# Increasing total fuel system pressure CIS?



## dohc (Sep 28, 2001)

*Increasing total fuel system pressure CIS (shimming pressure relief valve)?*

What does increasing fuel system pressure by shiming the pressure relief valve do in general?
Does it make the engine run richer in all conditions?
Thanks

[Modified by dohc, 10:40 AM 2-5-2003]


[Modified by dohc, 10:41 AM 2-5-2003]


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## dohc (Sep 28, 2001)

*Re: Increasing total fuel system pressure CIS (shimming pressure relief valve)? (dohc)*

Really? No ones done this?


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## Racer_X (Jul 14, 2002)

*Re: Increasing total fuel system pressure CIS? (dohc)*

Increasing the system pressure does a few things for CIS. 
In a CIS basic system, it will make the mixture richer under all operating conditions. On those systems, it's similar to adjusting the mixture screw. 
On CIS-Lambda and CIS-E systems, it won't enrich the mixture because the electronics will adjust the mixture back leaner to get the oxygen content of the exhaust back in optimum range.
The second side effect of increased system pressure is very important in a forced induction system. Higher system pressure will increase the pressure differential between the fuel injection system and the intake manifold. Basically, your fuel injection system is designed for a normally aspirated engine. 
In both CIS-E and port fuel injection systems like Digifant and most stand alone systems, the system makes some assumptions. One assumption that is usually made in fuel injection systems designed for normally aspirated engines. There are no "fuel flow sensors" in the injection system. Actually, there aren't any pressure sensors either in most systems. The design assumes a fixed pressure in the fuel system and an absolute pressure somewhere between a vacuum and atmospheric pressure in the intake manifold. Systems with a MAP sensor can tell what level of vacuum is present in the intake. The amount of fuel that flows through an injector is dependent on the difference in pressure between the fuel system and the intake manifold or port that the injector orifice is open to. A higher pressure difference will flow more fuel. A smaller pressure difference will flow less fuel. 
In a normally apsirated engine, the pressure difference stays close to the same in most operating conditions. You have ~70psi in the fuel injection system and somewhere between about 25 inches HG vacuum and atmospheric pressure in the intake. That's a range of 12 psi or so in the intake. One reason that there has to be enrichment at full throttle is because the pressure differential is less, and the metering orifices in the fuel distributor have to open a greater amount to flow the same volume (mass actually) of fuel with the reduced pressure differential. 
In a forced induction application, you could have an additional 10-25psi of pressure in the intake. That would further reduce fuel flow for a given opening in the CIS system, or a given duty cycle on the electronic injectors on another system. This reduction in flow will make the mixture leaner as the boost pressure rises. That's a really bad thing (tm). 
Raising the system pressure will increase the pressure differential and increase the fuel flow under these conditions.


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## Butcher (Jan 31, 2001)

*Re: Increasing total fuel system pressure CIS? (Racer_X)*

You make some sense but you are not all the way correct. The EFI system does have a way of measuring manifold pressure. The pressure differential is virtually the same because the fuel pressure is changed with the vacuum in the intake manifold. There is a vacuum line to the fuel regulator to make this happen. This is there so the ECU does not have to guess what the manifold is seeing and change the fuel injector time to compensate for this. 
Lambda systems are great but they do not compensate that much and are very slow in making that change. Sure if you raise the fuel pressure it will most likely cause a richer mixture but when the O2 sensor is not working [cold running, full throttle] it will most likely be too rich. When you do finally keep a contant throttle the O2 sensor should keep up and might keep the fuel mixture correct but everytime you change the throttle opening there will be some delay in the sensor correcting the fuel mixture. If there is a large change it has to make then the driveablity will suffer.
Raising the operating pressure for a CIS system and playing with the control pressure might get you a higher fuel delivery at full throttle/max load conditions and have a reasonable driveablity but you should know about the CIS system, how to change the warmup/control pressures, and have a good fuel pressure gauge to make the changes.


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## dohc (Sep 28, 2001)

*Re: Increasing total fuel system pressure CIS? (Butcher)*

There's a vacuum controlled FPR in the system? Where? I've only seen this on the CIS-E systems. And some Audi CPR have a vacuum control added into the regular temp controlled CPR.


