So, for a while now I've noticed something strange, and I mean for a couple of months. When I cold start the car, and the engine is cold, the idle is rather rough, but it's the sounds the exhaust manifold and cat make that are weird. It's almost as if the system is loose and is wiggling and banging the underside that subsides after about a minute. If I try to simplify this more, it's almost like it's snoring from somewhere around there. When it's doing this, I've noticed if I push the gas pedal a bit, and RPMs rise slightly, the issue disappears. I let go, it comes back. I looked at my OBD scanner and the only thing that changes during that time, is the Timing Advance.

Now my Timing Advance is 10 degrees when the car is warm and idling(and this is normal as per haynes manual), but is -3 when it's cold started and idling and rises slowly. And it's this negative offset that causes these weird sounds, which I believe may be backfire or knocking. I'm not that experienced to say what the difference is between them. The reason I believe it's the Timing Advance is because when I accelerate with the gas pedal just slightly(to raise the RPM by 100-200), the timing increases and the engine calms down in fact.

Now, I'm sure that when the car was built, these Timing Advances were working correctly. The question is, what has changed with age to make these settings cause this weird issue? Of course, I may be wrong, but this Timing thing really does correlate a lot to my issue.

S40 2002, 1.6l petrol, 109hp. Non-turbo.

 

its not uncommon for the flex pipe to break some of the weave and create an exhaust leak. I'd also check around for a loose heat shield (which may tighten up as the metal heats and expands. Can't help you with the timing advance - S40s only came with the 1.9T to the states and timing is controlled by the ECU (nothing to adjust or break). If your car is similar with the twin coils etc it doesn't have a distributor so there's nothing to adjust for timing, but something like a bad engine coolant temp sensor can fool the ECU into thinking the engine is warm when its not. You can do a simple test of the ECT resistance - it should be about 3000 ohms at room temp (ie 22C) and 5500 ohms at 10C, then drops down to about 300 ohms at 85C.

 

To be honest, I have not seen a Distributor. There are only 2 ignition coils so I guess my car is controlled by the ECU as well, I believe(EMS 2000 system according to haynes manual). My coolant temp sensor seems fine from the data reported. And this issue occurs when the engine is cold(like after a night on the parking lot). When the engine is still warm, this issue doesn't occur at all. So this only happens when the engine has cooled to ambient temps outside. According to my OBD graphs, the timing advance is negative till the engine gets to around 40C.

But my point is, that not that I need adjusting on the timing, but something is causing the exhaust manifold/cat to backfire(presumably) or rattle when the timing advance is negative and the engine is below 40C. Presumably when the car was new, it wouldn't have had this problem. I'll also look at the heat shield.

The only sensors that are currently weird, are my O2 sensors and the Air Intake Temp sensor. My AIT sensor creeps up to over 70C at times. My car is black, and it's leather inside, and it is sunny outside, but not yet the kind of heat that we will expect. It's currently more like 20C but temps will reach 40C in August.

As for an exhaust leak, it sounds plausible as well. My upstream O2 sensor is often reporting very high voltages(1.275v). Graph below. And because of these voltages, my ECU keeps adding more fuel.


And the image below is the Timing Advance versus Throttle position.

 

seems plausible that an air intake temp sensor could mess with your fuel trim putting the blame on the O2 sensor. For a $30 part, I'd be inclined to swap it out if its easily accessible.

 

Thanks, I'll try replacing that first and then move on to the next thing if it doesn't help. However I have no idea where the sensor is located on my 02 non-turbo petrol s40. The haynes manual says it's located in the air inlet ducting. Is this the big rubber hose connected between the throttle body and air filter?