'91 Fuel pressure readings?

I have questions about fuel pressure, what is normal?
here is what I have:
new fuel filter just installed
all readings with engine idling,
Test #1
FP resistor connected and FPR vacuum line connected 47psi.
If you pinch the return hose, no change in pressure; if the vacuum hose is disconnected from the FPR, no change in pressure.

Test #2
FP resistor bypassed by jumping the connector and FPR vacuum line connected 48 psi.
FP resistor bypassed by jumping the connector and FPR vacuum line disconnected 57 psi.
If you pinch the return hose, the pressure jumps to 72psi with the vacuum line connected or disconnected.


In test #1 , it seems like the fuel pump is putting out "all she's got captain" at the reduced voltage and there is nothing left for the FPR to regulate. I haven't done the flow test from the service manual yet because I am sure it would fail with the resistor connected, and that is how the test is performed from the instructions. I am thinking the fuel pump is weak and at full song, even with full voltage, cant keep up. knowing the fuel pump is not a quick job, I don't want to "throw a fuel pump at it".

Can someone fill in what is normal pressure readings for the same or similar conditions?

I am chasing a CEL code 2 & 44, "rear bank fuel supply system" that is intermittent, but happens on most trips. When the CEL illuminates, a quick engine off and back on will clear it. I have noticed it seems to illuminate when lifting off the throttle. I have replaced the O2 sensor, After looking into this, one of the things I found is the fuel pump resistor is bypassed with a jumper wire in the connector. . What also adds to the mix is an unknown chip in the ECU, high flow airbox, headers and exhaust.

Thanks in advance,

I don't have access to a early model years service manual. The service manual for my 2000 specifies that with the engine idling, the fuel pressure should be in the range 47 - 53 psi with the manifold pressure reference line (vacuum line) removed from the regulator and pinched off. When you reattach the vacuum line the fuel pressure should drop to the 36 - 44 psi range. The fact that your fuel pressure remains high with the vacuum line attached suggests that there is a leak or blockage in the vacuum line, the fuel pressure regulator is incorrectly adjusted or the fuel pressure regulator is faulty. Your fuel return line could also be blocked.

However, given that you have a mystery chip in the ECU, the fuel map may be set up to operate with a higher fuel rail pressure than stock (increases the flow rate through the injectors). In the absence of any documentation that applies to your mystery chip, you really have no idea what the fuel rail pressure should be.

If you had a stock ECU, I would suggest trying to set the fuel pressure regulator with the vacuum line attached to the correct range and then re check to confirm that you get the correct pressure with the vacuum line removed. If you end up with the correct values then your pump is probably OK (tests to be done with an un bypassed fuel pump resistor). With a non stock ECU, you are on your own!

Edit:

Did you flip the results in test #2 ? With the engine idling (vacuum in the intake manifold) the fuel rail pressure should always drop when the vacuum reference line is connected to the regulator. The objective of the vacuum reference line is to keep a constant pressure differential between the fuel rail pressure and the intake manifold pressure.

Thanks for the reply, it helps to bounce things off another person.
yes, the numbers were backwards, (I did edit them now) Thanks for pointing that out.

the '91 manual has exactly the same procedure as your '00

• Start the engine. measure the fuel pressure with the engine idling and vacuum hose of the pressure regulator disconnected. Pressure should be: 323-363 kPa (3.30-3.70 kg/cm2, 46-53 psi)
• Reconnect vacuum hose to the pressure regulator. Pressure should be: 250-304 kPa (2.55-3.10 kg/cm2, 36-44 psi)

the vacuum line and fuel return lines are okay, the return line was blown out with air and the vacuum line was replaced and has quite the flow when disconnected. the FRP is non-adjustable OEM looking.

Now I look at it, it might be a FPR, causing this problem. The pressure is defiantly too high, and the fact it won't go any lower than 47 is a problem.
Maybe the FPR was replaced with one of a higher setting by a previous owner? don't know at this point.

Guess I will get a new FPR installed and move onto the next issue.

You are correct about the non adjustability of the FPR if it is the Honda OEM regulator. I was thinking that maybe they retrofitted a different FPR with the modified ECU. The fact that the fuel rail pressure does not drop with the vacuum line connected is a pretty sure sign that the regulator is faulty.

