The fuel system was rerouted and new filter installed.
The problem is still there when warm and it also stalls by itself unless kept at a high idle.

I hooked up fuel pressure guage and vacuum guage.
The fuel pressure is always reading 47-48 while cranking and idling.
When it won't start two squirts of fuel in the TB always works to start it.

I spent some time watching the vacuum guage while idling.
The guage moves from 12-15 Hg (just in the green section) right after you start it warm but as soon as the ECM tries to settle the idle down, the vacuum drops to 5Hg and it stalls.
When I raise the idle to 1000-1100 by adjusting the throttle plate it doesn't
stall, but when the throttle plates are closed and the engine is drops below 900 rpm it stalls everytime once the engine is at operating temp.

I might be having problems with the PROM tune or perhaps a vacuum leak?
The engine rebuilder told me that the cam he chose would affect my vacuum but I submitted a new data sheet to have a new PROM made and it worked for almost a year.
Maybe the 48mm TB is too small?

I really am stuck on the diagnosis and don't know what to check next.
Any suggestions would be appreciated.

Ryan

 

possibly IAC, or TPC. This is only a guess. Use a DVM when the engine is cold and check the voltage ( go to shbox site for reference #s ) then once hot try doing the same again and see if the 3's are different.
GL

 

I removed and used shoebox's method to test the IAC and it needs replacement.
I ordered a new one and will report if that solves the problem.

 

IAC improve cold idle but still no start when warm.
Going to try unplugging MAP when warm and see if it starts.
It still starts warm with 2 squirts of fuel in the TB.
Anything else control the air/fuel when warm?
Hopefully I don't need another PROM tune...
I'll post a list of all the sensor outputs while the engine is running at idle warm.

 

I adjusted the TPS from .36V to .64V at 0% throttle but no change.
Starts cold but doesn't restart when operating temp unless gas sprayed into throttle body.

 

Idle lopes from 750-950 in park
Temp 191 degrees F (fan running after idling 10 minutes)
Voltage 12.5V
IAC 49 (but varies 50-60 as idle lopes)
Learned IAC 32
INJ P.W. 3.5 (both banks)
Spark advance 26 degrees (varies up to 33 as idle lopes)
Fuel Trim 18 cell yes enable
LT Bank 2 142
LT Bank 1 148
TPS 0.64v @ 0% throttle (I adjusted it up from 0.35v)

 

Well I spoke at length with Mark of Street and Performance today and he wants me to install new 24.9lb/hr fuel injectors.
I am also going to remove all header wrap and take digital laser temp readings of each header tube.
From there I can take more scanner readings and determine whether either of my PROM tunes will work or if a 3rd is required.
I haven't given up yet but the hard start when warm condition has been going on since last summer and I'm nearing the end of my patience

 

Injectors have been ordered and in the meantime I remedied a loose PROM chip and removed the header wrap.
I also found a small header / collector flange gasket leak that I am going to replace.

Ryan

 

96 Z28, Stock, 140,000 miles (I like to drive her), have replaced the alternator and radiator prior to issue and that is it.

I start her in the morning she runs perfectly fine as long as she's on. I go to work and she sits there, at the end of the day she starts and runs great. If drive to the gas station and run in for a candy bar I better grab a can of starting fluid or leave her running.

Watched your video on youtube, eerily the same sound. I am only a backyard mechanic and have never delved below the heads and my 71, 79, 86 camaros have all given me my share of hassles, but I am lost. Here is where my thinking took me...

There are different stages of the computers handling fuel air right? (open loop, closed loop, EP?)
Can there be something that doesn't reset to have the correct mixture for starting...that it stays at the mixture that the car needed while it was running?
I get fuel, I get spark, when it's hot I just need starting fluid (easier combustion, or more fuel?) Since Fiero2m8 has sprayed fuel and it worked I think the later.

But my issue is the same and it is a stock, unaltered 96 LT1. I'm bringing her out of winter storage and have saved almost 2k for upgrade/maintenance. I need to solve this issue so the problem can be fixed first, before the goodies go on.
Any advice would be so helpful. Thanks.

Oh, just wanted to say, when she is running I run her hard and you takes it all. I am not an easy driver, but I don't overly abuse her. While running I have had zero issues

 

Time for me to jump in here.
I have the same problem.

Cold motor fires perfectly EVEN if the gage on the rail shows 0 fuel pressure.

Drive the car and get it hot then park it. Try to start it 20-30 minutes after that and it cranks and cranks and spits and sputters. It is just a real struggle to get it to start. There can even be 40# of fuel pressure on the rail according to the gage and its still hard to start as described.

On a hot motor, I can turn the key off/on three times even when the the rail shows 40# and then it will fire perfectly.

My pump does NOT have a rapid bleed off and can actually hold 30# or > for over an hour. My FPR and injectors show no signs of leakage. Car has recently been dyno tuned and the problem has not gotten any better. Car runs like a champ in general. Motor cranks over plenty quick so its not an issue with the battery or starter.

I am going to swap the coil hopefully soon and try that. I think I do have a "Autozone" ICM BTW. Going to look into that too. I have wrapped my fuel lines in heat socks where they come close to the driver side header, that made no improvement. Replaced the fuel filter too. Coolant temp sensor on the water pump is reading normal. Nothing odd looking on the TB sensors.

WTF.

 

Going to test my fuel pressure this weekend and run the code finder on it again. I kind of think the fuel pressure tester will be a waste of time.

I was planning on doing the coil, distro, wires and plugs in the next week or two. I was kinda holding out to see if I could solve the issue, but if it is not fuel it has to be the computer or spark right? If it starts with starting fluid I am hoping that a much hotter spark will be the solution....and if not then I will still be upgrading something. Just hoping that I don't have to start changing sensors in the hopes that I stumble accross the problem.


I wish Fiero would have told posted after he did the injectors.


Did I understand you correctly? If you turn the key on and off a few times, while the car is hot, it'll start fine?

 

Yes thats right even when the rail shows 35# of pressure. Thats just wierd and leads me to believe it is a pressure problem but why the **** would it start perfectly cold with NO pressure on the rail

 

Please remember that pressure in the fuel rail can be caused by either liquid fuel (which can't be compressed and hence transfers any pressure applied to it), or fuel vapor. Next time it won't start when HOT .... take a ball point pen or some other pointed implement and depress the schrader valve at the back of the fuel rail and see if fuel vapor or liquid fuel comes out. If the schrader valve passes gaseous fuel, (i.e. vapor) ...... bleed it down until liquid fuel emerges from it ... and then try starting it.

 

I will DEFINITELY try this. Thanks.

 

When you use the search feature, often the end result isn't posted.

Yesterday I replaced the 8 26lb Accel injectors with 24.9lb stock ones and it fixed the problem.
It idles better, and starts cold quickly just by turning the key.
It also restarts well warm providing the throttle plate is opened slightly during cranking.

Thanks to Mark at Street and Performance who convinced me to spend the money on new injectors.