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93 NA ATX 3000gt DOHC
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Could be water or trash in tank that mixes with fuel when car moving or when gas added stirring things up when added. But then settles to bottom of tank after sitting, causing mostly water to be drawn in on startup stalling engine or if trash it could be covering pre-strainer on pump pickup stopping fuel flow.

It will be interesting to see what you find when draining tank. Lastly with yours being a ’92, there’s always possibility of ECU problem but the likelihood of that coinciding with adding gas seems slim.
 

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Next, I hooked up jumper wires and a multi-meter to the fuel pumps (-) Negative Post and the other was connected to the fuel pump 12V+ line. I started the car while monitoring the voltage I watched the car start up and the volts shoot right to 12v no problem, and then the volts go to zero, as I can hear the relay golden box thing attached behind the carpet on the right side of my centre console. as soon as it loses voltage, the box clicks, and the engine dies, then the box clicks again.
Not sure if I completely understand where meter was attached (at pump connector in hatch access panel opening maybe), instead of fuel pump test connector near wiper motor under hood. But the “golden box thing” is the MFI relay which half of it controls the fuel pump on/off, which as @beegeezy pointed out is done by ECU closing/opening that half of the MFI relay by supplying/removing control coil ground.

When key turned to start position pump half of MFI closes sending power to pump (thru pump speed relay), if ECU receives a run signal from engine it will keep pump running by holding that half of MFI closed. If ECU doesn’t receive run signal in a given amount of time it will open that half of MFI stopping the pump. So you could have problem with MFI relay, ECU not maintaining ground or ECU not getting a run signal from engine.

Does anybody know what the cause of the problem is or is the one fix fixes all the Hotwire/Relay Mod??
If you mean pump speed relay bypass with this statement, that won’t cure your problem, if you’re hearing MFI relay open (click) right before voltage drops to pump and engine stops. MFI relay sends power to fuel pump speed relay which then shifts between high and low speed when ECU determines which speed is needed. So if power is being lost leaving MFI relay, pump won’t run in either low or high speed.

Would anybody be kind enough to direct me to the guide most appropriate for the problem I am facing? Thank you all!
I’d start by monitoring voltage leaving MFI relay on Black wire w/blue stripe (B-L) when starting and running to see if power drops there before engine dies, if does you’d know it’s with MFI relay or ECU control of it.
 

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That black wire with the blue stripe is the same wire I tested at the fuel pump and monitored it loosing power as the engine dies, would that be an equivalent measurement or should I check directly at the MFi itself on that black wire w/blue stripes?
Testing it at MFI relay will confirm if MFI or ECU is causing the problem, or if it's after the MFI like maybe at fuel pump speed relay circuit.

This is the procedure I was wondering if this would fix my problem: Stealth 316 - Fuel Pump Re-Wire just to be specific. Would this procedure get my car running again, since i already tested it and it runs while directly hotwired so I just assumed that all I would need was the relay and switching circuit to turn it on and off. Or am I not undersanding something?
Yeah that would fix it, but the ECU wouldn't be able to turn pump off if engine quit unexpectedly. Like from busted fuel line or wreck, which if fire was involved pump would continue to feed it if you forget or not able to turn switch off.
 

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Unless I'm missing something, I think he asked if the procedure he linked from Stealth316 would fix his car? That's just a basic pump hotwire that we all use often, and works as stock. (minus the low speed)
OK… So you’re not missing anything, I was because never read that complete re-wire process that still uses the Black wire with blue stripe from fuel pump relay to triggered the added relay in trunk area, instead of another switch source as I assumed :oops:.

BUT….. If that wire is now dropping voltage as he says, it will still drop-out the added relay causing pump to stop. So nothing would be gained with that process, unless it’s simply open circuit on resistor side branch that’s causing the complete drop to 0 volts, instead of ~ 9 volts :unsure:. Which if that's the case, the relay by-pass you mentioned in post #9 would be simplest cure.
 

