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93 NA ATX 3000gt DOHC
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Grabbing at straws now, but since it stalls immediately after start that implies during open loop operation. One thing hits me that’s not on you list of things checked. Even though you mentioned “Coolant sensor reads a reasonable temp”, that sounds like the one for gauge display (single pin sensor) and not the one for ECU control, which is (two pin connector sensor) located next to one for gauge in coolant manifold. If that one’s resistance is off a large amount it could cause over or under fueling as soon as engine is started.

Below is bench test procedure for that sensor, but without going thru all that you could just unplugged harness connector and take ohm reading between two sensor pins, with engine at ambient temperature to see if it reads between 2.4Ω for 68°F or ~ 1.5Ω for 78°F or something calculated closer to your location. Basically if its in area of being correct at ambient engine temperature it wouldn't cause your issue at startup, but if off the chart it could make ECU cause fueling problem.
 

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Discussion Starter · #22 ·
Grabbing at straws now, but since it stalls immediately after start that implies during open loop operation. One thing hits me that’s not on you list of things checked. Even though you mentioned “Coolant sensor reads a reasonable temp”, that sounds like the one for gauge display (single pin sensor) and not the one for ECU control, which is (two pin connector sensor) located next to one for gauge in coolant manifold. If that one’s resistance is off a large amount it could cause over or under fueling as soon as engine is started.

Below is bench test procedure for that sensor, but without going thru all that you could just unplugged harness connector and take ohm reading between two sensor pins, with engine at ambient temperature to see if it reads between 2.4Ω for 68°F or ~ 1.5Ω for 78°F or something calculated closer to your location. Basically if its in area of being correct at ambient engine temperature it wouldn't cause your issue at startup, but if off the chart it could make ECU cause fueling problem.
View attachment 303407
I have checked it a few weeks ago now and it read pretty close to ambient temp, I don't remember what it read but I will definitely check it again. I'm wondering since I replaced both upstream o2 sensors if one of them is giving a funky reading but not enough to set a code. I'll check tomorrow once I'm back at the shop and post the finding. I found an injector reseal kit from another forum and I might pull and test each one.
 

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I have checked it a few weeks ago now and it read pretty close to ambient temp, I don't remember what it read but I will definitely check it again. I'm wondering since I replaced both upstream o2 sensors if one of them is giving a funky reading but not enough to set a code. I'll check tomorrow once I'm back at the shop and post the finding. I found an injector reseal kit from another forum and I might pull and test each one.
O2 sensors are not even used by ECU when in open loop operation right after start, takes a little time (not sure how long) before it switches to closed loop operation (probably at certain engine temp:unsure:).

Checking injectors is a logically step and so would monitoring fuel pressure (if gauge installed) at startup, when vacuum comes up reducing fuel pressure
 

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Discussion Starter · #24 ·
O2 sensors are not even used by ECU when in open loop operation right after start, takes a little time (not sure how long) before it switches to closed loop operation (probably at certain engine temp:unsure:).

Checking injectors is a logically step and so would monitoring fuel pressure (if gauge installed) at startup, when vacuum comes up reducing fuel pressure
I'll hook up a vacuum pump to the fpr and see if I can induce an appropriate vacuum to weed out any oddities in the vac system
 

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I'll hook up a vacuum pump to the fpr and see if I can induce an appropriate vacuum to weed out any oddities in the vac system
Main thing is when engine vacuum is high after start that the fuel pressure doesn’t drop below about 35 psi, which is recommended for turbo engine at idle. It will increase as vacuum goes down during acceleration and boost. So I’m not sure what vacuum pump test of FPR will show with a constant vacuum.
 

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I found an injector reseal kit from another forum and I might pull and test each one.
O2 sensors are not even used by ECU when in open loop operation right after start,
Got thinking about injectors and ECU. It appears you’re still using stock non-tunable ECU and after market FPR, but not sure if you’re still using the stock 360cc injectors. If you’re running larger injectors without a way to tune them for different rpm ranges, that could be your problem as I doubt the stock ECU could control them properly.
 

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Discussion Starter · #27 ·
Got thinking about injectors and ECU. It appears you’re still using stock non-tunable ECU and after market FPR, but not sure if you’re still using the stock 360cc injectors. If you’re running larger injectors without a way to tune them for different rpm ranges, that could be your problem as I doubt the stock ECU could control them properly.
I'm still using stock fpr and injectors. Only after market mods is the radiator and crank pulley on this one. I didn't specify earlier my bad
 

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Discussion Starter · #28 ·
Grabbing at straws now, but since it stalls immediately after start that implies during open loop operation. One thing hits me that’s not on you list of things checked. Even though you mentioned “Coolant sensor reads a reasonable temp”, that sounds like the one for gauge display (single pin sensor) and not the one for ECU control, which is (two pin connector sensor) located next to one for gauge in coolant manifold. If that one’s resistance is off a large amount it could cause over or under fueling as soon as engine is started.

Below is bench test procedure for that sensor, but without going thru all that you could just unplugged harness connector and take ohm reading between two sensor pins, with engine at ambient temperature to see if it reads between 2.4Ω for 68°F or ~ 1.5Ω for 78°F or something calculated closer to your location. Basically if its in area of being correct at ambient engine temperature it wouldn't cause your issue at startup, but if off the chart it could make ECU cause fueling problem.
View attachment 303407
Coolant temp reads about 75F which is fair with ambient temp.
 

