Alright, general consensus is that the culprits of the vapour lock could be the check valve at the fuel pump, the injectors, the fuel pressure regulator, or the fuel dampers (somewhat in that order given my car's symptoms).

The check valve was new, replaced with one from Roger. So I moved onto the next potential culprit - the fuel injectors.

To @mrmerlins point, I ran some cleaner through the fuel system. We can't get techron up north of the 49th, so I went with Royal Purple injector cleaner as it has a healthy dose of PEA.

As before the car had 2.5 bar when I started the test, and after 30 minutes it was still holding 2.5 bar. After 45 minutes, it was holding 1.5 bar. Could this be a small victory? Significantly better than before, and it may improve as I run more of the cleaner through the system. Which by default would lead to leaky injectors as the pressure culprit.

I may not have time to run it up to temperature and get it good and hot until the weekend. At that time I can try to reproduce the conditions under which I had the hot start issue. Fingers crossed.

 

figure you gonna run atleast one maybe two tanks of treated fuel first. do short rides ,
so the cleaners have time to work.

NOTE putting these chemicals into the fuel system should be SOP for all the cars,
it usually fixes starting and running issues.
NOTE consider doing the cleaning prior to oil changes,
this will assist in removing the dirt from the engine.

 

Update. Fuel injector cleaner made a difference in an unusual way. Where before the cleaner, the pressure dropped off fairly quickly, then held at 1 bar, now the pressure holds for about 20 mins, then starts dropping. Here are the comparative findings.

Before cleaner:

Initial: 2.5 Bar
5 mins: 2 Bar
15 mins: 1.5 Bar
40 mins: 1 Bar
60 mins: 1 Bar
75 mins: 1 Bar

After cleaner:

Initial 2.5 Bar
25 mins 2.3 Bar
28 mins 2 Bar
30 mins 1.8 Bar
32 mins 1.5 Bar
40 mins 0.8 Bar

Not to make too much of that, but it's not the result that I expected. Anyway, next will be breaking out the Mighty Vac and pulling vacuum on the fuel dampers and fuel pressure regulator. Will report back on what that reveals, hopefully this weekend.

Regarding the voltage, I did get it to act up when hot starting and the voltage at the dash gauge was low even after it was started and revved. I now have the cigar lighter gauge, but of course when I plugged it in there was no power to the lighter. After fixing some wiring I now have power to the lighter, so I can measure the voltage there. I suspect the MrMerlin is correct about the dash gauge because when it acted up and showed low voltage, I scrambled out of the car and tested the voltage at the jump post and it was a couple of volts higher than the dash gauge. Will be interesting to measure it at the lighter and compare that to the dash gauge. Which brings up an interesting question: The dash gauge on this car is usually pretty accurate, so why, only when hot starting, would the dash gauge read incorrectly?

 

it sounds like you have poor electrical connections at the CE panel and the hot post and the other ground points,
use Deoxit 100 ,
try replacing the fuse for the instruments.

 

I don't really understand on what you are basing any confidence in the dash gauge? Based on experience I'd say that you can use it as a relative measure only (subject to some outside influence) but for an absolute measurement use a DMM. The places that are most interesting to measure are:

1) The Jump post (closest easy to measure point to the alternator)
2) The (30) connections at the CE panel (this is the voltage the relays see)
3) The battery - directly

The lighter socket and the dashboard are really not so interesting here to the running of the car (and are potentially subject to other outside influences) - so I'd ignore those for now.

Alan

 

Alan the reason I suggested to use the lighter volt reader is because it is very accurate instead of the gauge.
Then once the hot post and cig lighter and battery readings are taken and compared,
he will have some kind of idea how much of a difference he is seeing as the system operates.
At this point the cig reader is more of trend unit than exact,
NOTE it will be a more accurate trend reading though as compared to the gauge.