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I agree with the reasoning provided to this point including beegeezy's. I think we can take it even 1 step further knowing that unplugging the airflow sensor (MAF) kept the pump and the car running. If crank/cam /run signal to ECU was dropping out or B+ voltage into MFI at pin 7 was lost, or MFI was bad and opening the circuit after only seconds,the car would shut down despite the MAF being unplugged...that didn't happen. So the ECU logic and/or some circuitry components failing is the branch point as to whether it gets ground to pin 8 W-R or not to keep the pump running. Confirmation done by seeing if ground suddenly lost on W-R at ECU end or MFI end. Pull ECU and post some good up close pics if that's happening.
This strikes me as perfectly good logic, that's easy for me to agree with especially after reading the below quote from your post #16
When I start the engine the volt meter reads 13.25 volts for a moment, maybe 1 or 2 seconds and then at the same time the voltage drops to zero I can hear the MFI relay click, the car continues to run for a few moments more (approximatly 2 and 1/2, or 3 seconds) then the engine stalls.

So the voltage does cut out before the engine dies.
I don't know why the ECU doesn't drop the ground with MAF unplugged (maybe @RealMcCoy does :unsure:), but it does seem apparent something is going wrong in ECU during some logic change. Pictures of ECU will only help if there's some obvious visual problems, unfortunately there can be problems with no visual evidence.

But it's obvious to me there's no need for added relays, jumpers, etc., until that ground being dropped by ECU is corrected.
 

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The ECU switches fuel pump speed based on load.(Airflow) Default is high speed. Once you start the engine and it establishes low load, it switches to low speed. If it dies when attempting to make the switch, that tells you the ECU is trying to do it's job, but there is a break in the low speed circuit. As I already posted, the most common issue is the relay full of water, but simply forgetting to plug the resistor back in will do the same... Unplugging the airflow meter simply puts in into limp mode and deprives the ECU any means of establishing load, so it defaults to high speed and never tries to switch.
While this explains why/when ECU switches between low and high speed operation, it still doesn't explain why when ECU switches pump relay to low speed it also drops ground for MFI pump relay coil. That half of MFI needs to stay closed no matter which state the pump speed relay is in, that's what I'm trying to wrap my head around.

Are you indicating there's logic in ECU that can sense a load problem when it shifts to low speed circuit side and drops out MFI relay pump side because of some kind of imbalance, similar to how a ground fault circuit breaker works? :unsure:
 

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Since we know it it the ECU that is the source of the problem, next I am going to attempt to diagnose the ECU itself. The question: is there an exact transistor or diode I should check or a certain circuit that you could show me or do I need to one-by-one test each and every component.
The most common problems are with the radial electrolytic capacitors failing from age causing erratic current in some circuits, which often have no visual signs. In many cases they leak electrolyte that can corrode and also damage near by components and/or circuit board traces, in these cases there’s usually visual signs of leakage as shown in link @OhioSpyderman linked about removal post #26. Sometimes the electrolyte will damage sub boards attached to main mother board because of ECU being mounted vertically on it’s side, see Important Note: Specific to repair for Stealth ECU (in red) ~ 1/3 down page of the below link.

No matter if there’s visual evidence of leakage or not, original electrolytic capacitors in a ’92 ECU needs to be replaced.


Maybe the malfunctioning IACV over time damaged the ECU which lead to the problems I am experiencing where my fuel pump cuts out? Or could it be the malfunctioning IACV with its terminals that are all under the required Ω28 - Ω30 (from what I have read at least since I have not been able to find specific IACV info for our cars.
The IACV readings you got of 25-26Ω is probably OK if it is the unit with tan colored top, which should be in ~30Ω rage @ 68°. The later solid black units run ~40Ω @ 68°. Ohm readings a little off or infinity reading from a open coil will not damage ECU, just casue a malfunctioning IAC unit. Damage to ECU from IAC usually occurs when one of four coils shorts to housing (ground), causing ohm readings of near zero ohms and complete continuity to IAC metal housing can be read from pin #s for that coil. When that occurs IIRC what happens is burn driver boards that can be seen as in below picture.

click on image to enlarge
 

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Thank you so much for all the info you have helped me with so far. I checked out the link and read the whole page, those radial cylindrical shape capacitors that bulge at the top and sometimes the bottoms leak which are obviously harder to detect than a top capacitor leak. Are these the same ones as in the links below? Or do other components fail often? Or just the faulty 42-year-old electrolytic capacitors that stop functioning I wonder...

