Hesitation/Fuel Pressure Issues and Resolution

[Lude_2_NSX]

I wanted to post up a a problem I had and the resolution for any future nsx'ers out there.

Since i bought the car (about a month ago), there has been a hesitation at below 4K RPMs, that went away above 4K.

Anyways, I began troubleshooting by installing a fuel pressure gauge on the fuel filter assembly. I recommend everyone do this. It only costs ~$50 and its an easy way to make sure you dont have any fuel pressure issues. What I found, is that my fuel pressure was at 30 PSI at idle and would not increase when the fuel pressure regulator was disconnected. I then disconnected the fuel pump resistor and jumpered the connector on the harness. Once started back up, my fuel pressure jumped up to 40 PSI at idle and around 45 or so when the FPR was disconnected from vacuum. I troubleshot this to a faulty fuel pump.

I replaced my fuel pump with a Walbro 255 from Science of Speed (no need to get in a debate about which is best, i've used Walbro pumps in the past with much success). Once replaced, I plugged my resistor back in and fuel pressure was perfect 42 PSI at idle. I hope this can help anyone going forward to may be experiencing the same symptoms.

Also, the fuel pump replacement was MUCH EASIER than I had expected. I was able to do the job by myself in only a few hours, but I found a few tips that are not necessarily outlined in the manual:
- Most importantly, drain your tank. There is a 17mm bolt on the bottom of the tank. It will make your life much easier.
- Remove the air cleaner housing to disconnect the hoses on the drivers side. You can see them from underneath, but they are a piece of cake from the top with the air cleaner removed.
- When re-installing the tank (if by yourself), lift one end and feed the hoses through the opening and hold that end with a jack. Then lift the other end while feeding the hoses through. It only took me one try and then the straps can go right back on once it is verified that the hoses are routed properly.

I hope this helps some of you guys!

 

[roybotnik]

Thanks for the tips, I'm going to try this sometime this week.

 

[Lude_2_NSX]

Good luck!

 

[Larry Bastanza]

Hey gald you are good to go. Nice tips......

 

[nsxto]

Thanks for the tips, appreciate you sharing your experience.

BTW, what FP gauge + adaptor did you use to hook up to the filter? Did you hook up an electronic sender to the filter and a gauge inside the cabin, or did you just install one of those analogue dial gauges that sit on top of the filter?

 

[Lude_2_NSX]

Thanks for the tips, appreciate you sharing your experience.

BTW, what FP gauge + adaptor did you use to hook up to the filter? Did you hook up an electronic sender to the filter and a gauge inside the cabin, or did you just install one of those analogue dial gauges that sit on top of the filter?

Just an analog gauge on top of the filter... Picked up an auto meter for like $50

 

[AcuraNSX]

Should I replace my Fuel Pump as Preventative Measure?

Lude, Larry, or any other fellow NSXer,

Should I replace my Fuel Pump as a Preventative Measure? My 91 Automatic NSX has about 120,000 miles on it and I don't know if it was ever done. Car shut off the other day while sitting idle in my driveway the other day.....I am thinking start with Main Relay. I am asking about the fuel pump because I just don't want to get stuck somewhere especially if the pump only costs around $118 for a Denso, or $129 for a SOS Walbro. Who made the OEM part for our cars? The OEM part costs around $400. Is the fuel pump more mileage dependent than time dependent? I had a 89 Mazda MX6 with 359,000 miles on it and I never once touched the fuel pump! I figured you guys are the most trustworthy guys to ask! Thanks always for your time and consideration guys!

 

[goldNSX]

I figured you guys are the most trustworthy guys to ask!

I guess we are.
The impellers of the fuel pump wear with miles mainly and how they've been driven. Weekend AT cars are kindlier to the fuelpump than MT cars with a lot of spirited driving and track events. Nobody wants to end up in the desert with a dying fuel pump, yes that's my fear too. My answer is to address the mail relay first if it hasn't been already.

There's no exact replacement pump for the NSX (Denso) but some of the mentioned pumps work, some better, some...

 

[Old Guy]

I want to respectfully amend [MENTION=10201]goldNSX[/MENTION] 's advice. I agree with the implied advice that the age of the pump is not a good indicator of impending wear out. Mileage is better than age; but, the true determinant would be operating time for the pump which is exceedingly hard to assess. A pump with 140,000 miles of highway only driving will have approximately 2300 hours of operation. 140,000 miles of urban driving would give you an operating time more than double that value. The number of starts is also a factor. When you first apply voltage to a pump the current is high until the rotor starts spinning and generating a counter EMF that reduces the current flow. That high current is not kind to the motor's commutator and contributes to wear. Unlike D9 Cats and the like NSXs do not come equipped with operating time meters, so you have to apply some subjective assessment as to how much time the pump has on it.

My experience with fuel pump failures has all been with European cars equipped with Bosch fuel pumps and none of them were built after 1990. So, its a sample set that may no longer have a lot of validity. I have yet to experience a fuel pump failure on more modern Japanese vehicles. Based upon my Bosch experience, none of them made it past 11 years of primarily urban driving. The failure mode was typically that the pump quit stone cold dead. If you were lucky, it would fail to start and then start on a second try once or twice before checking out permanently - leaving you in the proverbial desert (or in my case on the side of the Alaska highway). In my limited experience, I never had a Bosch fuel pump fail to deliver pressure. If it ran it made pressure.

