Beachin_240

My car runs absolutely terrible. It starts up fine, and then within several seconds, the idle drops to almost nothing, just barely enough to stay running. Sometimes it stalls out. Once it warms up a little bit, the idle stabilizes around ~850, but it's still a little unstable, warbling around a bit. If I give it any gas before it's fully warmed up, it stumbles a little, threatens to stall out.
Once it's warmed up and I'm driving, I only get to ~2K before it starts stumbling so bad I can hardly accelerate. Any load at all causes it to start sputtering and bucking. It takes ~30 seconds to limp it up to 55mph.

I pulled ECU codes, and got 34, for the knock sensor. I pulled the sensor and it looks perfectly fine. I used a multimeter to check the sensor, check continuity in the wiring all the way to the ECU...everything checks out fine. The FSM, at this point, says to check the ECU pins for damage (which there isn't), erase codes, and take a test drive. I'm going to do so, but I just can't imagine the thing running any better since I didn't find any issues.

Setup
S14 T28 @ 10psi, Walbro pump, FMIC, stock ECU/injectors/MAF, 3" turboback, new NGK BKR7E V-power copper plugs

It's an S13 convertible, with an S13 blacktop, #62 ECU. I never use anything less than 92-93 octane.


What's up? I know that ECU codes for one thing will sometimes get tripped by something else. Is there anything jumping out at you that might be wrong with my car that would trip code 34?

Is it possible the ECU code got tripped for some reason, so the car now thinks the knock sensor is bad and is constantly pulling timing, and it'll run fine once I clear the codes?

IN SHORT: Why am I throwing a knock sensor code when there's no problem with the knock sensor?

Please, chime in, I'm at a loss here.

Fabrik8

Bad ECU? Do you have the knock sensor cable shielded properly? I'm guessing that whatever problem you're having with stumbling doesn't have to do with the knock sensor and is more likely a problem with a MAF sensor, etc. Have you got it tuned, and have you checked to see what your fuel pressure is? The stock fuel regulator should be able to regulate with an aftermarket fuel pump, but that's not necessarily a sure thing.

One other question: Did you just complete the swap, and has it always ran like this? Basically, is this a problem that just showed up after running fine for a long time, or is this a problem that you've had since the beginning?

Beachin_240

Nope, it's a new problem. I'll break down the sequence of events a little better:

I bought the car last year, already swapped. It was basically a stock swap, with only the fuel pump and FMIC. I added everything else, including a GTiR T28 turbo. Didn't like the power curve, so recently I swapped it out for a BB S14 T28. Also replaced my NGK iridium plugs with fresh V-Power copper plugs. Boost is set with an HKS actuator.

Since then, the car hasn't run quite right. At first, it wasn't that bad; it would just stumble a little bit in the middle of the rev range, but would still pull nice and hard once it got past that. Idled perfectly fine on startup. It's gotten progressively worse, to the point it's at now where it barely goes anywhere and runs rough/stalls on startup.

Knock sensor cable is shielded properly; as best I can tell, it's just like it should be according to the FSM. I have continuity between the shielding wire (sticking out in this picture, on the right side) and the ground pin on the connector on the left side of the picture; also have continuity between the signal pin on the connector on the left, and the pin on the connector on the right. No cross-continuity (I don't know if that's a word, but basically, no shorts in the subharness). It does look like shit, but it seems as though it works fine.


I recently installed a Nismo FPR as well, I forgot to mention that. Pressure's at 36/43 with vacuum line off. I haven't checked the MAF.

It hasn't been tuned. But it ran perfectly fine with the GTiR turbo and no tune (albeit a little peaky). I suppose the ECU could have gone bad, but the thing that gets me is, the problem has been getting progressively worse; it wasn't that bad to begin with.


ALSO worth noting: It boosts fine, just doesn't go anywhere. Turbo has almost 0 shaft play. Haven't gotten any oil in intake piping, and I haven't noticed any smoke coming from the exhaust (signs of something like a blown ring).

-JB-

get rid of the Nismo FPR, they are complete junk and will cause fluctuation in pressure. Go back stock or go with a better brand.

If you don't believe me, search nismo fpr on nico


also I would change out your MAF to see if thats the problem. The MAF can cause problems just like what you are describing.

Beachin_240

Yeah, I still have the stock one so I might do that. The Nismo FPR was installed very recently though (after this problem got really bad; I thought the stock regulator might have gone out) so it wouldn't be the cause.

I'll check out the MAF. Any reason it would throw a knock code though?

Fabrik8

I don't know much about Nissan ECUs, but most ECUs have long- and short-term fuel trims, and some other stored parameters. If the problem you're having is getting progressively worse, the ECU may be trying to override something on the engine, such as a bad MAF or a fuel system change, etc. This should be easy to figure out by simply resetting the ECU. If that is the issue, you should see everything go back to the way it was. More likely though, you probably have a sensor failing. Grab the FSM and see if there are testing and troubleshooting procedures for that and the other critical sensors, like intake air temp (there is usually an IAT sensor on turbo cars with a MAF) and throttle position.

Beachin_240

Cool, I'll do that. I already troubleshot the TPS since that's the first thing that I thought of, and I don't see an IAT sensor in the FSM, but I'll clear the codes and see if it helps things. Then I'll test the MAF to see if it checks out.

EDIT: Looks like the stock SR uses an MAF sensor, while a lot of aftermarket engine management (like a Power FC D-Jetro) use a MAP and IAT sensor combo to replace the MAF.

Fabrik8

It may not have an IAT sensor, some turbo cars don't. I assume you've checked the calibration of the TPS during your troubleshooting?

Beachin_240

Yeah, the resistance checked within the specified range the way it was adjusted. At WOT though, it went up twice as high as it should have (10k ohms rather than 5k). I swapped it with a TPS from a known running car, and that sensor did the same thing.

It completely confused me, until I realized that the TPS probably reads a max of 5k ohms with the pedal to the floor, rather than with the throttle body opened all the way by hand; I probably rotated it too far. I felt dumb after that.

bkg

Yeah get fuel pressure gauge to check fuel pressure check maf, ecu, and tps and ignition. I had similiar problem and if was bad ecu bro. That's all I can come up with but I am by no means a sr20 guru.