Fuel Pressure Regulator vs. Fuel Pressure Dampener... How do they work together?

[phoenix_iii]

N00b question...

Researching some things, I may be replacing the Fuel Pressure Regulator.

How does the Fuel Pressure Dampener come into play?

I have an 85.5 944 NA.


I am looking to replace my fuel lines (Lindsey kit) and vacuum lines (again, kit, etc)



944-110-201-03-M14 --> Fuel Pressure Damper, 924S 1987-88, 944 1983-84, 944 02/85-88, 944 Turbo 1986-89
http://www.pelicanparts.com/cgi-bin/PartsLookup/search.cgi?command=search&part_number=94420106101

944-110-198-03-M14 --> Fuel Pressure Regulator, 924S 1987-88, 944 02/85-88, 944 Turbo 1986-89
http://www.pelicanparts.com/cgi-bin/PartsLookup/search.cgi?command=search&part_number=94411019803

[bholmes]

the fuel pressure dampener dampens the pulses coming into the fuel rail from the fuel pump so it doesn't surge really bad due to the pump. the fuel pressure regulator is on the outlet side of the rail and it regulates how much pressure is in the fuel rail at all times and lets the rest of the fuel go back to the fuel tank once the pressure is met.

[Tidybuoy]

There's also pulses caused by the injectors. Each time an injector opens, there is a slight drop in fuel pressure. The Damper compensates for this temporary drop in pressure. It's basically a spring loaded device that keeps pressure up.

Where as the fuel pressure regulator releases fuel (back to the tank) when pressure is too high. It keeps the pressure down to a prescribed, safe level.

They work together to keep pressure even.

[TibetanT]

^ You guys really know how this works!

I wouldn't add anything to what was just stated above. Good job on explaining the fuel pressure regulation and dampning.

[HondaDustR]

Not necessarily a "safe" level, but a consistent level, since the injectors will flow more or less fuel depending on the pressure.

[Tidybuoy]

There's a third part of the equation and that is the fuel pump check valve. This is a one way valve that prevents fuel pressure from leaking down after the car is shut off.

By keeping the pressure up, even when the car is off, it enables the car to start on the first few cranks - otherwise the system would have to build up pressure before having enough fuel pressure to squeeze fuel thru the injectors - taking 10 or 20 cranks before the engine will start.

[BoxsterGT]

Can someone explain about the "surging" that can occur?

Why might this damper be needed in a full on race car with full throttle then closed throttle repeatedly?

Is that where the "surging" happens?

Len

[v2rocket_aka944]

the "surge" described has no correlation to the throttle position.

the fuel pump may make pressure pulsations as it turns but the real culprit that the dampener tries to solve for is the water-hammer effect inside the fuel rail when the injectors open/close. imagine a tube full of liquid, at 45 psi, with 4 outlet valves that open and shut in milliseconds. that liquid in the tube wants to rush out the valves and starts moving but when the valves shut the fluid crashes against the closed valve and bounces around - this is the pressure spike inside the rail.

the dampener is a spring-loaded chamber that can expand slightly and quickly to absorb that pressure spike (since the fluid, fuel, is incompressible). this repeats a bajillion times a second (or whatever the injector open/close frequency actually is).

it helps keep the fueling more stable but is largely a NVH reducer. lots of cars with the same Bosch fuel injection in the 70s-90s did not use dampeners and were fine. and dampeners largely disappeared with the introduction of sequential fuel injection since only 1 injector was firing at a time. however some more luxury-oriented cars with sequential injection have since added back a dampener for the NVH reason mentioned above.

i have a non-Porsche engine in my car using the stock fuel pump, batch injection, and no dampener, and it runs fine but there is some taptaptaptap noise audible which i attribute to the injectors.

[BoxsterGT]

So how does this "Water-Hammer" effect the performance?

Why would one want the damper especially when running on the track?

Is there a shake or roughness in the motor under open & closed throttle?

The custom fittings and the $200 plus cost of a 3.2 Damper adds up fast. Why spend the money?

What is the benefit of adding a damper, especially when they are so expensive now?.

Len

[v2rocket_aka944]

it probably won't effect performance enough to make a big difference. pressure fluctuations can mean actual fuel going through the injector can vary vs the target. but it would be a little rich or a little lean for a given instant.

