Engine cutting out, backfire on hard acceleration

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desertq45
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Engine cutting out, backfire on hard acceleration

Postby desertq45 » Fri Nov 10, 2006 8:19 am

Car: 1990 Q w/chainguides, new fuel pump 2 yrs ago, fpcu replaced 2yrs ago, new fuel filter, plugs 3yrs ago, rebuilt tranny 2 yrs ago, 93 TCU.

Now the problem:

The car idles fine, runs fine under light to moderate acceleration... but when you pound the pedal the enging cuts out and seemed to even backfire once. Mainly happens under hard acceleration and wide open throttle, although it has cut out a time or two going from 1st to 2nd w/o wide open throttle... just fast climbing rpms.

Codes: 55 and knock sensor (I forgot the number... 36??)

As I understand it the ks will retard timing to prevent predetonation... but would a malfunctioning one cause the behavior I'm experiencing?

It just "feels" like fuel delivery... but there wasn't any humming coming from the back seat and the pump isn't very old. Still, it acts like a clogged filter (replaced that Saturday) or something similar.

Any ideas?

Thx.


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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby maxnix » Fri Nov 10, 2006 8:38 am

Timing correct? TPS sensor connection corrosion free?

Sounds like you are due to repeat a lot of your running minor maintenance.
Modified by maxnix at 11:07 AM 5/12/2007

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Re: Engine cutting out, backfire on hard acceleration (maxnix)

Postby desertq45 » Fri Nov 10, 2006 8:46 am

That's never ending.

I have the underplenum kit from Joe @ Scottsdale Infiniti and now that the Africa-hot temps have receded I no longer have an excuse not to pull the plenum and replace all the hoses... again.

I wonder if a vaccum leak will cause this?

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Postby pito11213 » Fri Nov 10, 2006 9:26 am

Have you changed the knock sensor?

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Re: (pito11213)

Postby elwesso » Fri Nov 10, 2006 9:32 am

doesnt really sound knock sensor related. The car will still run smooth and go, but it just will feel down on power, not really like its being choked....

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby Q45tech » Fri Nov 10, 2006 10:00 am

Measure the fuel pressure after the new filter under real road conditions make sure you have 43.4 psi at WOT all the way to redline. Then replace KS , how many miles on the 3year old plugs?

If it was in ATL I would just use my KS/temp sensor emulator plug to see if problem goes away.

Important to see if TPS goes to 3.8 volts and MAF voltage increases as rpm increase

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Re: (elwesso)

Postby desertq45 » Fri Nov 10, 2006 10:04 am

Right... that is what I thought Wes. It seems to be fuel delivery. I haven't ever changed the KS's so when I do the plenum I guess it may be good insurance.

I've had the car since it was new... this is the first time it has done this. I've been proactive on all the maintenance and replaced the pump/fpcu and injectors before they hinted of a problem. The only thing that caught me off guard was the tranny.

I think I'm going to go back to the fuel system and at least eliminate it from the works.

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Re: Engine cutting out, backfire on hard acceleration (Q45tech)

Postby desertq45 » Fri Nov 10, 2006 10:15 am

Thanks for the help Dennis... I wish Atlanta was in Arizona.

I'll check the fuel pressure and see where it is-- is that w/or w/o the regulator? 43psi with, 63 without? I can't remember.

The mileage on the plugs is about 35k.

TPS voltage checks out. I'll check the MAF also.

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Re: (desertq45)

Postby maxnix » Fri Nov 10, 2006 11:07 am

desertq45 wrote:I haven't ever changed the KS's so when I do the plenum I guess it may be good insurance.
You won't want to go under all the new hoses to replace them when they do fail.

There is always http://www.jerrytucker.net , but eliminating the fuel system is primary. Changing air and fuel filters annually is a good idea, especially with crappy AZ gas.
Modified by maxnix at 5:32 PM 3/10/2007

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Re: Engine cutting out, backfire on hard acceleration (Q45tech)

Postby desertq45 » Fri Nov 10, 2006 12:45 pm

Dennis--

At idle I have 40psi... WOT it drops to 38. I guess I found my problem??? Strange that a 2yr old pump would fail... have you ever seen clogged pickup screens (on the pump) or lines that were restricted?