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## imaradiostar (May 4, 2002)

*Re: Increasing total fuel system pressure CIS? (dohc)*

CIS-basic and CIS-lambda cars from VW (at least in the USA) didn't have a vacuum sensitive pressure regulator or even control pressure regulator (also known as a WUR). Audi, Saab, and Volvo turbo-CIS cars used a vacuum (and boost) sensitive WUR which decreased control pressure based on engine demand. This allowed the air meter to raise higher and meter more fuel to the engine for a given amount of air flow. Picture the control pressure as the force against air flow meter movement. The higher control pressure is, the less the flapper will move at a given amount of air flow.
Per the manual you should only shim the FD pressure regulator (little spring loaded thing in the FD) if it's out of adjustment and the system pressure in the FD isn't correct. Some performance guys advocate a slight increase in pressure to move the whole fuel curve upward. A slight increase in pressure can yield a slight increase in maximum fueling at the expense of increasing minimum fueling.
At one point I experimented with drasticly altering fuel pressures on my GTI- I shimmed the FD pressure reg using a small washer that was WAY too thick to use as a shim. I would back the mixture screw out to where the car would run decently and it would no longer idle because the FD would go from zero flow to too much flow too quickly. It's as if the system pressure in the FD (which I don't know because I didn't have a guage) was so high that if the plunger slit was exposed at all, too much fuel made it to the injectors. In the summer months when the car would heat up quickly it would run fine once the o2 sensor warmed up and the frequency valve compensated for the difference but throttle response never really seemed very good.
Some people have had good success with a slight increase in pressure in conjunction with the Mercedes-Benz 190E injectors. They're supposed to be a higher-flow part and perhap's they're better suited to a higher fuel pressure.
jamie


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## dohc (Sep 28, 2001)

*Re: Increasing total fuel system pressure CIS? (imaradiostar)*

Thank you!


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## Butcher (Jan 31, 2001)

*Re: Increasing total fuel system pressure CIS? (imaradiostar)*

After you raise the fuel pressure you must also increase the warm up regulator pressure too. This will add more pressure to the top of the fuel dist. and cause the dist to run leaner [for a better driveablity]. Adjusting the warmup regulator is not difficult but like I said in past posts you must have gauges or your better to leave things alone. I agree with someone that stated if you raise the system pressure everything gets raised. MB uses a vacuum operated warmup regulator to increase the fuel pressure [to the dist] under vacuum. This does make it much better to use as you drive around with a better fuel mixture and when you floor it, it will have a richer fuel mixture.


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## ToddA1 (Apr 22, 2002)

*Re: Increasing total fuel system pressure CIS? (Butcher)*

can you explain how to adjust the wur (pics would be nice). I'm using the audi 5000 turbo wur, and see no way of adjusting it other than taking it fully apart.
Thanks


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## Butcher (Jan 31, 2001)

*Re: Increasing total fuel system pressure CIS? (ToddA1)*

As a tech that has worked on CIS for many years I know this works but if you do not have any fuel gauges for the CIS system you will screw things up. The fuel lines attach to the WUR. The place where they attach to is made from some type of steel [not aluminum]. That piece is pressed into the WUR and if you move that piece up or down you will change the fuel pressures going to the fuel dist. This does not change the system pressure just the control pressure. The metal plug also does this but it is difficult to move that up since there is no way to grab it. The metal piece that is pressed into the WUR is very hard to move and you must be gentle with the hammer [or prybar if you go up]. I have done this many times to set the cold pressure just right to keep these customer happy so I know it works. If you have never done it you might want to have spares in case you screw something up. 
Most WUR change the presures only so far. By adjusting the WUR you can't change the amount of change but where the change is happening. By raising the system pressure and then raising the WUR pressure you can raise the entire amount of fuel delivered and still have a drivable car. 
DON'T FORGET TO HAVE A FUEL PRESSURE GAUGE AND THE WARMUP REGULATOR PRESSURE GRAPH. ADJUST THE WARM UP REGULATOR HOT OR COLD BUT NOT IN THE MIDDLE.


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## ToddA1 (Apr 22, 2002)

*Re: Increasing total fuel system pressure CIS? (Butcher)*

thanks, I have some spares...time to tinker.


