Car suddenly dies while driving

I was driving my car to work the other day (about 35-38 miles) and about 1 mile from work my car suddenly died. I was able to get the car to work by letting it sit for a while then restarting it, but every time after some period it would die. It appears to be a heat related issue where something is getting hot and letting it cool down allows the car to be driven for a little bit. When it's running it runs perfectly fine with no issue and when it dies there's no hesitation or anything it just dies flat on it's face.

The car is a 1991 stealth RT/TT with very few mods. I've deleted the clutch booster, replaced the BOV with an evo 9 BOV, replaced the fuel pump with a walboro 255 (I've put about 6000 miles on the car with this fuel pump with absolutely no issues until now), and I removed the relay for the fuel pump and put a jumper wire from the input to the pump output so it's always seeing 12 volts. The car had about 60,000 miles on it when I bought it back in 2018 and it now has about 68k. The first thing I did when I got it was the 60k service and drained the tank and put a new pump/fuel filter in it along with a few other maintenance items like spark plugs, ECU caps, rebuilt the throttle body with MIL spec throttle body shaft seals and all new gaskets, new ISC, etc. I also have a few other mods like a turbo timer, an aftermarket radio, and I may be forgetting a thing or two. The exhaust, intake, and other power mods are all pretty much stock.

I just so happened to have purchased a chromed ecu recently and thought I'd give that a shot to see if it fixes the issue. After installing the chromed ECU today it did not fix the issue. I'm fairly certain something has broken, and if I had to guess I'd say it's something electronic as I would expect a fuel issue to result in bucking/hesitation however this dies immediately as if I've shut the engine off even if I leave it in gear and try to coast. Every time the car dies it will not start back up if I try immediately. I have to wait for a while and then try again and it will start up as if nothing happened and then drive for a while before dying again. It is fairly repeatable it seems as even letting it idle in the driveway results in it dying after a period of time. The gas tank is a little low (~1/8) but again I would suspect a fuel issue would result in bucking/hesitation and not die as suddenly as it does.

Any help is appreciated, thanks.

I'm going to bet it's a spark issue. Instead of guessing you need to figure out what's missing when it won't start. Fuel, spark, air, timing (both ignition and cam) are required fir the engine to run. If one is missing or off, then it won't run.

Hey man, thanks for responding. Since this is an intermittent issue and I feel like it's pretty well bounded I was hoping to have some more concrete test plans whipped up before I carve out time to work on the car again. It's pretty rare that I have spare time to get to stuff like this anymore so planning things out goes a long way.

For example, it seems like the PTU failures are more common on this platform than other mitsubishis of the era, and that would line up with the vehicle suddenly dying as opposed to a spark plug/plug wire/coil since I would expect the vehicle to hesitate a lot if one of those were to fail. Is there something I can easily check on the chromed ecu to evaluate the health of the PTU when it won't start? I've seen the write up on stealth316's page but my concern is by the time I get all the pipes and stuff removed to get access to the PTU it will have cooled down again and will be functioning properly.

Another component that is common to all cylinders is the CPS/CAS. I believe this may be more accessible than the PTU and therefore more feasible for me to put a DMM to it while the vehicle is not functioning, is that correct? Have others seen failures like this in the past with this component where the vehicle works intermittently?

I don't think it would be an injector/fuel pump issue either as again the symptoms don't align with those components failing (losing only a single cylinder or slowly loosing fuel pressure).

Is there an easy check for the MPI relay to see if it is functioning? I know that is more accessible than the PTU. Is the MPI relay a shared part with other mitsubishis of the era? I have a ton of DSM parts laying around I could swap in for diagnostics. I could also whip up a relay to replace the MPI to see if that sorts things out.

Are there any other components which would result in the vehicle electrically shutting off suddenly?

It's not an air issue, it's either fuel or spark and it's as if someone is flipping a switch. The first time it happened I was on the freeway (basically one of the busiest interchanges in all of america) and after waiting a minute or two and trying to get it to fire again it fired right up and I was able to immediately put full power down to get back up to speed and join traffic so I could merge over and exit. So when it works it works 100% and when it doesn't it's completely dead.

