Backfire, no power, won't rev '96 Z28
 

I'd appreciate anyone's ideas on what to check.
I'm the original owner of a bone stock '96 Z28.
Car was running fine (daily driver).
Filled up with gas (premium), pulled out of the station and got on it (not WFO). Started to accelerate and then bang, bang, bang...exhaust backfires one after another and lost power. Pulled to the side of the road with car idling just fine. Tried to pull away and exhaust backfires galore, and no power.
Had the car towed home.
Starts up fine cold or hot and idles no problem. Starts to exhaust backfire when I give it throttle. If I open the throttle VERY slowly I can get up to 2500 rpm; any more throttle and the rpm falls off and the backfires start up.
Here's what I've checked/done:
Verified that it's gas NOT diesel (since I had just filled up).
Measured fuel pressure at idle and at 2500 rpm. Steady at 40 psi.
Manifold vacuum is 18" and steady at idle, rising to 20" at 2500 rpm (no load).
Pulled the AIR checkvalves from both exhaust manifolds and measured backpressure at less than 1 psi from idle to 2500 rpm.
SES/CEL is not lit and my scan tool shows NO codes set.
(If I run up to 2500 rpm and let it start backfiring for awhile then I'll get the 'random multiple misfire' code)
I honestly don't know if the exhaust backfires are due to lean misfire, or a too rich condition.
I'm at my wit's end here...any and all help would be most welcome!

Thanks,

Mike

Check the plugs/wires. Make sure all cylinders are getting spark. Has the opti ever been replaced?

You got bad gas. That, or by some HUGE coincidence, something unrelated just happened to strangely pop up. But under the circumstances (acting up right after gassing up), my money is on the gas being the culprit.

pull your fuel filter and dump contents into a clear container so you can see what comes out. gas will float water will sink. dump from the tank side of the filter

I've checked spark on a couple of cylinders and they're okay (checking all of them is a pain in the arse). May have to, though...
I replaced the opti about a year ago (maybe 10-15K miles). Symptom that time was the car completely died.

Thanks for the suggestions!

WOW!
:icon_doh:

Time to check the 'gas'
I've had bad gas before, but never to where the car wouldn't even drive...so, I wasn't even considering that as possible once I ruled out diesel.

Good plan.
Thanks.

sounds like a bad PSU.

OP here.
Sorry for the delay, but work and things...ya know.

I've used the bleed off my pressure gauge to fill a glass container with about 8 oz. of fuel. I let it sit for over an hour and no water separated out. I then added 8 oz. of water and let it settle, then made a felt pen mark at the water/gas line. Then shook the jar and let it settle for over an hour. Measured the new separation line and calculated right around 10% ethanol (pretty typical in southern California).
Took some fuel and soaked a white paper towel and let the gas evaporate. No oily stain residue, so not diesel contamination.
Bottom line, I'm not finding anything wrong with the gas despite the problem happening IMMEDIATELY after filling up a nearly empty tank. Any other tests for bad gas??

Next, I put an oscilloscope on both the low and high resolution Opti outputs: both signals look correct.

I've had the ICM tested 3 times. Checked okay.
No, it wasn't warm, but the problem occurs whether then engine is started stone cold or hot. So, I don't think testing the ICM cold makes any difference.

Next step is to put the 'scope on the injectors and see what is happening with pulse widths; my scanner doesn't seem to have a way to monitor injector pulse widths.

Any other ideas??
This one is really making me nuts!!

Thanks.

gona have to pull the cap off the opti and have a look see, sounds like the rotor may have come lose. need to put thread lock on the screws that hold it on or sometimes they come lose.