Car will not start unless fuel pump unplugged first

2001 viper w/ built engine and paxton. Fuel system is double RSI in-tank (2x walbro 267), external Weldon regulator, 80lb bosch injectors. Car was dyno tuned by Torrie ~2yrs ago (HP Tuners + stock ecu) and has run flawlessly since but today decided to "break".

If I try to start the car it cranks, sometimes sputters like it almost wants to start, but will not start.

If I remove the fuses for both fuel pumps, the car will start immediately, idle for a second, then die.

If I then put the fuses back in (either 1 or both pumps), the car will immediately start, idle fine and drive fine. Going WOT runs like a ***** ape and does not blow up. However, when I stop the car and turn it off, when I try to restart it it will not start unless i remove the fuses for the fuel pump, start it, let it die, then plug the fuses back in. It does not matter if, once it starts up, if I drive the car for 5 minutes, idle it for 5 minutes, or quickly turn it back off .... as soon as i key off it will not restart until I first run it briefly with the fuel pumps disconnected.

Regulator shows 56psi with vacuum hoses disconnected (exactly where I set it years ago) when cranking or idling. When I key on the pumps turn on (audible), regulator goes to 56psi, then back to 0 when the pumps shut off 1-2 seconds later.

It almost feels like the manifold/fuel rail is flooded with fuel, but that doesn't make sense because then the car wouldn't idle or drive fine once it starts, and when I can get it to start in that above sequence of events it does so immediately without any issues (e.g., coughing, hesitation, etc.).

edit: I also can't see it being something in the tune because it's been perfectly fine for 2 years, and if the tune were flooding it with fuel on start, then when I plug the fuel pumps back in it would re-flood.

edit 2: If i leave the car sit for ~2hrs it will start up on the first crank and idle fine. If I turn it off and immediately back on, it will go back to just cranking and coughing, until I unplug the fuel pump, let it fire up and idle for a second to clean the rails out, die, then plug the fuel pump back in and it will start up immediately. I suppose a solution is to put the fuel-pump-on wire on an off/on switch and first start the engine with it off then on, but that still leaves me wondering *** is going on

Thinking ground issue or possibly a vacuum leak?

It idles fine and fuel trims look fine so I don't think it's a vaccum leak. Last times I've had vacuum leaks either the idle was super high and/or short-term fuel trims were really off (like +/-25%). And all hoses are connected and visually look fine

Ground doesn't make sense because the symptoms occur like clockwork in an exact sequence.

It's almost like the intake manifold is getting flooded with fuel when I shut the car off, so it has to get drained out (either by time, starting w/o fuel pump, etc.) before it will work again, but that doesn't make any sense because one would think the car would be running like crap then

Does a gen 2 viper have a clear flood mode? I.e., press the gas pedal to the floor and the injectors won't fire when cranking the car?

If you let the car sit for a few days and you still have to unplug the fuse, chances are the Manifold is NOT gas saturated as it will have dried up by then

Some progress today, I'm just not sure what it means.

Car sat overnight. Started up on first crank, idled fine. I did on/off several times, no issues.

Drove it up/down my driveway then off/on several times, no issues.

I let it warm up to just before the fans turned on, no issues.

However, once it warmed up to the point where the high speed fan turned on, then when I keyed off it would not restart. It would just crank and sputter but no start.

If I floored the gas pedal (i.e., entered flood mode) it immediately started up on first crank. It would idle hang slightly high - ~1,500 rpm - for a second or two, then drop down to 750.

I looked at three plugs and they were fine (i.e., not fouled or smelling of fuel).

So when the car is warm, when starting either I'm getting too much fuel, or not enough air. Given I have no issues when the car is cold, I want to lean towards fuel because I image a cold engine loves lots of fuel and a hot engine doesn't.

I can't see tune because how could it be fine for a year and not act up? But I also can't see mechanical like leaking injector because it idles and drives perfectly fine.

Next time turn to key on/off to run the pumps 2 or 3 times then try to crank it.

Goggles Pizano said:

Next time turn to key on/off to run the pumps 2 or 3 times then try to crank it.

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No difference

So far what I've found is that on cold and semi-cold starts there is no issue (fires up right away).

Once water temperature is around 100*ish or so, then it will just crank and no start. If I floor it while cranking it starts up immediately (initially thought this meant I was in clear flood mode, but I can't find any reference to a Gen 2 Viper having this feature).

