Engine not reving over 4000 RPM?

[Denver90Q]

I recently resealed the valve covers and replaced the fuel pump and cleaned the egr and throttle body on my 1995 Q. Now the car will not rev over 4000 rpm. Otherwise it runs and idles good. I had a loose fuel injector connector and a check engine light and it was idling rough before I cleaned and tightened the connector. I also checked all the other connectors (MAF, CAS etc.) and replaced the fuel filter. I haven't checked the MAF or fuel pump but it was an OEM pump from Infiniti. It was on the Consult scope at the dealer recently and all they said was it had a bad injector. I guess the next step is to run a fuel pressure test and/or take it back to the dealer? Will Consult show if it has a bad MAF?

[jimbyjimb]

Does it seem as though the computer is limiting revs to 4g or does it just taper off and run out of power?

[Denver90Q]

It abruptly looses power at between about 3500 to 4000 rpm. The rpm cutout is not totally consistent like when the ECU does it.

[fixer]

TRAY A KNOWN GOOD MAF if you can swap it from another car for a few minutes. if not maf. ck for plugged exh. if rpm topps out after an extended revving and wants to die if you try to keep revving it. also you can ck the timing. did you remove the cam sensor at the front of the cam when doing valve covers?

[Denver90Q]

No, Didn't mess with sensors except to clean connectors. I have a almost new MAF in my 1990 but I don't believe it will fit the 1995 Q connector.

Engine idles fine even after extended revving.

What is "plugged exh." exhaust ?

These engines sure are finicky.

[Infinitiguy19]

Did you replace the fuel pump before it took out the FPCU (Fuel Pump Control Unit)?

Did you use a new or used pump?

Are all the connectors to everything you touched cleaned, tightened and plugged in?

A bad MAF sensor would tell the ECU limit the rev limit to 2000 RPM's

[jimbyjimb]

I'm thinking hard about it. The fuel pump is two speed, perhaps the FPCU isn't putting the pump in 2nd gear (figuratively)? Maybe something is causing spark to cut out at that RPM? CAS? Hmmmm. This is so strange. TPS not re-installed correctly? There should be a mark on it or it should be marked prior to removal. Don't see how it could be EGR. Look at TPS again and ensure it is lined up with the old markings, if not try adjusting it and seeing what happens.

[Denver90Q]

I'm thinking hard about it. The fuel pump is two speed, perhaps the FPCU isn't putting the pump in 2nd gear (figuratively)? Maybe something is causing spark to cut out at that RPM? CAS? Hmmmm. This is so strange. TPS not re-installed correctly? There should be a mark on it or it should be marked prior to removal. Don't see how it could be EGR. Look at TPS again and ensure it is lined up with the old markings, if not try adjusting it and seeing what happens.

Yea it's a tough one to figure out. The only thing I can think of is that the fuel pump is not putting out enough fuel. I'll try swapping the FPCU as it is easy to do and I think my 1990 FPCU interchanges with my 1995? Didn't mess with the TPS except to clean the connector so don't think I will need to worry about that. The car has only about 80k miles on it and this is the first maintenance work I have done on it.

Also, the car won't accelerate over 3000 RPM uphills. It loses power very suddenly. It is also very consistent so I don't think it is a loose wire or bad ground.

[jimbyjimb]

Probably fuel pressure.

[Q45tech]

"The fuel pump is two speed" ????????????

Even though the FSM clearly shows 3 speeds

Is the rpm limit different in a 1st gear WOT acceleration vs a slow steady mild climb above 3500? What is position of accelerator as the cut off appears? WOT or 1/2 throttle.

[Denver90Q]

I replaced the FPCU with one for my 1990 (the part numbers are different but it seems to work). Also put first gear start OEM TCU back in today. The rpm cutout has dropped to 2000 rpm at WOT in 1st and up hills and 3000 max. Is this a sign of a bad MAF. I wonder if there is anyway to test a 1990 MAF on a 1995?

I had second gear start TCU in for awhile and it was at 3000 WOT yesterday. Seems to be getting worse? Still idles and drives fine at less than 3000 rpm's but will cutout on acceleration at any rpm over 3000.

There was some corrosion in the tank (from ethanol) when I replaced the fuel pump with OEM unit from Infiniti a couple of months ago. Also, there was some fuel additive (BG 44K and a small amount of acetone, and Isopropyl) in the fuel. The combination of which, I attribute to the demise of my less than 80K mile original fuel pump. However, I have filled it up since then. Probably should change filter again and was planning to steam clean tank and coat it soon.

