Oppai

The title is mostly a joke but only because I'm not superstitious enough to believe it so.

My car Is acting seriously weird.

To preface, my car is running stock twins on PFC + xavier borg's dl-340xb with the base map fully tweaked to the fc tweak recommended defaults.
Air pump deleted, acv deleted, fpd deleted, emissions solenoids deleted, double throttle and fast warm up deleted, catless midpipe, HKS v-mount, koyo n-flow, Antigravity atx-30 battery relocated to rear, walbro 450 with its own circuit & relay, aem 0300 x series wideband.

Onto the symptoms...
The car turns on and runs fine, idles (mostly) fine. I drive off and get going without any drama. After warming up the car to operating temperature (about 10 minutes of stop & go driving) the car's idle becomes audibly worse. After about 15 minutes, the car starts misfiring and having trouble staying on. At this point, with constant throttle on the highway, the car lurches over and over. My data logs on pfc-connect show the afr, iacv, and injx spiking to max values over and over. Basically a few sensors in conjunction go absolutely ballistic even as the battery voltage stays steady. Car is lurching on and off power with constant throttle.


The running quality quickly deteriorates until the car is sputtering, unresponsive to all throttle inputs, barely able to idle, and eventually stalls itself out. After i let the car sit for about half an hour, it starts back up and runs like nothing ever happened. But eventually it repeats the same behavior of sputtering out and stalling. (I limped it home this way)

Here's a list of things that are NOT the problem.
I compression tested the car after limping it home today and it tested >110psi on all faces and rotors (phew). It's a new mazda motor i swapped in 3 years ago with about 1k miles on it.
Ive smoke tested the car and it shows no vacuum leaks.
Idle air control valve is cleaned and works. Evident by the fact that my pfc's electronic dashpot is working.
My fuel pump is new, on its own circuit, and i hear it prime when i turn the ignition on.
The injectors have been cleaned and flow tested and are fully functional.
the water temperature showing on my aftermarket gauge stabilises at around 200f in traffic and 180-190 at speed so it's not overheating.

All signs point to some kind of electrical or fuel delivery issue, but i cant see how it could be a fuel system issue when ive been through the entire fuel system already. Could it be a bad ecu, or bad ground or something??

any thoughts?

armans

iTrader: (2)

not sure if PFC supports it but do you have closed-loop on or off? Also your coolant temp sensor acts weirdly, might be the cause of your problem

antman0408

iTrader: (2)

Coolant temp is going from 175 to -38. I think that would be throwing everything else off when it’s doing that. I’d check that first

Oppai

The coolant temp sensor was replaced with a mazda OEM sensor about 2 years ago. I have a hard time believing it would go bad so quickly. Other culprits possible? Do these things going haywire all have a shared ground or something? Injectors, coolant temp sensor, idle air control valve...
Nonetheless i have ordered a new sensor to see if it will fix the coolant temp issue specifically...

(By the way, the actual coolant temp GAUGE inside my car which should be wired to that coolant sensor was displaying normal temps... could it just be a software glitch?)

Valkyrie

I suspect an intermittently bad connection (e.g., a broken wire).

Resistance goes down as the temperature increases, and the resistance curve probably bottoms out at something like -38 F when resistance goes to infinity.

The resistance curve of the stock IAT (which is basically just a water temperature sensor) bottoms out at around -30 C at ~13,000 Ohms.

Coincidentally, -38 degrees is around where the Fahrenheit and Celsius scales cross over...

Redbul

iTrader: (27)

I have heard before that the TPS might test in range, then go wonky when it gets hot. Have you swapped in another TPS?

Check the directionalty of the filter for the MAP sensor.

Redbul

iTrader: (27)

Are you LHD or RHD? If RHD are you Version 4 or newer? Position of the coils was changed at the end of 1995 for JDM cars. It has been on ongoing problem that people who do not realize this and either hook up the coil harness or the plug leads in the wrong order.

Pete_89T2

iTrader: (4)

Your PFC uses the OEM wiring harness, correct? And is your OEM harness still the original one that came with the car from the factory some ~30 years ago now? If so, it's been thru enough years of aging + heat cycles and wear & tear to most likely have at least a few poor/intermittent opens and possibly some intermittent shorts that will wreak havoc on normal operations. When my S5T2 was still on the OEM ECU, and around 20+ years old, I had lots of intermittent electrical gremlins - OMP would sometimes fault into limp home mode; could never keep the TPS set for long, wonky idel, etc... Replacing my OEM wiring harness killed all the gremlins and everything just WORKED after that.

Oppai

tps is fine. You can see the tps signal voltage in the logs video is steady.

Oppai

I guess once ive exhausted my troubleshooting options the nuclear option is a harness replacement. My lazy *** really doesnt want to go there yet though..