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## imaradiostar (May 4, 2002)

*Re: Increasing total fuel system pressure CIS? (Butcher)*

Mark- I've often wondered how CIS pressure can be adjusted. I'm guessing that you adjust them by taking them apart and bending the bi-metallic strip to vary its resting place. Bending it closer to the diaphragm would lower control pressure and away would increase control pressure. Is this what you're doing?
I've read about a mod where you can thread a bolt into the WUR and use it to vary the control pressure. Have you ever tried that method?
It would seem to be that a bi-metallic strip should last for many years longer than the average WUR so maybe it's a matter of cleaning and adjusting a non-working WUR to make it functional again.
Any more tips for us?
Oh yeah, for the record, JC Whitney sells a CIS pressure guage set for 66 bucks.
jamie


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## Butcher (Jan 31, 2001)

*Re: Increasing total fuel system pressure CIS? (imaradiostar)*

As easy as changing the pressure of the WUR I would stick with the 'butch' method that was explained earlier [and do not bend the spring]. I have disassembled a few WUR in the past but it has been so long ago that I do not remember what is really inside to talk about. When I worked with BMW the early 320 [with the 1.8 motors] ran poorly cold and we used to push in the small plug on the top of the regulator to adjust the cold pressure. If you went to far in we disassembled the regulator and pushed the pin back up. You learned very quick not to hit it to hard since it was a PITA to remove and repair your screw up. When I went to MB I learned that moving the entire pressed in piece [where the fuel fittings are] did the same thing but you could move it up and down [the pin could only be moved down with out disassembly]. By making some small slide hammer or using a 'Lady Finger prybar' we could make it go up. Remember it is pressed in there and it was not designed for a daily adjustment. If your cold pressures change constantly then it is time to replace the warmup regulator. But to 'fine tune' your new one to the warm up regulator graph that is in many Bentley and Bosch books that it does work.


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## imaradiostar (May 4, 2002)

*Re: Increasing total fuel system pressure CIS? (Butcher)*

I see what you're saying- I wasn't paying attention before. Thanks for the info and hopefully you won't give me extra homework for zoning out in class!!
jamie


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## dohc (Sep 28, 2001)

*Re: Increasing total fuel system pressure CIS? (imaradiostar)*

Someone I was talking to recently told me I could adjust pressure by moving the WUR away from the block so it remains cooler...
Any thoughts on that?


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## imaradiostar (May 4, 2002)

*Re: Increasing total fuel system pressure CIS? (dohc)*

quote:[HR][/HR]Someone I was talking to recently told me I could adjust pressure by moving the WUR away from the block so it remains cooler...
Any thoughts on that?[HR][/HR]​This will defeat the purpose of the WUR- you won't have a variation in control pressure from cold to warm running.
This assumes you unplug the WUR heating element, of course.
jamie


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## Butcher (Jan 31, 2001)

*Re: Increasing total fuel system pressure CIS? (dohc)*

The WUR works with heat. They have a heating element that bends a spring that causes the fuel pressure to go higher. If you add space to the mounting of the warmup regulator then that will just cause your hot engine starts to be too rich and it will run poor. Like the other guy said you would have to remove the electrical connector so the heating element will not be working.


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## A20 PVW (Nov 15, 2002)

*Re: Increasing total fuel system pressure CIS? (Butcher)*

How about piping cold fresh air straight to it(the WUR). That way the faster u go the more cool air it will get meaking (in theory) it run richer???


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## superl8 (Dec 31, 2002)

*Re: Increasing total fuel system pressure CIS? (A20 PVW)*

I have made the WUR adjustable - take a look
http://superl8.tripod.com/ 
do a google for---rsche control pressure regulator
that's where I got the idea and the details.
BTW mine is off an audi with the vacuum compensation as well as altitude adj.
volvos and sabb use CIS as well just look till you find the one that looks similar to the one I've used as it adjusts for load via vacuum
good luck. 
BTW this thing should mount on the block or head somewhere that gets hot.


[Modified by superl8, 3:06 AM 2-15-2003]


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## Mr Black (Jan 20, 2002)

*Re: Increasing total fuel system pressure CIS? (superl8)*

Can someone explain to me how a WUR, which is supposedly design to enhance warm-up (ie-at idle) is connected to full throttle enrichment? I don't understand the relationship.


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