Hey guys, im new here, and just got my first 240sx, its a 1996 ka24de and it definitely has its fair share of issues. If someone could help me figure this one out i would greatly appreciate it.
The car will randomly start bucking, similar to when you're first learning to drive a stick, but well after you're into gear. Its a constant on and off bucking, with no rhythm to it. Pushing the clutch in will stop the bucking, but the cars RPMS will start going up and down like crazy, and when releasing the clutch it returns to the same issue. Once the bucking starts, it will not stop until the car is turned off, let sit, and then turned on again. While bucking, pressing the gas has no affect on engine rpm, so i think ive narrowed this down to a fuel issue. Lucky me this is the only part of a car i know nothing about.
So, when i bought the car, the guy told me it had a ground issue, and that all it would need is a new negative terminal to fix it. Now, im not exactly a car-guru, but i was pretty sure that info was complete bull, but i played his game and bought the car, knowing well what i was about to throw myself into. Well now that i have the car and im playing around with it, i truly cant find out what the issue is. I am unable to recreate the issue at will, it only happens when you think its not going to happen. As you can imagine this makes driving the car a bit of a horrifying experience. I have floored the car going uphill in every gear, as well as coasting at high rpm. One of the spark plugs had a bit of oil on the threads, but other than that the car seems to be getting good spark. I have already replaced the ground cables, and cleaned all the other car grounds, albeit the wiring on this car was heavily F****d up, so its kinda hard to tell whats going on sometimes. I have gotten as far as tearing apart the trunk to get to the fuel pump, and oh boy is it a mess. Heres pics, if anyone can help me figure out whats going on here:
https://drive.google.com/open?id=16UP1t ... 1wzCT2iY-k
https://drive.google.com/open?id=1-2NIA ... oTdlSWyX1s
Sorry if the pictures suck, as thats all i had time to snap today, i can post more tomorrow if needed.
From what i can tell, it looks like the previous owner has hung a bottle of gear oil, with a line leading into the top of the fuel pump, and i have no idea why, but ok. And there are 3 additional connectors not hooked up back here. Does anyone know what these connectors do, or are supposed to go to? The fuel pump has two connectors on it, both look stock, and 3 fuel lines going out, one into the gear oil container, and two to the front.

If that was confusing, my main questions are:
Why the hecc is there gear oil going to my fuel pump?
Is the bucking even a fuel issue? if so where should i start?
Is this how im supposed to post things here?

Thanks in advance for your guys' vast knowledge of these cars, i look forward to learning everything i can.

This is a duesy.

First of all the gear oil jug is there in place of the charcoal canister. That's also what those connectors are for. For our purposes we'll ignore them, it's just a vent. It's not ideal, but as long as it's not plugged, it won't cause any issues beyond a fire risk.

Next is the actual problem.

Trying to diagnose over the Internet is tricky. It requires a lot of data. First of all you need OBDII codes. My guess is you'll find codes for the vent solenoid and purge solenoid, as well as EVAP leak, which we'll ignore. Any other codes may lead to the root cause.

If you think it's fuel, then the first thing to test is fuel pressure, buy a cheap test gauge, plumb it to the feed line and tape it to the windshield, then go drive. Pressure should be 36ish psi at idle and should rise to 42ish at WOT.

If fuel pressure remains within that window for the most part you can move on.

My guess is it's either wiring, or sensor related.

Hey Ian, thanks for the reply,
Ive been playing around with the car alot for the past couple of weeks, and have learned quite a bit, heres an update.
I have been able to somewhat reliably recreate the issue by driving the car hard for a minute or two. This causes the engine rpms to begin surging, both at idle and while driving, and this is causing the bucking. After the issue starts it will persist even after turning the car off and back on, and does not go away until a while after it sits for at least a few hours.
I have attempted scanning codes with obd2 reader but to no avail. Mine was able to get power from the obd2 port but no codes, even though the CEL is on. It said something along lines of link error. Went to auto parts store who used $160+ reader that definitely supported the car, still had a link error. SO yay, something new to fix! Until i get that fixed, i am currently reading up on how to check codes manually with the light on the ecu, albeit a bit confusing.
I have ordered alot of new parts, listed here:
Dist cap/ rotor button
Coolant temp sensor and sender
Fuel filter
spark plugs
coolant for a flush
oil and filter
new lights for a few bulbs that are out
new PCV valve
ordering a cheap Fuel pressure gauge today

I have already installed the Dist cap/button, as it arrived today. This saw a noticable improvement of the cars performance, specifically with starting. The bucking issue did go away for almost 30min after replacing these parts, rather than the usual 10min, but after driving the car hard for a little while it started bucking just as bad as before.