Anecdotal evidence is a notoriously bad way to make a quantitative decision because its primarily based upon self reporting (if you are using the PRIME forum to collect the data). However, when its the only evidence you have, you 'go with what you got'. My take on the fuel pump events reported on Prime is that the most commonly reported event is failure to deliver adequate pressure rather than outright quitting. That would suggest that if you do the fuel pressure test in the service manual and the pressure is in spec, the pump may be just fine. It also suggests that you get some warning before the pump quits. However, that evidence may reflect the fact that perhaps nobody makes a post on Prime when a fuel pump fails to run on a 15 year old car because they figure that it is a normal wear item and not worthy of a post. Hence you end up with a biased data set.

If you do the fuel pressure test and it fails to meet spec then there is no question that you should replace the pump. If the pressure is within spec, then you have a more subjective risk assessment to make about the possible inconvenience of being caught with a dead pump. To put that in context, if you are one of those NSX owners that drives around without a spare tire to reduce weight, then you probably should not worry about a pump failure leaving you stranded (I have had more tire punctures than fuel pump failures in my driving experience). That said, at 27 years of age and 120,000 miles, if I were risk adverse I would not necessarily feel bad about doing the fuel pump replacement as a preventative maintenance item.

Oh, and as goldNSX said, replace that Main FI relay if it hasn't been done already. Much more anecdotal evidence to support it as a failure item and much easier and cheaper to do than a fuel pump replacement.

 

[AcuraNSX]

You Guys are Great.....

You guys are great! Thank you for the advice and extremely well explained responses. I believe the Main Relay you are referring to is that little box in the car behind the passenger seat, next to the radiator fan control unit? Question is should I do Walbro or Denso.....hmmm....? SOS sells the Walbro, just didn't know if they made superior pumps than Denso. Should I even bother changing the resistor box/relay in the engine compartment passenger side on the firewall? Or just hold off on that?

 

[goldNSX]

Thanks to [MENTION=26435]Old Guy[/MENTION] for the extended answer. He's right that wear is correlated mainly to running hours (and pressure) in general which correlates to mileage (but not with factor 1).
The car gives you several hints if the fuel pump is weak:
- less torque below 4k rpm. Hard to detect (I've had the chance to drive another NSX with a good fuel pump and was surprised how slow mine was back then) unless you have also...
- hesitation below 4k rpm (other possible causes here as well)
- VTEC-like power delivery above 4-4.2k rpm.
- less power at the top end
- left stranded
1-3 can be detected.

From my experience (both CTSC) I'd avoid the Walbros, some people rave about them but they might work for a few years but the reliability is far from a Denso IMVHO.

RockAuto suggests a Denso fuel pump I think nobody has ever tested. The other one ...155 (Supra) is a little bit overkill for a standard NSX.

 

[Old Guy]

I believe the Main Relay you are referring to is that little box in the car behind the passenger seat, next to the radiator fan control unit?

Should I even bother changing the resistor box/relay in the engine compartment passenger side on the firewall? Or just hold off on that?

Yes, on early cars the main FI relay is between the ECU and the cooling fan control unit.

I assume the resistor you are talking about is the fuel pump resistor? I only recall one post of somebody experiencing a failure of the fuel pump resistor so I would put it further down your trouble shooting list. Similarly, based on the number of Prime posts, the fuel pump relay (which is really the fuel pump resistor by-pass relay) does not have a significant history of failures so I would leave it. The fuel pump resistor does generate a fair amount of heat and it is on most of the time (for most drivers). Heat is not a friend of electrical insulation and your car is 27 years old, so a useful no-cost activity would be to examine the wiring at the resistor and the plugs. Check for signs of heat damage to the insulation and heat damage to the plugs - plastic housing melting and allowing the pins to move resulting in poor or intermittent connections. If the wiring around the resistor looks OK, it probably is OK.

The DENSO 9510011 that Rock Auto sells for less than $100 is Denso's catalog pump for the NSX. So far, my experience with Denso automotive electrical stuff in general has been nothing but positive. Can't say that for some of the other manufacturers.

 

[goldNSX]

The DENSO 9510011 that Rock Auto sells for less than $100 is Denso's catalog pump for the NSX.

Not sure if it's the same as the one used by Honda esp. regarding the current draw. It would be interesting to have a feedback from someone who went with that one. It would be nice to have a 100% substitute to the expensive OEM one.

 

[Lude_2_NSX]

Do your main relay first. Its an easy fix. You can go the "poor mans route" by just re-soldering it, but I would just buy a new one, repair the old one and keep it in your glove box. Maybe you'll help someone out at NSXPO someday...

In regards to the fuel pump, it may show signs of failing or just completely fail. You can replace it as preventative maintenance, or just make sure you have towing coverage and consider that your peace of mind. It's a slippery slope once you start thinking about EVERY part that can leave you stranded. Then again, some folks have more expendable income than me... :frown: