High CIS System Pressure - How Does it Affect Our Fuel Systems?

[mikedsilva]

I'm on a never ending quest to learn, it seems.

Just rebuilt a 3.0SC engine, to be a 3.2SS.

The supplied engine and fuel system had sat for years so the WUR and Fuel Dist were sent off for rebuilding by a local specialist. While there, I asked about the extra capacity of the engine, and he offered to give it a 'power adder' to cope with the extra cc.

On the test stand, it was difficult to get started. But once started and warmed up, I tweaked the cis mixture screw richer (barely an 1/8 turn) so it was idling at afr 14. This is after the wur and aar had completely warmed up etc.

But when increasing rpm, the afr would dip VERY rich... my AEM gauge only shows 10 at the bottom scale and thats what it reads when rpm is increased. Holding 2000 rpm, it was reading 10AFR... blipping the throttle, it was producing some very rich smoke out the exhaust tips. It's clearly running very rich off idle.


I connected my fuel pressure gauge and noted the below.

WUR 0438 140 045
FD 0438 100 097

System pressure – 6.1 BAR (factory spec around 4.5-5.2 according to Bentley)

[ is this how you mod the distributor for the power adder?]

Control pressure for WUR after it had been running a bit (I can do it in the morning to get colder temps)
Infrared thermos 35C
Cold and running = 2.8 bar (spec according to graph 2.6 – 3) (seems in spec)

Warm and running with vac line attached 3.3 (spec 3.2-3.6) (seems in spec)
Warm and running with vac detached 2.6 (spec 2.7-3.1) (a bit out of spec)


I've not pulled the pressure regulator plug out of the FD yet, but I'm thinking I need to get my system pressure in spec first...

Thoughts?

[PeteKz]

6.1 bar is too high. Without changing anything else, I would reset the SP to the upper end of the spec--5.1-5.2. Then see if that reduces the richness.

Adding 6-7% to the displacement does not require changing the CIS. It has more than enough excess capacity to handle the slight additional displacement.

[boyt911sc]

Mike,

I have two (2) people right now with almost exact situation like you requesting for customized fuel distributor and warm up regulator for their bigger CIS engines. One thing I noticed in your post is the excessive system pressure at 6.1 bar/88.5 psi. The system pressure is too much for normally aspirated motor. You are using a slightly lower SP than an early 930 (90~95 psi.).

Tony

[mikedsilva]

Originally Posted by boyt911sc

Mike,

I have two (2) people right now with almost exact situation like you requesting for customized fuel distributor and warm up regulator for their bigger CIS engines. One thing I noticed in your post is the excessive system pressure at 6.1 bar/88.5 psi. The system pressure is too much for normally aspirated motor. You are using a slightly lower SP than an early 930 (90~95 psi.).

Tony

yeah
so the weird part is, I just spoke to the rebuillder and he said my FD was showing 5.1 on his gauges so he didnt touch it.

How do I alter the sys pressure? It looks like a hex bolt and a large nut

[PeteKz]

Yes, that's the SP valve. It has a spring and several washers under it. The washers are how you adjust SP.

Since your rebuilder thinks the SP is in spec, check your gauge against a known pressure source, like an air compressor or your tire.

[mikedsilva]

Originally Posted by PeteKz

Yes, that's the SP valve. It has a spring and several washers under it. The washers are how you adjust SP.

Since your rebuilder thinks the SP is in spec, check your gauge against a known pressure source, like an air compressor or your tire.

That's a fair point.
However, the pressures my WUR show concur with his settings so we're a bit confused. I think I have a second gauge.. will try it out.

On another note, this engine had been sitting unloved for ages, and it was noted that the fD was badly contaminated inside. I ran brake cleaner through all the fuel hoses and blew through them with compressed air. Is it possible that something got missed and could a blockage somewhere be affecting the pressure readings?
(I'm still an infant in understanding CIS and it's intracacies).

Today, I noted that the rebuilt WUR was not increasing control pressure as it warmed up. It was sitting at 1.1B cold (25deg C) and only got to maybe 1.5 Bar warm. It was running pig rich.

I fitted a WUR from my other enine and it was controlling pressure fine.

Here's an unlisted vid of it running with my old WUR.
https://youtu.be/8nE-InqDyIs

Ah, the joys of trying to bring back to life, a pile of poo.

[mikedsilva]

So I just pulled out my other fuel gauge... and I am getting the exact same system pressure.. 6 Bar.

I pulled out the plug on the FD and here is what I found.


the large copper washer, just out of shot, is approx 0.8mm thick.
There are 3 thin washers between the needle, and the spring..