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby lino » Sat Mar 10, 2007 3:19 pm

Have you solved your problem?

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Re: Engine cutting out, backfire on hard acceleration (lino)

Postby desertq45 » Sat Mar 10, 2007 3:35 pm

Not yet...

I changed the fuel pump-- no difference. The pump and screen looked pristine, not even discolored. So I blew the lines out with air back to the fuel filter, replaced the filter w/new, left the new pump in and feel confident I've eliminated fuel delivery as the problem.

Work then interrupted for a while and I've been traveling a lot.

However, your timing is good, since I just picked up a set of knock sensors this week. I think I'll dive into the plenum shortly. I have all the hoses, so it is now just a matter of finding the time.

The strange think is that the car is fine other than the occasional bucking and cutting out under acceleration... but then that is the fun part of driving the Q.


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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby lino » Sat Mar 10, 2007 4:49 pm

Good luck and please keep us posted with your progress. I have a problem, may be similar in some respect that is driving me crazy. I just posted it in the Infiniti Online Mechanic.

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Re: Engine cutting out, backfire on hard acceleration (lino)

Postby Q45tech » Sun Mar 11, 2007 5:30 am

Fuel pressure measured after the fuel filter should be 34 psi at warm idle and 43.4 psi at WOT at any rpm including 7300 [if ecu allows].

This will reduce based on atmospheric pressure [hurricane or high altitude] roughly 1.0 psi per 3,000 feet above AMSL.

If below these numbers FIRST change the fuel filter, then diagnose the pump.

The Q only needs 92 liters per hour to make 300 HP and even with enriched AF for cooling only 110 liters flow thru injectors almost half is returned to tank thru fuel pressure regulator to cool the rails [constant excess flow].


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Re: Engine cutting out, backfire on hard acceleration (Q45tech)

Postby desertq45 » Sat May 12, 2007 7:53 am

Hey Guys,

I've been slowly working through my problem with little change. The current symptoms are more pronounced and easier to replicate. Quick accelerator pedal movement (attempted hard acceleration) results in the engine cutting out... it stays running most of the time but almost stalls. Some times check engine light comes on.

Repeated effort results in complete stall and the check engine light always comes on. Occasionally, the Q will accelerate like you would expect until the top end and then cut in and out... immediate decelleration results in normal drivability. In fact if you gingerly apply the throttle, it will not exhibit the described symptoms (although it is absolutely no fun to drive it that way).

What I've done (in chronological order):

- Replaced Fuel pump with new from IofS.- New Fuel filter.- Switched to a known good FPCU w/new pump... no change... opened case of original, no burn marks... put original back in.

No change in symptoms.

Replaced all underplenum hoses, etc.Cleaned IAC, plenum, TB, EGR, etc.Replaced KS w/new.Cleaned MAF per Q45.org and did the harness "wiggle test" as well as cleaning the contacts until shiney and adding dielectric grease. Checked harness wiring and still very fexible down to the loom on the ornamental cover.

After this, Q started right up, idled smoothly, but... same wot problem. Severity of cut out and stall is random... but normally on hard acceleration. Basically, status quo.

I read hours worth of posts on MAF and then purchased a used one for $80 to see if it made any difference. Installed it, no perceived changes. I opened the top of my original MAF to check the solder connections. They looked fine, but I resoldered them anyway. Ran continuity tests across the races (not sure it why) and it was good from the connector all the way through.

I once again cleaned the harness connections and reassembled.

No change.

So... today I ohmed the injectors. I live in Phx and every mechanic I know tells me the gas kills Q injectors. I found that all injectors ohm between 12 and 14, but number 3 is 17 and 7 is 20. Would this be enough to cause my problem?

I've also recall someone here mentioning that they went through 2 MAFs before finding one that really worked. Are these things that volitile? I just didn't want to drop $400 on an experiment for a new one from Joe, but if it is the likely problem it would be worth it to put an end to this pain.

Any thoughts or pointers would be very welcome here... I think I've ruled out fuel delivery other than possibly the two out of range injectors. I'm also wondering about spark plugs?? It's been 40k since last change.

Any ideas?

Thanks

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby maxnix » Sat May 12, 2007 8:10 am

New is new, used is somebody else's problem.

