I'm completely stumped with what's happening here, My 91 formula 305 tpi (MAP) bone stock runs amazing idles perfect etc. But seemingly at completely random times, my car will sputter and struggle to idle (almost like a misfire) at about 300-200 rpm till it bogs down and dies. And I could crank it forever and it wont start. But if I hold down the gas pedal/ open the throttle blades, it wall start after a few seconds of cranking and will stall the same way unless I keep my foot on the gas for about 30 seconds till it idles and runs completely normal. It will not stall while driving in gear, (manual) and has great power but if I put in the clutch after a hard pull sometimes it will trigger a stall. it also never stalls while idling, only if I'm moving it and manipulating the gas like when parking or moving it around a lot with my foot in the clutch a lot. Also Had Diagnostics pulled on it with a snap on scan tool and everything looked perfect at idle, fp, afr, idle rpm etc. I replaced the tps and map sensor with ac delco replacements and I've completely eliminated the possibility of the tps being bad because now it idles perfect, good ground signal, resistance etc. also lots of other stuff replaced: IAC, ICM, distributer cap and rotor. any Idea what could cause this? 99% of the time the car runs perfect and never has any issues and the stall condition seems to be completely random and happens when the car is stone cold and when hot. I can answer basically anything that I might have forgot, Thankyou for any responses!

 

I'd be curious to see what the fuel pressure looked like when it's acting up.

 

see that's the issue, During idle the fuel pressure is perfect, but i cannot replicate the stall, i would have to drive around with a live scan tool basically plugged in 24/7. or a Fuel pressure gauge permanently lined in on my windshield.

 

This indicates engine is flooded. When TPS > 87.5% while cranking, it's called "clear flood mode". The flooded condition is cleared in a few seconds by providing no cranking fuel.

 

Leaking fpr or injectors

 

Hmm, cause when I crank it I have my foot ever so slightly pushed on the throttle. It's maybe less than 25% open. And I don't know how it would flood in the first place.

 

see, this is why im stumped! Fuel pressure was fine at idle and it has a brand new south bay fpr with new rubbers and everything as well as 19lb bosch fuel injectors from southbay. they are most definitely not leaking.

 

Does it die on rapid deceleration?
your description of putting your foot down to open throttle thought it was clear flood mode but it’s not.

 

Could this be a timing issue? Is there any factor that could change my timing intermittently or a bad pickup coil or something?

 

by rapid deceleration do you mean engine rpm? Cause yes it happens occasionally after winding it out to high rpm when driving and then putting the clutch in or neutral. If its in gear and I'm on the throttle i usually dont notice anything. That could be why it happens when I'm moving the car and I Rev it up and it stalls. But most of the time I can get done doing a lil' brake stand and put it in neutral and it will run just fine.

 

Had a similar issue....

My fuel tank vent valve was not operating correctly.

It would create a huge vacuum and would cause fuel starvation,....then sometimes shortly after it stalled... it would let air in and allow fuel to flow and the car would run fine

When it stalls, try open the gas cap and see if its under heavy vacuum / suction.

 

I was thinking dispite the new iac a issue there. Can you catalog the event? Or like mentioned vent issues

 

There was somebody on here who was having problems specifically with a new southbay fpr.

Also fyi you shouldn't need to use the throttle to start a fuel injected car, that's what the iac is for.

 

Ill definitely try that next time, and is the fuel tank vent valve located on the pump?

 

hmm, not sure how i would without a scan tool plugged in 24/7. also would a bad replacement Iac cause an issue? like a non ac delco factory part? I searched for months and it turned out my tps sensor just was not of the quality of an original part and it fixed my issue. and I'd need some clarification for the iac valve, most instructions say it can't stick out a certain length when installed, and only recently I heard that the valve must be pushed all the way shut before installing so the computer knows its at zero. which one is the correct procedure?

 

Exactly, I know there is not an accelerator pump and I never touch the throttle when starting, and I actually couldn't figure out how to start the car after a stall for months until I tapped the gas and i found that without fail it will restart if I just put my foot on the gas. It would just crank forever and I'd have to wait a day or so before it would start. But it seems that opening the throttle blades lets it start so im thinking there could be an issue with the IAC control.

 

Stupid question, but are you sure TPS/IAC are properly set and that TPS voltage at closed throttle is 0.55v-.0.65v. If OK, I'd say tape the FP gauge to the windshield.
Any chance you could export a .csv log from TunerPro that captures the problem and post it? That will tell a lot. Otherwise, working blind.

 

no stupid questions here! My tps is reading correctly, and since I have a 91 tpi setup it cannot be adjusted. and the IAC is set, although im not sure if the valve is good, im pretty sure its some knock off. But I know that the 85-87 IAC valves are slightly different from the 89-92 valves I just have no idea which one is correct. There is AC Delco part number 217-415 and 217-437. Do you know which one is correct for a 91? And I will try to get tuner Pro working as I've had a lot of issues with it in the past.

 

Don't know about IAC part numbers, but any reference to part numbers should provide proper fitment.

But do know that TPS MUST be adjusted to provide proper voltage at closed throttle. That may require slightly enlarging mounting holes. See: https://www.thirdgen.org/tpimod2/

 

I thought that 90-92 SD third gens have self zeroing tps sensors? Also Mine is reading the correct voltages and such right now I already went through that chasing an erratic idle that I finally solved with the correct tps sensor so I 100% have ruled that out. Why I'm confused about the part numbers is that a few members on this board have had issues with the wrong year IAC and didn't specify the part numbers. and I've tried many websites and even AC Delco's official website doesn't specify which one is for my car as it brings up those 2 different part numbers for my 91 bird.

 

As others have noted I would do some troubleshooting on the fuel system - specifically the fuel pump and tank vent. (hangs down from top of tank near rear driver side spring) .You CAN drive around with a fuel pressure gauge taped to your windshield if you really need to to check this.

 

I'll try using a fp gauge on the windshield, ive been through 3 gauges and the all puked fuel out the top so I'll have to a buy a quality one lol. And I'll inspect that vent too. I'll report back soon!

 

mmm, kinda..... the, as it's called, "UFO vent valve" is attached by a rubber hose to the sending unit. Its on the driver side. kinda right in front of the wheel. Its a plastic valve that attaches to a metal bracket on the body. It lets air into the tank as fuel is sucked out. You can;t miss it!

Mine would prevent air from getting in the tank and cause an intense vacuum.....eventually it would prevent the fuel pump from being able to pump fuel and the car wou;dn;t start until I guess the valve unstuck itself or I went and manually released the vacuum by opening the gas cap............it was so strong you could hear the tank "pop" when air was allowed in and the rush of air when I released the cap was massive..... I did the rollover valve mod butHawks sells reproductions of them if needed.