FPCU diagnosis question

[91QAZ]

I need help diagnosing a non-start condition on my 91 Q, 92K miles, original FP and FPCU as far as I know. The engine cranks, but acts like it gets no fuel. I've read up on the FPCU and how a failing FP will cause the FPCU to overheat, etc. I know my FP needs replacing - it's humming - don't know for how long, but I noticed it now that I've been running diagnostics. I want to make sure that I don't have a more immediate problem/failure causing the no start condition.

So I disconnected the fuel line at the fuel rail (after the fuel filter), and turned the key to "on" - no fuel, cranked the engine - no fuel. Then I used the alligator clip jumper from the FPCU harness to ground to bypass teh FPCU and tried the same thing. This time I got fuel as soon as the key hit "on". So I concluded my FPCU must be bad. I removed the FPCU and looked for signs of it frying. I see NOTHING. And according to my ohm meter, the #4 connector does not have an open circuit to any other point on the board that appears to be connected to that trace. Now I'm confused. What am I missing?

Tom

[elwesso]

sometimes the ground to the FPCU is bad...

[91QAZ]

Hmm. Do you know how the FPCU is grounded? Is it grounded thru the body of the unit, or thru the harness?

edit:

The first time I ran this test, I had taken the FPCU out (unbolted from the body) and had only plugged the harness in, leaving the FPCU dangling in the trunk. After reading Wes' post I decided to re-test with FPCU bolted to the body just in case it is grounded thru the body. This time I got fuel with the FPCU installed when I key is 'on'. So either the FPCU is grounded thru the body and I just screwed up the test the first time, or I have an intermittent problem with the FPCU. Unless there is a 3rd option that escapes me right now, I'm going to have to start looking for a no spark condition. The car cranks well, but I get no start - it doesn't even try to start... I'll pull ECU codes next and see if that leads me anywhere.

Thanks,Tom

Modified by 91QAZ at 6:28 PM 2/20/2006
Modified by 91QAZ at 7:56 PM 2/20/2006

[maxnix]

Be sure to check fuel pressure before jumping to that conclusion.

[3Q Jay]

the obvious.....guides done?

[Q45tech]

FPCU is grounded by 4 mounting screws use a jumper with alligator clips if testing without mounting.

While most show signs of overcurrent/heating stress, have seen a dozen where transistors are open without visable signs.

Over the years must have looked at ~~50 units.

I always keeep a spare in trunk, along with ecu and tcu and the tools to change them when I go out of town. Plus a mini FSM...........never know when you have to use the lift at Joe's garage in the boondocks.

Never happened in 15 years because I was prepared.

Never used the spare tire either in 305,000 miles. Over 10 sets of Michelin tires.

[91QAZ]

Jay, Infiniti Service History indicates that the guides were done by the dealer when owned by the prior owner.

Dennis, thanks for the confirmation on the FPCU ground. That confirms that my first test was a bad test, as teh FPCU was not grounded at the time.

Brian, what are you thinking? Not questioning you - just want to understand. I did get fuel flow with the FPCU installed, but sounds like you're saying it may not have been sufficient pressure to get the car started?

Thanks for the input, guys.

[Q45tech]

Understand that while cranking and for the first few seconds after the crank the FPCU is in the WOT [highest flow position] as during cranking [when coolant is not warm] the injectors fire in the BATCH MODE....all are open simultaneously to get a jolt of excess fuel into the chamber.

The FPCU may not work [blown transistor in WOT mode] yet work in idle and cruise or any combination depending upon which transistor circuit fails.

[DrewQ45]

Got one if you end up needing it....

$60 + Ship.



...Drew....

[elwesso]

Drew "4 first names" E. Worlds greatest online Q45 parts salesman.....

[91QAZ]

OK, I've done some more reading and some more testing. Dennis, I think I now understand what you are saying - FPCU controls FP via 3 different fuel pressure ranges. FPCU may not be working in one or more of these ranges, but may work in some. I think this is what Brian was getting at as well.

I just ran the jumper test again - the one that bypasses the FPCU by grounding the harness. My understanding is that this operated the FP at it's max pressure. In this condition, I still have no start, so seems to me I've ruled out the FPCU at least as the immediate reason for my no start, yes? Battery is now hurting - it's on the charger - and I may not be able to get to the car again until the weekend, but I did pull ECU codes and got a "55", so no help there...

I poked around the engine compartment hoping to find some electrical connection that I bumped or worked loose after my underplenum/KS job (about 100 miles ago) Nothing jumped out at me .

[3Q Jay]

1. I just ran the jumper test again - the one that bypasses the FPCU by grounding the harness. My understanding is that this operated the FP at it's max pressure. In this condition, I still have no start, so seems to me I've ruled out the FPCU at least as the immediate reason for my no start, yes? 2. I poked around the engine compartment hoping to find some electrical connection that I bumped or worked loose after my underplenum/KS job (about 100 miles ago) Nothing jumped out at me .

