Pinging under load is all about not getting enough of or the right type of fuel. And there a alot of good reason this happens:
1. There is one fuel pump , one fuel filter and as many as three fuel pressure regulators on our beasts. A problem in any of these components can starve the engine at peak load and therefore cause pinging/knocking.
2. Even with them operating properly, the air temp sensor in the air filter assembly or the mass air flow sensor can misinform the fuel computer to deliver less fuel than necessary.
3. Carbon build up in the intake system is also a known contributor.
4. Vacuum or lack of it may play havok. There is a vacuum hose that connects to the throttle body to the air pump valve.
5. Slightly blocked injectors restrict fuel delivery.
6. A damaged or bad crankshaft sensor can advance or retard timing.
7. Bad gas.
8. Injector wiring harness shorts.
9. Dirty injector connections.
The list goes on and on. I am sure I missed a bunch more.
Kevin Berez (firstname.lastname@example.org)
86.5 928S Auto Meteormetallic
If you hear pinging, you may want to check your knock sensors and/or their connections.
Part # 911.606.141.00 (sorry for the 911 part number, not my fault ;-)
Retail cost $43.30 and as usual can be bought cheaper.
The test procedure outlined in the Factory Workshop Manuals are in Vol. 1A Section 28 page D28-8 and D28-9
Other indicators of defective Knock sensors are "poor pickup", "high fuel consumption" and "low power output".
Sometimes failure of the Knock Sensors can be caused by antifreeze and/or oil present on or around the knock sensors.
For the better part of the last year and a half I have been plagued with pinging at ~3000 rpm. I replaced all the usual suspects like ignition and vacuum, etc. Finally, with a spare pair of computers provided by DR I discovered that the ignition computer is the culprit. The down side is the cost of its replacement. But, it's nice to know.
Kevin Berez (email@example.com)
'86.5 928S Auto Meteormetallic
I discovered that my 90GT was pinging under heavy acceleration once hot. All systems seem to check-out fine. I then did an autocross where the fuel pump gave out. It turns out that all GT and GTS' have two fuel pumps (BTW, all Euro models do too).
Here is what happened. My internal pump had died some time earlier, (apparently this is not too uncommon) and the external pump was working a bit harder to compensate. Under normal driving conditions, the external pump was adequate, but under heavy loads and high lateral cornering situations, it just was not up to the task. By pinging, the engine was trying to tell me there was a fuel delivery problem. The high lateral stresses during an autocross caused it to give up the ghost.
Once I repaired both pumps, the car ran fine. Later, I found that the fuel pressure dampers and the fuel pressure regulator had ruptured diaphrams. This affected low RPM scenarios, but would probably contribute to poorer top-end performance. Fortunately, I was able to fix the problem before the performance driving season begins. YMMV.
P.S. I recall Marc Thomas mentioning, at the last 928 Frenzy, that the rubber hose fitted to the internal fuel pump is affected by the oxygenated fuels used here in the States. (For that matter, I suspect all the rubber lines and diaphrams that come in contact with these fuels are affected.) He told us that there is one type of rubber hose that is not affected by the fuel, and that he routinely replaces the OEM hose with this type to fix the problem. Marc, if you're out there, could you please elaborate?
Knock sensors are a little microphone sensor (piezo I would expect). They can fail. The Bosch "Hammer" diagnostic tool allows testing of both knock sensors. I currently have a failed one in my 91 GT that was diagnosed with the Bosch "Hammer".
Jonathan M Rackowe
1991 Porsche 928 GT, Manual, Baltic Blue/Linen Leather
928 Tips Home Greg's Home