Volvo S40 2007 hesitation / "misfire" during acceleration at low RPM
 

Hey guys,

I recently bought a 2007 S40 1.6 petrol. During the trial run with a mechanic, I felt some hesitation / sputtering, sort of like a small "misfire" when accelerating until we reached about 2000-2200 RPM, then it went away. It can be felt in all gears. The mechanic said it's just spark plugs, so I bought the car.

Now, spark plugs, ignition coil, the cables that go to the spark plugs, oil, oil filter, air filter is replaced. The problem still remains, although these replacements did help a bit. I took it to a Volvo shop, and they found that one of the pipes on the exhaust manifold that come down from the valves is broken, so they advised me to fix that (either by replacement or welding).

The car does have this louder, sort of rattling engine sound that you would get when your exhaust is broken, but could this cause sputtering?
Also, when I refuel, the pump keeps clicking off. Could there be some kind of clogging in the system, or maybe the fumes build up?

Even the engine shut off / stalled once while I was driving. Fortunately I was close to my house but I couldn't start it again. Next day, the mechanics in the shop couldn't replicate the problem, it revved up on the first try. When I came home from the Volvo shop, it almost died on me again when I stopped at a red light, but I noticed the RPM going down and revved it up, then it kept going like nothing happened.

Any ideas?
Thanks in advance,

Exhaust manifolds aren't easy to weld(cast metal sucks) and there are plenty of used replacements available.

A good first step would be to get the car checked by a scanner that can communicate with all the modules in the car, not just CEL checks.

Thanks! In a few weeks another mechanic with an electrician will look at it, hopefully they will find something. I was just curious if anyone had any idea what would cause this. I have a hunch it could be several different problems producing this one symptom.

Yeah, the mechanic guy said he needs to disassemble the valve cover to reach it, and he may have to replace instead of welding.

Edit: So you mean something more detailed than an On-board Diagnostic? Is there a possibility that there is some fault but the computer does not detect it?

There are well over 10 "modules" in the car. Examples include the TCM (transmission), Climate control module, Radio Module, Door modules, REM (rear electronics), CEM (Central electronics), etc. This all are capable of storing codes that may not trigger CELs and also provide their own data related to their functions.

As far as Manifold replacement on Euro-spec cars, I am unsure. But they can be easier to replace than weld in most cases. Especially since you would often need to remove it to weld it anyways.

For the rattle, check the heat shield on the manifold as well as the heat shields under the car. Might be loose and can make a lot of noise.

not sure what is meant by "one of the pipes on the exhaust manifold" means. Did you have a look around? Do you hear any "puffing" coming from the manifold area or downpipe when the car is idling (may be difficult to do without having the car on a lift - not advising to stick your head under a running car :-) ). I'm wondering if they were referring to the EGR tube which comes off the header/downpipe and they can crack/break which is an easier fix than having a crack in the cast iron of the header.

As I understood, it should be this (sorry for the random image, I tried to circle it with a snipping tool):
https://cimg6.ibsrv.net/gimg/volvofo...405b0aedd3.png

Yes, you do want to replace that manifold / cat converter. And yes, that can cause the problems you're having, since the crack will cause very inaccurate readings at the upstream O2 sensor (also known as the fuel-air sensor since it allows the engine management computer to set the amount of fuel based on the sensor's analysis of the exhaust gasses).

Ah yes, I did not realize this was one piece on this car. +1 to what habbyguy above me said

Great, thanks! Hopefully this will solve it at last.