KA24E, occasionally down on power...?

O

orion

New member
Hey guys...need some brainstorming help.

I posted an "intro", but in case you missed it: I have a used ITA S13, with a SOHC KA that has required a headgasket, new bearings, new injectors, and many cleaned up plugs (for sensors).

It never felt like it made power above ~5000...it pulled great until thereabouts, but then just stayed flat.

It's taken a lot of work, but I got it running *perfect* a couple weeks back - I did a "redneck dyno" (quarter mile), and it trapped 84.4mph...which would estimate 130RWHP given a weight of ~2800 with me in it.

It ran it's best that night. Pulled hard and strong until redline.

Took it to the semi-local short road course (Gainesville Raceway), and it was flat again? I thought maybe it was the heat, but the data shows a max speed of ~68mph on the back straight - I should be doing around 74-75mph, easy...even in 100F weather. And again, it had no pull over 4500-5000rpm.

This was 2 days after it ran perfect.

I have to think it's a sensor, or wiring: MAF, TPS, Coolant temp...one of the ones the ECU would use to determine load and timing advance.

Maybe fuel pressure, but I don't think so...it's got a fuel cell with a Bosch pump.

I think it's a poor connection, or intermittent short.

Here where you come in:

Have you ever been to the track, and had your KA24E feel down on power...only to find the CTS plug had come loose?

Ever had a bad TPS, and the car ran...but felt slow?

I've tested all these sensors at idle, and they all check out. CTS resistance is good, within spec for the ambient temperature. TPS shows ~0.5v with the throttle closed (idle), and +4v with it WOT.

Base timing has been checked and checked again, and it dead-on at 15 BDTC.

Maybe a bad distributor, or more specifically...a bad crank position senor within the dizzy?

Give me some ideas, please.

I feel like I've checked and double-checked everything...this is my 11th 240SX, and I had to do my fair share of troubleshooting over the years.

But on this one, I'm stumped.

Thanks for any suggestions.

- Brian
 
Still looking for ideas.

Coolant temp sensor has been triple-checked, and it A-OK.

1.6k Ohms at ~80 F, .547k Ohms at ~150F...within spec.

The hard starting makes me thing base timing is off - I might just try some random advance/retard to see if that helps (that's actually an FSM-approved diagnostic tool...hehe)...and go from there.

Then, I guess I'll start tracing wires and checking continuity.

??? - Brian
 
Brian, I'd also double check that the timing chain/cam gear are properly aligned. What's your distrubuter cam gear look like? They are notorious for wearing out, especially the way we twist these motors.
 
Thanks for the reply...

Cam timing is dead-on...like I said, it ran great ONE afternoon. Dont think these motors could skip a tooth.

I will check the dizzy gear...

- Brian
 
Sorry, to clarify, not just the gear on the distributer. The gear on the crank that drives it. Nismo use to make a gear of alternative material (I want to say a bronze silicate or something like that) because of that issue. I don't believe they are still made and are hard to find now a days. Good luck.
 
I had issues with the injector wires. The rubber boots get hard with age and can rub through the wires causing a short. Had this happen a couple times. Dr. Tristan helped me find it at the track once. Showed up as a miss in the motor. Sounds like you've gone through and checked all the connectors, though.

You said the motor is hard to start? Mine fires right up once the fuel rail is primed. Have you checked the fuel pressure just to be sure the pump is ok? Maybe a faulty fp regulator? Might replace the dist with a known good one just to rule it out. I've read of some issues with the dist having problems when hot, but I've never experienced it.

As an aside, I have no idea how my car ran with the wiring harness it had. That thing was so rigged I'm surprised the car even started. The harness in the new tub is near perfect. :)

David
 
Thanks for the reply.

A different dizzy will be my first parts swap...and then I'll check fuel pressure while driving (it's fine at idle / no load).

My daughter was born 3weeks early (she's just fine...she's perfect!) on Monday...so this may be on hold a short while.

- Brian
 
Get your priorities in order man!!!



Go get yourself a shotgun and start training for those boys she'll bring home in high school...
 
Haha...I got the guns covered...I'm a Sig Sauer guy...and I'm stocked up :)

...I always think of that scene from 'Bad Boys 2' with Will Smith...hehe.

- Brian
 
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Took an hour today to mess with the car...

Checked continuity on several circuits, tested at the ECU - CTS, IAT, TPS, MAF...all good, even when wiggling the harness.

