Oh, also verified comments above re: alternator. With engine idling at ~800, fan was occasionally stopping. Was fine at ~2000. When I revved the engine manually and watched the alternator housing and pulley, I could see housing/fan moving down slightly (rotating out and down slightly) - I know belt tension increases when you increase rpm but not sure there should be any play?

After winter (too cold to work on cars) and a bunch of travel and other things that kept me from working on cars, I finally had time to go back to this in recent weeks. At first, at the beginning of summer, I was getting the same results as before. The car would either be a crank-no-start, or it would start and run for a couple of seconds, or it would start and run like **** and die when you gave it throttle. Eventually it was crank-no-start all the time, then I left the key on one too many times and the battery wouldn't recharge very well, so since it was 8-9 years old I just got a new one.

Put the new battery in, no change. Not that I was expecting any.

As someone pointed out, the alternator belt was too loose and since that needs addressing no matter what, might as well do it. Took pulley off and instead of six shims, most but not all behind the pulley, I found three shims and one washer that was 4 shims thick (I measured.) Odd. I ordered some new shims from a local dealer and in the meantime just moved shims around until tension was appropriate. Also around this time, I accidentally left the ignition on again with the new battery so I had to take it out and charge it again. Dang, David, pay attention.

Decided next thing I was going to do was test fuel pump. Yes, pressure in rail indicates normal but still. I had already verified spark. Gauge at fuel rail was reading around 35psi when running, which is about right. Unplug vacuum to FPR, idle would go down from ~1400-2000 to 800 (odd) and fuel pressure would go up to 40, OK that's good too. Still, I wanted to run the pump directly. So I made a jumper wire with a switch on it (right out of the manual) to jump from the two pins on the FP relay. Meanwhile I had charged the new battery and put it in, so why not try it again before testing the fuel pump. Tried cranking the car and...it fired right up, and stayed running, more or less normally. Revved normally. Shut it off and started it again, same result - basically normal. Next day, normal. Took it for a drive around the neighborhood.

So now I have a car that was driving along one day last year until I downshifted and it backfired, fell on its face and shortly thereafter died; it would then run barely or not at all; and now it has seemingly cured itself. Took it for a short freeway drive a week ago and drove normally. Subjectively it is maybe slightly down on power.
Also the the low undertone
(http://forums.pelicanparts.com/porsche-911-technical-forum/1098047-what-low-undertone-my-engine-sound.html)
is still there, and it's louder. This has been gradually growing since early 2021, when the car was fine. But now I have a car that is back to being normal, but I don't understand what happened last year, so I can't consider it 'fixed'.

I never had a reason to wonder about compression, but I've checked fuel and spark, so why not compression. And let's pull the valve covers and measure valve clearances again. Maybe the wub-wub is from a cylinder with super-loose valves? I dunno, I'm just guessing.

I pull the #4 plug (right bank, rear of car) first and it's black and damp (it was the first one I pulled, right after running the car) and smells like fuel. I assume it's fouled because of all the testing I was doing, where I was dumping lots of fuel into the cylinder but it wasn't really running. I pull the other five, however, and they're all normal-looking.

A few days later I check compression on #4 first and I get like 110psi. huh, that's not right. Test it again, same result. I should say that this is the first time I've ever done a compression test so I assume there is some significant non-zero chance I'm doing some stupid noob thing. Test was done on a cold engine, WOT (using foot), then hop out and read gauge. I test the other five cylinders and four of the rest are the same result, 110psi, and #3 (left bank, rear of car) is 90. I test it again, same, I check an adjacent cylinder again, get the same 110 I got the first time. I still don't entirely trust the results or myself, so tomorrow I'm going to go test my Sentra SE-R, which I know to be healthy, with the same gauge as a reference check. (I have a second gauge but I can't get the hose started in the plug hole because it's so long, so I can test with two gauge heads but only one hose.) If results on that engine are also abnormally low, then I'm the problem and the question to answer on the 911 is why is #4 low? If results on that engine are expected then the question to answer is why are all cylinders so low? IIRC it should be in the 130-170 range (engine was rebuilt about ten years ago and ran great right up to last September). It seems highly unlikely that every cylinder actually has low compression by the same amount.

