40 seconds then dead

[Mciamp99]

Hello Again everyone

for those who have folowed my trials with the LT-4 transplant to my wifes 75 vette, I am glad to say it is finally running. I am sorry to say it acts like her husband. That is it runs great for 40 seconds, then dies out(as if the fuel has stopped). Unlike her Husband, the car lights right back up.

Now I know 40 seconds is better than the 0 seconds we had before, but not enough to get me out of the driveway. my scanner show something I think interesting and that is the Fuel Trim Learn is disabled. I am also showing a code for a skip shift but expected some codes like that when I started this.

I had heard of a friends LT-1 doing similar things this summer and we changed the Ignition Module and the Fuel Regulator and that cleared it up. I am going to try that tommorrow. I have plenty of Fuel Pressure at the head and it never dies off. There is plenty of spark, that is not stopping either. It definately seems like a fuel delivery problem.

Has anyone experienced something like this and if so How did you solve it? the Opti Spark is new. Could this also be a computer problem or rlated the the Fuel Trim Disabled Feature?

I am running with the MAF system as it would be in the 96. I have programmed out the AIR Purge and EGR. Any suggestions would be appreciated.

 

[rrubel]

I had an identical problem in my wife's Legacy this weekend after playing with the intake. Turns out a return hose had popped off on the engine side of the MAF, allowing more air to enter the intake than the MAF was accounting for. After the initial start and idle, it would go to adjust itself and stall. Took about 30 seconds to happen.

So maybe air is getting in to the plenum after the MAF, or the MAF isn't hooked up right.

[RICHR]

 

[Vettelt193]

is it 40 seconds no matter what? can you rev up the car and keep it running, or will it still die? If it always dies, I wonder if the computer senses something drastically wrong and is killing the engine to protect it?

 

[Mciamp99]

I dont know what we can do for the 40 seconds, I am taking this on the word of the man who finally got the car running. I am thinking about the computer as well. I am wondering if something is off that should be on or vice versa. I am ramping the car back to my house today and plan on hooking up a scanner and seeing exactly what is happening. I also have the LT-1 Edit program and plan on taking a good look at the current settings. I will let you know how I make out.

 

[Vettelt193]

I was just trying to isolate the problem... If it stays running if you get on the gas a bit, then your problem is more likely idle related... if it shuts down though, while revving to say, 2000 or 2500 rpm... the computer is shutting you down

 

[Mciamp99]

ok it will take the throttle, but it seems the gas runs out. I am wondering if the return line might be clogged. I was also wondering if the fuel tank needed to presurize . I was pretty sure I checked the return line before the body went back on the vehicle, but it may have become kinked or pinched when the body was dropped. It is a full length braided line. I will have more info tonight.

 

[Vettelt193]

so it doesn't just shut down, it slowly stalls out? you can tell it is a fuel problem then because if you rev it up, it should stall out faster.... I would understand fuel would be a problem if you lost fuel pressure... then the fuel pump would be acting up (or only pumping during pre-start, and not while the engine is actually running)... what really makes no sense is it starts right back up... even if you had a fuel problem, it would at least crank until it got gas again... let me know if it stalls slowly, or just cuts off

 

[BlackNBlue95]

We had an interesting one like that.. it was an 85 w/tpi and it was a no start condition if you sprayed a little starting fluid it would run for a bit then die.. we found the fuel to be contaminated with water the drain in back was clogged and the rain water got into the tank.. but what is really important is if it won't stay running even with the foot part on the gas then you either have a fuel or iginition system issue and the computer could be cutting these out for some odd reason like a sensor not hooked up or something..

 

[Mciamp99]

ok here is the hole scoop as I know it.

The motor starts and runs for 10 to 15 seconds, then dies. It will start again with out cycling the key and run for 8 to 10 seconds then die out. It will start again with out cycling the key and run for 4-6 seconds. Then it will just fire and shut off. If I cycle the key it will run the longest.

Fuel Pressure at the head is still 45 psi at the time it cuts out. with a noid light attached, you see the injector stop working then the motor starves. Basically for some reasons the fuel injectors are being shut off. It is confusing becuase it will light back up so we have ruled out a loose connection or corrosion in the wiring.

