The first generations of the intake
manifold gaskets were paper based and will fail. Parts #217793 and #233974 have now been replaced by a much better gasket #322509.
A failing intake gasket is not something to underestimate. Well before getting a CEL, a lot dirt can be sucked directly inside the engine and will definitely contaminate the intake valves. Also, the intake manifold will struggle to create vacuum which will have an impact on the brake booster, EVAP and exhaust valves. With more air sucked in, the engine will run leaner (=hotter) especially at open loop. At open loop, the Motronic ECUs don't adjust the A/F ratios related to the lambda signals.
An easy trick to check if you have leaking gaskets can be done with a simple small pipe while the engine is idling after a cold start. Place one end in your ear and with the other end, move slowly the pipe around each of the intake ports of the engine. If you hear at some point a hissing noise, you have a failing gasket. If the gasket is already failing since a while, you'll see also traces of grease and dirt right in front where the gasket has an open gap. As a lot air is sucked through this gap, it will attract all possible grease and dirt towards it.
Changing the intake gaskets is not a difficult job but requires some attention and experience. All intake manifold pipes, main intake, cable ground connections and injection rails need to be removed before the manifold can be taken out. The most time consuming part will be cleaning up everything, especially if the intake valves got contaminated with dirt. It took me a good day to do the entire job.
A full reset procedure is required to make the Motronic ECUs relearn the engine.
If you still have the old intake gaskets, prepare to get them replaced because they will eventually fail without you noticing it.
With a simple pipe, search for hissing noises
Pictures of broken intake gasket where you can clearly see the open gaps where air got directly sucked in
Intake valves got contaminated at each port where the gasket failed
Old intake gaskets vs new gaskets. See how the paper gaskets are soaked with fuel which made them more fragile and caused them to be torn apart with the high depression created inside the intake plenum.
Intake plenum cleaned up
Intake ports cleaned up and new gaskets installed
Intake manifold back in order