94 Q45 bucking under load

[tomanderson]

I have a 94 Q45 (113K) that bucks when going up a long grade. Problem off and on for about 3 years. Have spent much time and money chasing this problem. New fuel pump, fuel pump regulator, transmission, tcu, knock sensors, MAF, plugs, subharness. Have had the car 9 years. Replaced injectors about 60K. No codes. The MAF connector has been repaired and is grounded to the body. Don't know who or when this was done. Most recent mechanic soldered the repair and "tightened the contacts" but no good results. Could problem be in the connector? Does replacing the connector require a whole new engine harness? Could it possibly be the ECU? Any suggestions you guys might have would be greatly appreciated.Love the car, hating this problem.Thanks,Tom

[96Qowner]

Wow, sounds like you covered all the bases!

If it were me, I'd find a used MAF through car-part.com and swap it in. Sometimes they just fry internally.

zerothread?id=176918

zerothread?id=209523

Or if you're electronics/soldering inclined, pry the top off yours and see what it looks like in there.

[tomanderson]

Thanks for the replay. MAF is oem new yesterday. Same problem.Tom

[96Qowner]

Rats!

Hmmm, ... I do remember some past threads discussing bucking under load ... lemme see if I can find them.

Ok, this is the best I could do:

I had a bad harness, and the most noticeable symptom was a slight bucking under a load (say 40-45MPH up hill in 4th) that would show up right before the transmission would downshift. Keep in mind that the ECU is retarding the timing advance when it's not getting a signal from the knock sensors, so the power difference is like having the timing off a few degrees BTDC. This is the ECU's way of protecting the engine while it can't "hear" what's going on - it just assumes the worst and compensates as much as possible.

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It's a long thread, dealing with engine temp and timing and knock sensors/harness.

Let's hope Dennis sees this thread, and Wes can probably help.

[Edit] It also occurs to me that it could be fuel quality related. Our fuel is not what it once was, and not what the engine was designed around. A couple point drop in octane (91 vs 93) might produce your symptoms. The timing chain can also stretch slightly over time/miles and throw the timing off a degree or two. You might get your timing checked.
Modified by 96Qowner at 11:50 AM 1/10/2007

[Q451990]

Did your fuel pump actually fail? My next step would be to check the fuel pump control unit (FPCU) - unless that's what you meant by "regulator" (vs. the vacuum controlled one under the hood). If your old pump pulled too much current, it can damage the FPCU internally. Also, was the knock sensor subharness replaced when you replaced the sensors? It's a common failure item.

I wouldn't rule out an ECU issue given all of the other parts you have already replaced - but ECU failures are very rare. I vaguely remember T3 replacing one for a similar symptom... apparently there was water leaking into that area from a poorly sealed windshield or body damage that was improperly repaired.

Can you recreate the condition on demand, or is it so intermittant that you think it's fixed for a while - only to have it come back? If you can recreate it, I would T a fuel pressure gauge into the line going into the rail (after the filter) and tape it to the windshield so that you or an assistant can read it while driving. At least that way you can know what sort of fuel pressure you're dealing with.

Good luck and Welcome to NICO!

Heath

[Q45tech]

Chasing something similar on mine so far 2 MAFS, new connector and pins, yesterday new fuel pump and FPCU [old one looked perfect].

I have ecu, and cas next on my list.

[tomanderson]

Thanks for the reply. My FPCU has been replaced and I replaced the fuel pump even though it didn't actually fail. 96QOwner was kind enough to chase down this old post:

I had a bad harness, and the most noticeable symptom was a slight bucking under a load (say 40-45MPH up hill in 4th) that would show up right before the transmission would downshift. Keep in mind that the ECU is retarding the timing advance when it's not getting a signal from the knock sensors, so the power difference is like having the timing off a few degrees BTDC. This is the ECU's way of protecting the engine while it can't "hear" what's going on - it just assumes the worst and compensates as much as possible.

Do you think that because the the connector has been repaired and grounded to the body that the connector or harness could be the problem?My mechanic says the connector has two silver and one brass contact (female) ends. Is this normal?

Can one purchase just the connector to try that first as opposed to replacing the whole harness?

Best regardsTom

[Q451990]

That quote that 96Qowner found is my discussion about the knock sensor harness, not the MAF harness.

The connector can be replaced separately. Q45Tech says that they're available at the dealership, but when I asked Joe (parts guru at Infiniti of Scottsdale) he said he didn't have them - so I'm not sure where to purchase it, but it's available somewhere.

