Any fuelling experts in the house?

[KS]

I have a recurring problem with the 914 which ONLY rears its head at Wiscombe, and which has all the hallmarks of fuel starvation. It only happens when pulling out of Sawbench (uphill right-hand hairpin) and Martini (steeper uphill 90 degree left-hander). Both are second gear pulls (others with more user-friendly gearboxes will often use 1st gear). It used to happen when the engine was on carbs, which I put down to fuel starvation, so I changed the float height and increased the length of the float bowl breathers, but to no avail. I expected the problem to go away with the swap to fuel injection, but not so. Here's a short video from the weekend which highlights the problem at its worst.

https://youtu.be/CyFWN2iTlZ4?si=4G7_u5EUWM3vunOP

The factory fuel tank is baffled and has a basic swirl pot surrounding the fuel outlet and return pipes. In addition, I now have a lift pump (Facet solid-state) feeding a 1.5-litre swirl pot which in turn supplies a high-pressure Bosch 044 pump. This feeds to the factory fuel lines back to the engine bay where the Bosch injectors are fed by -6AN hoses. The return line goes to the pressure regulator (set at 3 bar) and then back to the swirl pot. The overflow for this returns to the main tank.

The stumbling feels like classic fuel starvation but I only experience it on these two bends at Wiscombe. On the road, it is impossible to replicate the symptoms - the car runs great without any problems. And on hillclimbs at Werrington, Tregrehan, Castle (when last running there) and St Gouéno it doesn't happen. I've run the car with a full tank, half a tank and quarter tank, but nothing changes.

Two thoughts/ideas: stock fuel lines need to go (but as jwhillracer says, they're like those used in a 911, so shouldn't be a problem) or the fuel pressure regulator and/or gauge is telling lies and fuel pressure is too low.

I'm open to any suggestions from people with experience with EFI fuel systems as I'm getting tired of coughing and spluttering my way out of these two corners!

[911hillclimber]

Might help:
My 73T with the 3.2/DME system using Porsche 3.2 fuel lines and 964 pump never ever faltered on any hillclimb or road driving for 13 years on the hills and 30 years on the road.
On the hills I had around 1/2 tank of fuel or less. Often the fuel low red light would come on for ages as the fuel surged in the tank round left turns in particular.

The 911 has nothing fancy, just about everything is 1985 Porsche 3.2 except the pump which is 100 mm or less from the tank, stock fuel filter in the engine bay. All hoses as per 3.2.
Pressure regulators are also stock 1985 original parts.

Does the 3.2 design / system have something that irons-out this issue?

[Sam]

Obviously it’s difficult to tell, but it sounds a bit like it’s struggling before you apply significant steering angle, like it could be the braking that’s the cause.

[911hillclimber]

It is very abrupt.
Maybe check all the looms connections.
My Lola tick over speeds up 500 rpm going round tight bends, it has a simple throttle cable and there is a bit of slack in the route, but i think that is chassis flex pulling on it somewhere. Not a problem, but on tight corners thing flex and move that road conditions never show up.

[Sam]

Similarly on an old Caterham I had worn mounts cause the drivetrain to shift under heavy load and short the ignition to the chassis.

Don’t think it’s that, as that caused a total loss of spark followed by an abrupt return to full power, but you know what they say about 99% of fuelling issues being electrical…

[KS]

I, too, am starting to think it's an electrical problem - just had a long chat with Adrian Crawford and his suggestion makes sense: jury rig both switched and ignition controlled 12v feeds to the ECU, feeding straight off the battery thereby bypassing both the master cut-off switch and the ignition wiring that goes all the way from the ECU in the engine bay forward to the ignition switch. I've had a master switch intermittently fail on me before, and the rest of the ignition wiring is now 50 years old... Just need a dry day before the weekend for me to explore.

[rhd racer]

Hi Keith

I had similar issues when I converted my 3.0sc CIS FI 914 to carbs. I was talking to Chris Flavell and he told me to junk the high pressure pump and regulator. I ran a purpose built baffled tank, and a swirl pot (both from Bob at Concept) fed each by a lower pressure pump in series. So tank to pump to swirl pot, then to a 2nd pump to carbs. He was absolutely robust in his preference for the pump to carbs, which was OE for a carbed 911 (can’t timber the name, maybe it was a Webcon)? This meant everything was low pressure, lots of reserve capacity in the swirl pot and -6 lines throughout.

I have to say running 4O IDA 3c carbs which would be a little thirstier than yours it never let me down.

Hope that helps
Wayne


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[KS]

But I'm on ITB fuel-injection, so needs to be around 3 bar pressure.

It only occurs on rapid changes of direction - out on the road, you can run wide-open throttle at will, or pull from 1500rpm without a hiccup.

[rhd racer]

Ah - in that case all the above is total bobbins! Sorry, thought you were on carbs.

In that case, like you say, unless you are getting a false pressure reading it does not sound like a fuelling problem, even though the symptoms point right to it

I agree that it could be a failing electrical connection when you corner hard. Assume the pumps are on rubber mounts etc?


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[neilbardsley]

Natively put seeing you have a constant 3bars I think this is unlikely to be a fuel issue and strange that it will be an electric issue that is only present in 2 corners. Chris Turner once sprayed water over the ruining engine of my 912 saying it was a good test for bad connections?

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[Ollie]

My money's on a rogue electrical connection or wire that's worn through and shorting out on something. Good luck sorting!!

[KS]

Yes, it seems to me that everything is pointing towards an electrical glitch. I just wish it would stop raining so I can go out and take a look (I don’t have a garage…).

[christaylorsound]

I had the same issue a few years ago with my SC on tight roundabouts etc. Like you I assumed it was fuel starvation but while I never precisely located the cause, a new rotor arm/cap and thorough check of all cables and HT leads cured it.

[Bruce M]

The rev counter appears to be working. Seems to pick up a little as the misfire clears but that could easily be the revs increasing. So that signal source (ECU or dissy) is still working.

What is the ignition set up?

[KS]

Crank fire with DTA T2 ecu. Don't think it's a crank pick-up problem, though, as it only happens under very specific circumstances (ie, sharp turn, uphill) – all good on read-out of ecu data.