Not sure they are meant to be there?

Out of curiosity, I removed the 3 thin washers and put it back together.
That hex bolt and large nut is deceiving" it appears as if it is an adjustable bolt with a locknut, but no.. it's all 1 piece I think.

Ran the pump and now system pressure is a little lower at around 5.5 bar.
Still too high.

I ran the engine anyway, and it needed a tad more fuel at idle (richen the mixture) to make it happy and with throttle open, it plummets to 10.

I guess I will send the FD back to be built back to stock spec.

The part I dont quite understand, is if it's flowing too much fuel, then wouldnt it be flowing too much fuel all the time? I'm setting AFR to approx 14 at idle but then it goes pig rich with throttle.

Whats the WUR job with throttle open? There is only 1 vac port on the rear of my TB and that is plumbed to the T on top of the decel valve and then to the side port on the WUR (thermo valve isnt fitted here).
Is it meant to get a lot more vacuum from the TB with open throttle which I believe in turn, increases control pressure, and leans out the fuel mixture?

[boyt911sc]

Mike,

Are you aware that there is a piston that goes with the PPV? You inadvertently left it inside the FD. Pull it out using an appropriate sized magnetic pick up tool and inspect the condition of the sealing ring. The 3 shims you found with your PPV affect the system pressure. You need an assortment of different shim thickness from 0.0010” to 0:0300” to adjust your system pressure. Use a micrometer to determine the calculated shim thickness.

Tony

[AndrewCologne]

Originally Posted by mikedsilva

How Does it Affect Our Fuel Systems?

To give an answer to your initial question on "how" it affects the system:
One is: The system pressure is present in the lower chambers of the fuel distributor, therefore with an abnormal high system pressure, the pressure in the lower chambers will push the diaphragm upwards against the entries of the fuel lines in the upper chambers ...



... this does reduce the fuel flow amount towards the injection valves and therefore the combustion chambers and this will ... simply lean out the mixture.

Exactly this is btw the logic how the Frequency valve controls the mixture on lambda engine models: simply via controlling the pressure in the lower chambers of the Fuel Distributor .
FV opens = pressure releases = more fuel = richer mixture
FV closes = pressure rises = less fuel = leaner mixture

[Old H2S]

This is why you get rid of the 2 lines going to the WUR and put an adjustable fuel pressure regulator in line back to the FD. Then you can set any AFR you want = ICARP CIS.

[mikedsilva]

Originally Posted by boyt911sc

Mike,

Are you aware that there is a piston that goes with the PPV? You inadvertently left it inside the FD. Pull it out using an appropriate sized magnetic pick up tool and inspect the condition of the sealing ring. The 3 shims you found with your PPV affect the system pressure. You need an assortment of different shim thickness from 0.0010” to 0:0300” to adjust your system pressure. Use a micrometer to determine the calculated shim thickness.

Tony

Ah, no I was not aware.
I've taken the FD back to the shop to put the stock springs in to set it to factory specs.

While he has it, I have fitted another FD and WUR from my old engine that ran well. My system pressure is still high at 6 bar. and I have used 2 different gauges that present the same.

Tomorrow I will remove the fuel return line and switch out with another to see if maybe there is an obstruction? I believe an obstruction here might cause higher system pressure?

I still ran the engine with this stock SC FD and WUR and the engine ran great. Revved nice and quick, etc. I need to do a cold start in the morning to see if my now set mixture means the warm up curve and aar are all working happily. Again, this is a replacement FD and WUR but they are from a 3.0SC. Same WUR part number as the original. Not 100% on the FD.

I suspect when I get the other FD back and it's built back to stock specs, the engine will run well.

[PeteKz]

Did you ask what your rebuilder did to "add power"?

[mikedsilva]

Update- sorry to waste everyone's time with this one.
The higher system pressure in this case, was due to the return line fitting to my engine stand tank. The diameter of the orifice was a tad smaller which resulted in higher "back pressure".
When I changed it to an open hose to the tank, the system pressure dropped to normal 5 bar.

whoops

I have however, sent the FD back to be rebuilt to stock specs.
Apparently they put some different springs in the FD ... I didnt really understand it but he said he will put it back to stock.

I've been running the engine today with a stock FD from a known good engine and it has been performing flawlessly.

[mikedsilva]

Update- sorry to waste everyone's time with this one.
The higher system pressure in this case, was due to the return line fitting to my engine stand tank. The diameter of the orifice was a tad smaller which resulted in higher "back pressure".
When I changed it to an open hose to the tank, the system pressure dropped to normal 5 bar.

whoops

I have however, sent the FD back to be rebuilt to stock specs.
Apparently they put some different springs in the FD ... I didnt really understand it but he said he will put it back to stock.

