Driving then engine cuts off. Checked interior #2 fuse and it's blown.... Ideas?

[Veleno]

So I was driving and the engine suddenly cuts off. It turns over/cranks fine, but just doesn't start. I checked the interior fuse panel and the #2 fuse is blown. I replaced the fuse, start car, car starts for a couple of seconds (around 3-5), then dies again. Checked the fused and again it's blown. I did this about 3 times.
Any ideas what it could be?

I replaced both the main relay and fuel pump resistor with no change (fuse keeps blowing). I'm guessing something is shorting but don't know where to start. I have a feeling I'll be dropping the gas tank to check the fuel pump but before I do that I'd like to get some recommendations. My car is a '92

Thanks for the help!

 

[JD Cross]

If you go to the shop manual page 23-53 you'll see the circuits covered by fuse #2.
Perhaps it would be best to start by checking the circuits protected by that fuse.

 

[Old Guy]

As Jim points out, the #2 fuse supplies a lot of different things (see attached clip from shop manual). The O2 sensors would probably be the first thing that I would check just because the wiring can become compromised because of the location and heat; however, all of the devices supplied by fuse #2 are potential candidates. Start with the O2 sensors (I expect that this is the heating circuit) and then work through the rest of them if it is not the O2 sensors.

 

[Veleno]

Thanks guys. I just found that diagram on page 23-51 of the above image. I guess going through each one would be the right thing to do, but man I wish there was a quicker way to identify the problem!

 

[angus]

The fuel pump is a good guess since it's the only thing protected by that fuse that can stop the engine.

 

[Old Guy]

The fuel pump is a good guess since it's the only thing protected by that fuse that can stop the engine.

Yes and no!

A short to ground or over loading problem on any of the circuits that are supplied by fuse #2 can cause the fuse to blow which then cuts power to the fuel pump. The original poster stated that after replacing the fuse, they could get the car to run for a couple of seconds before the fuse would blow again. This suggests that the pump is operating and might be OK.

I also recognize that the pump could be going bad and drawing too much current which is what is causing the fuse to blow. Unfortunately, unless the original poster has an amp meter, there is no way to test this short of replacing the pump. Also, in my limited experience, that is not how electric fuel pumps fail. My experience (which I admit is not with pumps on the NSX) has been that they start to fail intermittently for a short period and then die permanently in open circuit mode.

Working through each of the circuits individually to try and identify the problem may be tedious; however, dropping the tank to replace the pump as the first step is a pain and expensive. The fuel pump might be the problem; however, I would leave it to the last after I had ruled out the other, easier to fix possibilities. The problem might be due to some failed insulation on the wiring harness or a bad connector which is allowing excess current to flow in the circuit and pop the fuse. Those types of problems can be a real pain to track down and repair.

 

[Honcho]

Just out of curiosity, do we know what the average fail time of electronic fuel pumps is? 100k? 200k? Might be worthwhile to investigate for owners of older years (91-94) for preventative maintenance.

 

[ftuhy]

A suggestion on testing for if its the fuel pump without having an ammeter. Disconnect the fuel pump at some accessible connector. (I have no idea where that is & didn't look in the manual) Get another 15A fuse, and using clip leads, etc, wire it from the battery +12 V lead to the fuel pump connector. Replace the original 15A fuse that was blown.

Start the car. Does any fuse blow? If the fuel pump one blows, you have the culprit.
If no fuse blows, decrease the size of the clip-leaded fuse to 10A. Does it blow now? etc.

 

[Veleno]

Yes and no!

A short to ground or over loading problem on any of the circuits that are supplied by fuse #2 can cause the fuse to blow which then cuts power to the fuel pump. The original poster stated that after replacing the fuse, they could get the car to run for a couple of seconds before the fuse would blow again. This suggests that the pump is operating and might be OK.

I also recognize that the pump could be going bad and drawing too much current which is what is causing the fuse to blow. Unfortunately, unless the original poster has an amp meter, there is no way to test this short of replacing the pump. Also, in my limited experience, that is not how electric fuel pumps fail. My experience (which I admit is not with pumps on the NSX) has been that they start to fail intermittently for a short period and then die permanently in open circuit mode.

Working through each of the circuits individually to try and identify the problem may be tedious; however, dropping the tank to replace the pump as the first step is a pain and expensive. The fuel pump might be the problem; however, I would leave it to the last after I had ruled out the other, easier to fix possibilities. The problem might be due to some failed insulation on the wiring harness or a bad connector which is allowing excess current to flow in the circuit and pop the fuse. Those types of problems can be a real pain to track down and repair.

Just thinking about going through the process of elimination is making me think this will take forever! However, I think it will be a good learning process for me.

Just out of curiosity, do we know what the average fail time of electronic fuel pumps is? 100k? 200k? Might be worthwhile to investigate for owners of older years (91-94) for preventative maintenance.

Good idea, but this probably won't apply to very low mileage early cars (which I'm part of). In general though, it should still be good info for the masses.

A suggestion on testing for if its the fuel pump without having an ammeter. Disconnect the fuel pump at some accessible connector. (I have no idea where that is & didn't look in the manual) Get another 15A fuse, and using clip leads, etc, wire it from the battery +12 V lead to the fuel pump connector. Replace the original 15A fuse that was blown.

