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99 QX4 Intermittent Stalling


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I posted this on the FB group but I am fairly long-winded so likely didn't receive much help because it was a lengthy read.


So my girlfriend's niece, whom I'll refer to as my niece from here on out, has a 1999 Infiniti QX4 with 265k miles on the clock. Recently I rebuilt the AC system. Had a few hiccups along the way, but it is back up and blowing cold air! For quite a long time, she has been having some issues with intermittent stalling. Lately it had gotten worse, so she asked me to take a look at it. She said it had also started blowing smoke out the tail pipe. The head gasket had been changed less than 6 months ago, so I asked her what color the smoke was, light or dark. First she said dark, then she said light, and I thought if it was the head gasket again, the shop that replaced it would have to warranty the work.


She brought it over, and it was obvious that it was a fuel issue. The smoke was dark, and it smelled like fuel. It was so bad I currently have a line of dark soot across my garage floor from where the tail pipe was located at the time. I started it up and lifted the hood. The distributor was obviously on it's way out - lots of bearing whine. So I ordered a distributor, hoping that it had just skipped a tooth and was running rich because timing was off. I knew in the back of my mind that there was little chance that the dizzy would solve the over fueling, but it needed changed anyway. Along with the dizzy, I also did spark plugs. New NGK Iridium IXs went in after a serious amount of cursing at cylinder #6 and Nissan for stupid placement of that plug. The Nissan spark plug tool was the only thing that kept me sane, though even locating the plug was almost impossible. Just a note on the distributor install. I had checked top dead center with cylinder #1 plug out, but it wouldn't crank over no matter what I did. At the advice of my girlfriend's brother, I sprayed carb cleaner into the throttle body and got a small backfire. So he said to take the cap off the dizzy, mark the location of the rotor, pull the dizzy, and rotate the rotor 180 degrees. Turns out I had been at TDC on the exhaust stroke. I had considered this, but I would have pulled the dizzy and rotated the crank 360 degrees. Rotating the rotor was a million times easier! It started up after a few cranks!


The truck was still running extremely rich, so I did a ton of research and realized that the most likely issue was the fuel pressure regulator. Dripping fuel out the tail pipe, Lots of smoke, and the oil smelled like gasoline. Bought a replacement, and then attempted to locate the stupid thing. On my VQ it is easily accessible, but I couldn't even see the regulator on the VG. Looked it up and realized I would have to pull the upper intake manifold. What a stupid design! Oh, the codes that were stored were the knock sensor (surprise surprise), and B2S2 O2 sensor. Neither of these should be an issue for the intermittent stalling, correct me if I am wrong.


I should have waited to do the plugs until I had determined the fuel regulator was bad. Changing them with the plenum off would have been cake! Anyway, I removed the plenum, struggled with the rear coolant lines that are also stupidly placed by the moronic engineers, cut those and bought replacement hose. Cleaned out every item I removed, including EGR and throttle body (still on plenum). Finally I could see the regulator. I realized I would need to pull the fuel rail because you cannot get at the screws holding the regulator to the end of the rail, also a stupid design. Stripped the heads of those idiotic screws, and used a screw puller to finally remove them. Found some replacement screws with the same thread, Put plenty of anti-seize on them, and plopped the rail back in with the new regulator. I was going to do the knock sensor, but removing the lower plenum was not going to happen, so I plan to relocate it if it keeps throwing the code. Got everything buttoned back up, with more cursing at those rear coolant lines, as well as the stupid EGR bracket that is easy to remove, but dang near impossible to get back in place without removing even more parts from the plenum.


Started it up and BOOM, no more rich condition! Exhaust looked clean after burning off what was left over from before. Finally I was able to pull the brown plug on the TPS and set timing with a light. During this process I also bought a OBD2 reader that will show and record/graph real time sensor data. After setting timing, I plugged back in the brown TPS plug, which controls advance/retard of the engine in real time to keep the RPMs stable at idle and improve fuel economy while driving. After letting it warm up at idle, I checked values with my code reader. Everything looked good, fuel trims were responding normally, MAF data moved with the throttle, O2 sensors were responding and approximately the same for B1/B2 (even the bad one was responding now). The only odd thing was the timing was at 15 degrees and not really changing at idle, whereas before it was all over the place.. Prior to the regulator change, the injectors cycled between 'closed loop' and 'open loop - fault' over and over. It now stayed on closed loop. I should have watched the scanner during driving or at least modulating the throttle to see if timing changed, but considering everything else looked good, I took it for a test drive.


