Engine dies immediately after starting. Apparently caused by TPS.

[swimmingpie]

Today, I was having a problem starting my car. More specifically, I was having a problem keeping it running. It would crank just fine, and the rpms would rise to 2000 or so, like normal, but it would immediately stall within 1 second. If I gave it some throttle, I could keep the engine running for maybe 2-3 seconds, but it would still stall every time. I tried cranking it probably 50 times, and the same thing happened every time.

To the troubleshooting. (tldr: it was the throttle position sensor)

I could hear the fuel pump make its signature whining sound when I turned the ignition on, so I ruled out a bad pump.

I check the battery and all my ground connections. Everything is fine.

I tried pulling the clock fuse to reset the ECU. No luck.

Naturally, my next thoughts were the infamous main relay and ignition switch. However, I fixed a bad solder joint on the main relay a long time ago, and I cleaned the contacts on the ignition switch less than a year ago. It seemed unlikely, but I thought I might as well start with the obvious things.

I took out the main relay and inspected it. Everything looked fine, but maybe there was a tiny crack I couldn't see. I resoldered every joint on the board and reinstalled. No luck.

The ignition switch was not behaving as if it was bad, but I took it out anyway. It looked fine. I cleaned the contacts off a little bit and reinstalled. No luck.

I'm getting puzzled here. I open up the ECU and remove my aftermarket ROM and put the factory ROM back in. No luck.

I pulled off the intake to check the throttle body. Looked clean. I cleaned off a small amount of dirt with an alcohol soaked towel and put everything back. No luck.

Finally, I think to disconnect the throttle position sensor. Maybe I can get the ECU to go into limp mode or something. I disconnect the sensor, and, lo and behold, the engine starts up. The idle is a bit rough, but it runs. I connected the sensor again, and the engine starts up just like normal and runs fine.

I don't understand why disconnecting and reconnecting the TPS fixed my problem. Should I be worried about the sensor going bad?

 

[solidol]

Given symptoms I would check map sensor first. But since you're saying you cannot keep it running at WOT and tps test result why not r&r it with working unit? What year is your car?

 

[Jinks]

From what I have read here, it sounds like a bad ignition switch.

 

[robbiedawg]

From what I have read here, it sounds like a bad ignition switch.

That's what I thought,too.

 

[Old Guy]

If the engine was really running for 2-3 seconds 'after' you disengaged the starter motor (rotate from start to run), it is not the starter switch. If the motor quits immediately when turning the key from start to run, then yes it could be an ignition switch problem. If the fuel pump is priming, you are also pretty much guaranteed that the problem is not the main FI relay.

The TPS is kind of a peripheral device on a speed density EFI like the NSX. Most speed density systems will operate pretty well with the TPS disconnected. As solidol alludes to, the MAP sensor, not the TPS, is the super critical sensor on a speed density system. No MAP sensor signal - dead in the water! However, both the TPS and the MAP sensor get a regulated 5 volt supply (likely a common supply to both sensors) from the ECU. If you had a short circuiting connection on the TPS plug, it could have been dragging the voltage down on the 5 v supply to the MAP sensor which may mean that the ECU is not getting a MAP signal . When you unplugged / plugged in the TPS, you may have cleared the ground fault. It is also possible that the TPS could have an internal short circuit that may be intermittent. Check out the service manual procedure for testing the TPS. It would also be good to test your MAP sensor as per the service manual.

The only problem with my theory is that if the voltage on the MAP sensor goes out of range, the ECU should generate an error code if it is OBDII ECU. You did not specify your model year and I don't know whether the pre OBDII ECUs also have error codes for abnormal voltages. It would be worthwhile to check to see if you have any stored error codes which might point you in the correct direction.

On the early cars, I have read comments that a mal functioning TCS can sometimes cause interesting throttle problems. I have a 2000 so I have never investigated the operation of the early TCS system - might be something to rule out although it seems unlikely given your symptoms.

 

[whrdnsx]

Fuel pump resistor? If disconnected will allow the engine and start and run for a while until the fuel pressure runs out.

 

[Larry Bastanza]

I would not completely rule out the Main Relay until you can swap it out with a known good one. Honestly I have not had this happen on an NSX, but a few years ago my daughter's Honda Del Sol had this issue and after resoldering the main relay it still did not work, and once I replaced it all was good. Although rare, the relays on the board can go bad, so regardless of the solder connections, it could still be your issue. Worth a shot, IMO

HTH,
LarryB