Ok so I have a 1981 Vette that I put an LT-1 in from a '94 Vette. The harness is from a '96 Trans Am and the computer is also from a '94 Vette. I used Datamaster to take out the VATS program, and checked my wiring over and over again. I have 12v constant power going to the computer along with the 12 v switched and grounds. I also re-wired the harness to use the '94 coil and module.

Here is the problem. The car starts right up but stops after about 3-4 seconds. I thought the VATS might be the problem but again I programmed it out. The engine runs a little rough and I have spark and fuel (not sure the how much pressure though but I am waiting on the fuel pressure gauge to come in) and I just changed out the gas and the fuel filter (had been sitting for a long time) and decided to try some water treatment just to make sure. Well the car starts right up, I can pump the gas and it reacts instantly but then stops no matter how much gas I give it.
I blocked off all vacuum ports for trouble shooting and tried the propane trick but nothing seems to help.
I had put the heads and intake back on about a month ago and followed along with the book while adjusting my rockers for valve lash but was thinking that perhaps I put them on too tight. Was also thinking that the opti might be going bad but it was running fine when I had the engine in my '94 Vette altough the engine has been sitting for about 2 years now. But like i said, I seem to have good spark.

Any toughts or recommendations would be great.

Thanks for any help

Dave

 

Hmmm - does the opti system have a timing disconnect/EST bypass wire? (sorry - I'm not That familiar with the opti years) If it does then try starting it with that disconnected - see if the ignition is the problem and if it will stay running if the ECM doesn't have timing control.

But - More likely - Fuel. Sounds to me like the oil pressure switch "takeover" of the fuel pressure relay isn't happening. Try just jumpering the relay and thereby Force the pump to stay running - see if that "cures" it. It's possible that the pressure switch isn't signaling the relay to stay closed. The ECM doesn't Hold the fuel pump relay On after the motor fires up - the Oil pressure switch does that... At least that's how the circuit is supposed to work on the L-98 cars - I'll guess (again - not real familiar) that the LTx cars are the same way. It's a GM thing...

 

Sounds like the Fuel Pump. A FP Guage is $12 at any local parts store. I have done this swap and did all my own wiring but I used the correct harness and it looks as you have a cobo of OBDII and ZOBDI parts. THe 96 Harness is from and OBDII car and the ECM is from an OBDI car. But to your imidiate problem it sounds as if the ECM is providing the initial signal to the FP as it is supposed to but not telling the FP to run once the car has started. withgout knowing what wiring was done and what if any codes you have it is a complete crap shoot.

 

Well I am still trouble shooting. The engine starts up fine runs for about 3 seconds and dies. Sounds just like the VATS but when the engine dies abruptly the fuel pump continues to run for another second or two. I went ahead and put a jumper wire across the fuel pump relay and it kicks right in. Started the engine and the same thing, engine just dies after 3 seconds. Doesn't bog down and then die, just cuts off.
Sounds like the dang VATS but I programed it out.

As for the oil pressure switch, the one on the back of the engine behind the intake manifold is the one you are talking about right, the only one I know of, is new and plugged in.

Any other inputs anyone.

Dave

 

Ok more information helps. If the FP is running then it is not the original problem I thought it could be. If the ECM is a 94 and you have disabled the VATS in the ECM then it should not be causing you any problems. What tuning software are you using? I may have an old bin file for tunercat from my swap motor I could e-mail you I would just have to pull an old harddrive I have in the closet. I know that bin file is set up to run in a stand alone configuration and ran great with my LT1 swap. It is for a Hotcam motor and should work. If you have the ability to tune then you should be able to log data and see what is going on as well.

 

Well thanks for the offer. I am using my '94 Vette to do the programing right now. This is getting annoying. I had an LT-1 in this car before. Did a cam swap, had the heads and intake port and polished and 1.6 rockers.

At the time I was running a 700r4, converted my own factory harness and all was great. When I got my '94 Vette out here she had high miles on here so I pulled the LT-1 out of the '81 and put into the '94 since I wanted to do more work on the '81 and she would be down for awhile. Well my landlord is going to sell the place, I am going to Fl for training for 3 months then straight on deployment. I am running out of time here.

Well anyway I took the OBD I computer I used to have in the '81 and put into the '94 to data log. The dang thing wouldn't read. Took forever in a day, several failed attempts and aborts. Finally got one good read from it and it turned out that the VATS was enabled. How the heck was it enabled when I disabled it before. Either way after that one read I couldn't read it anymore.
Gave up on that OBD I for now and reprogrammed my '94 OBD I. Took out the VATS and nothing. Same symptoms. Starts for 3 seconds, sounds pretty good then dies and the fuel pump stays on for 2 seconds.

Well I got my fuel pressure guage today and I will try that.
On the side note. The engine was sitting for about 1.5 years so I am wondering if the OPTI is bad. Hawaii does get pretty humid. Has great spark while running but just dies which I guess sounds typical of a bad OPTI.

 

Oh, by the way I am using Tunercat.