91 NSX Crank No Start, No Fuel Pump Prime

[NSXninja]

Hello fellow NSX enthusiasts. I have owned my NSX for almost 9 years now. It's hard to believe that it's been that long and even harder to believe that I have never had to get it towed until recently. I’m somewhat knowledgeable about working on the NSX and I know how to do some basic maintenance, but when it comes to the more complicated problems I could use some help. I’m hoping someone here can steer me in the right direction.

I recently went for a short drive to visit with family and when I was leaving, the car took a very long time to start. The engine just kept cranking, but wouldn't start. It eventually started and ran fine until I foolishly made a stop on the way home. I decided to get gas, thinking that maybe the fuel gauge was off or something fuel related was going on. I tried starting the car again and I immediately noticed the fuel pump wasn't priming. I tried starting it numerous times with long breaks in between to no avail.

After having the car towed home, it sat for over a week before I finally had time to work on it. I should mention that the CEL never came on. I was looking through the service manual and I noticed that it references using a jumper to count the CEL flashes. I turned on the ignition to see if the CEL had come on and of course now the fuel pump primes and the car starts just fine.

Obviously something is failing, but whatever it is, it's working at the moment. Some other symptoms that may or may not be related: the car often has trouble starting when parked on an incline and sometimes it takes longer than usual to start when the engine is already hot. I have been suspecting the fuel pump might be failing for a while, but the fact that it always runs fine after it starts makes me think it might be something else. Any suggestions on what the most likely culprit is or what to test/check would be greatly appreciated.

 

[drew]

Main Relay. Tap moderately hard above the center speaker just below the rear window when starting.

That is presuming the dash lights are bright.

If not bright: then you have an ignition switch issue.

 

[docjohn]

what he said^^^ my car had some glitchy no start/stall issues and after a new ignition mechanism in the dash its like auto Viagra..

 

[NSXninja]

Thanks for the suggestions. I don't recall the dash lights looking dim, so I will start with the main relay.

 

[Old Guy]

This is pretty much a reprise of Doc & Drew's comments.

When you turn the ignition switch to the run position, the CEL should come on for 2 or 3 seconds and then go out. That is the self check routine for the CEL. If the CEL does not illuminate then the ECU is not getting powered up and the likely candidates are:
- the main EFI relay
- the ignition switch (the dash lights will likely not come on if its the switch)

If the CEL does come on for the self check; but, the pump does not prime then the likely candidates are:
- the main EFI relay
- the fuel pump

The main EFI relay is a 2 for 1 special with two relays in it - an ECU control relay and a fuel pump control relay and failure of either of those relay circuits can kill the car. If your 1991 is still running with the original main relay that would be the #1 candidate. The relays typically fail by a fracture of the solder joints on the relay circuit board which is why Drew's 'hit it test' can sometimes get them to work again. However, sometimes they stay dead so the rap test does not always work. In most cases, reflowing the solder on the circuit board with a soldering iron is an effective repair.

Unless the CEL light is not illuminating, don't immediately jump to replacing the main EFI relay. Check the fuel pump by testing for voltage by back probing the connector at the fuel pump resistor. If you switch the ignition to run you should measure 12 volts at the fuel pump resistor for 2 seconds - the fuel pump prime pulse. If you don't get that 2 second prime pulse then its likely the main EFI relay. If you do measure 12 volts and the fuel pump does not run then the fuel pump is likely dead.

Depending on the mileage, on a 1991 the main EFI relay, the ignition switch and the fuel pump are all candidates for age and wear replacement. On my 2000 the main relay died in 2021 and I preemptively replaced the ignition switch around the same time. I have a new Denso fuel pump sitting in a box to cover the time when it eventually fails.

 

[NSXninja]

Thanks @Old Guy for all the info. My CEL does come on like normal when I turn the ignition to run, so I will do the checks you described to narrow the problem down. I'm really hoping it's the main relay, because I heard replacing the fuel pump is not fun.

 

[drew]

One super important thing: you're not going to find "CEL" in the Manual... it's a MIL- Malfunction Indicator Light

MIL (aka Money Light)

 

[Old Guy]

One super important thing: you're not going to find "CEL" in the Manual... it's a MIL- Malfunction Indicator Light

MIL (aka Money Light)

I admit that I don't recall the three letter abreviation 'CEL' in the Honda manuals. However, in the service manual section covering trouble shooting the fuel injection they refer to "Check Engine Light" and the light does have the little word 'CHECK' on it if you have a US model (my 2000 is a USDM car). Canada being officially bilingual gets the bilingual light which has nothing written on it. Just for fun, the Electrical Trouble Shooting manual does refer to it as the Malfunction Indicator Light and uses the abbreviation MIL - even though the light symbol in the ETS says 'CHECK'. The owner's manual does refer to it as a malfunction indicator light.

So, you may not find CEL; but, you can find Check Engine Light depending on which manual you look at. NSX trivia!

 

[FlopShot]

Carrying a spare Main Relay is part of the NSX starter kit. Original relays are known to fail. Tapping on it (and hopefully getting your car started by doing that) is a short-term solution.

 

[wavy]

This is pretty much a reprise of Doc & Drew's comments.

When you turn the ignition switch to the run position, the CEL should come on for 2 or 3 seconds and then go out. That is the self check routine for the CEL. If the CEL does not illuminate then the ECU is not getting powered up and the likely candidates are:
- the main EFI relay
- the ignition switch (the dash lights will likely not come on if its the switch)

If the CEL does come on for the self check; but, the pump does not prime then the likely candidates are:
- the main EFI relay
- the fuel pump

The main EFI relay is a 2 for 1 special with two relays in it - an ECU control relay and a fuel pump control relay and failure of either of those relay circuits can kill the car. If your 1991 is still running with the original main relay that would be the #1 candidate. The relays typically fail by a fracture of the solder joints on the relay circuit board which is why Drew's 'hit it test' can sometimes get them to work again. However, sometimes they stay dead so the rap test does not always work. In most cases, reflowing the solder on the circuit board with a soldering iron is an effective repair.

Unless the CEL light is not illuminating, don't immediately jump to replacing the main EFI relay. Check the fuel pump by testing for voltage by back probing the connector at the fuel pump resistor. If you switch the ignition to run you should measure 12 volts at the fuel pump resistor for 2 seconds - the fuel pump prime pulse. If you don't get that 2 second prime pulse then its likely the main EFI relay. If you do measure 12 volts and the fuel pump does not run then the fuel pump is likely dead.

Depending on the mileage, on a 1991 the main EFI relay, the ignition switch and the fuel pump are all candidates for age and wear replacement. On my 2000 the main relay died in 2021 and I preemptively replaced the ignition switch around the same time. I have a new Denso fuel pump sitting in a box to cover the time when it eventually fails.

I have a 1993 that's been sitting for a few years, trying to get it running and I am having similar issues.


Am I checking for voltage after the sending unit correct?

Also is there a way to feed power to the pump directly to check if its seized or not?

 

[Old Guy]

Not sure what 'sending unit' refers to.

Yes, you can access the fuel pump + supply wire at the main EFI relay or the fuel pump relay or the fuel pump resistor. Go to NSX Prime Library and follow the links to find a .pdf of the 1991 service manual. The wiring diagrams start on page 23-350 of the SM and wiring diagram #14 shows the fuel pump circuit with the wire colors. Section 13 of the service manual shows where all those parts are located.