The car has 124K on it, and it's really only seen a couple hundred miles in the last several years (I suck). But in those last few hundred miles, out of necessity, I'd put on a new Racetronix fuel pump kit, and a new vented opti (with Mitsubishi optical sensor) and the car ran great. But then I let it sit some more, and now it doesn't want to idle when cold and just stalls after running for 12-ish seconds. You can give it gas to keep it running, and once it warms up, it idles fine and runs perfect. When up to temp, it will start up every time and continue to idle. It's clearly running rich until it warms up, because it's puffing smoke and reeks of gas...which ceases when up to temp.

The paperclip trick reveals no stored codes. The SES light does come on when the car stalls. The air filter is clean. It has a tank of fresh 93 octane. I've checked it for vacuum leaks with a carb cleaner cleaner spritzing/visual. I think the car idles/runs too well when hot to have a vacuum leak, but I replaced a few hoses that were starting to dry rot. I changed the coolant temperature sensor (Delco) in the water pump. The plugs were relatively new and looked good, proper gap. I did run a new set of MSD wires over the valve covers, because I saw some arc in the dark. Also, swapped in a new fuel filter (and main fuel line, because it was rust welded to the filter). I checked the injectors for leaks visually, by pulling them and priming the fuel pump. None. All the injectors pulsed when I bench tested them with a transformer. I changed the fuel pressure regulator and the IAC.

Here's a potential point of interest that I'm stuck on: Currently, as stated, the car will just stall after running for 12-ish seconds, but before I changed the fuel pressure regulator and IAC, the car would rough idle when cold and only stall if you tried to move it in gear, then act totally normal once warmed up. I smelled fuel in the vacuum line of the fuel pressure regulator (could that just be from rich cold starts?), so I changed the fuel pressure regulator (Standard Motor Products). Also at this time, I cleaned the throttle body, which was pretty dirty and replaced the gaskets in it. I also cleaned the very dirty IAC pintle with contact cleaner, trying not to get any down in the connecter part. When I started the car, it idled perfectly and you could move it in gear...so, I thought I fixed it.

I went to drive it the next morning, and now instead of idling rough when cold, the car idled fine for 12-ish seconds, then stalls if you don't nurse the throttle. I figured the IAC had some issue after being cleaned, so I replaced it (Delphi), and reset it. The pintle on the replacement wasn't sticking out as far as the stock one, but I've read it self-adjusts. BTW, '93 has a different looking IAC than other 4th gens, it's like a TBI third gen type.

I bought a fuel pressure gauge. On initial prime of the pump, it went up to 41-42 psi, but then dropped to 35 psi a second or two later. Over ten minutes, it slowly bleeds down to 30 psi. So not in spec of holding 41-47 psi with the key on. Since my Racetronix fuel pump only has a few hundred miles on it, and my injectors are all good...I assumed that the Standard Motor Products FPR is junk, and swap it for an AC Delco Gold/Professional FPR. Forgot the exact psi numbers (don't get old) when the Standard Motor Products FPR was on the car idling for the 12-ish seconds, it behaves similar to the Delco numbers below .

The Delco FPR, while the fuel pump audibly primes for 3 seconds, shoots up 44-45 psi, and when the pump stops priming, drops right down to 30 psi. When you crank the car, it's 40 psi, when it starts, it drops to 38 psi, and the shuttering needle of the gauge slowly climbs to 45 psi, running progressively rougher over 12-13 seconds before stalling, the fuel pump is still running after the stall, and then, stops a few seconds later.

I'm stumped. I'd appreciate any thoughts. Thanks in advance.

 

Are you shorting pins A and B? Do you get the code 12 “handshake” indicating the system has entered diagnostic mode?

http://shbox.com/1/dlc2.jpg

Did you add any injector cleaner to the fresh fuel?

The fuel system does not need to hold 41-47 PSI when the pump shuts down. That's the spec for the engine running, vacuum hose off the regulator. It is acceptable for the pressure to drop slowly over a 10 minute period. In addition to a faulty FPR, rapid drop can be due to a faulty check valve in the pump, leaking injector, leaking fuel line, including the flex line in the tank. Normal for the fuel pump to shut down when the ECM loses the cam position pulse signal from the Opti, because the engine stopped turning.

Fuel pressure dropping when the engine starts is normal, that's the vacuum compensation. The fact the pressure gauge stutters represents the gradual and erratic loss of intake manifold vacuum due to the misfires.

Did you follow Shoebox's procedure for installing (adjust pintle to same length as old one), and the reset?

4th Gen LT1 F-body Tech Articles

Random thought - If the long term fuel trims (LTFT) are extremely high, due to an A/F ratio problem (e.g. - false lean?), it might run excessively rich in open loop (cold), but when it enters closed loop (warm) the short term fuel trims (STFT) are then available to pull fuel and also compensate for the elevated LTFT’s. Have you tried pulling power to the ECM to reset fuel trims (pull PCM BAT fuse for 30 seconds)? Won't correct the problem, but if it runs better cold initially, may be pointing to a problem with A/F ratio.

 

Yes, I was trying to flash codes the using shoebox's method, and it kicked the fans on and showed code 12, but that's it, no other codes.

I did not add any injector cleaner to the fuel, but I can pick some up.

Thanks for clarifying the fuel pressure readings. Makes sense. And yeah, the engine is definitely misfiring when I'm trying to keep it running with the gas pedal.

Tomorrow, I'll try and adjust the IAC pintle per shoebox's tutorial and reset it again (this is the method I used to reset the it upon install), and I'll report back.

I just tried your idea of pulling the ECM BAT fuse for 30 seconds. It didn't seem to change anything.

