I frustrated!
Finished a timing belt change 2 months ago it drove great for 300 miles including 2 trips up over the mountain that I live on. Drove down to Gas it up (chevron premium)
it stumbled once like a hickup on way down but seemed fine, left gas station and car started running very rough and no power to it, when I got home (1 mile trip) the exhaust pipe was hot enough to set a discarded rag under car on fire and exhaust system smoking badly. Since this I have replaced plugs, wires, distributor caps and rotors without perceptibal change. (had planed on this work anyway and had parts in house) I have pulled passenger side cam cover and the timing mark on the cam drive gear was still where it should be with the crank at TDC, Compression is 160 to 175 across all cylinders so not that either. So I either have a jammed up exhaust or something wrong in the fuel system causing me to burn raw gas in the cats? Right? Anyone want to weigh in on this, before I start pulling the exhaust apart and ordering new cats My mechanic/best friend is just as perplexed though not a Porsche mechanic he had his own Jaguar/foreign auto shop for 20 years and was a mechanic on McClaren F1 team.
what are we missing HELP

Did a quick Google search and this article seems to cover a lot of the possibilities.

https://rennlist.com/how-tos/a/porsche-928-why-is-my-car-losing-power-383830

I'd likely start by pulling the vacuum lines from the fuel pressure dampers and fuel pressure regulator to determine if there is any fuel leaking passed any of the diaphragms, do the vacuum check for testing for a clogged catalytic converter, and look into the O2 sensors.

Be sure to check that you've got spark from both distributors. The 928 S4 has two separate 4 cylinder spark systems controlled by one EZK spark unit. The engine will run with just 4 cylinders firing but the raw fuel from the non-firing cylinders will ignite in the catalytic converter and could cause a car fire.

As mentioned, a vacuum leak could cause a loss in power.

Good luck,
Rich

Give Roger Tyson a call. I had a similar situation o my 91. 928s r us had a EZK bypass( think that's what it was called) which corrected the problem right away. I also replaced the cats with an x pipe.

The 928 S4 and newer engines have two separate ignition circuits. Ignition circuit 1 is comprised of cylinders 1,4,6,7, the right side coil and distributor. Ignition circuit 2 is comprised of cylinders 2,3,5,8, the left side coil and distributor. If there is a failure in one of the ignition circuits, raw fuel is pumped out of the cylinders and into the exhaust system. When the raw fuel hits the hot catalytic converters a fire may occur – OUCH!

From model year 1989 onwards, Porsche added the Ignition Circuit Monitoring Relay as a safety feature. This circuit is completely independent of the LH injection system. It is identified in the workshop manual wiring diagram as the Ignition Control Circuit. The function of the system is to turn off the fuel injection to the failing ignition circuit. The inputs to the relay are two exhaust temperature sensors that provide input to the relay. If one of the ignition circuits fails, the “Ignition Circuit Monitoring Relay” shuts off the pulse signal to all of the fuel injectors of the affected circuit.

As you might imagine, the symptoms of a shut down ignition circuit are a significant reduction in power and a rough running engine. For trouble shooting, you need to determine if the shut-down is due to a failed ignition circuit or a failure of the Ignition Circuit Monitoring Relay itself.

The first step is to quickly identify which circuit is affected. Look at the clear relay module that is fitted next to the EZK spark control unit in the passenger compartment.

When ignition circuit 1 (cylinders 1,4,6,7) is shut-down, a red LED is illuminated.

When ignition circuit 2 (cylinders 2,3,5, is shut-down, a green LED is illuminated.

Check the ignition circuits and repair as necessary. If no problem is found with the ignition circuit, there may be a failure of the Ignition Circuit Monitoring Relay circuit, the signals to test are:

1. Terminal 31: ground
2. Ground must be present at terminal AL when the ignition switch is in the off position.
3. Battery voltage must be present at terminals A1, A2, 15 and 87 when the ignition is in the on position.
4. Battery voltage must be present at terminals AL and 61 when the engine is running.
5. A Voltage value of approximately 2.7 V must be present at both terminals E1 and E2 when the ignition is in the on position.
6. The resistance between E1 and E2 is approximately 5 – 10 Ohms (measured at the disconnected relay socket).

The temperature sensors that fit in the exhaust system should also be inspected.

If there is a failure in the temperature sensors, four of the fuel injectors will not open and engine power will sharply decrease. However, this is an IMPORTANT safety system. If a "bypass" of the sensor circuit is done, the protection of shutting off fuel if one of the two sparks systems fail is lost and a MAJOR FIRE may occur and you can lose your Porsche 928 and worse, you may possibly be injured in a car fire.

Any time there is a major loss in power. Do NOT drive the car. Have it towed in and correct the problem.

Fixed it a while back but just to let everyone know it was a bad coil so only running on 4 cylinders replaced both

If a bad coil was indeed your problem then look out for a burnt valve or two in your next 1000 miles. They do that.