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Now I'm nearly certain this is a fuel pressure problem. It could be that the fuel pump pickup is clogged (often happens with old/bad fuel and decaying plastic/rubber bits in the tank). A longer shot is that the return line from the fuel distributor to the tank is restricted which will drive the pressure higher and make the system run lean (which is the likely cause of the bucking). The fuel pressure gauges will tell you.
If you want to take a shot a just replacing a part without doing the pressure diagnosis you can try replacing the warm up regulator, but that's an expensive test without knowing for sure. Also, the fact that it runs differently cold > warm suggests that is working at least to some extent (usually when they fail they stick in one position).
One important caveat here - if this is a Lambda system (oxygen sensor/catalyst) the engine will run very poorly if the lambda system is not working. You should hear the modulating valve (located near the fuel distributor) buzzing when the fuel pump is running. Let us know if you have Lambda, that's a different troubleshooting issue.
On the electrical side, I would check the voltage at the fuel pump when it is running (please be careful about anything that would make a spark near the fuel tank!!). If it's above 10 volts then the pump is likely working ok. If it's below that then either the pump has a problem (they tend to stick brushes with old fuel) or the wiring/fuse/relay has a problem. The pump is either on (assuming sufficient current which is what you are looking for with the voltage test) and producing full flow and pressure or it's off, there's no in between.
Based on the 1978 non-turbo wiring diagram, power for the pump should go from the battery to the starter to the alternator to fuse 10 to the fuel pump relay to the pump. You can also check the voltage at the relay when the pump is running. The wiring diagram does show 5 wires at the relay, they are power from the fuse, pump, TSI socket (the round test socket on the relay panel), ignition module, and ignition relay. It goes to the ignition module because the engine has to be turning for the fuel pump to turn on. The diagram in the service manual is pretty tiny and hard to make out, but I think I've got that right. A very common problem is for the connectors from the fuse to the relay to develop high resistance and get hot and melt things and then stop working all together, so that probably explains why the circuit has been modified. But as long as you have more than 10 volts at the pump when it's running then that modification is probably OK.
I hope that helps!
Best,
Gary
_______________________________________
Gary Stottler
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(#1463) [Profile/Gallery]
(more from Gary Stottler) on Sun, 13 Apr 2025 06:03:13
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