Car stalling AGAIN, Updates- none of them good. Options

[rjames]

Drove around for 30 minutes in 80+ degree weather. All arterial roads, so several stop lights, etc. No problems until I was almost home. Car stalled when I stopped at a light. RPMs just dropped to zero. Was able to start it again with a little difficulty and had to keep my foot on the gas to keep it from stalling out again. Power seemed ok though the rest of the way home (I think). It stalled once again at a stop light a couple of blocks from home and then again when I pulled in the driveway. I let it sit for 30 seconds, started it again and it idled just fine. Rock solid at 1000 RPMs. Pulled it into the garage where it sits now.

Prior to today the car has been running and idling perfectly since I went through the FI parts last year, which included installing a 123 distributor, new CHT sensor, a newly rebuilt mps (calibrated for correct afr on my engine), new vacuum hoses, NOS throttle body, rebuilt injectors, new fuel pump, valve adjustment and timing.

I haven’t checked the AFR since last year, not sure why it would change though.
It’s a ‘75 so the fuel pump is in the front, so I can rule out vapor lock I think.

Anyone have ideas as to why it would be stalling?

[rjames]

Ok I'm back to updating my thread that seemingly never ends. I thought I had my problem of high and/or surging idle and/or intermittent stalling issue fixed with the new ignition and new injector harnesses.

Nope.

Car hasn't stalled again, but almost did. Started the car the other day and after it warmed up the idle was a perfect and steady 1000rpm. Which was strange since I couldn't get it below 1200rpm previously. AFR was 13.5 too, even though the last time I had driven it it was 10.5 at idle. (IMG:style_emoticons/default/huh.gif)

Drove ~ 15 minutes to the grocery store. All good. When I got back to the car the idle was now at 1100rpm. Another 15 minutes of driving and idle increased to 1200rpm.
More time driving and the idle started fluctuating from 600-1200rpm. At one point at a stop sign the engine almost stalled- idle just dropped to almost 0 like the engine had just been turned off, but the idle came back up before that could happen.

I did notice that when the idle is fluctuating (usually going from ~600-1200 rpm) it will continue to fluctuate even if I step on the gas a bit and hold it there- say increase the RPMs to 1500rpm, the rpms will continue to drop to about 1200 and then go back to 1500. So this issue is happening even when the ECU is not seeing an 'idle' state because the TPS is seeing idle anymore.

The nagging feeling that maybe there is a vacuum leak wouldn't go away so I connected my vacuum gauge via a T off of the throttle body retard port and it shows 17.inHg at 1100rpm and when it surges to 1200rpm it reads 18.5in.Hg. Of course when I did this test the idle wasn't dropping below 1100rpm so I don't know what the reading would be then, will try and capture that. My guess is that it will be less than 17in.Hg just by virtue of the slower engine speed. In other words- I don't think i have a vacuum leak because I would expect the vacuum to drop at higher idle RPMs if that was the case.

This feels like an issue with the FI system or the engine itself. But I've got a new ignition harness, new FI harness, new CHT, 123 distributor, injectors tested, verified fuel pressure, etc. I've tried swapping ECUs and I've got a rebuilt MPS that holds vacuum. I suppose I could try putting the vacuum gauge between the manifold and and the MPS and see if the MPS diaphragm is intermittently failing? Could this be a thing?

What about the engine itself? What are the symptoms of leak at the heads? Compression issue? (Current #s are 110, 110, 130, 131) Other? Car seems to have plenty of power.