TargaT,
The fact that the condition changes back and forth is both an advantage and disadvantage. We don’t want the problem to simply disappear for some unknown reason only to reappear at some inopportune time. Let’s start with the least disturbing tests first.
Ignition timing is an easy test and you can drive with the timing light in place. You want to look at the steadiness of the timing, the repeatability up & down of the advance curve, etc. A dwell meter could also be installed to see if the points change.
Double check that the fuel lines are plumbed correctly at the fuel filter console. Porsche published an incorrect diagram early in MFI history. Check
HERE.
Given the situation where the fuel pressure is suspect, you can plumb a
temporary fuel pressure gauge. Safety is your prime concern. Fuel pressure is measured at the fuel filter console banjo fitting closest to the rear of the car. It is easy to install a “T” fitting in that hose and run a measuring hose to a fuel pressure gauge in the cockpit. Run the hose out the back of the engine compartment and tape it to the side of the rear window and in your open door window.
Use real fuel hose and clamps at every connection. Don’t tolerate any leaks. Even the slightest fuel leak can lead to disaster. Make the hose long enough to easily reach the fuel filler on the LF fender as you may want to also use it for fuel flow testing.
The
minimum fuel pressure spec is 0.8 ±0.2 bar (11.8 ±3.0 psi). Under normal operating conditions the fuel is returned to the tank by the over-flow valve (in the middle fitting on the fuel filter console) and the fuel pressure is about 1.0 bar (14 psi). The running pressure is not critical, only the minimum pressure.
You can use a flashlight and mirror to inspect the MFI pump timing belt. Milt is correct; the belt teeth can quickly fail from misalignment or too much tension. Changing pump timing can be frustrating to diagnose.
Please report your findings.
Congratulations on your rebuild. These are minor (yet frustrating) gremlins that will get sorted out. Once running correctly all the time you will want to do a through set of CMA checks and adjustments. This will both get the MFI adjusted the best possible and establish your baseline for future troubleshooting. A complete and extended CMA is part of your regular maintenance.
Best,
Grady