We have already changed the Opti, Crank Sensor, Ignition Module and Fuel Regulator. VATS is disabled. What signals could the PCM be geting that would force a shut off, but allow a restart without cycling the key?

I almost believe the PCM or the programming is bad. I wonder if the computer heats up and just stops working. It sounds stupid to say that, but all the harnesses check out, the O2 Sensors are sending the right signals to the scanner and no codes appear. The only thing the scanner shows that concerns me is that the Fuel Trim Learn is disabled throughout this process.

It will take the throttle until the fuel injectors shut off.

On the Bright side, this motor sounds great without the cats and throught he Flowmaster exhuast. Little things excite me at this point. I know this is solvable so I keep smiling, but if I knew I were going to have this much trouble I would have rebuilt the original motor.

 

[BlackNBlue95]

I almost believe the PCM or the programming is bad. I wonder if the computer heats up and just stops working. It sounds stupid to say that, but all the harnesses check out, the O2 Sensors are sending the right signals to the scanner and no codes appear. The only thing the scanner shows that concerns me is that the Fuel Trim Learn is disabled throughout this process.

It will take the throttle until the fuel injectors shut off.

Yea sounds like definitely something PCM related... Who did the programming? I take it you atleast have a scanner.. I'm wondering if fuel trim learn is disabled if it's dyeing because it doesn't know what injector on time it should be at.. i dunno I'm just guessing.. is there anyway for you to enable Fuel Trim Learn.. I know most the time they need to go through and idle relearn once you re connect the battery..

 

[Vettelt193]

could it be a tach problem?? I am not 100% sure what your PCM uses, but if it is getting a faulty tach signal, let's say it thinks it is running at 2000 RPM idle, then it may wind up killing itself... (this theory does not foot too well with revving up and still stalling out, but it is an idea)

I don't think it is a problem with the PCM hardware, more likely the PCM thinks something is wrong and is shutting down the engine, or is trying to correct the way the engine is running from a faulty sensor reading.

 

[dmrodco]

Oil Pressure

I am not sure if you have checked this however the ECM has a subroutine that shuts down the injectores if it does not show oil pressure at the ecm. If your ECM is a ODBII the data is real time and you should be able to monitor it while you run an start it. Good Luck

 

[Vettelt193]

Re: Oil Pressure

I am not sure if you have checked this however the ECM has a subroutine that shuts down the injectores if it does not show oil pressure at the ecm. If your ECM is a ODBII the data is real time and you should be able to monitor it while you run an start it. Good Luck

that is what I was trying to come up with... why I couldn't think of oil pressure is beyond me since it is the most obvious thing! This is definitely the first thing to check

 

[cntrhub]

Hook up a timing light to any one of the spark plug wires. Keep the timing light trigger on and watch the light as the engine shuts off. Did the timing light keep flickering as the engine shut off? Then, it's fuel. When you can crank the engine and there is no start, but the timing light keeps flickering.......then it's still fuel related.
When you can crank the engine and there is no timing light flicker, then it's the ignition.
In theory, this timing light trick will diagnose one or the other.

 

[Mciamp99]

We have already eliminated spark as an issue. I have a tool that allows you to watch the spark and the spark keeps going. The noid light on the other hand is a diferent story. I do think our next step will be to check the Oil switch. The wiring was made to include gauge and computer infor on one sensor. We will go after that next. I will keep you all up to date.

 

[Mciamp99]

I'm Alive!!!!!! 9 Months after the fact I am alive. for all those who want to know, my problem was in the PCM. I finally got my hands on an F-Body Program, Loaded it in and Wa-La, I ran for as long as I wanted. I now can go through and make sure everything works as planned. I already found my next problem but can correct it with a piece of hose and a couple of clamps.

Thank you all for all your help and I am sure I will be back with future problems.
Michael

 

[Mciamp99]

I want to thank all who helped get me going. My project is fianlly on the fast track to completion and I look forward to driving it come spring time. If anyone is interested in seeing pictures of this project I posted at

http://www.corvetteactioncenter.com/forums/showthread.php?s=&threadid=17899&perpage=15&pagenumber=3

It is a C-3 Mod Page in another part of the forum. Once again thank you all.