# '90 b12 is stalling, need help



## greenhousedrummer (Jan 29, 2017)

Hey guys, I bought a 90 Sentra automatic with about 161k miles just under a month ago. The car was in decent shape when I got it, it just had an issue with power loss when giving it gas in 3rd gear at low rpm. If I let up on the throttle a bit, it would accelerate fine. Punching it seemed to starve the engine. I found that manually shifting at higher rpm's worked well. Well after a while, the car started to fluctuate at idle, and then a week later it started to stall. My mechanic has been at a loss, and here's everything that's been done to the car so far.

Exhaust pressure test-within specs
A loose fuel line was tightened after it was found jiggling it would cause the car to stall. This was noticed before stalling became an issue. It fixed it for a while.s
Replaced fuel filter
Changed out MAF with a 89 unit, changed nothing
Changed out ECU with 90 unit, changed nothing
Cleaned all connectors relating to the MAF, IAC, and TPS, changed nothing
Cleaned the EGR valve, ran a bit longer then stalled again, now exactly the same, probably inconsequential or coincidental?
The car has brand new spark plugs, wires, and distributor cap courtesy of a new head gasket job.

The best test I've been able to do is: I'll hold the throttle steady after starting (there's no tach but roughly around 2kish rpm I'd say) in which the car will hold rpm fine for about 8 seconds, then start to die. At this point I give the throttle a few jabs, then hold it steady again. Same thing, it will start to die after a few seconds, then I can give it a jab and it will keep running. The moment I let off of the throttle it stalls, but tries to stay running. 

If I drive it, it lurches and loses power in a more serious manner and wants to stall, so it's not drive-able at this point. I'm at a loss. Literally. I now have two cars that do not drive, and this one was supposed to make up for my other one until I got it fixed.. Any help would be greatly appreciated, and I am more than willing to supply more info.


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## Car guy (Mar 31, 2016)

Okay, this is a TBI engine, with a single FI.

Sounds like any of the following:

overdue FI - and, bear in mind, if replacing this, there are 4 o-rings involved (2 on the FI and 2 on the FI cover), a white plastic FI hold-down washer ... and critically (I repeat) ... a single teeny phillips screw that holds down the FI (against the top FI o-ring) into the FI cover (black plastic round-ish cover, these covers crack over time and lead to FI seal leakage and stumble). If replacing the FI (and o-rings or cracked cover), cover the TB so nothing falls down inside. Whatever you do, protect and hang onto that phillips screw.

was the rotor replaced? Depending upon how long it's been since you looked, I would remove the dizzy cap (be careful not to strip the 2 opposing phillips screws that hold it on), check its top center spring brush (for rebound), all 4 cap brushes (and gently clean and mouth-blow-away and slightly damp-paper-towel-wipe-away the debris out of the cap after the brushes cleaning), and the rotor leading edge.

was the timing reset after the dizzy was re-installed?

were the timing chains replaced - they wear and the front most one (right as you face them) is known for plastic breakage and ultimately failure (interference engine failure)?

was the timing chain tensioner replaced? 

Also, there is a dash pot on the d's side of the TB, which is controlled by a wax pintle. That could explain the idle, as it (partially) controls that.

And, then there's the head gasket and head (and block top) itself ... 

Finally, you can check the ECU's red and green lights for any possible code (though they generally report none, especially if its the FI or rotor or wax or dash pot. If there's no code, they will flash 55 (normal). There is a process for that. If you can't find googling, let me know, and I'll track it down. It involves carefully rotating the ECU dial a certain way (c-c-w or c-w, can't remember which), then ignition ON but car not running, then count the flashes, then rotate the dial back to the beginning. 

Let us know how you get along.


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## greenhousedrummer (Jan 29, 2017)

Hey thanks so much for your post. Things have been hectic so I didn't have time to reply, but I took your advice and had a new injector installed. The car seemed to run a lot better after installation and didn't stall right away. I cranked up the idle settings on the ecu, and tightened the throttle cable to where it's just barely giving throttle to the engine and now no stalls. I'm still getting an intermittent fluctuating idle, and every now and then the engine will bog down when going into gear, but if I pump the throttle it will continue without issues. But no lurching, much less power loss, and I can actually drive the car around which is a huge relief.


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## Car guy (Mar 31, 2016)

Great to hear. Thanks for letting folks know.

When the MAF was slid out and the new one slid in, was its perimeter surface sealed (to prevent air flow into the TB)?

Also, there is an IACV on the firewall side of the TB, a pair of emissions solenoids on the p's side of the TB (not on it) - one controls the EGR/BPT/canister vapors and another that controls the AIV, and those associated pipings and hose routings. If any of these is allowing unintended air/vapor flow, that would explain the continuing idle fluctuation. Still feel like there may be an ignition-related component to this too.


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## greenhousedrummer (Jan 29, 2017)

I changed the MAF out myself, and no I did not seal it. I'm kind of a noob when it comes to working on cars. What can I use to give it a proper seal? I'll check out everything around the iacv, thank you. Is it possible for the issue to be caused by a bad tps?


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## Car guy (Mar 31, 2016)

There should have been a small baggie in the air flow meter box; in that baggie, there would have been a small amount of white silicon grease; that was to be applied to the mating surface where the (inner) metal part of the air flow meter abuts the p's side of the TB. Also, there should have been an o-ring on the outer end of the hot wire meter's extended housing.

Yes, the TPS on this car could be a factor, as it is both an idle switch and a T position sensor.

Would recommend checking the AB hose to ensure it is secure and has full air tight integrity on both ends.

On the IAC configuration, there is an AAC (p's side of the TB - large round size one), a FICD inward of that (A/C add-on), and an idle up (cold start) on d's side (of the TB). The AAC is normal operating temp one. Gasket inward of that (as I recall).

Are we sure the rotor head is good - new, clean, no carbon build-up? And, where is the leading edge of spark burn on the rotor, precisely (as you examine it face-on reading from left to right)? Is it early, right on, or late?

Burn kVs on this engine are .2 to 1.5 with burn times of .4 to 1.7 ms; so, the slightest timing error is important.

What is idle RPM? Right at 800, or ?

Checked each and every hose? Lots on this car.


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