[Monkey Wrench]

like in a plumbing system '" water hammer" causes shock waves this rattles your pipes. a good plumber may install a dampener or even just add a T and run a line upward and cap it off, then the air gets stuck inside and does the same, because air can compress, also a plumber will affix rigid lines with the knowledge that even minor flex can cause fatigue as its repeated many times.
they do move. even if you are gentle with taps, there is the valves in your dishwasher and washing machine , they do similar and may be abrupt. a good comparison is your pipes moving because the washing machine is filling and stopping. the weight of water in a pipe is considerable and so if the water is stopped and started , that causes a shock.

In a car. ok it is higher frequency but it still may cause fatigue in the fuel lines, or at least in rigid lines. I believe that's the reason for the short rubber line after the pump and before the rigid lines. The engine of course moves so it needs rubber lines there too. rubber may absorb some shock if the capacity is basically able to change, but the lines are generally quite rigid in a fuel system.

the dampener is not essential to make the car run but some reported some noticeable noise if it is removed.

its easy to ascertain that vibrations come from the pump but there is a little thing here, in general a pump does not cause pressure, it causes flow. an open pump produces no pressure.

Pressure is the resistance to flow and similar for vibrations, the varied resistance to flow by way of the injectors opening and closing is the cause of the vibration.. the argument can be a bit like a chicken and egg theory but that is how it is normally approached in hydraulic theory.


so what can fail on a fuel pressure dampener? well it has no exit, maybe it could get full and become ineffective, I suppose it could leak, I bet that most that are being replaced are just mechanics being thorough and perhaps wanting the cosmetics of a new one. Ive never heard of a failure myself, has anyone else here seen hard evidence of theirs actually failing? the pressure regulator it can fail, if it fails fuel can get through the diaphram and come into the sensing line which should not contain fuel, if you find fuel in that line its a sign the diaphragm has ruptured..

I just replaced the FPR in my ford van with a straight six even though it tested fine and fuel pressure was ok.. this was because replacement is very difficult and I had to remove the giant intake manifold. I basically shotgunned a few parts during a disassembly and reassembly.. I have a few 80's volvos and a ford van, they have bosche fuel injection and no fuel pressure dampener.

my feelings are that a fuel pressure regulator could fail and maybe cause a problem if you were out on a long trip. if it starts to fail it could also confuse you and led you into some other direction before you located the cause.

I personally feel the chances of such an event around the dampener failing are much less likely.

I'm not planning to replace mine, If I did I'd put in an adjustable one , or one from some other car. because ( in my opinion) it's overpriced..

I have in the past posted a link to a spreadsheet showing different makes and models and their fuel rail pressure. I believe that any one would work so long as its designed for the same pressure as the Porsche,or adjustable to that range, and maybe for an engine that is of somewhat comparable size.

the rest is just plumbing. the only reason I'd gravitate to the OEM one is if your car is original and pristine enough for that OEM look, to matter enough to pay the porsche price. many here are in that boat and keep their cars really nice and that is worth something. If you have the wallet for it, go for it ! if not, I'd google fuel dampeners, there is a lot of info on the web.

i'm basically not rich , so its not on my list. If you are, keep the economy flowing ! ;-)

[Monkey Wrench]

Originally Posted by Tidybuoy

There's a third part of the equation and that is the fuel pump check valve. This is a one way valve that prevents fuel pressure from leaking down after the car is shut off.

By keeping the pressure up, even when the car is off, it enables the car to start on the first few cranks - otherwise the system would have to build up pressure before having enough fuel pressure to squeeze fuel thru the injectors - taking 10 or 20 cranks before the engine will start.

the system does retain fuel pressure, provided the injectors close properly and the check valve is working as it should. you can do a test install a guage run it then watch how long it maintains pressure..

10-20 cranks , I dont think i agree with that..


actually the fuel pump comes on with key on and primes the system anyway. it might help it start a tad quicker. I dont think it's unusual for the pressure to leak down overnight past the check valve.

it is possible though, if an injector doesn't close properly to get a bunch of white smoke upon startup if you do have an injector stuck open. that can dump a 1/2 teacup or so of fuel into a cylinder... it is possible if a cylinder gets full of fuel , to break the engine with the starter.. check for that if you notice a white cloud of raw fuel smoke upon startup. I've had that happen. when I saw it my volvo would blow white smoke for a block, then run normal most likely just one injector gets stuck so it is localized. I also had one stick closed, no fuel and the engine then basically ran on 3.. the symptom was a rough idle and lacked performance. Its surprising how well an engine can run on 3 though..

[BoxsterGT]

Thank you MW, really appreciate the information.


Yes, the replacement cost of the original 3.2 FPR and Dampener is up there, and function is most important.

I have started to make up adapter fittings (M16 x M12) to install a used 3.2 Dampener in the 930 FD to WUR circuit for testing.....



The original uses 3mm Polyamide tube with a rubber cover. Once complete a final version will be shown.

Len

[v2rocket_aka944]

Originally Posted by Monkey Wrench

actually the fuel pump comes on with key on and primes the system anyway. it might help it start a tad quicker. I dont think it's unusual for the pressure to leak down overnight past the check valve.
.

as stock a 944 fuel pump does NOT prime at key on.
the fuel pump only turns on when the engine is cranking due to the way the DME/DME relay are wired.

you can get an aftermarket DME relay with a prime function if you want though.

[Monkey Wrench]

that makes sense. In my Volvos and ford van the key on does prime the pump, interesting that the stock Porsche does not. often owners will get used to the sound of the pump upon key on, and use that as a means to notice if the pump or relay fails. it is harder to hear it when cranking, or when engine is running.

I stand corrected.

[Monkey Wrench]

the pressure guages use the bourdon principle
they may vary a lot in quality,

I used one for air pressure, rated at 200 PSI , a bit too high to be really accurate but its what I had in my junk box. you can buy better quality stuff, a good gauge is probably water filled.

careful with the lines and dirt because the fuel injectors dont have big holes so any dirt or stuff could potentially get trapped in here.. its usually got the fuel filter to protect it.

my ford van had an easy way , it has a pipe thread and basically a tire valve with a heavy duty valve cap.. in my volvo and maybe the porsche I thought I'd connect after the line to the rail and I might sacrifice the end of used fuel line and the end of a used fuel rail to get those metric fittings. I wonder if they are the same size, maybe, probably metric . I sure you can buy them..

I asked at the auto parts counter and he had some stuff in a kit but it looked like Chinese plastic junk to me so I just used an air guage.. not the best thing for accuracy but ok as a quick and dirty check I guess.

[TheSecondChris]

I tested my 87 951 fuel pressure when the engine is running. The gauge reads a little above 2.0 Bar, which corresponds to what is should be from Clark's Garage.

The needle wiggles a little bit between 2.0 and 2.2 Bar. This seems to indicate to me that the FPR and Damper are both working.

I suppose that these readings could be very different when under boost condition, but I did not simulate that.

How much bounce do you see when the damper is bad?

[Monkey Wrench]

the shocks are caused by the injectors opening and closing and interrupting fuel flow. that happens at a high frequency. a mechanical gauge couldn't keep up with the frequency to report the pulses, so its showing an average basically.

the type of gauge usually used is a mechanical device that works on the Bourdon principle. if you wanted a faster response rate you could look into electronic pressure transducers.

a pressure transducer reports electronically so it can be coupled to a computer with a digital input, a mechanical gauge is connected with a hose.

I believe the ECU is triggering the injectors and does so in time with engine position and RPMs. I think they are fired in banks of 2 and not singularly as you may expect but I'm a bit grey on the details of that. they are basically running at a fairly high frequency so the pulsations may make a sort of buzzing sound or a whine that would increase with RPM.
a fairly high frequency vibration.

it could probably be graphed and analyzed if you set up the appropriate measuring devices, but this is not a normal mechanical check.

better quality fuel gauges will be likely fluid filled, water is enclosed to slow the needle movement to allow a more accurate average reading

you may find that if you look into newer cars they may have gone towards the injectors firing in sequence and you can look up "sequential fuel injection". in that system the injectors would fire individually and spread the vibration over more smaller pulsations.

in general the fuel systems in cars began as fairly simple carbs which were easy to maintain by farmers of back yard mechanics,, and toward more proprietary and expensive to repair systems, some may basically be designed to send you to the dealer to make them money and require sophisticated software and tools to analyze service.