Tom- glad to hear guides have been done!1. you shoulda heard the fuel pump hummin like a turbine in this condition--yes?2. you were running fine after the plenum? did you already try the flooded condition start (i.e. foot to the floor)? got a timing light to check spark?

[Q45tech]

The FPCU doesn't control PSI it controls VOLUME [pump speed- liters per minute]. It makes sure that there is enough VOLUME to allow an excess for Pressure regulator to work.

If the pump is not pushing enough VOLUME into the rail the pressure regulator cannot work it closes off the return line partially but what was designed is X volume flowing thru it creates a minimum of 34 psi back pressure..............less volume less backpressure thus rail pressure falls.

The ecu KNOWS what the excess VOLUME must be for each of 3 load situations and rpm..........thus it commands the pump via FPCU to increase volume.

The rub is as pumps age the commands are carried out by FPCU but the pump doesn't run as fast thus volume is lower than brand new.

Measure the pump speed [look at commutator waveform and count revs].

As pump slows down with a given voltage the current increases thus this melts the circuit board solder.

[maxnix]

All of which ties in to texasoil's comments on armature corrosion causing pump failure. See his posts on this topic and his pictures in Member's Rides.

[91QAZ]

Jay,

1. When jumpered (is that a word?), the FP definately hummed a tune - louder than with just the FPCU connected. 2. The car ran well after the underplenum job - I also installed a NICO ECU from Robert at the same time. I did get a little bit of stumbling under hard accel right after the underplenum job/ECU install - but it has not repeated that since. Probably related to my current problem, but I'm not sure how at this point...3. I have not tried the flooded start procedure yet, but I will now that you've mentioned it. My battery hates me right now.4. I have an old school timing light - I need to get an inductive type light - mine has the cheapo spring that goes between the plug and wire...

Dennis and Brian,

I follow you regarding the pressure vs psi and armature corrosion. If I understand correctly, my old pump may not be delivering enough volume to hit the 34 psi minimum, regardless of what the FPCU is telling it to do. I don't have access to tools that would show waveform, but I was already considering getting a fuel pressure gauge. If I tee a gauge between the fuel filter and fuel rail and measure pressure while cranking (with the FPCU bypassed), this should tell me if I'm hitting the 34 psi minimum, yes? Will this test accomplish the same thing as the waveform test? i.e. my pump is too worn to push the volume required by FPCU?

I guess my next question is: If the pump cannot push the volume required to hit 34 psi at cranking will the ECU (or some other factor) keep the car from starting (via no spark or some other method)? So far, it's clear to me that my FP needs to be replaced, and undetermined about the FPCU, but I'm wondering if the FP alone is the cause (or COULD be the sole cause) of the no start.

Thanks for your patience and experience,Tom

[maxnix]

Don't think the ECU monitors FP pressure. Just assumes it is working correctly. Whether car will start or not dependent on fuel spray and other engine system conditions.

Might be difficult to measure all three stages of operation without running engine.

[91QAZ]

That's what I was afraid of - which indicaates that even though I have a worn FP, I have more immediate problems causing teh no start. I guess I'll have to start poking around with electrical components. Is anyone aware of a way to test whether I'm getting spark - was so much easier with old school distributor and wires...

[squeefoo]

- was so much easier with old school distributor and wires...

I agree !Don't feel bad, the learning curve is rather steep with this car. I remember opening the hood on my '82 Olds 98 5.7 diesel (what seems like a million years ago) and wondering There was no NICO or anyone who could tell me anything when I got my Q... just an FSM.

You'll feel better about it after you've screwed with it and read more here.

I've seen inductive timing lights for ~ $20 but Wes's technique only shows signal not spark. ...but it's something.

[RobertsnewQ]

Do you have a fuel pressure gauge?If not, it's worth the $35 at Autozone.

Hook it up and check your cranking fuel pressure.

If you're not seeing 42psi something is wrong.

At this point I would try mashing the throttle and doing the flooded start thing. The Q's injectors are HUGE, and this is a very, very common cause of no-starts. I'm sure Dennis can cooroborate that...

[91QAZ]

Squee, I learned to drive in my dad's 82 Buick 5.7 diesel - talk about bad memories. That is a car that should have NEVER been built...

Robert, I'm off to Autozone to get a fuel pressure gauge. I'll try the flooded start procedure first though, now that my battery is charged up again.

It really seems like a fuel problem because when this problem started I was driving and the car just stuttered and died (of course, in the middle of an intersection - what are the chances!?). So i'll check fuel pressure first if that's not it I'll get a timing light and start attacking the spark possibility..

Thanks guys

[91QAZ]

Well, I tested fuel pressure - I'll post results in a new thread that is more appropriately named...