I have a Volvo s40 1996 1.8 (manual), and the problem is that when the engine gets warm it starts to have idling problems and when driving and trying to accelerate fast it starts to jerk.

When the engine is cold it idles around 1000rpm but when it gets warm it drops to near 700 (which is okay'ish) but you can see from the tachometer at random time periods it drops to 500-600 rpm. From this, I could think a cylinder misfire or bad spark plug.

When the car is ideling and you try to floor it, it feels like it coughing and the engine starts to shake a lot and you even might get like a cylinder backfire (like a gunshot)

Now when you are going to drive it, the jerking occurs mostly on lower RPMs (I think up to 2000-2500) when pressing the throttle full down. When accelerating slowly its hard to notice it but you could feel it is there. This could refer to faulty air/fuel mixture readings.

When I went to a garage where a friend of mine works we had an opportunity to swap some parts around from the same model/year car for troubleshooting.

Things we tried

• Only code what we could get from the scan was that the Crankshaft Sensor, so we swapped it out and the code went away.
• Tested fuel pressure and the fuel rail injectors pressures which were all normal
• Replaced spark plugs
• Swapped the coil and spark plug wiring
• Checked all the vacuum pressures and all were normal

We checked the MAF sensor in the plugin/out style and the difference was noticeable so I would cross that out of the list too.

When doing all that the issue remained still present and we run out of ideas what to do next.

 

You have the same problems as me! My car also jerks when I floor the gas pedal(when driving; not idle), but not when I do it slowly. But mine is 1.6, 80kw 2002 manual. If you manage to find the problem, I'd be very very interested to find what is causing it.

You should also check for head gasket leaks and cylinder compression. I've heard the pistons on any car wear out with age and don't create a strong compression.

 

The last post I made is a little old text, so for more information I can say that there are no head gasket leaks and also all of the cylinder compressions are ok.
I will try and replace the ignition coils, because that seemed to fix the issue for someone in another forum.

 

Btw, since your car is closer to mine in terms of power. When you are driving uphill, what is the highest gear you can be in? Cause my S40 struggles on some inclinations to accelerate uphill, even in 3rd gear. I just think my S40 is rather underpowered. And I am not sure if I am imagining things or there is some issue.

 

It really depends on the speed, I guess. In general there is no problem with going uphill. I just have to press the pedal slowly, if I slam it, then it would probably have problems going up.