I just replaced the fuel pressure regulator with a new OEM part, and now the pressure is 41-42 psi and is within specifications of 36-44 psi (FP resistor connected, idle with vacuum line connected)

But, the pressure still does not change when the vacuum line is removed. Next will be the flow test. But I don't want to do that in the garage, until the weather lets me keep the door open to ventilate the fumes.
The old FPR was exactly the same as the new part. so I believe it was a correct part to begin with.

You do have a bit of a mystery.

As a simple suggestion, try replicating your original test #2 with the fuel pump resistor by-passed. If that gives you the higher pressure with the reference line disconnected like it did originally, then you know that your new regulator is good as opposed to getting a faulty one by some incredibly bad stroke of luck. It may also suggest that your pump is getting tired and can't deliver the required pressure when the fuel pump resistor is switched in (this comment applies on the assumption that the NSX has a turbine style as opposed to a positive displacement style pump). Your plan to do the flow test would be the appropriate next step.

However, before doing the flow test, inspect your connections to the fuel pump resistor and check the resistors resistance. You want to make sure that the poor fuel pump performance is not due to a bad connection at the resistor which is dropping the voltage more than it should.

There is very little information about the resistor in the '91 FSM. Mine reads about 1 ohm. I would love to see what the operating voltage is for the fuel pump running in low is for a known good system. Maybe someone has done this?
I am guessing that the flow test will fail. But we will see. A little to cold here to play with it much. Who knows, my car could have had a cheap knock-off pump installed by a previous owner! Only one way to find out......

A question: mileage on car(fuel pump)?

Since the fuel pressure will not go up with the pinched return line/resistor connected, is does seem to indicate that fuel flow from the pump is impeded. There is a filter(sort of like a sock) on the fuel pump internally in the fuel tank. The results make me think it is dirty. Unfortunately it would mean tank removal, and if so I think I would install a new pump for sure. I have had this code before, and in the case I had, after all pressures looked correct, I sent out the fuel injectors for cleaning. It was an amazing difference, code cleared. So this does seem like a lack of fuel for sure.

Regards,
LarryB

GraemeD said:

There is very little information about the resistor in the '91 FSM. Mine reads about 1 ohm. I would love to see what the operating voltage is for the fuel pump running in low is for a known good system. Maybe someone has done this?
I am guessing that the flow test will fail. But we will see. A little to cold here to play with it much. Who knows, my car could have had a cheap knock-off pump installed by a previous owner! Only one way to find out......

Click to expand...

After taking a look, the service manual for my 2000 is equally silent on a test value for the fuel pump resistor. In addition, unless it is well hidden in some other section, the fuel / emissions section of the manual does not provide any values for flow testing the fuel pump.

I am like Larry B. If you are considering dropping the tank to inspect the fuel pump assembly, I would have a new pump on hand for replacement. Even if the existing pump turned out to be OK, I would be so burned if that pump failed in a couple of years and I had to go through the tank removal process again!

Thanks for replying Larry,
For clearity, I just changed the FPR and had the injectors cleaned. I also removed and cleaned the throttle body and idle valves. I have not driven the car yet to see if the CEL code reappears. I have just idled the engine to leak check the injectors, FPR. I also set the base idle and checked the fuel presssure.

As for the sock being clogged, I would think that I would see very low pressure when the pump is run in high. I did drive the car with the with the presure gauge hooked up and the presure was in the 50s at full throttle. That was before this last round of work.

If the pressure goes to 50 psi at WOT, it seems very odd that it wouldn't also go to 50 psi with the vacuum line disconnected from the FPR. Aren't those inconsistent results?

Differance is it goes into "high" above 4000 rpm. Thus the resistor is removed from the circuit and the pump gets full electrical power.

One last fling!

Since problems with the main FI relay in the NSX are not uncommon, with the car running check the voltage on the supply side of the fuel pump resistor (black / yellow wire). You will have to use a fine piece of wire to back probe the connector to get the voltage measurement. The voltage should be very close to your supply voltage which should be greater than 13 volts with the car running. If the voltage at this point is materially lower than the supply voltage, then you have some bad connections or the main FI relay solder joints or the contacts are failing. The reason I suggest doing this with the car running is that the current drawn by the pump will flag the effect of the poor connections which may not show up if there is no current flowing.