Have you checked the FPCU? Seems like you should be getting more pressure at WOT>

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Re: Engine cutting out, backfire on hard acceleration (maxnix)

Postby desertq45 » Sat May 12, 2007 8:32 am

Yeah, I hear ya... used is iffy at best... but all I was looking for was some "change" by switching. But the Q didn't even blink.

My logic was that if my original MAF was the problem, I needed to rule it out. Even if the used part was bad (which the guy I bought it from swears it is not) I'd see some sort of change in the symptoms. Since no change, I thought harness... but most members feel the harness issues are localized with the connector, which I also checked out.

So I don't know. I'm just trying to troubleshoot w/o burning coin on parts I may not need. If I can get 80% certain, I'd flip for the new one in a second.

Regarding the WOT pressure... my numbers earlier in the thread were pre-new pump. I've since had a mechanic check and it is 43 psi.

Do you think the problem could still be the MAF?

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby db_autotek » Sat May 12, 2007 8:51 am

Sometimes a bad PCV valve will cause wierd problems like this. I don't have any experience on the Q45s PCV valve, but just on experience with my many other cars. I bet you've already tried / checked / replaced this though... just adding my two cents.

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby 96Qowner » Sat May 12, 2007 9:20 am

You've probably considered the obvious, but why not take it to Infiniti of Scottsdale (Joe's place) and having them put it on the Consult. It sounds easy to replicate, and that would tell you what's going on with combustion.

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Re: Engine cutting out, backfire on hard acceleration (96Qowner)

Postby desertq45 » Sat May 12, 2007 9:53 am

Thanks for chiming in guys...

I did change the PCV valve... in fact the new oem pcv hose that came with the plenum kit actually split at the fitting. It must have been defective because it was the last thing I hooked up after doing the plenum and there is no stress on it. I heard it hissing when starting the car one morning. But, no affect on the problem.

If I can't get any further on my own, I will probably be forced to take it to IofS and see what they say... I just hate to admit defeat.

I did email Isaac about his software, but haven't heard back yet. I'm not sure it would give me the info I need, but it sounds cool.

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby maxnix » Sat May 12, 2007 10:43 am

So the new pump increased the pressure by 3psi?

Do you have the early style connector or the later 1994-1995 conector on the MAF?

NGK plugs?
Modified by maxnix at 3:10 PM 5/14/2007

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Re: Engine cutting out, backfire on hard acceleration (maxnix)

Postby desertq45 » Sat May 12, 2007 11:04 am

Actually 5psi at WOT... but then that is also on two different guages. I'd trust the second more than the first. In fact I think my initial measurements with the first pump were wrong since switching pumps didn't make any difference . Plus the first pump was only in the car 2 years.

Early style connector-- 3 prong.

Yes, NGK plugs.

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby desertq45 » Mon May 14, 2007 10:40 am

bump.

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby elwesso » Mon May 14, 2007 10:45 am

I would check your FPCU. You might just try grounding the fuel pump and seeing if it being on full speed helps...

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Re: Engine cutting out, backfire on hard acceleration (elwesso)

Postby lakeq45 » Mon May 14, 2007 1:48 pm

Non-mechanic putting in my two cents...

I had an 88 Regal once that was doing something that sounds similar and it turned out to be the cat restricting flow and causing back pressure at high throttle/RPM.

Given the discussion so far, I assume that you are way beyond this. Just a thought though.

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Re: Engine cutting out, backfire on hard acceleration (lakeq45)

Postby desertq45 » Mon May 14, 2007 1:59 pm

lakeq45 wrote:Non-mechanic putting in my two cents...

I had an 88 Regal once that was doing something that sounds similar and it turned out to be the cat restricting flow and causing back pressure at high throttle/RPM.

Given the discussion so far, I assume that you are way beyond this. Just a thought though.
Thanks lakeq45, I am a bit past that, but I appreciate the input nonetheless.

I actually re-read Jesda's post from 2004 and I'm beginning to lean towards just putting a new MAF in and see what it does. This is friggin maddening and the temps in PHX are already in triple digits. Even with A/C in the garage, wrenching is no joy.

I'm going to try Wes' fp ground bypass just for grins and giggles but if that doesn't make a difference, I think I'm giving Joe a call tomorrow.

