Maddening Running Issue

[jb]

Did this issue get resolved?
If so what was wrong?

[RobFrost]

Will it hold at 3k revs, given sufficient throttle?

[sfh3l]

I know this is a different car ('72 T, running on Webers), but I had similar symptoms with what turned out to be a largely blocked fuel filter. I am not saying your filter is blocked, but more just echoing your hunch that it is fuel and not sparks. It is easy to forget the old rule, but check the really obvious things first. Good luck with this. I know how annoying a small thing can be when it stands in the way of enjoying your car!

[willbrown]

JB, not resolved yet as have been away - looking to do the 30 sec fuel pump test next week.

Sam, it's a good point re. fuel filter - will check that too.

Rob, no it wont hold revs at all. It just carries on as per the video.

[RobFrost]

It appears to be a problem which accumulates over a period of two to three seconds while the engine's running, and resolves itself over a similar period when the engine's not running.

That points to a "insufficient rate of flow" problem, which is most likely fuel.

Most likely some crap in the system. That's made more likely if the problem started abruptly.

I'd be curious if you turn the key fully off in-between starts - so the fuel pump's not running - does it fail to recover between starts?

There's a theoretical possibility the cdi has a bad earth. If so, I could imagine charge building up on the supposedly earthed plate of the capacitor, which might take a few seconds. But I think that's less likely than fuel. Ignition is testable by spraying carb cleaner into the intakes as you crank. If it runs better with that, you have ignition and your problem is fuel.

Sent from my SM-G988B using Tapatalk

[willbrown]

Rob, thanks.

Re "turning fully off between starts" - it doesn't fully start - i'm having to crank it over in the video.

It hasn't been an abrupt fault - seems intermittent. When the car was running fine after re-built it did exactly as per the video one night after a fairmile meet, although it did start and eventually it cleared after a lot of revving and then ran fine.

Bad earth is still a possibility I guess, but all connections are good.

[knapmann]

I had issues like this that I traced to fuel delivery on my MFI, for me the issue was variable fuel pressure caused by sticking pressure control valve in the return fitting of the fuel filter assembly, pressure would stick high and low randomly when driving around. I found this by fitting an inline analogue pressure gauge in the fuel line in the engine bay. I think this is a great addition to have as I can see the fuel pressure is good an consistent now whenever I want, this would also help diagnose blockage problems from crud etc mine runs around 11psi and is consistant now, the car was instantly improved. Theres more details in my resto thread showing how i checked the valve operation and how I fixed it without replacing it

[willbrown]

knapmann - thank you - i will definitely check that out. i think the control valve was mentioned earlier as a potential problem. very interested to see how you fixed without replacing!

[willbrown]

here's a link to knapmann's resto post with in-line gauge etc for those following: viewtopic.php?t=66841&start=465

[knapmann]

my comments around the pressure release valve are here posted 14th & 15/03/2021, pages 24 and 25 of the thread.
viewtopic.php?t=66841&hilit=square&start=345

[willbrown]

...thanks i was just looking at that.

[knapmann]

it might be totally different issue, but if the OP suspects fuel delivery is part of the problem for me the best way to check is to get a pressure gauge then if that shows a problem it can only be a blockage at the intake in the tank, a blocked filter canister (unlikely), a problem with the pressure release valve, or a failing electric pump.

This assumes it is piped up correctly. There are/were issues with incorrect fuel line diagrams from the factory depicting incorrect routings of the filter canister to & from the MFI pump and tank return etc If it has been piped up incorrectly then who knows!

The error is shown by Grady on post #19 here

https://forums.pelicanparts.com/porsche ... blems.html

[willbrown]

knapmann, this sounds like a good first option. do you have a link to the gauge you installed please?

have checked the line plumbing - it's all correct.

[RobFrost]

It'll probably run with this squirted in the intakes if it's a fuelling problem.

https://amzn.eu/d/inJdQQO

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[knapmann]

knapmann, this sounds like a good first option. do you have a link to the gauge you installed please?

have checked the line plumbing - it's all correct.

i bought this one... it was only £50 back then!
https://www.ebay.co.uk/itm/111704283762

this looks much the same and cheaper
https://www.ebay.co.uk/itm/284123850859