 

Alan, agreed, that the jump post, battery and CE panel are the points of interest. The challenge I have is that the problem is intermittent, and when it happens, over the course of a couple minutes it starts to sort itself out. When it does the hot no-start, to get the best electrical data, I have to scramble out of the car, open the hood, grab my DMM and test. Being north of 60 years old, those acrobatics don't happen so fast any more I did take MrMerlin's advice about the cigar lighter gauge, and indeed, it reads higher than the dash gauge, so for now, I'll treat the dash gauge as decoration only.

MrMerlin, I've done some Deoxit work already under the dash, but will endeavor to go over everything again. As mentioned before, the CE panel is unbelievably clean and all the brass looks factory shiny with no corrosion or oxidation - almost like it just came off the showroom floor. But I won't assume that the connections are all without resistance.

So, regarding the fuel pressure loss, I grabbed the MightyVac and pulled 20" of vacuum on each of the 2 dampers and the FPR and all three hold vacuum like a champ. Unless I'm misapprehending how they work, that would suggest that the diaphragms inside of them are all intact and we can conclude the they're not the issue. Would I be correct in that? If so, then we're back to the fuel injectors or the fuel pump check valve (which is new). Anything else that I could test?

 

well it sounds like the fuel injectors are the highest probability for pressure loss.
That said do short drives to let the chemicals work,
its possible they are just dirty and frequent use will clean them and make them seal ,
if not then have your injectors cleaned or replaced.

 

I had a similar problem with mine when I first got my 928. It turned out to be the injectors leaking and causing hard starts after running. Sent them to cleaned and tested.
That was the issue for mine. I had one that was bad and couldn't be fixed.

Good luck!

 

I've decided to pull the injectors out and have them cleaned. I'm too impatient to wait for the injector cleaner to work, and if they're cleaned and tested good, then I can rule them out as the cause of a vapour lock.

 

You can always add some very light gauge wiring so you can monitor these things easily from the driver seat. You can use very light gauge (24AWG or lighter) so its easy to fit around things, through body gaps, through doors etc. Do all the set-up once and leave it in place temporarily - as long as you need - to monitor this. Using very thin wires doesn't compromise the voltage measurement (current is negligible) and it can act as its own fuse just in case.

Alan

 

Update: I had the fuel injectors professionally cleaned and post-cleaning there was no evidence of leaking. I reinstalled them with new seals. But here's the interesting bit. When I pulled the injectors out, clearly someone had been in there before and replaced the short rubber hoses that connect the injectors to the fuel rail. BUT, they had just pushed them onto the barbed fittings with no clamps, or brass escutcheons like the factory did. So there is some possibility the that problem was not the injectors, but rather an imperfect seal to the rubber hose. We'll never know.

So I added fuel injection specific clamps to all the hose connections and put it all back together.

Now I have static pressure of about 36PSI and after sitting for 3 hours, I still have about 29PSI. This is a vast improvement over the previous numbers.

I'll run the car for a few heat cycles and see if the hot start issue is resolved.

I may also change the name of this thread, since the low voltage, while it is still a thing, didn't end up being the true subject.

 

I cannot over estimate the benefit of refurbishing injectors. I had an 80 five speed in here, had the injectors cleaned and replaced the rubber hoses.

The difference after the injectors were rebuilt was night and day.

Before the idle was steady but sounds like a 60's VW - more of a putt - putt - putt.

After idle was stronger and throttle response was great. I could not stop driving it - sadly I had to return it to the owner.

 

Well, it appears that I posted too soon. The fuel system still refused to hold pressure. To recap:

Check Valve at the fuel pump has been replaced.
Fuel injectors cleaned and tested and all hold pressure
The 8 short fuel lines between the fuel rail and injectors were clamped with fuel injector specific clamps.
Each of the 2 dampers and the FPR were tested with 20" of vacuum and all three hold vacuum

There is no fuel leaving the system. Everything is dry and there's no odour of gasoline. So I'd presume that the pressure leak is within the closed system.

So all of that said, where should I look next for the source of the fuel pressure loss?