Thanks for the info on the canister, i had no clue haha.

EDIT: also, the fuel tank leaks over 1/4 tank, and seems to have alot of pressure in it when i open the tank to fill it up. These are issues im not too worried about, as i will be replacing the fuel tank with a cell in a couple weeks.

I will update thread once fuel gauge arrives.
Thank you so much for the help!

I'd focus on getting access to the ECU. Being able to view live data and check codes is the first real step in the diagnostic process. It could be something as odd as a failing throttle position sensor causing rapid changes in timing.

Why a fuel cell? The S14 saddle tank is optimally located for good center of gravity, and low polar moment of inertia. It's one of it's key benifits over the S13.

Update
Got to the ECU, checked the diagnosis light, found codes 84, 87, and either 93 or 903. None of these seem to be something that could cause this.

And my friend included a fuel cell with the car. It's super hard to find non-cracked gas tanks for this car in my area, plus id hate to have to drop the sub frame just to fix a leak that will probably happen again. Although if you think the stock fuel tank is a good option, and know any aftermarket alternatives or ways to keep the stock tanks from leaking I'd be all about it. The only three s14s in the area I know of all have the same leak..

I have installed spark plugs, as well as all my new lights, and coolant temp sensor, and then bled coolant system for any air. The engine now misfires just as frequently, but where it would be bucking for almost a second before, now it's down around 1/2 a second for each buck/misfires.
Still waiting on fuel gauge to check pressure. A mechanic I know also mentioned that it could be the distributor could, and it would probably be a good idea to get a new distributor. May try that later on if I can't figure anything else out.

You'll have to refresh my memory on the codes. I haven't had a stock ECU in an S chassis in nearly a decade.

Distributer is a possibility, but that's an expensive gamble.

Where's the tank leaking from? If it's crack, yah you're SOL, and a cell is probably the way to go. What are your plans for the car?

Code 84 is Automatic trans-to-fuel injection communication, which is odd as I didn't think this was an auto to manual conversation, but maybe it is. Will look into it tomorrow, it's always something new with this babe. 87 is for evap, and 93 isn't a code listed in service manual, but I have seen some posts saying it's actually 903, and it's evap related. I may try going to a Nissan dealership and seeing if they can test the consult port, maybe test it while it's doing its odd misfire, idk.
Tank is leaking from right above the strap, and it's a pretty big crack. The fuel cell my friend gave me is a 15 gal, and I was hoping to put it in the trunk. I'm planning on using the car for my first drift car, to get into the sport. No hardcore build or anything like that yet haha.

Okay, so i got the fuel pressure test kit, i Tee'd it in after the pump and before the fuel rail, and to my surprise everything was pretty good. At idle the fuel pressure was 35psi, and at WOT it was 39. That is a bit lower on the high end than what youre getting, so i suppose im running lean at high rpm then? On a side note, the pressure went back down to 10psi in just a couple minutes, and from what ive heard that a bit fast to lose fuel pressure, although im not sure its affecting this issue, so i will look into that more later. Ive added some extra ground wires going to from MAF ground to battery, cleaned the MAF sensor (even though my friend who sold the car to me said it was new), and added an extra ground to both the intake maniforld at factory ground, and then one more on the block to the firewall. There has been no noticeable improvement with any of these.
Yesterday, i put premium in the car rather than regular (Ive been using regular since it has a gas tank leak and didnt wanna waste money) and the issue lessened a bit, the bucking became slightly less frequent, but still random. After how much im driving it, ive noticed the issue is occurring after taking the car over 3k rpm usually. It doesn't happen every time, but usually if the car is warmed up and i take it over 3k, let it hover there a minute, then drive normal again, the issue will start. Ive been able to reliably recreate the issue this way 3 times now. In my mind this narrows it down to probably 3 possibilities, correct me if im wrong:
1. Distributor is going out (issue hasn't been getting worse though, so im iffy about this, especially with how expensive they are)
2. O2 sensor (im not sure how much the o2 sensor controls the air/fuel ratios on a 240sx, as ive mostly owned hondas where you dont even need o2 sensors as they dont control much of the air/fuel mix. but ive been reading that they control quite a bit of it on these cars)
3. A bad ground somewhere (ive added grounds but they were only 10 gauge, so i may have not helped at all with those, and since the issue is when the car is warmed up, an electrical issue is most likely the cause i think)
4. I know you told me it was an electrical issue to start, im sorry i didn't listen :0

SO as far as next steps, when i get home tonight im going to try driving with the o2 sensor unplugged, see what it changes, and maybe order a new one if necessary. I have a cheap multi-meter, maybe i can set it to ohms and check continuity on a few grounds as well. The car also has an aftermarket exhaust installed by previous owner, so they may not have grounded the o2 sensor or something, who knows. I will update thread again once i have made more progress. If you have any other ideas for me to try please let me know!