So is the issue something you can predictably replicate, or is it very random? If it's predictable it might be worth your while to have someone do some real time data logging with a Consult. Too bad you're about as far away from T3 in Atlanta as possible without involving a boat in your travel route!

Heath

[maxnix]

a slight bucking under a load (say 40-45MPH up hill in 4th) that would show up right before the transmission would downshift.

Check your RPM. Your are lugging your engine in D at that speed.

Depending on the incline, altitude, fuel and temperature, you should not be in D until you are at least 60 mph and it is flat or down hill. Try to keep at least 2500 RPM minimum when extracting non-level road accellerating performance from the engine. It's a heavy car.

Unless your fuel and intake system is spotless and no deposits are on the valve and combustion chambers, your car will hesitate. And you are wearing the lock-up clutch at an accelerated rate.

So, do you have and auxiliary ATF cooler? Do you use BG44K the tank before each oil change? Do you go WOT to redline at least once a week? Ever had a fuel rail flush? Manual plenum, EGR and IAC valves cleaning?

[96Qowner]

My mechanic says the connector has two silver and one brass contact (female) ends. Is this normal?

Can one purchase just the connector to try that first as opposed to replacing the whole harness?

I don't know if Joe still has the connector stuff, but at one time he did

Many thanks to Aus94Q45, without which I would not have called Inf/Scottsdale the 2nd time and found the part I needed. Hopefully with these part numbers anyone can order these items successfully.

****Connector Part Numbers****

38751-123 Pins (you need 3) $1.54 ea38751-112 Connector Shell $17.08Total: $21.70

The invoice shows Location/List as BC1A

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[tomanderson]

Thanks, guys. I called Joe and he is overnighting the connector and pins. I'll let you know how it turns out.

Brian: Thanks for the input. I haven't tried any of your suggestions except I do use Lucas injector cleaner every other tank. What is WOT? I'll get on those suggestions after new connector is installed.

This problem started four years ago with stalling at coast down idle which I understand is usually an MAF or MAF connector problem. The tech found the car running rich and the ECU leaning out to compenstate. He stated that when he moved the connector, the mixture "jumped around". He cleaned the connector and ran a parallel ground. This was almost four years ago and the car hasn't run right since. Makes me think it might be the connector.

Many thanks,Tom

[Q451990]

WOT = Wide Open Throttle (e.g. Pedal to the metal)

Hopefully the connector fixes your problems! If not maybe Brian is on to something here... pretty much any problem that keeps the engine from running as new can cause issues under a heavy load. Might be useful to know what sort of Power Balance readings and resistance readings you're getting from the injectors.

Heath

[tomanderson]

Thanks Q451990. I had the windsheild replaced shortly after I bought the car. Just went out and looked closely at the molding. At the bottom center of the windshield where the black molding should meet, there is a small gap. I can see the bottom edge of the windshield in the space. I am not familiar enough with the location of the ECU but could water penetrate to it from that location? There was nearly a five year period between replacing the windsheild and the emergence of this problem. Could this still be the issue?

Appreciate any responses.

Thanks,Tom

[maxnix]

Skip the Lucas especially and even Techron (unless nothing else is available) and run with BG44K in your last tank before each oil change.

[maxnix]

ECU is in passenger kick panel, so check it. Yes, water can get anywhere once it is in the cabin.

[tomanderson]

Hi guys,The new MAF connector appears to have solved the problem. I have tested it five times on the grade of death and it runs perfectly. Four thousand dollars later, it turns out to be a $27 dollar part. Maybe I shouldn't start partying just yet but the car has never made it five times up that grade without failing. The tech said that the parallel ground my local Infiniti dealer's tech ran wasn't run from the ground connection. He said it appeared that the MAF wasn't operational at all and that the ECU was doing its best to compenstate. Is this possible?

I am going to continue to test it through this week and if it holds up I'll do the valve covers and the rack. Should be good to go after that.

Thanks for all your help. I'll keep you posted.

Tom

[maxnix]

The tech said that the parallel ground my local Infiniti dealer's tech ran wasn't run from the ground connection. He said it appeared that the MAF wasn't operational at all and that the ECU was doing its best to compenstate. Is this possible?

I believe it would be in a fail safe mode if the ECU didn't see it. Lucky you didn't blow the ECU by cross connecting the wires.

You still need to modify your driving habits, though.