Start the car. Does any fuse blow? If the fuel pump one blows, you have the culprit.
If no fuse blows, decrease the size of the clip-leaded fuse to 10A. Does it blow now? etc.

I think I follow you, but I'm not sure. You're saying to run a wire from the battery to the fuel pump connector but with a 15A fuse in between? If the fuel pump (#2 under dash) fuse blows after restarting, it's the pump? If not, then it's something else? Why would I reduce the fuse to 10A if it doesn't blow? Please correct me if I misunderstood.

 

[ftuhy]

Just thinking about going through the process of elimination is making me think this will take forever! However, I think it will be a good learning process for me.



Good idea, but this probably won't apply to very low mileage early cars (which I'm part of). In general though, it should still be good info for the masses.



I think I follow you, but I'm not sure. You're saying to run a wire from the battery to the fuel pump connector but with a 15A fuse in between? If the fuel pump (#2 under dash) fuse blows after restarting, it's the pump? If not, then it's something else? Why would I reduce the fuse to 10A if it doesn't blow? Please correct me if I misunderstood.

If #2 under the dash blows after starting, its something else. Only if the rigged-up fuse blows is it the pump.
Why reduce the fuse size if all works well? 15A is supposed to handle everything, including the pump. Unfortunately, the shop & ETM manuals don't give a current drain or resistance spec for the pump (at least one I could find). My WAG is it should be less than 10A, maybe even less than 5 A drain. Without a meter, the way to test that is to simply keep reducing the rigged-up fuse size until it blows. (You can get a cheapie digital multimeter at Harbor Freight for under $10 that should work fine. But the maximum current scale is 10A. If the current is higher than that, it may blow the 10A current scale. No big loss and you'll still know the current was too high.)

 

[Jeff@Carbon6]

Also, you may want to check and see if your O2 sensor wires are shorting out on the heads, headers or exhaust.

 

[davidf]

I did this at 75,000. Yes call me Mr. anal but the car was 21 years old and ethanol is tough on fuel systems on older cars. With almost no techs within 80 miles of me {zero in my town} I didn't want to get stuck on the side of the road somewhere. Pricey, yes. Do I feel better, YES!

Just out of curiosity, do we know what the average fail time of electronic fuel pumps is? 100k? 200k? Might be worthwhile to investigate for owners of older years (91-94) for preventative maintenance.

 

[Larry Bastanza]

Guys,

I thought about this and took a further review of the schematics. I do not think it is the fuel pump, and here is why. When you turn the ignition key to "on", not "start" the fuel pump is activated my the main relay and it is running(about 3-5 seconds) building fuel pressure in anticipation of the engine starting, so if the fuel pump is active and running, prior to going to the start position on the ignition switch, the fuse would blow then, not once the starter is activated. First thing to do is confirm the fuel pump is running in the "on" position. If so, you will now need to systematically disconnect each circuit that the #2 fuse feeds to determine the culprit.

My $.02

Regards,
LarryB

 

[Veleno]

Guys,

I thought about this and took a further review of the schematics. I do not think it is the fuel pump, and here is why. When you turn the ignition key to "on", not "start" the fuel pump is activated my the main relay and it is running(about 3-5 seconds) building fuel pressure in anticipation of the engine starting, so if the fuel pump is active and running, prior to going to the start position on the ignition switch, the fuse would blow then, not once the starter is activated. First thing to do is confirm the fuel pump is running in the "on" position. If so, you will now need to systematically disconnect each circuit that the #2 fuse feeds to determine the culprit.

My $.02

Regards,
LarryB

So I had a little time yesterday to try something. I disconnected the fuel pump resistor, started the car, and had the same results (blown fuse). I'm guessing, as Larry pointed out, that it's not the fuel pump. The fuel pump does run/cycle in the "on" position prior to starting.

Where/what do I disconnect next? Based on the picture above, it's one of the following:

-SRS control unit
-indicator lights
-voltage regulator
-front/rear oxygen sensors
-speed sensor
-a/t/control unit (is this for automatic cars? I have a 5 speed)
-emissions control box

Do I simply find these things in the service manual and disconnect them at the harness then try to start the car to see if the fuse blows? Also, if I disconnect the oxygen sensors, does the check engine light come on?

 

[Aluminsx]

If it was me I would check out the service manual and start by disconnecting the easiest to access to the most difficult. I'm pretty sure the CEL will come on if you disconnect the O2 sensors but if you don't blow a fuse when you do that you'll have the culprit. Replace the faulty sensor, reconnect and the CEL should clear after a while. Good luck.

 

[Old Guy]

As suggested by Aluminsx, isolating each end device (O2 sensor, speed sensor ...) and sequentially testing for fuse failure is probably the best place to start. However, if the failure is in the wiring harness between the fuse box and the end devices, you can disconnect all the end devices and you will still blow the fuse. As a test procedure tip, as you disconnect each end device, if the fuse still blows when you test energize the circuit, leave the device disconnected and move to the next device on the list. If you end up with all the devices disconnected and you are still blowing fuses, you most likely have a fault in the wiring harness.

As a suggestion, start the isolation procedure with the O2 sensors. The wiring for the O2 sensors is in a location with high potential for 'abuse'.