Need tech advice on weird elec/fuel problem
02-21-2004, 12:45 AM
#16
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Realist D,
Other listers have provided good suggestions. I though of some other potential causes of your problem:
a) The L-Jet and LH-Jetronic injection systems have an idle switch on the throttle linkage. For LH equipped 928s, if the idle switch doesn't work the LH controller doesn't know that the car is in idle mode so the idle speed regulator will not engage. The switch is located next to the throttle body, so it isn't readily accessible. It can be checked by removing the connectors from both the LH and EZK module and measuring the resistance between pins 3 and 5 on the LH connector (not on the module itself). With the throttle closed, resistance should be 0 ohms, off of idle (about 1 degree rotation) the resistance should jump to infinite (open circuit).
While you’re in there, a similar test is performed for full throttle position. This time check the resistance between LH connector pins 12 and 5. In idle position, the resistance should be 0 ohms, and at full throttle position (more than 3/4th throttle opening) the resistance should be infinite. If the wide-open circuit isn’t working, both fuel injection enrichment and spark timing change will not function correctly.
b) A vacuum leak can cause a situation where the idle speed regulator cannot keep the idle speed within the proper RPM range.
c) The pesky hybrid circuit that is the cause of many LH injection controller failures is responsible for idle speed regulation. It also controls lambda fuel mixture regulation. A common failure mode is the fuel mixture going excessively rich. It also controls the fuel pump relay, so a bad LH may cause the fuel pump relay not to engage. “Smart” mechanics may wire in an external relay to compensate for this problem and turn on the pump. But they are just delaying the inevitable. Hybrid failures often cascade as the circuit continues to oxidize and eventually the engine will not run.
d) I drafted an overview about the thrust bearing failure issue and show how to release the pressure on the flex plate. Pictures of the procedure performed on my GTS are included. If anyone would like to see the overview, send me a note at info@electronikrepair.com and I’ll send you a PDF.
Other listers have provided good suggestions. I though of some other potential causes of your problem:
a) The L-Jet and LH-Jetronic injection systems have an idle switch on the throttle linkage. For LH equipped 928s, if the idle switch doesn't work the LH controller doesn't know that the car is in idle mode so the idle speed regulator will not engage. The switch is located next to the throttle body, so it isn't readily accessible. It can be checked by removing the connectors from both the LH and EZK module and measuring the resistance between pins 3 and 5 on the LH connector (not on the module itself). With the throttle closed, resistance should be 0 ohms, off of idle (about 1 degree rotation) the resistance should jump to infinite (open circuit).
While you’re in there, a similar test is performed for full throttle position. This time check the resistance between LH connector pins 12 and 5. In idle position, the resistance should be 0 ohms, and at full throttle position (more than 3/4th throttle opening) the resistance should be infinite. If the wide-open circuit isn’t working, both fuel injection enrichment and spark timing change will not function correctly.
b) A vacuum leak can cause a situation where the idle speed regulator cannot keep the idle speed within the proper RPM range.
c) The pesky hybrid circuit that is the cause of many LH injection controller failures is responsible for idle speed regulation. It also controls lambda fuel mixture regulation. A common failure mode is the fuel mixture going excessively rich. It also controls the fuel pump relay, so a bad LH may cause the fuel pump relay not to engage. “Smart” mechanics may wire in an external relay to compensate for this problem and turn on the pump. But they are just delaying the inevitable. Hybrid failures often cascade as the circuit continues to oxidize and eventually the engine will not run.
d) I drafted an overview about the thrust bearing failure issue and show how to release the pressure on the flex plate. Pictures of the procedure performed on my GTS are included. If anyone would like to see the overview, send me a note at info@electronikrepair.com and I’ll send you a PDF.