Oppai

92 rhd and coils are hooked up correctly. Otherwise the car probably wouldnt run fine initially

Oppai

so i tried tweaking and yanking the harnesses all over to try to induce this behavior while the car is idling stationary and had no luck. The bad wiring theory seems the most plausible right now but dont seem to be having much luck locating where my wiring went bad.

DaveW

If it's not an electrical issue...
Have you logged fuel pressure when this happens? I'm suspecting fuel pressure drops off with heat. Not that this applies directly to you, but on carbureted cars this is usually vapor-lock. I'm wondering if the there is a leak or restriction in the hoses/filter in fuel tank before the pump which would cause it to not pump properly when it and/or the fuel gets warm.

Oppai

i havent. I dont have a fuel pressure sensor on the car since its a stock system apart from the fpd delete and walbro 450. I also suspect fuel pressure loss as the symptoms seem to point towards that. could a clogged charcoal canister also induce vapor lock? Ill try swapping the fuel filter just as a precautionary measure. But the fuel filter doesnt strike me as something that would cause a temperature-sensitive restriction. More like just a constant restriction.

Maybe its worthwhile for me to try installing a temporary inline fuel pressure gauge to confirm this, but inducing these symptoms on the street is also really sketchy and not something i want to repeat...

j9fd3s

iTrader: (3)

its not vapor lock, but a clogged canister will cause problems, when the engine pulls fuel out of the tank, it needs to be able to let air in to take up the space, if it can't, then the pump can't pump.
it gets worse as the fuel level drops, so if you have a full tank its usually fine, but if its near empty its bad.
you can diagnose by taking the gas cap off

i also had a car where the fuel filter rusted out internally, it would start and run ok, but the more you drove it the less power you had.

j9fd3s

iTrader: (3)

+1, sensor, connector and or wiring is bad.

R-R-Rx7

iTrader: (24)

i would zero out the coolant temp fuel enrichment so this variation in temperature does not contribute to any fuel adjustments. write down your numbers before you zero everything out and see the behavior

Oppai

im on a full tank so doesnt sound like an issue with my charcoal canister. Ill be replacing the fuel filter regardless because its probably overdue anyway. Guess ill take it apart to see if it's a possible culprit.

Oppai

good idea thank you

boostin13b

Check the plug to that sensor. Those break fairly often with age and heat. Could be a poor connection to that sensor that looses an ok connection as heat builds. I have seen the break where the pin backs out from the connector.

Molotovman

iTrader: (49)

+1
Replace the harness, free yourself of the headaches.

What you're describing sounds like a connection or signal ground loss issue due to a poor connection which is causing several things to go haywire at once. Replacing the harness is considerably easy, don't trip yourself up due to intimidation.

Redbul

iTrader: (27)

With his modifications, I wonder how much of the original harness is actually there, or whether it was already re-wired?

There is a NIB emissions harness on Buyee for about $800 atm.

My point on the TPS was that owner found his TPS would appear in spec when cold and when warmed up, but would go out of spec when the engine got hot.

I am not sure if that would be a readable condition.

But it sort of fits your described problem, so might be worth a try, to try a different TPS.

(See recent posts by Tom Smith on this issue for his car.)

Also read up on the Fuel Pressure regulator. The FD FPR has a solenoid controlling it which the ecu activates under certain engine temperature conditions. Often on "reliability " mods" that solenoid is deleted. Be aware that if the FPR lacks a vacuum it will close, raising your fuel pressure. My vacuum hose to the FPR had turned to dust, likely contributing to flooding sensitivity issues I was having.

Oppai

The TPS voltage being reported in the logs is in spec for the duration of the drive & also roughly correlated to what I recall my throttle inputs being. I'll probably forego trying to swap out TPSes since they are rare as hens teeth.

My harness has had the IAT sensor and rad fan thermoswitch connectors rewired for aftermarket sensors. There may also be a couple of other things I'm forgetting about.

Redbul

iTrader: (27)

With a emission delete (and possibly a change to parralled turbos) a good deal of the "Emission Harness" (often referred to as the "Engine Harness') would likely have been deleted as well. So replacing the harness with a custom harness may be a more cost effective route than buying an entirely new EM harness and chopping it up.

I am assuming your Power FC is the proper spec one for your year and model.

There is a remote chance someone swapped in a newer set of coils. Even with the plugs hooked up wrong the motor will run, but as you get into higher revs (above 6000) trouble appears....in a big way.

Pete_89T2

iTrader: (4)

^And now that I watched the OP's video, I'm even more convinced it's a wiring issue. The coolant temp displayed randomly jumps back & forth from an indicated 175*C to -38*C; that tells me the common sensor ground and/or common +5VDC sensor power supply wiring is buggered up, at least intermittently.