G50: Fuel Pump? Nope - FPCU? Nope - FP Relay? Nope...

[MJC]

Guys, I come to you as a last resort, I don't like to post repair questions on the forum because I'm almost positive it has been covered previously on the forums. I've tried to search and it has been a great help with replacing the fuel pump, jumping the FPCU, etc.

I wouldn't be surprised if there is a thread that tells me exactly what I need to know, but I admit I must not be too proficient with the search engine on this forum, 80 percent of the time I search I return no results.

Anyways....

Car: 94 Q45t, 135k miles

Situation:

Driving home one day about 5 weeks ago, car starts bucking...first a single cut as in a fuel cut, then it stutters and all together shuts off, I wheel the 4400lb yacht into a parking lot. I manage to get her started again and limp it home with it bucking sporadically throughout the trip.

Figured it was a fuel problem, jump online here, seems to be somewhat common that the cars need a new fuel pump, and at 130+ k, I figure, no worries. It was a few weeks before I could get around to changing the FP due to travel, work, and a few other things, long story short, I put in a new fuel pump and start driving.

Drives fine, then the intermittent bucking comes back on the first drive. I jump back online and see the posts referencing the fact that when the pump dies the extra current it draws often can have ill effects on the FPCU. I read up on the jumper modification and jump the harness to a ground giving the pump 12 volts.

Start driving...nope itermittent cut appears again. At this point I'm seriously confused because I figure the pump has 12 volts constant, what could possibly cause the pump to (what seems to) randomly shut off for a split second.

I think MAYBE I have a bad relay that somehow works fine whenever I'm in the driveway to hear it switch, but possibly acts up when driving. So I bypass the relay. Drive the car last night, acts perfectly, drive the car to work today, acts perfectly. I drive the car home tonight...about 5 miles in, cut...another mile (just trying to get home now), cut repeatedly. Leaving one stop light it hit a repetitious cut I can almost phrase as a rev limiter at approximately 2000rpm. CEL comes on during this, luckily I was one street before home and was able to make it.

I would LOVE to hear any advice anyone on here has for this problem. At this point I'm wondering how this could be a fuel pump issue considering that the FPCU is jumped and the relay is bypassed.

My thoughts are possibly a TPS sensor or MAF because when I hammer the throttle in this situation the car tries to 'wake up' if you will and will jump back and forth.

The fuel filter I changed a few months back at 130k or so and I had NO problems until now.

I hope I'm not asking a question that has been answered on here before (if so, very sorry) but I admit I wouldn't mind having a simple answer thrown in my face at this point I'm pulling my hair out on this one.

Thanks in advance!

Regards,

Mark

[AZhitman]

Well, if you don't get a solution out of that description, we're ALL hosed!

Mark, do you have a spare TPS you could test? I'm thinking you have an electrical, not fuel, problem.

[MJC]

Haha sorry for the novel! I don't have a spare TPS, but I think I have a spare MAF laying around from my father's old 96, however something in the back of my mind is telling me that I remember reading the MAF's are different on the 95-96 models.

I'll see if I can locate that and check out to see if they are indeed different. I'll also try doing a voltage sweep on the TPS to see if I can see any dead spots or anything fishy.

[Rex]

Will it start and idle by itself?

Has the fuel filter ever been changed, that you're aware of?

Does the cut happen when "crusing" or just when accerlating?

I'm loggin off, so I won't be able to respond to your answers, but they should help others help you.

[MJC]

Well given that I have about 24 hours since I bypassed the relay - and maybe 30 miles total since the "FP went out" the first time - I can't give you a whole ton of feedback on the driving habits as of late.

That said, sometimes it starts and idles on it's own, other times it takes one or two 'starts' - again this is a new development and never did this beforehand.

The fuel filter, I don't know if it was changed prior to approx 110k miles as that is when I purchased the car, but I changed the fuel filter about 4000 miles ago.

In regards to where the 'cut' happens, it seems to primarily happen when accelerating while in traffic. It has happened to me under cruise (actually the first time it bucked at all was technically at a slow 35ish mph cruise in traffic), however it seems to be more prevalent in lower speed (or stoplight) conditions when trying to accelerate. I'm thinking it's possible the warmer air from the low speed conditions is getting the best of what may be a faulty MAF.

Both last night and early this morning on the way to work, it was cooler air and relative-high-speed cruising, whereas on the way home I was in traffic again. Under high speed cruise, when I stand on the throttle, the car responds just as it would normally with no ill-effects to speak of.

The more I think about it, the more I'm leaning towards a faulty MAF, and all feedback is appreciated on the matter.

Thanks again.

[Q451990]

I'm leaning toward the MAF as well... seems to be more and more common lately.

Heath

[maxnix]

Two questions:

How much fuel in the tank during this experience? Less than a quarter full?

What is the condition of the FPCU? I am favoring damge at the solder joint at least as the pump was probably cooked and the increased resistance will melt the solder, which will then reform solid when it cools.

[MJC]

Two questions:

How much fuel in the tank during this experience? Less than a quarter full?