It drove fine. I drove for about 15 miles with no issues, so I thought it was fine. I text my niece the next morning to see how it was doing, and she said it was running better, but had stalled on her once on the way to work. It always starts right back up, but I need to solve the stalling problem. The stalling only happens when slowing down or stopped, after driving for awhile.


TL/DR - 1999 Infiniti QX4 VG33E Intermittent Stalling - New Distributor, Spark Plugs, Fuel Regulator - Sensor data looks good to me, but I do not know what values should be reported for each sensor. Idles fine at start-up, stalls during driving when slowing down or stopped. Starts right back up. Prior codes stored for Knock Sensor and Bank 2 Sensor 2 O2 sensor. 


So I am thinking it may be the following:


MAF - The sensor data seemed to respond correctly with throttle input - Does anyone know the value range that should be reported?


Coolant Temp Sensor - Reported values are believable, but I do not know what it is reporting when the stalling happens.


Throttle Position Sensor - Was advancing/retarding timing prior to Fuel Pressure Regulator change, seems to respond fine during throttle input. I did have some issues with dying as RPMs dropped after changing everything, but I had also put in a good amount of Sea Foam and every vehicle I have Sea Foamed has the issue until it burns that stuff off.


Crankshaft Position Sensor - This is a likely candidate. The oddness with the static 15 degree advance output reporting on the scanner leads me to believe this may be the issue.


Anyone have any more items to add to the list?



Edited by drewp29
Edited for clarity
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If the idle's steady, it makes sense that the timing would be, too. So long as it changes when load changes I'd assume that's how it's supposed to be. It was probably all over the place before because the computer was trying to make sense of the failed regulator barfing fuel through the vac line.


I had an intermittent stalling issue with my '93 a while ago where it would stall out while stopped or coming to a stop, but only when warmed up (probably due to lower idle when warm). It was intermittent enough to be a pain to track down. Sometimes just turning the truck off and on again would clear it up, other times wiggling various plug seemed to do it, but then it got bad enough that I could barely drive it. I noticed that the plug for one of the idle air components felt a little loose, so I cleaned it up a little and squished the female pins so they'd be tighter on the male pins. It hasn't acted up since!

Edited by Slartibartfast
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I wiggled a bunch of connectors the other day to see if I could get it to change at all, but none of them made any difference. Definitely worth checking though!


I went ahead and changed the fuel filter. The old one wasn't crazy dirty, but it had been on there a long time as evidenced by the amount of corrosion (which we don't really see in Colorado). It definitely helped the idle a little, but it was pretty smooth before. Then I pulled the new codes that cropped up yesterday. It had thrown a MAF code and had the same code pending. So I think that seals it, or at least I hope. I ordered a Hitachi replacement which should be here this weekend, so we shall see if it solves it!


What's odd is that the MAF follows the throttle quite well when viewing real time data on the scanner, but it must have some overshoot or or delay at some point while driving since it tripped the code. Both codes were lower RPMs right above idle, so I am guessing it errored when trying to keep it idling while slowing down or stopped.


Hopefully we have the answer! I will report back when I get it changed.

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Hmm. I guess if the MAF is flaking out, that could be making it stall. Also have a look at the MAF filaments. If they're caked with schmoo the curve would still be smooth but the values would be wrong. A filthy MAF solved a lot of issues on a friend's Buick, but that one was leaning out and going into limp mode if you pushed it rather than dying at idle.

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I did clean the MAF with CRC MAF cleaner, and it looked good, but I'm thinking there is some internal issue with the filaments. I replaced the MAF yesterday, so we will see if it truly did solve the problem. I do know the idle was better than ever after changing it. Engine was still up to operating temp when I swapped it and the last couple times I had started it with the engine warm the RPMs dropped after the crank and bogged down, almost killing it before it caught itself and bumped them back up. Started it up and it was solid, let it idle for a bit and no change to the idle RPMs once they settled at about 750-800 RPMs. Shut it off and started it a few more times without issue.


Now I just need to fix the rear coolant line off the intake. I think in the process of trying to get the pinch clamp back on the hose I make have sliced it a little. It has a slow drip, and I can see the coolant shining on the hose. It is really slow though, so I'll take care of it here soon, but it shouldn't matter if she drives it for a couple days. Stupid location for a coolant line anyhow, just about an inch higher on the plenum and you could remove the top of the hose without issue, but the location they went with makes those two hoses ridiculously hard to get at. One of them is almost impossible, which is why I cut it to get the plenum off, and consequently why there is a small hole I accidentally made in the replacement hose while putting it back on.

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