I have a 79 transam and have used this quarantine time to finish my almost 15 year long project. (yea i know its a long time).

I have a 2004 6.0 LQ9 block with LS1 pulleys/intake. DBW throttle body, painless harness. Stage 1 thumpr comp cam.

HERE IS MY PROBLEM.

the car first started up abut 1 month ago and idled and started perfectly. I have also been working on the ridetech air ride suspension and the vintage air AC set up throughout this time. The car would run and drive fine, i would test drive it around the block and car would run fine but never pushed it too much due to needing a re-program on the computer because of the cam. The longest distance i drove is about 1 mile. I would also let the car idle to build engine temp so i can test my electric fans. no issues there either. I trailed the car to a muffler shop to install the mufflers and O2 sensors about 2 weeks ago and car ran the same with the O2 plugs and exhaust installed. I would regularly plug and unplug my battery to work on wiring for both the air ride and AC system. Car would continue to run and idle fine with no issues.

This week i drove the car about 2 miles (longest i have ever driven it since it fired up) to a local AC shop to evacuate and pressurize my AC system. The car acted normal and i drove it to the shop without issue. Temps/idle were perfect. I pulled into the shop and the car suddenly turned off. after i looked in the engine bay and found nothing out of the ordinary, I found that the negative battery cable in my truck fell off the terminal and thought that was the problem but it was not. The car build fuel pressure at the regulator and fuel system appears to operate like normal, but the car will no longer idle. LIKE AT ALL. I try to turn on and it sounds like the VATS turned back on. Turns on and shuts off immediately about 1 second later. I can get the car to idle rough if i hold the gas pedal about 1/4 of the way, but it is a very rough idle. it is no longer drivable.

I have no idea where to start looking as the car ran fine up to this point. any thoughts or suggestions will be appreciated.

 

video of startup

 

attempting to idle with 1/4 throttle

 

Try feathering for a few minutes. See if it gets happy.

 

Sounds like an issue I had.
disconnect the MAF sensor and retry.
if that don't work try disconnecting the battery for a few minutes

 

tried feathering it for a while and also disconnected the battery overnight and left it on a trickle charger. Still no luck. I need to check the MAF.

 

I think you are somehow losing "keep alive" 12 volt power to the PCM. This same problem occurs with my 70 LS swapped El Camino anytime I disconnect the battery. Basically, without full time 12 volt keep alive power, the PCM forgets its learned parameters and has to relearn what it takes to hold an idle. It does not matter whether I have a 0411 PCM installed or an 896 PCM, it'll struggle to hold an idle for a few minutes.

But the good news is that these PCM devices are quick learners and they even adapt well to the changing engine load caused by an AC compressor.

Rick

 

Check the easy stuff first. If its not been tuned yet, check your plugs and make sure they're not gas fouled.

 

i appreciate all the suggestions. I unplugged my MAF sensor and IAT sensor (my harness has the old two plug style) and it was able to idle for a long period of time. Should I clean both plugs that are new? Or replace them? Should i drive around to program the computer without the sensors?

I havent gone in for a tune, but will do that next week.

 

There is probably something wrong with the MAF sensor (like my issue)
plug the IAT sensor back in. The engine will run on "speed density" without the MAF sensor.
are you using the MAF sensor that came with the donor engine

 

No i picked up a brand new AC delco part. I maybe over oiled my K&N and put it in when it was still wet and got the sensor dirty.

 

UPDATE:

I replaced my MAF with a Delphi unit (AF10055). idles like a champ now. THANKS FOR THE HELP EVERYONE!

the housing on this model is 3 inches. I am running a 4 inch intake tube and will add a bung to the MAF to have a continuous 4 inch tube to the throttle body.