That's a good idea to check the main relay. For the fuel-pump resistor, you can pretty easily bypass it inside the firewall with a piece of wire and a couple spade connectors.

Old Guy,
Good suggestion, but I have already done that. There is system voltage "to" the resistor using the back pin method. I even took the main relay apart and inspected the solder joints under magnification.
I would love to know what the voltage is on both sides of the resistor and or even the current draw, of a known good car.
I have just been very busy and haven't had the time to chase this down. Next time I get out there I will measure all the voltages and measure the current draw. Stay tuned

Seems like you have covered the obvious and not so obvious stuff already.

I would offer to take the voltage measurements; but, my car is wrapped up in a storage bag in a secure facility. That and the fact that its way below zero here - hard to take measurements when you are wearing mits!

Update:
The weather finally agreed with my schedule, and I got a chance to drive the car. I put about 100 miles on it, and I am happy to report no CEL. It looks like the injector cleaning might have solved that problem (code 2 & 44). It idled better also and it seems like the fuel consumption is better also.

I did some voltage measurements and here is what I got with the engine not running, main relay removed and a couple of wires with a switch in its place. Battery charger connected to keep the system voltage up.

Resistor bypassed, fuel pressure 52 psi. Voltage to pump was 11.41v. The pump pulled 5.4 amps
Resistor connected, voltage was 8.4v and around 3.5 amps.
Resistor is .71 ohms

interesting observation, is the amount of voltage drop with the pump running. The system voltage was 12.7v - 12.8v but dropped to 11.4 with the pump running. This is measuring at the feed side of the main relay. The wiring between the fuse and the relay is where I think the bottle neck is. To me a voltage drop of over 1v in the supply sideis too much. I am not done with this and plan on testing this further.

As as for the fuel pressure, I think my next item for replacement is the resistor, my thinking is: the system should always run at the pressure that the FPR dictates, with is about 50psi relative to manifold pressrue. Since my fuel pump with full voltage will output enough to allow the FPR to regulate the pressure by bypassing the excess, it should be good. But with the resistor inline, the power is not enough to allow the pump to put put enough pressure to let the FPR do it's thing.

When I get a chance to play with it some more, I will passs along what I find out.

Update:
I did the flow test per the service manual, ZERO flow. ( I knew this was going to be the case) this is with the system fully intact (resistor connected). I repeated the test with the resistor bypassed, and only got maybe 50cc of fuel when it should be around 330cc.
So it might be time to drop the tank.
I have read here that the pumps are mostly reliable, Has anyone dissected a removed weak pump?

As for my fuel pump voltage drops, I did a little more testing with the engine idling. voltage at:
Jump start block, 14.4v
white terminal on back of ignition switch, 14.04v
fuse #2 supply side; 13.84v
fuse #2 load side, 13.80v
fuel pump resistor (bypassed), 12.4v

I am still not happy with the readings and will keep searching for the reasons why the voltage drops.

The OEM pumps are very reliable. As the cars get up there in age this is to be expected with the brushes wearing out along with the resin turbine.

Statistically, Walbro and Aeromotive make OK replacements. However, if you do a search on here you will find a number of people that have had problems with them. It's not much fun to replace the fuel pump twice on these cars.

I have 10ga directly from the stud on my alternator going to my Denso "OEM Supra" fuel pump. Racetronix has a nice kit for in-tank fuel-resistant wiring and a leak-proof bulkhead fitting, part number BCWS-001 (I ran their two 14-ga wires in parallel). I also used their FPWH-009 kit for the ex-tank wiring, fuse, and relay. They make nice stuff and it is very reasonable.

Might be a little overkill for an OEM application, but I don't worry about excessive resistance anymore!

Dave

GraemeD said:

Update:
I did the flow test per the service manual, ZERO flow. ( I knew this was going to be the case) this is with the system fully intact (resistor connected). I repeated the test with the resistor bypassed, and only got maybe 50cc of fuel when it should be around 330cc.
So it might be time to drop the tank.
I have read here that the pumps are mostly reliable, Has anyone dissected a removed weak pump?