I had Torrie adjust the startup tune (less fuel more air) and now when water temperature is 160* or above it starts up fine. However, it would still crank/no starts when temperatures are around 100-150ish unless I give the gas pedal a slight push (i.e., not to the floor, just a little push, like maybe 10% of the way down). Cut back a little bit more fuel and it starts up fine with no pedal action in this range. The amount of fuel cut back isn't abnormal/alarming to him for 80lb injectors.

I don't know what to make of this.

I don't think it is the IAC because it idles perfectly fine.

I don't think it's the fuel pumps because there's 2 of them and my fuel pressure is rock solid when cranking/engine running.

The regulator is not leaking fuel from the boost line (i.e., diagram issue)

It can't be a stuck/leaking injector because it idles/drives fine.

I can't be flooding my engine because the plugs looks fine and it responds to less fuel, either by tune modification or unplugging the fuel pump and cranking it.

This makes no sense - everything is perfectly fine for over a year, then one day it decides it has too much fuel on startup but everything checks out mechanically and nothing in log files suggests any issue.

If you adjust the fuel in the tables that's your problem. Better to run rich then lean. Plus I bet there is a gap in the fuel table at that temperature range.

But then you contradict yourself that if you 10% the throttle it starts but if you adjust the tune and cut back on fuel it starts?

Goggles Pizano said:

If you adjust the fuel in the tables that's your problem. Better to run rich then lean. Plus I bet there is a gap in the fuel table at that temperature range.

But then you contradict yourself that if you 10% the throttle it starts but if you adjust the tune and cut back on fuel it starts?

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I know - it's all very weird.

The initial adjustments Torrie made pulled back startup fuelling a little bit, which meant that instead of having to hold the pedal to the floor to restart it I could just feather it and it would start. He made additional startup fuelling adjustments and now it starts in all water temperature ranges without any pedal action, just like it used to.

I drove the car and STFT are +/- 5%, and LTFT is basically 0. No smoke, pulls hard, smooth idle, absolutely perfect to idle and drive.

So now I'm left with a quandry. It can't be electrical (e.g., bad ground) because electrical does not just impact startup. It can't be mechnical (e.g., leaking injector, dead IAC) because something would have to show up during driving. So how after a year of using my car on the old tune does the car go from starting perfectly all the time to one day decide it's being over-fueled only at startup to the point it will not start other than when the engine is basically cold.....

Unfortunately I've had weird tuning shit like this happen before. I have a built LS3 in one of my cars. Tuned it on the dyno. For 5 years it ran perfectly. Then one day after I started it it started spitting flames and blue smoke out of the exhaust and ran like absolute dogshit. Couldn't find anything mechanically wrong. Put it on the dyno and the fueling was absolute garabage. We re-tuned it and immediately the engine started working properly again and has been issue free for the the last 10 years. Nobody can explain that one - literally no changes were made to the car for 5yrs and overnight it went from being perfectly fine to the point I was certain the engine was finished, and the culprit was that its fueling requirement somehow decided to change itself.

Corrupted tune file.

Goggles Pizano said:

Corrupted tune file.

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then re-loading it would have fixed the issue (it didn't). With my new revised tune (reduced cranking fuel) cars starts up perfectly. If I re-load my original dyno tune from a year ago, it goes back to refusing to start. Re-load my new revised tune and it starts up perfectly.

Battery and/or battery related connections.

Crank fueling is both voltage offset and temperature based, and base offset reference and fueling are also pressure based, both fuel and atmospheric. Cranking voltage changes as battery ages. Very few tuners have the ability or knowledge base to properly set up voltage offsets. G3's tend to be very, very finicky on this because of how they opted to set up the voltage offsets and base tables. The fact you have 80lb injectors immediately tells me this is a possibility as proper information on these injectors is limited at best.

You *fixed* the issue, but chances are next time you change the battery out, it will create issues in other ranges instead.

*this* is why if anyone contacts me for tuning work, I wont take the job unless its based on stock injectors or exactly what type I tell them to install as I have the proper data.

Fuel pressure is only half the battle. Fuel volume is also part of it. Do you have an inline filter? Has it been replaced?
Also, the symptoms sound very much like vapor lock in the fuel rails. Once the engine is up to temp, the rails are getting filled with vapor instead of fuel and acts like an air pocket, which won’t burn. Killing the pumps and cranking till it starts allows pressure in the rails to lower and will allow actual liquid to get to the rail. Then kicking the pumps back on allows liquid to continue.
If the startup injector pw wasn’t edited for the 80lb injectors, that would have a major effect as well. Most tuners don’t edit those and especially when going to e85, you end up WAYYY rich at startup (ie, have to open the throttle a bit to correct the mixture ratio).