Thanks for your help, you're the best!

I doubt it's a grounding issue. If it were, it would happen all the time. If the fuel pressure is the same, whether it's doing it or not, you can mostly rule that out as well.

Unplugging the o2 isn't a bad idea, it'll force full time open loop, but that doesn't always mean the o2 is the issue if the problem goes away.

I really think you should focus on gaining access to the live data. That may quickly tell you what's not reading properly.

The more you describe the issue, the more it sounds like an issue with the TPS to me. I've had a lot of KA TPS sensors go bad over the last 2 years, as they're getting quite old now.

https://youtu.be/DtxA03Yc3kc
Thanks for your quick replies!
Here is a video of the issue, after the car has been driven for about 15 minutes and warmed up. It is raining today and the issue seems to be worse than usual, maybe it's just my imagination. I tried driving with o2 plugged and unplugged - no change at all. I wish it were that simple tho.
I will try to get my hands on a consult reader, although I'm a bit unfamiliar with the readers/software. If you know any special brand or software that you'd recommend please let me know. the car definitely does not have the stock tps in it, although I don't know how old the one in it is. I will try testing the tps here soon with this multimeter I have.
You're idea sounds really good though, live data would be super helpful with this, since the car doesn't want to give me any codes for some reason. I've exhausted my spending money for this week though, so this car will have to wait about 3 days until I get paid again, but if you have any good software in mind or specific readers let me know and I will order/buy them

Okay, so I unplugged the tps and tested the car. With the tps unplugged, the issue begins immediately, even when cold. Idle searches and settles up to 1500rpm, and stays there. After about 10 seconds it starts surging erratically. I don't know if this narrows it down or not, since the issue is still happening. Still trying to learn more about consult at this point

Update: Just replaced distributor, and rechecked fuel pressure. Fuel pressure definitely remains steady other than the slight variation from the rpms flying around. New distributor has improved but not eliminated the issue. Should I try replacing the tps? What else could this be?

I would stop shotgunning parts and save yourself some headache and frustration in the long run. I'll post a link but the TPS is adjustable. I'd recommend adjusting it and if that doesn't resolve the issue, testing your MAF. Borrow a timing light and check your base timing while you're at it.

[edit, i see you pulled codes and MAF wasn't one of them. disregard that part.]

You didn't replace your distributor with a Spectra brand unit did you? Have used a couple and nothing but headaches.

All of this stuff is testable with a digital multimeter and FSM. While not a catch-all, the FSM will tell you EXACTLY how to check/resolve problems based on each of the codes the ECU is giving. That doesn't mean you can't test other things but its a good place to start. Sometimes your issue is with the wiring harness and not the sensor itself. The FSM has you test the wiring too. I see you have a digital multimeter so you should be able to test for this stuff. All you need to read are DC voltage and continuity (resistance) in some of the wires where directed.

[edited for links]

https://www.nicoclub.com/archives/nissa ... nuals.html - NICO link to the FSMs. Download a copy to a thumb drive. This handy thing has saved my butt so many times. If you learn how to use this, you can do LITERALLY anything to this car with the right tools. Your MIL/ECC code stuff will be in the EC (engine control) section. Each one tells you how to test it with the diagnostic tool the dealership has... OR with a multimeter for the rest of us.

https://www.nicoclub.com/FSM/240sx/1996/ec.pdf So there's the EC section for a 96' 240sx w/ KA engine. Go to page 126 for testing/adjusting your TPS. I'll throw a link with pics below for you but it's easy. Warm engine up, check voltage at a point. Adjust TPS to achieve target voltage or replace if you can't get it adjusted in spec. On the S14 there are 2 three-wire plugs at the TPS. you'll want the one on bottom for the TPS voltage here.




https://www.nicoclub.com/archives/adjus ... 240sx.html - guide to setting your ignition timing. I used a paint marker while I had my crank pulley off to mark the 20 degree notch. You can use basic chalk if you want to, in order to make it more visible. If you already did this when replacing the distributor then kudos.

https://www.nicoclub.com/archives/ka24d ... -idle.html - How to test/adjust your TPS.

https://www.240sxtech.com/ - General tech articles for our cars


Let us know what the outcome with the TPS is but I'd put my money on it being a contributor to your issue here.