I will check fuel pressure tommorrow if I can find the right adapter for my Sears fuel pressure gauge and let you know. Might also swap out ECU just for kicks. All signs point towards fuel pump again. If inconclusive, I'll have to go back to the dealer for another consult hookup. Could have bought a couple of Consults by now.

Thanks for the help.

[jimbyjimb]

There is software and a patch cord available for your laptop for under 500 usd through some Australian outfit as I recall. Past threads would have that info. Can't say I have a wide range of expertise with Q45 fuel pump control unit, never read FSM on it since I haven't had trouble with it and had NO idea it was 3 speed. That is just odd. Can't say I've ever heard of that before. I've seen dual pump systems and two speed pumps, but never a three. What exactly is the point? Pump longevity? Seems a waste of resources when there is a pump replacement that does not require an FPCU. I'd like to know the reasoning behind multi-speed pumps.

[Q45tech]

AS with any electric motor the pump life is a direct function of RPM x hours of use. Thus 3 speeds is better than 2 which is better than 1 speed.

The other function is to maintain the rail pressure in the ideal midpoint under 3 broad conditions, After all the injector flow dynamic range is almost 30:1 from idle to peak. [1.1 ms>11ms] [34 psi vs 43.4 psi] [650 vs 6500 rpm]

You can read the ecu to FPCU signal control voltage to see which speed range the pump is supposed to be in [based on TPS load index aka throttle position].

PUMP Condition is a function of rpm [since gasoline density is a function of chemistry and temperature] , back pressure [fuel filter].

The 90-96 300zx tt uses same system.

I use an amplified current probe to view each commutator's peak current and calculate the pump rpm while the fuel pressure gauge is reading input rail pressure.

You can make a current loop to substitute for pump fuse and view the current draw and waveform just like with fuel injector, coil on plug, etc fuse.

http://www.lindertech.com/fuelpump.htmh ... ri....html

[jimbyjimb]

Who needs an FSM with you around? J/k. Thanks for breaking it down.

[fixer]

the fpcu controls the grounding of the pump to vary the speed, it is only for noise. when the eng does not need a lot of fuel, it will vary the voltage to the pump to reduce the noise. if fpcu is suspect you can bypass by installing a ground to the white wire at fpcu. if you got a bad pump you need to ck with a press gauge .

[jimbyjimb]

Noise? That'd have to be one hell of a pump to hear even from the back seat. I'm sticking with Qtechs explanation there. Pump life and maintaining rail pressure seems to make alot more sense than noise. If Nissan pumps are so loud they have to be speed controlled it sounds like there are some serious design issues that Nissan could have worked out cheaper than using and FPCU, like a different pump.

[qship96]

Yes, the Q pump is noisy when running full speed......winter start runs pump at full speed for 30 seconds or so, and it is CLEARLY audible from drivers seat.If you have never heard that in these conditions, you have mechanical or hearing issues!!!!

[Q45tech]

If you mount a 255 lph pump of any brand in the Q45 tank it will be noisey without speed reduction.

Remember the typical round analog fuel pressure gauge is very damped to average out the short pulses......................if you look at the midpoint [rubber front hose] with a digital undamped fuel pressure sensor and oscilloscope you will see much more variation than shows on an analog gauge.

Depending on rail pressure 34>43.4 and the injector open time [1>11 msecs] the actual pressure will vary at different injectors by 1-3 psi from open to closed.................this can mean a 3.5% difference in fuel flow.All about reducing these spikes to minimum.

The slower the pump the less flow and the less gasoline gets sent to ultra hot engine compartment in Summer to get heated up.

The SAE did a study of gasoline temperature vs fuel pump speed. Install a temp probe and read gasoline temperature in rail across the year.

2002 OBDII has a fuel temp sensor in tank to help with fine adjustments to injectors on returnless systems.

http://www.aa1car.com/library/2003/us60 ... es/131.jpg

[jimbyjimb]

I meant under a driving condition. Its implied. You have comprehension issues. There is a clear difference from running constantly to charging the fuel line for intial startup.

[qship96]

you could hear pump running at full speed, even while driving.

No problem, typical newbie mistake on your part.

