what kind of symptoms would a broken FPCU cause?

[Evanarc]

I was getting the code 12 and 34 for MAF and knock sensor problems.my mechanic tested the resistance on the fuel pump and injectors and they seemed to have checked out A.o.k. He says the engine is burning too lean and the most likely cause could be a broken/malfunctioning fuel pump control unit. A brand spankin' new one will run a lil over 500$.when it comes to mechanics theres none better, but a technician he is not and computers are a lil beyond him. Im very far away from an infiniti stealership nor do i feel like biting the bullet on the 125$ diagnostic fee.what do you guys think? what symptoms would a broken FPCU create?

thanks in advanceEvan

[Infinitiguy19]

Well what was the the voltage and ohms of the fuel injectors and fuel pump?

Did he test the fuel pressure at idle (33 PSI) and WOT AKA Wide Open Throttle (44 PSI)?

Does it jerk when you do a WOT run?

Does the car feel like its starved for fuel?

Was the fuel filter replace recently with in the year?

I have one by the way if you need it.

Good luck

[Evanarc]

the car definitely feels like it wants to go but its starved for the fuel it jerks and bucks alot in drive, the fuel filter was replaced less than a yr ago, i thought this was an MAF problem but i replaced that, the wires and connector and there still seems to be an electrical problem ( the interior lights shutter when the idle is jumpin around)

[Q451990]

PPastos is on the right track... next step is to check your fuel pressure and volume.

Heath

[Infinitiguy19]

The lights flicker, That strange. How is the alternator or the battery?

Thanks Q451990

[Evanarc]

both r brand new for me as of january

[Infinitiguy19]

I would have to say like some other people have told me on that issue, It might be the switches on the door (when you open the door, the lights go on because of those little black switches).

[maxnix]

Nissan rebuilt alternator and OEm minimum specification battery?

The pump is not measured in resistance but rather current draw. Check the volume after it is T off after your brand new OEM fuel filter.

A failed FPCU will deliver no voltage either constantly or intermittently to the FP.

[Q45tech]

Like an alternator the FPCU has 3 circuits in parallel and it is possible for 1,2, or all to fail. Idle, Cruise, WOT acceleration.

Because at idle the engine only uses 1 gram of gasoline per second, 3-4 grams at cruise, and 16-20 grams at WOT [depending on rpm].

To maintain a steady pressure at the injector the fuel rail must have 3-5 times more fuel flowing than the amount squirted out the injector.

Bucking and snorting occurs when the volume and thus pressure falls below these limits and different cylinders get different and low amounts of fuel.

The fuel pump always gets the same +12-14 volts from batterry just that the FPCU inserts a resistance value in the pump ground line. This is a variable speed control.

The exact pump speed depends on the wear and tear on pump commutator and brushes and the resistance [voltage dropping effect] from FPCU.

[goody90q45]

I was getting the code 12 and 34 for MAF and knock sensor problems.

Is the CEL still on (for code 12) or have the codes cleared? Until you get through this leave your ECU on the floorboard so you can quickly read codes. Don't worry about the KS for now.

....i thought this was an MAF problem but i replaced that, the wires and connector and there still seems to be an electrical problem (the interior lights shutter when the idle is jumpin around)

There's no reason for the lights to flicker unless something is shorting out or there's a bad ground. Corrosion is a huge issue for owners in the Northeast. Are the battery clamps in good shape and fitting tightly (OEM ones tend to wear out)? Posts clean? Look at the positive clamp closely and make sure everything is intact. Follow the negative cable to ground and make sure it's secure and uncorroded.

......i thought this was an MAF problem but i replaced that, the wires and connector and there still seems to be an electrical problem......

Do you have the 4th wire on your MAF connector going to ground (originally a dealer mod)? Have you checked the wire splices on the new pigtail to make sure there's no shorts? Is it secure and uncorroded? Your MAF connector is new but shouldn't wiggle more than 1/8" up and down. The pins should be clean and lubricated with dielectric grease, and fit tight on the MAF. They should look like the center pin in the pic.

.......He says the engine is burning too lean and the most likely cause could be a broken/malfunctioning fuel pump control unit...... what symptoms would a broken FPCU create?

Tech's post explains it all but in layman's terms if you lose the WOT setting (melted solder joints inside the FPCU?) the car will buck when you accelerate or climb hills. When your speed levels out or you reach the top of the hill the FPCU goes back to cruise mode and the engine runs fine. If your FPCU has lost both cruise and WOT modes your Q won't run worth a darn as soon as you put it in drive.

A fuel pressure check at idle, cruise and WOT is the best advice (tee in and tape the gauge to your windshield) but there are a couple of quick and easy things to check in the meantime. Go in through the trunk by removing 1 or 2 plastic trim fasteners and unplug the FPCU. It's in the background in the picture. Is there any signs of burning or charring on the FPCU or connector? If so it's time to replace the FPCU and probably the fuel pump. If you're a DIY'er pull out the back seat and rear deck to remove the FPCU. Open it up and see if there's any signs of melted solder connections. I understand the internal ground connections can have problems.

You can also ground the fuel pump to run on full speed only and take the FPCU out of the circuit. From the FPCU connector run a jumper wire from the pin with the alligator clip to a ground on the chassis (gas tank shield). If the problem goes away and the engine runs good it's definately the FPCU.

Good luck Evan. Don't give up on your Q. Once you find the problem you'll probably see that it was something very simple.

[Unnatural1]

Excellent post from Goody.

I would start with bypassing the FPCU with a jumper wire as Goody shows in the last picture above. I believe that wire is solid white. It is the ground wire from the fuel pump to the FPCU. This is very easy to do yourself as you simply need a piece of wire and no special tools (such as a fuel pressure gauge). If your Q begins to run as it should, then you know your FPCU is the culprit and should be replaced.

I would recommend finding a known good FPCU from one of the members here at NICO in the classified section. Keep in mind it is a used part and may also fail in the future, but I've seen them for under $100. If you do find that the FPCU is at fault you need to check the fuel pump for excessive amperage draw. This is usually what kills these units. They over heat from excessive amperage.

Just checking fuel pump resistance as specified by the factory service manual is really not useful, in most cases, in determining the fuel pump's health status. If by chance your mechanic has a lab scope or graphing meter, this can be used to inspect the wave form produced by the fuel pump to make a determination.