92 NSX with error code 44 - Bad O2?

I'm trying to resurrect bad father's NSX and after 20ish minutes of driving I get a CEL. I jumped the connector, and it flashes 44, Rear Fuel Supply System. I replaced all the coils with aftermarket, but it still shows up, again after 20 minutes. The car recently passed smog (the light goes away when you turn the car off, only to show up again after 20 minutes), but didn't pass easily (The NO at 15 mph was in the 500's). If you allow the car to idle for 15 minutes, the light still doesn't illuminate after 10 minutes of cautious driving. I get NO performance change with the light, and the car doesn't seem to have any rough running. Many people with a bad O2 seem to have a stumble around 3k, I do not experience this.

This all leads me to believe that my rear bank upstream O2 sensor is bad. Not the heater element, but the sensor itself and this is why it takes so long to pop the light? Does this sound right, or is it something else? I don't exactly want to throw money at it if the problem is something else.

I don't get great gas mileage (low 20's), and I drive the car like a parade float. The car is stock, except for air filter and a cat back exhaust. The manifold is stock. It sounds like Denso is the preferred item.

I appreciate any input.

Do you have access to a fuel pressure gauge? Checking fuel pressure to rule it out as a problem is always useful, although fuel pressure problems usually manifest them selves as mixture codes on both banks. It may be possible that your fuel trim is hitting a limit on the back causing the error code and the front ones are just sneaking under the error limit for fuel trim.

If you are resurrecting an NSX does that mean it has sat for a long time in storage? If so, stale gasoline would be an obvious culprit. If it has suffered from non use and the injectors have never been cleaned, then you may have dirty / clogged injectors resulting in a lean condition or you may have dirty / stuck open injectors resulting in a rich fuel condition. Pulling the injectors (be sure to put in new seals) and sending them out to be cleaned and flow tested would never be a bad thing on a 28 year old car.

If you have access to a high input impedance voltmeter or an oscilloscope, it is possible to check the operation of the O2 sensor. Rather than describe the process here, you can find the test procedures on the web. This will definitely tell you whether the sensor is completely dead. The voltage on the sensor may also give you a clue as to whether you have a fuel mixture problem if the sensor is working. The 1992 only has the one sensor in front of the cat (no downstream sensor). Be aware that I think there might be a problem with replacements. You may have to use a later model sensor and fiddle the mounting brackets a bit. There was a thread on Prime about a year + back describing problems finding exact replacement sensors on some earlier cars.

I had a similar problem, ended up being a failing fuel pump. The low fuel pressure was causing a lean condition, throwing a CEL for an o2 sensor. I'd check your fuel pressure (you can also install a fuel pressure gauge on your fuel filter permanently).

o2 sensors are only about $100, I ended up replacing both of mine just to rule them out. They are also a wear item that can affect future efficiency and drive-ability, so it's never a BAD idea to have new o2 sensors.

So it sounds like my list of suspects is as follows: o2 sensors, dirty injectors, fuel pump. I'm going to add fuel filter and FPR to that list too.

Is the fuel pump wiring reason for concern? I have a 4G63T that are notorious for failing wiring and losing volts to the fuel pump.

My plan is: replace o2's. Rock Auto has Denso's for stupid cheap. Also going to check fuel pressure with loaner tools. Assuming the pressure test doesn't reveal anything, I'm going to replace the fuel filter and then send out my injectors. Then FPR, mostly because I'm not looking forward to dropping the tank. Then, if I have to, replace the fuel pump.

I do not suspect the resistor on the fuel pump, as nothing seems to be rpm dependent.

Old Guy, I think your plan is pretty spot on.

Fuel Pressure test showed 40 at idle with vacuum connected, 48 without vacuum. If I play with the throttle a bit, it goes to 42ish. This seems to be in range, so I'm NOT going to focus on the FPR or the pump for right now. I'm still going to change the filter because I know it's been due.

I got the o2, so that's going in today. If that doesn't do it, then it's on to injectors. I've seen aftermarket new (Autoline, Ultrapower, Beck/Arnley, Standard Motor) for 22-50 bucks, or not much different from cleaning the original ones at RC. Is there any prevailing wisdom on the subject? New is new, but oem is oem.

EDIT: In a pinch, a 13/16 open end can be used to help loosen the banjo bolts on the fuel filter.

I hesitate to say it, but I think this is solved for now. The new Denso o2 (pn 234-4065) went in, and after 30 minutes of decent driving I have no CEL. This o2 did not have the rubber grommets around the wire that clip in under the cam box, but I made some from electrical tape.

The trick for me to get the old o2 off was PB Blaster, and to take off the shock mount brace. Once I did that, I slipped a box end wrench over the end of the socket wrench for more leverage and she unloosened right up.

Thank you all for your help. Hopefully this stays solved.

mk2davis said:

I hesitate to say it, but I think this is solved for now. The new Denso o2 (pn 234-4065) went in, and after 30 minutes of decent driving I have no CEL.

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A win is a win, embrace it!

If the injectors are 28 years old, you might still want to put cleaning on the to-do list if the car has sat for extended periods of time. Also, how is that other frequently deferred item - the timing belt?

Old Guy said:

A win is a win, embrace it!

If the injectors are 28 years old, you might still want to put cleaning on the to-do list if the car has sat for extended periods of time. Also, how is that other frequently deferred item - the timing belt?

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Salt in the wound. SALT IN THE WOUND!!!

The timing belt needs to be done too, and that's a job I'm not looking forward to. And the manual steering rack needs to be addressed, I have an email out to D1_guy for that as he seems to be the resident expert. The AC needs to get fixed, the chromed wheels leave a bit to be desired....

But d@mn, she is a fun car to drive.

mk2davis said:

Salt in the wound. SALT IN THE WOUND!!!

The timing belt needs to be done too, and that's a job I'm not looking forward to. And the manual steering rack needs to be addressed, I have an email out to D1_guy for that as he seems to be the resident expert. The AC needs to get fixed, the chromed wheels leave a bit to be desired....

But d@mn, she is a fun car to drive.

Click to expand...

Glad you got it fixed!

Prioritize the t-belt. And take your time lining up the belt, especially the rear exhaust cam.

I looked, but didn't see any guide for additional info re: the t belt. I'm surprised I didn't find a suggested part number list for replacement or a list of suggestions in the how to?

Have you found the TB/WP DIY pdf?

http://www.nsxprime.com/FAQ/DIY/timing-belt.pdf