[Denver90Q]

I had some more diagnosis to report. I think I have it narrowed it down to the TPS but probably need to run some more tests to confirm. I have not adjusted TPS but did recently remove and clean the throttle body. TPS is still lined up with old markings. The situation is complicated because the throttle body also includes the TCS assembly. Disconnecting the TCS sensor at the throttle body or switching it off had no effect on driveability issues.

Since about a week ago I swapped out the MAF and ran a fuel pressure check. Readings are 5 psi with engine off, 10 psi with ignition on, 35 psi with engine idling and 36 to 25 psi when revving with throttle cable in engine compartment (the vacuum line to the fuel pressure regulator remained connected when testing). Don't think it is the MAF or fuel pump because the engine runs perfect when revving it with the accelerator cable inside the engine compartment. However, as soon as you attempt to drive it or rev it with the gas pedal it will bog down and occasionally backfire thru the air intake. Driveablity seems to be getting worse and now the car will not exceed 1500 rpm going up hills or grade.

I should be able to get a used throttle body assembly from a junk yard or would it be better to just replace the TPS with a new one? I don't ever recall hearing that these wear out? Is installation of TPS possible without a Consult?

[Infinitiguy19]

From my understanding TPS do not wear out.They either get missalinged or the connectors get dirty.

Used is OK or even remanufactured on this one, just make sure the pins are clean and bent down a little like the MAF connectos should be.

And installing the TPS would be eaiser with a laptop Consult or real Consult, but with a accurate VOM you should be fine.

[Denver90Q]

Thanks for the reply.

The TPS does not appear to be misaligned and I have cleaned the connector twice.

After further tests, the fuel pump does appear to be the problem. Now the idle PSI is less than 30 and it quickly drops off to almost zero when revving the engine. Not sure if temperature is a factor as today is 70's and sunny and yesterday was 30's and snowing.

Modified by Denver90Q at 7:41 PM 4/19/2009
Modified by Denver90Q at 7:48 PM 4/19/2009

[maxnix]

Since about a week ago I swapped out the MAF and ran a fuel pressure check. Readings are 5 psi with engine off, 10 psi with ignition on, 35 psi with engine idling and 36 to 25 psi when revving with throttle cable in engine compartment (the vacuum line to the fuel pressure regulator remained connected when testing).

I think readings should be at 34 psi at idle and 43.4 psi at WOT.

Check Q45tech's posts.
Modified by maxnix at 12:19 PM 4/23/2009

[Q45tech]

Rising rate fuel pressure regulator which adds 1 psi of fuel presure from idle or cruise depending on plenum pressure.

At idle/cruise 18" HG is ~~ 8.8 psi of atmospheric wheas at WOT it is 14 PSI.................5 psi air pressure vs 10 psi of fuel variance.

So depending how you look at it 1 psi gain can yield 2 psi of fuel gain.

The fuel pressure is infinitely variable from 34> 43.4 depending on the position of throttle plate [as read by TPS but that has little to do with fuel pressure] EXCEPT TPS voltage SELECTS the FPCU pump speed.

[Denver90Q]

So fuel pressure should never drop below 34 psi on these engines when running?

Wish they would make these pumps more compatible with fuel additives as this is the fourth OEM pump I will have replaced in the past few months. The Carter pump on my 1990 still seems to be holding up well.

[qship96]

Genuine Infiniti Q45 fuel pumps are very reliable,even with ethanol laced gasoline- my factory installed original was replaced at 110,000 miles, and this one currently has 116,660 miles on it.My Q has run MTBE laced fuel from 96-05, and ethanol laced since 2005.

[Denver90Q]

Genuine Infiniti Q45 fuel pumps are very reliable,even with ethanol laced gasoline- my factory installed original was replaced at 110,000 miles, and this one currently has 116,660 miles on it.My Q has run MTBE laced fuel from 96-05, and ethanol laced since 2005.

Maybe my car is jinxed. Today I am getting 34 PSI at idle which briefly climbs to 37 PSI when revving engine and then gradually drops to 30 to 25 PSI. Engine still bogs down when revving it from the accelerator pedal a few times. Any reason why the readings would vary so much?

[qship96]

Check entire fuel path- filter, pressure regulator, damper, FPCU ,etc. Pump should give long life if tank is free of rust /dirt particles.

[maxnix]

Houston, we have a fuel pump problem.

Don't forget to examine FPCU also.