Like this kind??? HD 100 Pcs 8 X 11mm Radial Aluminum Electrolytic Capacitors 470uf 25v for sale online | eBay
The radial cylindrical shape electrolytic capacitors are the biggest problems because of age and heat cycles. The ones Mitsubishi used in early ‘90’s started failing after ~ 20 or so years. Other components even including ceramic capacitors hold up much better and don’t fail often when not affected by some other failure.

Yes similar to ones you linked but those have wrong micro-farad (µF) and voltage wouldn’t work as replacement for the one 47 µF 50 volt capacitor you need. The µF rating MUST be same as one being replaced and the voltage needs to be same as or higher than one replaced. The three correct µF and voltage currently in you ECU is shown at bottom of bellow link.


Just to clarify for me, which relay is the "speed relay" is this the MFI or the Relay and accompanying resistor located beneath the air box? I keep hearing the speed relay referanced and I'm always wondering which one it is
I'm sure he's talking about one below air box as that one only controls the speed of fuel pump and I also often refer to it as pump speed relay. Mitsubishi literature only refers to it as fuel pump relay, which is sometimes confusing since one side of MFI relay actually starts and stops fuel pump by supplying or removing battery voltage.
 

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So far, it is obvious to me that the leaking capacitor(s?) needs to be replaced, as well as that resister labeled "C6" that is circled in red in one of the photos from different angles. The other damage from the same leaking capacitor you will also see from multiple angles and circled in red. You'll be able to see what I am talking about.
That’s classic electrolytic capacitor leakage, which requires capacitor replacement and board cleaning with alcohol. That 100 µF, the 22 µF and the 47 µF need to be replaced, with correct polarity installation. You would need to lightly scrap around/under side board legs pins and around C6, then use isopropyl alcohol and tooth brush to scrub those ares good to remove corrosion residue.

I’m not sure if componet in C6 location is damaged or not, possibly just covered with corrosion and/or residue. I can’t tell for sure but it is probably a surface mounted (SMD) capacitor, simular to one above your red circle of C6 and on other side of side-board in location C27.
 

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Thank you for the advice on cleaning the board. I am already aware of polarity on electrolytic capacitors, it's indicated by the grey line running down one side of the capacitor corresponding with the side of the capacitor where the negative pin resides. I would make note of these features before I began.

As far as C6 SMD Ceramic Capacitor being just covered in leakage from the 100uf capacitor and perhaps still functioning fine once cleaned up, that would be a bonus.

I look at it all like this. I could probably fix the ECU myself for $15 or $20. Perhaps even free considering I have hoarded motherboards from all manner of electronics for years now and they are full of those style Capacitors. May just be able to salvage high quality ones if I get lucky and they are of the same Ultra Farad and Voltage rating as the ones I need.

The alternative is spending possibly $200 on sending my ECU away to have it fixed by professionals.

I mean; replacing capacitors and cleaning boards is fairly simple procedure if you have the equipment to do so (which I happen to have). Have other members succeeded in fixing they're ECUs themselves? Even if I didn't succeed in fixing it myself, I could always turn around and send it off to be fixed or go the CHROME ECU way as was suggested just 2 posts ago. I do not know how smart of an idea that would be. Is there a lot of work in converting to chrome ecu?
Sounds like you have tools and knowledge to do the radial capacitor replacements. Knowing that, I’d say just do replacement, clean around C6 and pins for side-boards, then just try ECU to see if it works correctly.

I understand you wanting to do it yourself and could save yourself a lot of money, at worst you’d just have to send it in for repair or replace (as you said) if your repair doesn’t correct it. I’ve fixed my ECU, TCU and Tach by simply doing the same, though I did have to buy a spare used TCU just to see where trace went that was completely burnt away when capacitor burnt up along with burning hole in PCB of my original board. Repaired trace, replaced capacitors in both units, now have one in car and a working spare.

If you don’t have correct capacitors or a electronics store near by where you could purchase them, you could just order set from 3SX for $8.96 that comes with an extra 47 µF that wouldn’t be needed in your TT ECU.

The Chrome units are expensive and not needed if your not going to be doing performance tuning and stock re-manufactured units can be purchased for half the price from some local auto parts stores.
 
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