Any clues as to what might be going on?

Any reason to check valve clearances? Could super-packed clearances cause any or all of this? Maybe an adjuster nut was loose and came off when I downshifted last year and now that valve is god knows what and that's my whole problem? I have no idea, I'm just guessing.

Separately, if I had spares to swap in for an A vs B, I'd swap out the cat and muffler to exclude that there is some obstruction that is causing the problem(s).

I'm not sure if I have one problem (that caused the car to die last year and also is causing the sound indicated in the other thread), or two. But I definitely have at least one.

thanks,
David

I forgot how to data log on TunerStudio (durrrr...) but I did take a video of the TS dashboard when the car was running normally at idle. I'll link to that and provide some more background info in a bit.

When was the last time you checked the strainer in the tank? I'm assuming it has one.

This was mine a couple weeks ago.

http://forums.pelicanparts.com/uploa...1656565073.jpg

Check #4 plug wire for resistance and compare with the other wires. It should be about the same. A bad wire may cause the wet plug.

I would get a leak down tester, buy or build one. I find leak down number much better at determining the condition of the engine.

You also might swap #4 injector with another injector and see if the wet plug follows the injector.

So it idles? I thought it wouldnt start?
Doesn't sound like fuel is the issue. You have sufficient fuel pressure for the engine run well.

What is you ignition setup again? Still using the CDI and distributor?

An intake backfire is what took out my airbox and led me to ITBs and efi conversion. You still have that 40 yr old plastic plenum and it could very well have cracked after that backfire you heard. Essentially a vacuum leak.

Never checked the tank strainer - never even thought of it - thanks.

Previously it was alternating between not starting, and starting and barely running. Wouldn't run well enough for me to drive it, and wouldn't rev past maybe 3k. Whether it would crank and not start or start and run badly wasn't random, but I could never figure out enough clues to figure out why it would do one and not the other.

Ignition is Ford EDIS. I do have a pop off valve on the remains of the original intake plenum. I do think I have some kind of vacuum leak, as sometime around the end of 2020 it developed an intermittent high idle - sometimes normal, sometimes around 1400, sometimes around 2000. Had not been able to chase it down before this happened. Carb spray didn't tell me anything, the intake runner boots seem OK.

When checking compression i use a momentary swich clipped onto the starter with power so i am not running back and forth from the drivers seat. You should do a smoke test and make sure your intake manifold gaskets are good. Along with breather hoses etc. i have used a propane tank with a regulator and blower from air compressor kit with a long tube to check for vacuum leaks but smoke works best.

Oh and blower and alternator should not move!

Checked the compression on my SE-R with the same gauge and hose, and got around 120psi, vs. the expected value of 150-175 (9.5:1 CR). I know that motor is healthy, so I can assume that for whatever reason the gauge reads low.

Next I will put the plugs back in the 911, test the resistance of the plug wires, and unplug the #4 injector to see if, and how, that changes how the car runs. Will swap injectors depending on results, but should look up what injectors are from and get O-rings first. I've had experience with that.

I am wondering if, for instance, in #4 I have valves (or valve, if one more likely exhaust) that are really really loose - this would account for low compression if lash is so high that the valve never fully closes, and that would probably also account for the exhaust sound, which sounds vaguely like a much lower-magnitude version of what old gasoline heavy-truck engines sounded like (I'm remembering the school buses that picked me up in Caracas).

So after I do the above I'll pull the left-side valve covers first and check the clearances.

Following up on this for the sake of future searches, even though I fixed this quite a while ago. This was, in the end, some kid of fueling issue, mostly a failing fuel pump and secondarily perhaps being low on gas. Replacing the fuel pump and later adding yet another gallon to the tank returned everything to normal.

Thanks for the update!

It's awesome when people come back to threads and say what actually worked.

If you are having running issues with low fuel level, this indicates either a clogged fuel filter or tank screen.