The only real change I made was disconnecting the neutral position switch...and that seems to have made all the difference in the world...????? Car ran GREAT this afternoon?

Begs the question: Anyone else encounter a similar issue? The neutral sensor is listed in the FSM diagnostics under the diagnostic sdction for 'hard to start', and 'stall at idle'...so it must be more important than a cruise control sensor.

??? - Brian
 
Weird. Got me. I'm pretty sure none of the stuff connected to the transmission is really needed. My new tub was an auto so there's no harness for the manual transmission. Most folks that do an auto to manual swap don't seem to bother to hook anything up.

David
 
At least for the S14 chassis, the 5th gear and speedo indicator both go to the ECU for a 120mph speed limiter. both of those wires have to be cut to remove the 120 limit.

I'd love to see the logic used on the ECU that involves all of those sensors so we can find a way around them without having to 'southern engineer' jumpers and resistors in place of sensors.
 
No real surprise, but the changes to the neutral switch made no lasting change. The improved performance is gone, and was clearly a coincidence. Dammit.

Has to be fuel pressure dropping as the revs climb. At least, that's my focus...for now.

I'll report back with results.

- Brian
 
orion said:
No real surprise, but the changes to the neutral switch made no lasting change. The improved performance is gone, and was clearly a coincidence. Dammit.

Has to be fuel pressure dropping as the revs climb. At least, that's my focus...for now.

I'll report back with results.

- Brian
Click to expand...

just had a voltage drop issue to the fuel pump in mine :/ had to run an external relay... Found the issue on the dyno, temp mounted my snap-on fuel pressure gauge in the car and drove it on track during the diagnosis.

speaking of dyno, at this point it might be worth some dyno time to track this one down... its amazing what you can accomplish on a dyno :)
 
^^^ Good input. The fuel pump was hardwired (non-OEM wiring) in the fuel cell...and the voltage and resistance of the power wires tested OK.

But you're comment on the dyno - I wish I would have taken it to one. Might have seen something to indicate that this was a fuel pressure issue before testing so much other crap.

Here's the update from this afternoon:

So...after testing ALL the sensors and finding everything within spec (checked at the pins of the ECU, to ensure continuity of the wiring...and proper voltage or resistance)...I was really starting to question fuel pressure.

At idle, and revving under no load...it was within spec. However, I tested it the other day by crimping off the return line...and it only got to ~45psi (???). I was wondering if the fuel pump was getting enough voltage under load, so I re-wired the pump and ran a new ground tonight (just to see)...with no change.

Decided to pull the cover off the fuel cell, and I think I MIGHT have found the source of the problem - The fuel line coming off the pump was folded over in 2 places, crimped closed. It was also 3/8" line (should be 5/16"), and probably 4 times longer than it needed to be (hence, the loops and the line folding over itself). I pulled it out, and the outside of the hose was all wavy (fuel or ethanol damage???). Where it was crimped/folded over...it felt closed off inside.

I cut it open, to see how it looked...and here's what I got:

crushed-fuel-hose.jpg


This motor's been fed by a 1/8" ID fuel line, basically..and that's best case scenario, where it was NOT folded over itself.

I have to think this is the problem with the power dropping off at high RPM...HAS TO BE.

I put it back together with 5/16" high pressure hose...which is short, and straight.

I'll test tomorrow, and let you guys know.

- Brian
 
FINALLY!...That was, without a doubt, the problem...and it's fixed!!!

It runs better than ever, in all respects.

- Brian
 
^^^ Thanks. And I agree...diagnosing weird issues is the worst.

I'll spend 5hrs welding up a new exhaust with a smile on my face. 30 minutes of probing wires and sensors and I want to commit seppuku...haha.

- Brian
 
Glad you found it! I'm the same way. building 'new' stuff is just fine and dandy, but chasing weird issues just makes me want to burn the car and start over..
 
How's your fuel cell hooked up? Mine has AN bulkhead fittings on the cell. It's AN fittings with race hose on either side of the bulkhead. I'd have to check inside to see how it's hooked to the pump. I don't recall off the top of my head.

I have a similar issue with the 3/8" hose instead of 5/16, but on the outside of the cell. 3/8" had been used with a 6AN connector to go to the factory hard lines. It worked, but it had to be double clamped to keep it from leaking. I found a 6AN female connector with a 5/16 hose barb. In case anyone else has that issue.

David
 
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