Alright, I've made some progress on this issue and I'm now back to the drawing board a bit. Here are some updates, if anyone could help that would be greatly appreciated.

ECU replaced with chrome ecu (already mentioned) - no change
Coil pack and PTU replaced (it was on sale wee!) - no change

Last weekend while trying to datalog the event of the car dying I realized the walbro fuel pump I installed was audibly turning off a half second to a second before the car was shutting down, which clearly is the problem. Either the pump is dying or something is cutting power to the pump which is causing the car to die.

Today I decided to remove the pump to inspect it. I jacked up the car and disconnected the fuel line and removed the pump, I mention this because the gas in the tank shifts forward when you do this. It appeared to be fine (pre filter isn't clogged or anything, no rust or disconnected electrical connections) and I put it back in the car. I then went to measure voltage at the pump and see what cut first, the voltage to the pump or the noise from the pump. I didn't lower the car back onto the ground and strangely enough the pump never died. So either submerging the pump in more gas or removing it and putting it back together appears to have fixed it, although I haven't actually driven the car any distance to verify this. I'm now very suspect of the pump (I've never liked walbros) and trying to decide what best to do moving forward.

Some additional background information. My car had not been driven since 2001 when I bought it in early 2018. I drained all the fuel and replaced it but the factory pump was shot from the bad gas. This is why my car has a walbro in it in the first place. At the time I debated pulling the tank and cleaning it however decided against it. I'm now debating that again. The car has driven about 6-8k miles since I got it in 2018 including some fairly long trips up and down the coast of california. The only other "fuel mod" the car has is the removal of the relay and resistor and rewiring the relay such that the "high voltage" signal is always sent. So I did not "rewire" my fuel pump, just eliminated the relay which kicks voltage down at low engine speeds to ~10 volts. I am on stock injectors, stock boost, etc. I also replaced the fuel filter with a new OEM fuel filter back in 2018.

Would really appreciate some help on this. As you can tell I don't get too much time to work on this thing as I've got too many project cars and not enough time in the day so any advice is appreciated. I'll probably start driving it around on short trips to see if the problem comes up again and then get the tank down as low as I can so I can pull it and clean it more easily before swapping the pump to something else, like perhaps an evo 8 denso pump or an MK4 supra TT pump. I'm still on the stock fpr but have chrome now so I could tune out the rich conditions at low speed/cold when the FPR is getting overrun.

Finally got around to pulling the tank, draining all the gas out, thoroughly cleaning it and putting it back in. While going through that process I found the connector for the walbro fuel pump had melted to the pump on the positive side, but only slightly. So for some reason the fuel pump was drawing an unusually high current. I got a new denso supra MKIV pump to replace it, and am waiting on a new fuel strainer to go with it. The fuel strainer 3sx recommends is not the correct one by the way. I expect the car to be up and running again once I get the new fuel pump in and some gas in it, but I'm debating if I should replace the main fuel filter at this point in time as well. The fuel tank was coated in a light layer of white film stuff, but it didn't seem crazy bad. It seems like this is commonly from ethanol fuels that get water in them. The tank definitely had water in it when I bought the car in 2018 and it killed two pumps in quick succession, but I drained the tank at that time and the car ran for 5 years without issue until now.

Will update once the issue is fully resolved to confirm for anyone in the future with the same issue.

Installing a new fuel pump seemed to have worked. I put about 500 miles on the car this weekend with no issues. The Denso fuel pump is INFINITELY quiter than the walbro and I will never consider buying another walbro ever again. Also, I struggled to find a new fuel pump sock for the supra pump (same as the factory one.) The delphi unit recommended by 3sx (FS0093) does not fit, nor does the denso one listed for the supra (952-0006). The airtex unit listed by 3sx does work (FS132) as does F233S from autobest (what I ended up installing). If you want a denso strainer I believe 952-0036 would work (I have one and could test fit if someone needs it).