess.

 

The O2 sensors don't usually just go bad. It's a wear item and they degrade over a long time. That's why 100k miles is the recommended change interval.

When my 2001 set the P0420 code I simply added a CEL Boss from IPD to the bottom O2 sensor since my state doesn't have emissions testing. I did replace the top O2 sensor to ensure the car wasn't running too lean and maybe roasting the cat convertor.