I posted a problem about a week ago about my ECM problem and I replaced it curing my problems. Here's the link:

https://www.thirdgen.org/techbb2/sho...hreadid=138609

I've done a search about backfire on this site, but none that I've read has a backfire when no throttle is applied and when rolling downhill.

Here is what has happened. Driving home on a slightly rainy night with headlights and fog lights on as well as A/C and stereo (no amplifiers in my car), I was rolling downhill with no throttle applied. I noticed as I was rounding a turn about 40MPH and braking, my steering got tight and then eased and got tight again. This told me my engine speed wasn't high enough to keep the power steering going. Thus, my engine RPM was dropping, my voltage dropped below 13 on the volt meter, and then BANG a major backfire that was super loud. The engine quit.

I had to steer while rolling down hill with no power steering and no power brakes. I safely got to the bottom of the hill and pulled over. i turned off the ignition key and waited.

I can now pull codes out of my computer since I replaced it a few days ago, but just a code 12. I try to start it, it runs then dies.

I didn't mention that I have a Jacobs Ultra Team igntion system on this 1988 IROC 5.7 TPI. It has the Ulta coil and Energy Pack (ignition box) on it. I bypassed the ignition box and the car started right up.

I drove home, shut the engine off and reconnected the ignition box. Started the engine and the engine runs fine like nothing happened.

I haven't started diagnosing the problem yet. I'll do it in the morning. Maybe my ignition box got wet? It's mounted in front the radiator on the cool side to keep the box cool.

Could my alternator output have caused the engine to stall and cause a backfire?

Does the IAC valve affect engine idle at highway speeds? Could it be faltering and the copmuter will say it's "ok" just like how faltering MAF sensors do not throw a code?

 

It seems that you've already isolated the problem (Jake Box).

The IAC does try to control engine RPM whenever the TPS is in the closed position, so even when coasting on the highway with the accelerator released, the IAC system is active. "ACTIVE" doesn't mean that it is trying to raise the idle, but just that it is monitoring the RPM and ready to adjust bypass air as necessary to maintain the target RPM. When the TPS voltage is at some level above idle position (I haven't been able to determine that exact number yet) the IAC goes basically inactive by stepping closed.

The ECM monitoring of the IAC is constant, just like the MAF, but the ECM is very tolerant of the IAC. Generating an IAC error code require that something is really out of whack, whereas the MAF out of range will set a DTC in 600mS or less. Speaking of the MAF, I'm hoping that you were getting AFTERFIRE (out the exhaust) instead of BACKFIRE (through the intake), since true backfire can take out a MAF hot wire sensor in no time at all. One good bang and it's a $450.00 paper weight. That's one of the main reasons the rear screen is in the MAF housing.

 

When I started working on my car today, I found out that the vaccuum hose to the fuel pressure regulator wasn't connected. I guess I forgot to connect it last wek while troubleshooting my car.

Also, the Jacob's ingnition box is fine. No blown fuses or broken wires.