the problem for the consumer and the environment. is that they manufacturers use a system of constantly changing sophisticated parts with a limited lifespan, this is because the auto industry is not interested in longevity nor the environment, but is instead driven by profitability..

one way to accomplish that is to keep reducing the expected lifespan of cars to force more need for sales. when a new car reaches an age where the cost of all these unique proprietary parts and dealer service exceeds the used vehicles worth, they get to sell another vehicle. this is called planed obsolescence.

some older cars that came with carbs, and evolved to be fuel injected and electronically controlled , could be upgraded to the older carb systems, reducing cost of maintenance often with some trade off in mileage or power, but the 944 was never made with carbs. Its an option if you own a common older muscle car..

wen the automakers started with fuel injections they also used proprietary PCB boards. The printed circuit boards could have been standardized but I know of no manufacturers that chose that path. Instead each manufacturer created their own proprietary system and copied the technologies of others. the boards became proprietary patented technologies.

some of the older cars were being converted to run on natural gas, its cleaner burning, makes less power , needs a big tank. we have so much propane and natural gas there we are starting to sell it on an international level.

natural gas doesn]t need to go through an oil refinery. I believe a practical approach would be to use small engine hybrid designs, that would eliminate the need for batteries, Also cars could be standardized and made with the intent of being repairable products. If batteries did need to be used they could be made easily swapped for fast charging and they could be standardized. CNC technologies to dismantle and re-use materials could be involved and designed at the design stages of products. its wasn't a mistake that these technologies dont exist. If robots can efficently assemble cars they could also be made to efficiently disassemble their parts for recycling of materials. the reason we do not have disassembly plants is because the industry is driven by profit and not common sense.

I have a friend that drives a crane in a scrap yard, I asked what hey do whrn they get a Tesla? drop it in the shredding machine so it is ground to bits, batteries and all . only now are batter recycling plants being designed. this is because the profitability of recycling plants is taking a back seat to manufacturing more products.





if the auto manufacturers became more interested in the environmental impacts of their products. great strides could be made with respect to the environmental concerns.

car recycling was pretty much made obsolete intentionally by the auto manufacturers through use of cheaply made proprietary parts and complex procedures.

Henry ford used standardization, he copied what the gun manufacturers did and made millions of model T's using standardization principles. and they were well loved due to being easily repairable by the farmers and back yard mechanics.. VW and Harley davidson and volvo did well at making repairable products.

now the auto manufacturers are fooling consumers into paying for high priced electric cars which are constantly being changed and altered. the ideal situation for an auto manufacturer is that the consumer pays the dealer constantly because he keeps buying cars designed for a very limited and controlled lifespan. buy or lease , doesn't matter, you are basically renting the car and you'll be back on schedule with your wallet open again as intended.

the problem is not that the manufacturers have found technologies that cannot be standardized, the same technology that was used to build a model T can be applied to modern hybrids or electric cars.
the problem is that the auto manufacturers see standardization as a threat to their profit margins. Longevity is not their goal , fast fashion cars which hurt the environment are the result.

The problem was intentionally created because of of automaker profitability making the decisions instead of common sense.

[Monkey Wrench]

what you can see by driving your car with the fuel pressure gauge connected is any dip in pressure. lets say your fuel line or the screen in your tank or your filter is partially blocked

under that condition you may find that because the fuel pressure regulator is doing its job, you get a satisfactory reading as compared to whats expected. now if you drive the car flat out up a hill and see the fuel pressure drop then you would know the fuel delivery rate is lacking and as a result could see a drop in fuel pressure..