 

I adjusted the pintle on the IAC. It was only about an 1/8" or less off from the old one, which is less than I remember (sometimes I remember wrong), so maybe it does self-adjust. But it did not impact the no start situation.

I'm thinking I should check out the opti. I was reading shoebox's diagnostic for it, and it said a low resolution code doesn't throw an SES code and can cause stumbling. Thoughts? I would say the car runs too well when warmed up to have a bad opti, but that's just been my experience. Every time it's had a failing opti, it didn't run at all (x4 bad optis).

 

DTC 16 for loss of the low resolution does not turn on the SES light. But if it is an active code, the engine will not run. If it were intermittent, it should show on your scan. The high resolution code can cause issues, although the engine is suppose to be able to run without it.

 

I don't know if the SES light comes on anytime the engine stalls? But it does come on when it stalls in its current state, just doesn't show anything stored when I paperclip it right after.

So on the ICM harness, Terminal B shows 0vac when cranking. But how does the car start for for a few seconds? Everything else on the harness checked out.

On the optispark harness, Terminals A & B said 4.7vdc, everything else was checked out.

 

The Shoebox test of the Opti harness just verifies the PCM is supplying 5 volt reference to the Opti. Doesn’t tell if the Opti is working.

When you measured volts to pin “B” on the ICM harness, are you sure you had the correct pin? As Shoebox cautions, the pin arrangement on the schematic diagram does bot match the actual order of the pins in the connector. And you had the connector off the ICM?

 

I had the connector off the ICM and in terminal B, but I actually don't think the meter was on the proper setting. I double checked it today on AC while cranking with 2 different meters, and it had between 1.0 – 2.0vac

 

Progress: I checked out the EGR valve. When I tried pushing on it, it was frozen. Finally, it freed up and I could pop it in and out with my thumb, so I ordered one from Autozone (Duralast) that I haven't installed yet.

I thought I should try and start the car now that the EGR isn't completely stuck. So I did. And it did not stall. It smelled rich, and idles rough when cold...kinda wants to stall sometimes, so occasionally you have to give it a little gas, but it runs. When it warms up, it idles smooth, a little high 1200-1300 rpms, pushing in the EGR with my finger maybe drops it like 200 rpms.

I think 93 has more stuff behind the intake manifold than 94+ LT1s, most notably the crossover line for the fuel rails. I cannot get any kind of wrench/socket on the 1/2" nut for the old EGR that's the further one away from the driver fender, more behind the intake. I've tried several lengths/types of box wrench down to stubby and can't get a bite, and there's also no room to turn a wrench. A short socket on an extension isn't deep enough to go over the stud, and a deep well can't clear the dome of the EGR to get on the nut. I'm thinking about cutting a 1/4" inch drive deep well socket. Any other ideas?

 

Update:

I replaced the EGR valve (Standard Motor Products, the Autozone one was wrong "P", not "N"). This didn't fix it, still wants to stall and you have to throttle it to keep it alive while the car misses and runs terribly. Sometimes, starting it in clear flood mode after it stalls, sometimes once, sometimes a few times, makes it idle pretty smooth even though it's still in open loop.

It could be worth noting that now that it's cold out, (30-40F), the car is harder to start. It takes more starts/clear floods before it smoothes out, if it smoothes out before closed loop. Stumbles way more when giving it gas while it's cold, and trying to get it to idle.

I also changed the TPS sensor (Delco), because I thought it was odd that if I unplugged it when the car was cold and started the car, it would idle smoothly at 1500 rpms and was easy to warm up to closed loop...where it runs fine. If you try to hold the throttle at 1500 rpms with your foot and the TPS plugged in, it does not stay there smoothly. The car stumbles and shakes. I try not to unplug the TPS, because I feel like it throws the PCM a curve, and then I have to unplug the battery to clear the SES light before plugging the TPS back in. Sometimes though, it just won't start and idle now that it's cold.

Often after cold starting, when the car goes into closed loop, it's idling a little high 1200-1400 rpms. If you just restart it, it idles nicely at 1,000 rpms, per the tach,

I've run a few tanks of Techron laced premium through the car, per Injuneer's recommendation. I ran a can of CRC valve cleaner through the intake manifold, and gave the car an Italian tune up.

Using a vacuum pump, I figured out my Evap solenoid didn't hold vacuum, so I replaced it (Duralast). No effect. I inspected the whole Evap system, replaced a few hoses that were starting to dry rot.

I noticed the last bolt for the header on the #8 cylinder was missing. I replaced it. I have copper header gaskets, they look fine of what I can see. I found 4 small holes on the bottom side of my y-pipe, where condensation would build up. The biggest hole was the size of a pencil eraser, so I epoxied them. Not my favorite repair, but until I buy another MAC y-pipe, it works...no effect on the starting condition, but the car idled a little smoother when warmed up (obviously quieter, too and the slight exhaust tick was gone).

I replaced the MAP sensor, because I think the other one was the 124K original, and it was pretty dirty. No effect.

THIS is weird:

The other day, I was tired of reinstalling the Holley air foil into my throttle body whenever I was messing with the intake (all the time), so I left it off. It's designed with a divet in the side to let air into the IAC passage of the TB. When I started the car, instead of wanting to stall, the idle would surge between 500-1500 rpms every few seconds, but it kept running. This continued when the car went into closed loop, and it started backfiring loudly from the exhaust every so often. I tried resetting the IAC, made no difference. So I put the air foil back in, and the car stopped the surging/backfiring, and went back to it's normal way of hard starting/rough idle in closed loop. Air foil? What? Why?

The car runs so well once it's warmed up.

Any thoughts appreciated. Thank you in advance.