Thanks for the two cents guys... I hope to get this resolved and the thread will be worth something to someone else down the line.

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Re: Engine cutting out, backfire on hard acceleration (lakeq45)

Postby maxnix » Mon May 14, 2007 1:59 pm

That might be a good possibility, but I don't recall anyone reporting back firing with bad catalytic converters.

But there has to be fuel there, so the converters could be melted and clogged, which has been reported. Still, just a result of another malfunction before the gasoline arrived there.

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Re: Engine cutting out, backfire on hard acceleration (maxnix)

Postby desertq45 » Mon May 14, 2007 2:17 pm

The thing is... it hasn't backfired since I first experienced the problem several months ago. And to top that off, it was a mechanic who told me it backfired... I never personally experienced it (I'd like to rename the title of the post as a result, because I don't think it's relevant). Also, I would think that restricted cats lead to consistent symptoms at a specific rpm range (assuming the same amount of exhaust flow and thus back pressure).

I'm getting the cut out and bucking at a number of different rpms. It is really inconsistent which makes me wonder about electrical. The other thing is, I can now make it happen at will... and the check engine light comes on when I do.

A mechanic friend of mine (who had a 92Q) that he thinks it's the fpcu... but I have 2 of them and tried both when I put in the new pump as I started this exercise... additionally, when the original pump failed 2-3 years ago, I bought the extra fpcu just in case, but swapping pumps cured the problem... however, I tested the fpcu just to be sure it was good. So now I know I have at least ONE good fpcu.

I guess I'll bypass the fpcu and short to ground to full pump pressure and see what happens.


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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby desertq45 » Mon May 14, 2007 3:40 pm

Ok Wes,

That was easy enough... followed your instructions from zer...84460

to bypass the fpcu to ground the pump directly. As first, I thought it made a difference. I stomped on the pedal and no cut out/check engine light. However, as the rpms neared 5000 it started stuttering and cutting out.

I pulled over, removed my alligator clip ground and plugged the harness back into the fpcu and repeated the driving habit... it behaved just as when bypassed.

It seems that in the morning when cold (ok relatively speaking since mornings now in Phx are 70 degrees) the cut out and check engine light are more consisent. In fact it seems that whenever the car has been sitting for a while this problem manifests itself. Tonight, I drove 15 miles or so home from work, it sat in a hot garage for an hour and then I found it would behave better. Not sure why... but at least I think I can totally rule out fuel pump and controller can't I??

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Re: Engine cutting out, backfire on hard acceleration (desertq45)

Postby elwesso » Mon May 14, 2007 4:08 pm

What code is it throwing?

Honestly I dont thinky ou will get anywhere with infiniti dealership. I doubt IoS despite their volume of G50 parts sales actually knows enough about G50s to diagnose a goofy problem like this.

You can always check the exhaust pressure by hooking a pressure gauge to one of the o2 sensor ports... There is an old thread where Tech talks about what kind of pressure you should se. The long and short of that is you should see very little back pressure. I tend to agree with your analysis on that though that it would be more consistent..

I think we can safely say that its not a fuel delivery problem and all the usual suspects... This is either going to be something really obscure or something overlooked.

I will its unlikely that with 3 MAFs that all 3 are bad. USED MAF's are usually not a bad idea, and for the money you can buy 3 for the price of one brand new and probably have 2 good ones! I am very inclined not to think its MAF related. plus in my experience its more of a bucking/jerking randomly, more or less at low load. When my MAF went bad it would make the car stall or the car would just start randomly bucking and jerking and then go back to normal, and would go to WOT fine.

Allow me to recap:

Symptoms- Under light or anything shy of lets say 1/2 throttle the car seems to behave. Under heavy acceleration, the car feels choked, and apparently has backfired.. However sometimes it acts as it should, sometimes it doesnt. It seems that its worse when its cold and once its warm its better.

You've replaced a whole bunch of crap, including KS, fuel pump, MAF all to no avail.

Im wondering if possibly the car is running super rich... maybe that one injector is dumping fuel but still thats only one cylinder, that wouldnt caust something like that. The Q can still run pretty good bangin on 7, so take that for what its worth.

Do you have a consult? you could do some data logging and we could look and at least see if its electrical in nature (there would be some funky voltage numbers)...


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