I do not think you are seeing any of that so I'd say you are done checking that. That's not to say you couldn't have some other issue like a air leak or a dirty or plugged up injector. the injectors each have a little gasoline screen and can be back-flushed. you can also look at the spray pattern or check if it has a leak or is stuck closed.

you can do that if you pull the fuel rail and watch but many will not recommend it as you are then atomizing gasoline and could cause a fire or explosion.

a stuck open injector will cause fuel pressure to drop quickly but your gauge seems to be holding pressure over some duration of time. if it did leak, the fuel in the fuel rail would continue to flow into a cylinder, and then when you started you'd get a bunch of white smoke , excess fuel in the the cylinder clears itself.

with a leaky injector, you'd see the gauge drop in pressure due to the leak so the gauge is a safer way to see if the injectors are able to close properly.

if the injector sticks closed it basically won't fire in that cylinder and youd be running around with an engine that lacks power, won't idle well, but probably still runs.

when you pull the plugs do a side by side comparison , if you ever see any drastic differences you can look for why that is. if they all look tan colored and OK then I wouldn't suspect a fatal issue with the injectors


an injector could spray but maybe with a bad spray pattern, much like a old spray can with a half plugged nozzle. that may be harder to pick up on

Im just replacing the 6 injectors in my ford van, it was running terrible so I put a bunch of parts in.. access is not good, I had to spend a lot of time removing the throttle body and upper manifold.. Its not so hard to gain access to in a porsche.

i'm hoping that installing reconditioned injectors will help my van run better. I chose not to clean them myself because access is very time consuming , otherwise I'd check then and clean them myself.

I think what surprises me most is that they seldom actually fail, there are lots of 30 year old cars running around with no new injectors.
My 944 sat so long the gas was basically turned into goo so I wont crank it and I will clean them and flush the fuel system before I even try to crank the engine..

I have a new filter pump ready and may take the fuel lines out and replace most of that..

the OEM fuel lines can be expensive, and there are aftermarket parts that will fit. some use a type of anodized aluminum fittings, some buy OEM or there is a guy on this forum that can make them up to look OEM,

I have a hydraulics shop that i take hydraulic lines to and have them crimped, I believe these are metric lines. I called and they told me they wont work on fuel lines, only hydraulics.

someone here did a nice write up with all the part numbers to convert to anodized aluminum fittings and there is a marine fuel hose. I bought some samples. it is a really thick heavy rubber fuel rated hose with an inner plastic core where the fuel runs. I think those are perfectly adequate. I do not believe it is metric line but may fit the type of fittings youstd need to convert it.

if the car is very original you may want OEM. I'm swaying to replacing mine with the aluminum fittings and marine grade fuel like. I contacted a local fittings place. I was planing to remove all the fuel lines and let them help me order them so I can get all the correct parts.

the diagram with the part numbers is a good start, most will be identical parts, bear in mind there were some changes, so you want the correct stuff.

on some cars the fuel lines run above the hot exhaust manifold, on mine they do not , you can consider relocating them. the problem is , that if there is a fuel leak. it can cause a big nasty gasoline fire and wreck your whole day. this has happened with other 944's


I think if you put a tap in the line before the dampener, when you closed it you may sense some noise emanating from the fuel line. I don't think you'd see any difference at a gauge or see much difference in performance..

over time fuel line vibrations could possibly cause some undesirable metal fatigue.. whether you'd ever see a problem with that I dont know, but when things vibrate there is stress upon the metal. repeated fatigue could possibly cause metal hardening and cracking like what happens with airplanes. It might be unlikely to see a failure, perhaps you'd hear the noise and it may be slightly annoying.

I suggest keeping yours if it hasn't failed. a new one might look better cosmetically

my 66 volvo has a similar part in it's braking system, its a piston with a spring behind it that can move somewhat if I stomp on the brakes and it is mounted underneath the car.. In its' lifetime, no leaks, no failures. single circuit braking system too.

[Monkey Wrench]

im not sure n the actual numbers but the fuel presume regulator obtains vacuum from the manifold.

the manifold vacuum will be higher or lower under certain conditions so it may be designed to be slightly variable.
when you are hard down on the gas, one may think well its sucking hard then so it has higher vacuum..

now that assumption may be actually quite untrue, I'm thinking back to an old international scout truck I had with a vacuum driven wiper that was a little worn and stubborn, sometimes it would stop wiping.

sometimes I'd have to let up on the gas to allow the windows to wipe better.. so I think it actually has more vacuum when the engine is slowing the vehicle with the throttle closed. its trying to suck air, but the throttle is not open so there is a higher vacuum level..


how the vacuum reacts may be slightly different than you first imagine. someone else can explain that better..

the way the engine increases the fuel rate is by increasing the open time ( dwell time) of the fuel injectors. I do believe you will see some fluctuations in fuel pressure and that may be normal.