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98 NSX-T Starts, idles but won't drive. CEL Multiple misfires due to fuel system malfunction.

24 September 2023
MY 98 quit running last week. Came out of the store and after multiple attempts it started and got me home. From then on it would not start. CEL gives multiple misfire codes all due to fuel system malfunction. I replaced the main relay now it will start and idle but when you put it into gear it stumbles and will not come off idle. I believe the main relay contains a fuel pump relay as well. Where do I go from here? Fuel pump?

Thanks for your help.
Your car is OBDII. Did you use a code reader or the service check connector to determine that you had misfire codes? If so, what were the specific codes? If you had fuel supply problem causing lean misfires I would expect that you would eventually generate a fuel mixture out of range error code.

Main EFI relay failures tend to be "its working" / "its not working" failures. You are correct that it is a two in one relay containing a relay to power up the ECU and a separate relay to power up the fuel pump. It is possible that you have a bad electrical connections in the supply to the fuel pump which is resulting in low voltage to the fuel pump resulting in low fuel pressure resulting in lean misfires. It is possible that you might have disturbed and partially restored the electrical circuit when you replaced the relay.

The fastest check to see if you have a fuel supply issue is a fuel pressure check. This is described in the service manual in the fuel section. The engine does have to be running to do this check. If the pressure is below spec you could have:
- a failing fuel pump
- a failing fuel pressure regulator
- a problem with the fuel pump resistor
- some other problem in the electrical supply resulting in low voltage to the pump
The service manual has a procedure for a basic test of the fuel pressure regulator. Do that at the same time as the fuel system pressure check to see if the problem originates there.

You can do a basic check of the fuel pump resistor which has been known to fail. Use a paper clip to short out the resistor connector on the back firewall. If shorting out the resistor returns pressure to spec then you need to replace the resistor or check for damage at that connection. If pressure does not return to spec, with the engine running measure the voltage at the paper clip. The voltage should be above 14 volts. If it isn't then you have some kind of problem in the electrical supply up stream of the fuel pump resistor. If the voltage to the fuel pump is above 14 volts and you are not reaching the pressure spec that does point a finger at the fuel pump.

If the fuel pressure checks out OK then the problem is obviously elsewhere and the specific codes may help determine the possible causes.