I have searched and I could not find a 'good' thread about VATS on this forum. Seems there about a million different opinions on the subject. I am searching for the TRUTH.

I have a '91 Z28 (1LE/LB9) car that only has 37,000 original miles on it. The car is unmolested in every sense of the word - about as close to new condition as you can get. Anyway, today I decided to take it out for a drive and it left me stranded. The first leg of my trip was about 14 miles and I stopped to eat breakfast. Got back in the car and drove it about another mile and a half and it just quit running at highway speed. I let it coast to a stop and I tried to re-start it...it would fire and run just briefly but then it would stall out. My first deduction was that maybe it ran out of gas and that the fuel gauge was faulty as a result of a bad sender. The car sits a lot. That was not the problem. Then I checked the fuel pressure, its right on spec with NO bleed down - 45psi. So next I decided to put another known good ignition module in it (it still has the original module in it) and that also was not the problem. It will still start and run briefly, but it stalls after about 2 or 3 seconds. Occasionally it will act like its going to stall and then pick back up again, but just briefly, and then it'll stall.

This is a VATS car, it has a resistor key. It also does have an aftermarket PROM in the computer (this is the only modification to the car). Previously, I always thought that VATS was a function of programmed logic in the PROM...and if you had an aftermarket PROM that the VATS was automatically disabled. Now I'm not so sure. I tried measuring the resistance across the key and couldn't get a good reading on it. It seemed like it was intermittent. I am now suspecting I have a bad key. What I'm confused about though is just how VATS works. I always thought it would disable the starter....so if you didn't have the key you wouldn't get anything when you tried to start. I had an '89 RS car with a TBI 305 and I know for a fact that was how it worked on that car. Did this change in the later years? Do I have a VATS problem here?

 

Additional information:

When I turn on the key, the security light does come on but goes off after a few seconds. Best I can tell this is a normal indication. It does not come back on after the engine starts and/or stalls out. It only comes on when the ignition key is cycled.

 

Nobody?

 

Magnetic pickup in the dist maybe?

 

Vat's won't cause your call to stall.

IIRC (could look it up if needed) upon startup, VAT's allows the start relay to energize, as well as the quad drivers in the ECM for the injectors.

But once started, ECM doesn't look for VAT's...I know this to be true, as I can pull my key while the engine is running.

Jump the uppeer two right pins on the ALDL connector, and see what codes are set (if any) should be plenty of info available on a search to read the SES light and procedure.

 

Correct. If the car starts, its not VATS.

 

It isn't displaying a check engine light. But I reckon that doesn't mean anything. I am starting to lean towards the mag pickup in the dist. It was kinda hot this morning when I took it for a drive and then I parked it and it was able to heat soak for about 45 mins before I started it again. It only ran for about 2 mins before it quit running after that. I have had mag pickups fail like that in the past with similar symptoms. It's just been a very longtime.

 

Could be the mag pickup coil in the distributor. It could also be the injector coils are shorting.

RBob.

 

I'm going to change out the mag pickup in the dist. tomorrow. I suspect its the most likely culprit. Wouldn't be very likely that all the injectors would fail at once. I did examine the wiring harness incase there mighta been some rat and mouse damage, but I didn't find anything wrong. Still looks like new.

 

The Multec injectors don't need to fail all at once. There is only one driver in the ECM for all eight of them. As the coils start to short it only takes a few to prevent the ECM from firing all of them (one driver for all eight).

RBob.

 

Actually I believe there are two separate circuits - right bank and left bank. Either way, in all my years of mechanic'n, nearly 30 now, I can honestly say that I have never come across a fuel injector with a bad coil. I'll keep that in mind though if replacing the mag pickup doesn't solve the problem and I have to go back to the drawing board.

 

Two circuits with one driver in the ECM. That one driver is pinned out to two terminals on the ECM, with each bank having it's own fuse. But we are back to the one injector driver for all eight injectors.

As for never seeing this, a search here will show many examples along with resistance readings of the injector coils.

RBob.

 

Welp, changed out the mag pickup, didn't solve the problem. I was sure that was going to be it because the old one looked like pure hell. All rusted and corroded. Checked the injectors, no problems there either. Has to be the ECM. I suspect a loose solder joint somewhere on the board.

 

Pulled the ECM out to give it a look-see, the mystery deepens. It had greasy fingerprints all over it and it was missing the guide pins at the top. Leads me to believe it's a junkyard replacement. I also decided to go look through the boxes of replaced original parts that I got with the car and discovered that every single sensor on the engine has been replaced, including the ignition coil and about a half dozen distributor caps. I'm guessing now that this car has had this problem at some point in the past and someone threw parts at it until they got it fixed. I'm guessing the ECM was the culprit the last time and probably this time too. The circuit board looked ok. I examined it with a magnifying glass to look for burned spots or loose solder, didn't find anything. Looks new really.

 

Welp, as much as I hate to admit it - it was a bad fuel injector that was causing the problem. The meter I was using yesterday was acting up due to a low battery and I got frustrated with it before I thoroughly checked all 8 fuel injectors. So today after I checked the ECM and didn't really find what I was looking for I decided to go get a new battery for my meter and re-check the injectors. I found #1 to be reading only 1.8 ohms. One other was at 14.8 and all the rest were above 17 or so. If I leave #1 disconnected it starts and runs fine on 7 cyls. I'm definitely eating some crow here, this is a first for me. Never seen a fuel injector coil short out before.

 

The upside is that it is a straight forward fix. A fellow currently posting on the General Engine board here has the exact same symptoms. His is injector #2.

RBob.

 

I appreciate the insight - I probably would've never checked the fuel injectors had you not mentioned it.