964 Stall and no start mystery

[rnstout356]

I have a ‘91 C2 Coupe. 109K miles. I have put less than 5,000 miles on it in the 15+ years I’ve owned it. The only engine modification is a cut out air filter box cover. I used to do PCA DEs, but life interrupted and I have driven it very little in the last few years. In my location, we have to do emission checks every year. The last time I drove the car was for last year’s emission check. The car has always run really well and always passed the emission checks. It is due again so I hooked the battery back up and tried to start it.

It cranked for a few seconds and started. It is always smokey starting after it has sat for awhile due to oil accumulating in a couple cylinders. I let it idle for 20 or 30 seconds and rolled in a little throttle. It stalled. I tried to restart. It cranked for a few seconds before I heard a loud bang from the engine compartment. I turned the key off and went to the engine compartment. The left side intake manifold had blown off the plenum (that is what I am calling the plastic housing that contains the throttle butterfly and splits to go to each cylinder bank), the manifold had popped off the cylinder intake ports and gas was dripping out of the plenum - not a stream, but a steady drip for 15 - 20 seconds and a puddle on the cover.

After putting everything back together, the first thing I checked was the distributor belt. I took the distributor(s) out, took the rotor and cover off and turned the belt through several rotations. The belt looks almost factory new - sharp teeth and no apparent wear.

Based on the gas dripping from the plenum, I’m guessing the stall and loud bang is due to an excessively rich mixture, but I can’t figure out how gas got into the plenum so far from the injectors. One guess I have is the fuel pressure regulator has failed, but how does the fuel get up into the plenum?

Another possible clue is that I try to leave 1/4 to 1/2 tank of fuel. It is stored inside and I am in dry climate so condensation in the fuel has never been a problem. As I was trying to restart the engine, the low fuel light came on. BUT, except for the puddle in the engine compartment, I see no other fuel leaks. Where is the fuel going?

Thanks in advance for any suggestions of things to look at.

[rnstout356]

I finally got back to troubleshooting this. I pulled the vacuum line off the Fuel Pressure Regulator and gas poured out. This explains where the gas in the intake is coming from.

[Vasque55]

I have also pulled the vacuum line off the fuel but pressure is not regulator and gas poured out..

Aetna Medicare

[Faster993]

This sounds eerily familiar. Possible you may have a have a broken diaphram membrane in your regulator? See thread here https://rennlist.com/forums/964-forum/933017-when-starting-car-i-heard-loud-pop-intake-manifold-comes-disconnected-from-motor-3.html

[Faster993]

And if the pressure is fine, a leaking fuel injector may be the problem, as noted by Leonardo17. Hope this helps. Sad no one has responded here.