What is the condition of the FPCU? I am favoring damge at the solder joint at least as the pump was probably cooked and the increased resistance will melt the solder, which will then reform solid when it cools.

Ironically enough, I had filled the tank prior to the initial incident, and given my lovely last month and a half or so, I'm still over 3/4 tank.

That said, I'm assuming it wasn't one final 'cooking' of the FPCU that has sent me on my wonderous journey here since I had plenty of fuel and at that specific point in time the pump was not only (not) straining, but it also had the surrounding fuel to cool it.

However with me chasing my tail recently, maybe I'm just trying to convince myself (again) that I've found the problem (again).

[qsiguy]

Sounds very much like when the MAF plug on mine has got loose. Happened several times to me. All it takes is a wiggle of the MAF plug and it's all better. Give that a try before you replace it. Pretty common for the MAF plugs to get a little messed up especially if someone changed the oil filter from the top (or any other maintenance in that area for that matter) and put some pressure on the plug.

[maxnix]

Check both the MAF connector and the FPCU board very carefully, and if neither looks bad, replace each one at a time with known good units to diagnose further and eliminate them.

[Q45tech]

I had similiaer symptoms but I started by replacing the MAF, then FPCU, then CAS -------- each item created an improvement.

The problem was in the MAF all along as I replaced it with a known good that actually was bad and had its own new set of problems.

Started over, repaired the original MAF and reinstalled. It has been fine for a month. {EDIT fine today 3 months 5/11}

Now #2 cylinder is ocassionally dead. Finally happened when a Consult was handy so the power balance SHOWED ME where to concentrate.

Who knows how many new problems I created wiggling old brittle wires, harness, and connectors. discoing things while trying to solve the first MAF problem.

I am not immune and don't always follow my own advice written in a calm enviroment vs on the side of the road at night with friends in the car.

[goody90q45]

......Who knows how many new problems I created wiggling old brittle wires, harness, and connectors. discoing things while trying to solve the first MAF problem.

I am not immune and don't always follow my own advice written in a calm enviroment vs on the side of the road at night with friends in the car.

You should have posted up in the forum. We might have been able to help you.

[maxnix]

I guess "known good" isn't always true, even when new. But certainly the odds are better.

[Q45tech]

The bulk of the problems with MAF are from crystalized solder connections either on output connections or those which feed the actual element.

Stress from plugging and unplugging MAF or cleaning the element gets transmited eventually to the circuit board solder connections.

Just plain vibration has to be considered.

Watch out for this especially with changing symptoms cold>warm>hot and the converse.

Sometimes you get weeks of good behavior and then suddenly.

Don't ASSUME any used part is good much less 100% as brand new.

Now comes the diagnosis of #2 [coil, plug, injector, wiring, 6 connectors, and ecu] it only occurs once maybe twice per week and heals often when you turn engine off and restart.

It is so easy to diagnose a dead car vs intermittent ones.

[96Qowner]

Heheh, yeah, it's the %&#@ MAF. Your symptoms are indentical to mine when my MAF was bad. I went through the same steps you did - fuel pump, filter, FPCU.

Do a search for MAF and stalling or bucking, etc and you'll bring up a trove of information. Look for both Recent and Archived threads, and be sure to check old archived posts in Infiniti General - a lot of stuff from the older version of the NICO boards ended up in there. The rest will be in Infinit Q45 or Infiniti Online Mechanic.

I must not be too proficient with the search engine on this forum, 80 percent of the time I search I return no results.

Be sure to select "Post Bodies" when you search, otherwise you only get threads with your search terms in the titles.

The short answer is carefully check the MAF connector. Clean it up real well and apply some dielectric grease. While the MAF is disconnected, clean the element (carefully!) according to these instructions from Q45.org:

http://www.q45.org/maf.html

Also read this thread:

zerothread?id=82862

It's either a bad connector, which you can easily replace, or your MAF has failed internally and you'll need to swap in a new or used one.

Your solution is at hand!

Modified by 96Qowner at 8:17 AM 3/14/2007
Modified by 96Qowner at 8:18 AM 3/14/2007

[MJC]

Well you guys are life savers. I appreciate all the help and responses from everyone. I had my father's old MY96 MAF still and modified it to work, we even beefed up the soldering on the connections so hopefully it lasts a good long while.

I haven't had one issue at all with it bucking since the MAF swap so I'm going to go back and put the FPCU back into the mix. I know there was mention of a modification to the FPCU that helps it deal with heat better, something along the lines of an extra wire that can carry the current load better. I'll have to search for that and try to perform that modification before I bring it back into the system.

Now my only issue is that I think my transmission is about to go. My 2-3 shift is getting more and more sloppy by the week.

On a positive note - I just got in the springs I special ordered from Blitz Japan so I hope to be putting those on with my Tokicos that just came in as well. I'll try to take some pictures and throw them up when I get a chance.

Thanks again.

[maxnix]

I know there was mention of a modification to the FPCU that helps it deal with heat better, something along the lines of an extra wire that can carry the current load better. I'll have to search for that and try to perform that modification before I bring it back into the system.

Well, Tangalora did post a change to the ground wire location on the board.

Not too many have found that necessary.