As for my fuel pump voltage drops, I did a little more testing with the engine idling. voltage at:
Jump start block, 14.4v
white terminal on back of ignition switch, 14.04v
fuse #2 supply side; 13.84v
fuse #2 load side, 13.80v
fuel pump resistor (bypassed), 12.4v

I am still not happy with the readings and will keep searching for the reasons why the voltage drops.

Click to expand...

Given that your car is 24 years old, I don't think that the total voltage drop from the jump start terminal to the fuel pump resistor is unreasonable. With the fuel pump resistor by-passed, you said the current draw was 5.4 amps. 0.3 ohms of resistance at 5.4 amps would explain the 1.6 volt drop between the ignition switch and the fuel pump relay. With a couple of meters of wire, a few terminations plugs and the main FI relay in series its not too hard to get 0.3 ohms. You may be able to improve things slightly by using some conducting grease on the various terminations; but, you are not going to get a huge improvement.

Where did you find the flow test procedure for the fuel pump? I had looked for that in my service manual; but, came up empty.

The fuel flow test in the '91 service manual, section 11 fuel, page 102.


FYI: the resistor rating (resistance) is printed on the resistor, mine is .73 ohms, exactly what I measured.

Another update;
Before I pulled the trigger on a new fuel pump, I did some more testing.

I repeated the flow test, but this time, I used an external variable power supply connected directly to the fuel pump connector behind the drivers seat, this eliminated 90% of the car wiring.

• simulated "high" mode: 13.5 volts, 7.1 amps, produced 250ml of flow (not enough, 9.0 volts should produce 330ml)
• simulated "low" mode; with the resistor (.71 ohms) inline, 13.5V supply to the resistor, 9.0v out to the fuel pump, 5.2 amps. (Did not measure flow because it failed above.)

Playing with the voltage from 8 volts to 14.5 volts, you can hear when the FPR is opening and returning fuel back to the tank. My pump is running okay, but is not producing enough output. I believe the impeller is most likely worn.

I have ordered a new pump and will install it a little later.

PS, I did find that one of the female contacts in the harness side connector for the fuel pump resistor did not grip the male contact very tight in the FPR connector. disassembled the connector and reworked the contact. it is fine now. I believe it was from a PO jamming a piece of round wire in to bypass it.

If you have had a chance to replace the pump, did you take a look at the original pump? I am curious about the type of pump. Is it a turbine style or a positive displacement style pump? If it is a positive displacement pump, I can see that there could be wear after 24 years that would result in flow problems. If its a turbine pump its not so obvious what would wear to result in the flow problems.

Two observations. In my somewhat limited experience it has usually been the drive motors that die on fuel pumps and the corollary of this is that (also in my limited experience) you have set some kind of record for going 24 years on an original pump!

I am in the middle of changing the pump, I got caught in the "while you are in there". so I am replacing all the hoses. waiting on some more parts.

I do plan to take the pump apart to investigate it's internals. I will post the results.

I disassembled the pump, it appears to be a combination of positive displacement and a vane type pump. it has vanes that pull the fuel around the chamber, but the size of the chamber decreases. I don't think wear is the issue, because the upper half of the pump floats vertically and has pressure on the backside of it, thus acting to push it down on to the vane and the vane onto the bottom half. There is about a .3mm difference between the unworn center and the area that rides against the pump housings, but because of the design, the housing closes up any gaps from vane wear.

The fuel flow is: strainer, pump, thru the motor and out the top. there is a pressure relief valve in the top next to the outlet. Maybe that is the issue, letting fuel escape because it is defective? I'll look at that tomorrow.

I also am replacing the output hose from the pump to the filter. Instead of buying the $205 line assembly, I removed the hose from the line by carefully cutting the crimp fittings and removing the hose from the metal pipes. The hose is bonded to the pipes, so it does not just slide off, again be carful not to scratch the metal pipes. using a plastic scraper, I removed the remaining bonded on rubber. Around $12 for 175mm of modern 5/16" fuel injection hose, a pair of pinch/crimp clamps, some fuel proof sealant (bonding agent) you have a pipe/hose assembly that will last another 20 years. I do plan to cover the clamps with heat shrink and the hose with a braided material for aesthetics.

I am waiting on the 5.5mm vent hose now. I need this before I put the tank back in. I had to order 8000mm (26ft), so if anyone wants to buy some from me, I will have plenty left for sale.