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Coolant temp reads about 75F which is fair with ambient temp.
o_O :unsure:……. How does that relate to ohm reading across pins of ECU temp sensor? Does that mean you converted ohm reading into 75°F, like you might have gotten ~ 1.6Ω reading?

I gave you a calculated ohm reading of ~ 1.5 Ω at 78°F, the ECU doesn’t read temperature in degrees, it simply converts voltage reading from sensor (controlled by sensor resistance) into an injector control pulse ground signal.
 

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Discussion Starter · #30 ·
o_O :unsure:……. How does that relate to ohm reading across pins of ECU temp sensor? Does that mean you converted ohm reading into 75°F, like you might have gotten ~ 1.6Ω reading?

I gave you a calculated ohm reading of ~ 1.5 Ω at 78°F, the ECU doesn’t read temperature in degrees, it simply converts voltage reading from sensor (controlled by sensor resistance) into an injector control pulse ground signal.
Right sorry, it read 2.6kΩ= around 75 if 2.4kΩ was 68F
 

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Looks like 2.6KΩ would be ~ 65°F, I honestly don't know if that much difference from ambient temp would cause your problem but I don't suspect that would be enough to cause engine to stall, however ECU getting a colder conversion point would cause over fueling.

Edit: In any case that reading is not off the chart as I was wondering, so I’m fairly comfortable in saying that’s not your problem.
 

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From what I recall, when the that temp sensor goes bonkers, it REALLY goes bonkers.
You obviously have spark (or it wouldn't even start at all).
The only other components are COMPRESSION and FUEL......

Either is sparking at the wrong time or your injector(s) are not supplying enough fuel
(Captain OBVIOUS)
I think you said stock injectors, are they original?

Bob.
 
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Discussion Starter · #36 ·
From what I recall, when the that temp sensor goes bonkers, it REALLY goes bonkers.
You obviously have spark (or it wouldn't even start at all).
The only other components are COMPRESSION and FUEL......

Either is sparking at the wrong time or your injector(s) are not supplying enough fuel
(Captain OBVIOUS)
I think you said stock injectors, are they original?

Bob.
Correct they are stock injectors. I have cleaned out the tank and replaced the other fuel system componenets prior to attempting to start it the first time so I don't suspect they are clogged. Each time I took off the fuel supply line clean fuel came out without chunks or sediment.
 

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Let me share this with you....


Not the same issue, but it was fuel related...

Damn, the things you do for your kids.....

Bob.
 
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@Constantmeme ,The engine starting and immediately stalling has always been on my mind as to what could cause that. One thing that my thoughts keep retuning too, is that ECU starts fuel pump when ignition is turned to start position and if it doesn’t receive run signal after key released to ON position, ECU will stop pump after few seconds if no run signal is received. This is only true if some pump hot wire is not installed bypassing the MFI relay.

You didn’t say if you have a fuel pressure gauge installed, but if you do you could watch it when engine is started for pressure being lost few seconds after engine start and for engine stalling shortly after pressure drops. This goes along with me asking about fuel pressure staying at/above 34-35 psi at idle.

Only reason I haven’t brought this up prior is that ECU uses crank sensor for the run signal, and since engine wouldn’t even start if it wasn’t receiving crank sensor signal I couldn’t imagine how it could start and then crank signal be lost. :unsure:

Edit: Wait..... looking back I do see you have fuel pressure gauge, but concerned about you saying dead headed, which implies pressure not being controlled by FPR and gauge maxed out. :unsure:
 

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Hope it’s ok to throw my 2 cents in here. I have been interested in this thread since day 1.

The MFI relay has 2 coils. 1 for fuel pump other for ignition-sensors.

Have you checked and ensure that the voltage at pin 2 or 3 AND pin 1 ( or the fuel pump test connector ) is at least 10volts ? I say that because if it starts and runs at 300 rpm you will never get a correct charging voltage there’s but it needs to be enough to hold in the relay.

My inclination is that if the engine is only getting to 300 RPM that after you let go of the starter switch the ECM shuts off the MFI relay because it does not see that the engine is turning over fast enough.


Also are you positive that no damage occurred To the flag ( reluctor/ steel disc ) on cam # 4 that the camshaft position sensor uses and are you positive it is in the correct position so that the engine knows number 1 TDC ? meaning it didn’t get played with.

P
 

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It will start and stall immediately, and won't start unless I unplug the cam+crank sensor, turn it over for 10 seconds, then plug in the sensors before starting again.

Fuel pump hotwire test connector gave constant fuel supply but did not change idle. Fuel pressure gauge deadheaded on feed line tested okay
Rereading these two comments I’m starting to suspect a problem with fuel pressure gauge insulation, which might be causing excessive fuel pressure and flooding engine. Being that unplugging Cam+crank sensor, turn it over for 10 seconds, then plug in the sensors before starting again.

When you hotwire pump test connector what pressure do you get with engine off? Should be 43-45 psi. Then without connector hotwired and when engine starts it should fall to 34 psi, at least until engine starts to stall and then slowly increase back towards max of 43-45 psi until ECU shuts it off. But at no point should it be above the 45 psi max reading.
 
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