Viper Specialty said:

Battery and/or battery related connections.

Crank fueling is both voltage offset and temperature based, and base offset reference and fueling are also pressure based, both fuel and atmospheric. Cranking voltage changes as battery ages. Very few tuners have the ability or knowledge base to properly set up voltage offsets. G3's tend to be very, very finicky on this because of how they opted to set up the voltage offsets and base tables. The fact you have 80lb injectors immediately tells me this is a possibility as proper information on these injectors is limited at best.

You *fixed* the issue, but chances are next time you change the battery out, it will create issues in other ranges instead.

*this* is why if anyone contacts me for tuning work, I wont take the job unless its based on stock injectors or exactly what type I tell them to install as I have the proper data.

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Battery is only a year old (although since batteries are crap today I guess that doesn't mean much) and cranking/operating voltages seem as they've always been, but I will keep that in mind if I put in a new battery and it goes wonky

4bangerviperjr said:

Fuel pressure is only half the battle. Fuel volume is also part of it. Do you have an inline filter? Has it been replaced?
Also, the symptoms sound very much like vapor lock in the fuel rails. Once the engine is up to temp, the rails are getting filled with vapor instead of fuel and acts like an air pocket, which won’t burn. Killing the pumps and cranking till it starts allows pressure in the rails to lower and will allow actual liquid to get to the rail. Then kicking the pumps back on allows liquid to continue.
If the startup injector pw wasn’t edited for the 80lb injectors, that would have a major effect as well. Most tuners don’t edit those and especially when going to e85, you end up WAYYY rich at startup (ie, have to open the throttle a bit to correct the mixture ratio).

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in-tank Walbro fuel socks and inline 10 micron Aeromotive filter. I cleaned out the tank really well before doing the fuel system. 94 octane (0% ethanol content, I check religiously since i hate ethanol since a lot of my cars are aluminum fuel tanks and/or carb'd).

Not sure if startup injector pw was originally modified or not. It was in this pass. But it seems really weird that if it wasn't originally modified, it would work fine for a year and then decide to become problematic.

I thought the 2001 year had the wrong connecting rods for boost.

efnfast said:

Battery is only a year old (although since batteries are crap today I guess that doesn't mean much) and cranking/operating voltages seem as they've always been, but I will keep that in mind if I put in a new battery and it goes wonky

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All it takes is once letting the voltage drop too far and sit over the winter, and the voltage drop-off during cranking is going to be completely different than it was before. Without a high-end controller or specialized logging equipment, you are never going to "see" what its actually doing at the controller reference. The same can be said for battery connections. If anything between the battery and PCM is loose, loose grounds, etc... the conditions are entirely different.

efnfast said:

Not sure if startup injector pw was originally modified or not. It was in this pass. But it seems really weird that if it wasn't originally modified, it would work fine for a year and then decide to become problematic.

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It was definitely modified to even have a prayer of starting on 80's originally without flooding out.

There could be a million reasons things changed. One of a dozen sensors that is aging and shifting its output such as TPS or MAP, any number of battery related supply connections, grounds, etc.

Without having the original file in hand or being the original tuner knowing what was done, and knowing what has been checked/done to the car history wise, I wouldnt want to speculate beyond this point.

ACRric said:

I thought the 2001 year had the wrong connecting rods for boost.

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Built engine

Viper Specialty said:

There could be a million reasons things changed. One of a dozen sensors that is aging and shifting its output such as TPS or MAP, any number of battery related supply connections, grounds, etc.

Without having the original file in hand or being the original tuner knowing what was done, and knowing what has been checked/done to the car history wise, I wouldnt want to speculate beyond this point.

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I don't blame ya ... remote diagnosis is never fun ... now that it's working again all I can do is wait and watch for anything funny. But if something mechanical changed, you'd think there would be symptoms at either cold start, idle, cruise or WOT .... so weird that everything was working fine except warm restart.

I would venture the warm startup prime shot is too large. But if I recall correctly there is no prime shot above a certain coolant temperature. The injector fueling rate may have drifted over time and small pulsewidths may now be too large or they are leaking slightly. It certainly appears to be a rich fueling condition when hot that is the issue.