I've been running the engine today with a stock FD from a known good engine and it has been performing flawlessly.

[porschedude996]

Not a waste of time at all. Just more real world data. I would have thought this wouldn’t happen. It seemed to me that the valve of topic would have held off the pressure from the pump. Now we know it could happen.

[mikedsilva]

I have the FD back and he has put stock springs in etc.. the AFR is now NORMAL.

My new issue though, is that the rebuilt WUR is giving me lower control pressure for no reason.
Let me explain:
Last week when running it was exhibiting normal (within spec) control pressures warm and cold.
Then I remember starting it again and for whatever reason the control pressure was very low. So my mixture was very rich.
I pulled the fuel connections off and blew compressed air through at the request of the rebuilder. It then gave me correct control presssures. He thought perhaps some junk from the old fuel lines got in.

I took the FD back to be set back to the original specs (ie remove the power adding mods) and he also put the WUR on his test bench.

I picked both units up and he said the WUR was perfect.

Sure enough, fitting both units and running it is running nicely.
Shut it off, chat to a friend, start it up again and now low Control pressure again.

It's not the 12V power source that is the problem. I have checked and checked.

Also, it was only 2 minutes from shutting it off. It was showing 3.3bar and now it was around 2.5-2.6.

Blew air through the connections again and it's back to normal.

I have taken the WUR back to be inspected. I think something isn't right inside. Maybe some garbage got iinside and it cannot get out.

Not sure if this means anything: but when I undiid the hose that connects the WUR to the FD, (other line is still connected) bubbles kept rising up through the fuel that was sitting in the banjo bolt. Does that sound like some sort of intermittent air leak or vacuum leak?

[mikedsilva]

I have the FD back and he has put stock springs in etc.. the AFR is now NORMAL.

My new issue though, is that the rebuilt WUR is giving me lower control pressure for no reason.
Let me explain:
Last week when running it was exhibiting normal (within spec) control pressures warm and cold.
Then I remember starting it again and for whatever reason the control pressure was very low. So my mixture was very rich.
I pulled the fuel connections off and blew compressed air through at the request of the rebuilder. It then gave me correct control presssures. He thought perhaps some junk from the old fuel lines got in.

I took the FD back to be set back to the original specs (ie remove the power adding mods) and he also put the WUR on his test bench.

I picked both units up and he said the WUR was perfect.

Sure enough, fitting both units and running it is running nicely.
Shut it off, chat to a friend, start it up again and now low Control pressure again.

It's not the 12V power source that is the problem. I have checked and checked.

Also, it was only 2 minutes from shutting it off. It was showing 3.3bar and now it was around 2.5-2.6.

Blew air through the connections again and it's back to normal.

I have taken the WUR back to be inspected. I think something isn't right inside. Maybe some garbage got iinside and it cannot get out.

Not sure if this means anything: but when I undiid the hose that connects the WUR to the FD, (other line is still connected) bubbles kept rising up through the fuel that was sitting in the banjo bolt. Does that sound like some sort of intermittent air leak or vacuum leak?

https://youtube.com/shorts/jaD7nazOe08?si=1B2jQw0RiNf1Q87H

[boyt911sc]

Mike,

Ask your rebuilder to calibrate your WUR and FD using your CIS pressure tester. So you are using the same pressure gauge tester for your troubleshooting works. I would not be surprised if there are discrepancies between your gauges. The question now are your gauges giving similar readings? Test and verify.

Tony

[mikedsilva]

Originally Posted by boyt911sc

Mike,

Ask your rebuilder to calibrate your WUR and FD using your CIS pressure tester. So you are using the same pressure gauge tester for your troubleshooting works. I would not be surprised if there are discrepancies between your gauges. The question now are your gauges giving similar readings? Test and verify.

Tony

hi Tony
our gauges give the same readings. It's just that I am using a slightly smaller orifice on one of the fittings on my return line to the tank on my test stand. When I just plumb an open line into the top of the filler, my system pressures are identical.


This WUR intermittent issue is perplexing.

In this video, I disconnected the fitting that goes between the WUR and the FD (via the gauge). You can clearly see bubbles rising. There are fewer bubbles in the video as majority had come out before I had the phone out of my pocket.

Is this normal?

[PeteKz]

I don't know if that's normal or not. I have not noticed it, but then, I have not been looking for bubbles when the CP line is disconnected. With the CP line connected, that port on the WUR is always under pressure, so no air bubbles would come out that way.