Simon928

I was doing some work on my '90 S4 and I had my air filter box out and my MityVac handy so on a whim I thought that I'd vacuum test my fuel dampers and regulator. The damper on the front held vacuum but the damper and regulator at the back of the engine did not. I put a dipstick in each of them and I couldn't detect fuel on any of them. I also inspected the vacuum lines and couldn't detect any fuel in them either. Does the fact that they don't hold vacuum automatically mean that they are bad, or is the fuel test more relevant?

jeff spahn

They should hold. BTW, be prepared for sticker shock. They have moved to classic pricing.

GregBBRD

Retest.
I suspect your vacuum tester was leaking, as vacuum leakage is extremely rare.

FredR

Simon,

You should be able to get the thing to hold vacuum and if as you did, you can achieve this with one of the three items then logic says you should be able to achieve such with all three items.

There is a failure mode that folks generally do not appreciate and that is when the nipple on the instrument is not sealing with the body rare as it may [or may not] be. I had this happen on the rear damper not long ago. Upon careful examination it felt as though the nipple had a slight "looseness" about it- how this came about I have no idea- no memory of dropping the thing or anything else stupid but...? I put some non hardening sealant around the base and then pushed home one of those rubber connectors until it sat in the goo pile and left it overnight to vulcanise. Tested it the following day and then it held vacuum.

Not exactly a satisfactory situation but considering the cost to replace that set it is scary. I expected it might get me by until I could replace the thing. The last time I checked that part seemingly was not even available so I did another vac test and it still held and to date it has been like that for two years- I now test it every six months and if it continues to hold all well and good but the sad fact is those things probably have a reliable service life of 15 years and after that it is a crap shoot.

icsamerica

You can observe damper operation using by a fuel pressure gauge. The gauge should be undamped. If the vehicle's damper is bad the fuel pressure will osscialte or increase / decrease with injector firings. This would be more noticeable at low idle speeds. The 928 fires all its injectors at once in a batch, this will create a momentary decrease in fuel pressure that the damper is supposed to absorb. If the damper is bad the gauge needle will swing abruptly in time with the click of the injectors.

I have no idea why Porsche spec'd a vacuum reference on a damper...Perhaps it's a safety consideration, should the damper's diaphragm fail the fuel would safely be vacuumed into the engine.

Simon928

I retested and I am getting the same result--the damper on the front holds vacuum but the regulator and damper on the back do not. I'm not doing the test any differently on the rear ones compared to the front so I don't think that there is an issue with the vacuum tester or the way I am doing the test, though I'm sure that it is possible. I don't have a fuel pressure gauge on me right now so I can't do that test at the moment.

I plan on doing a full intake refresh this winter so that was one of the reasons I decided to do a check on these components. I'm sure that I already have a number of issues with vacuum leaks or sensors at the top of the engine that need addressing so it would be hard for me to attribute any symptoms directly to the FPR or FPD without making sure I eliminate any other potential problems first. I already have new GB fuel lines but I haven't checked the dampers and regulator until now. The prices have risen astronomically for these items so I wanted to confirm that they were indeed bad before throwing money at them unnecessarily.

And I admit that I haven't been too in the loop on this, but are there currently any other quality aftermarket alternatives to the Porsche FPR and FPD for S4s?

karl ruiter

I use a mighty-vac type tool with a clear hose. Crank in some vacuum and, if it is leaking, you will see fuel bubble up in the hose, coming out of the part. That, by the way, it the problem when they fail....the intake sucks unregulated fuel through the vacuum port causing the motor to run rich, especially at idle where vacuum is high and fuel needed is much less. Be a little careful as, once one of the regulators or dampers is leaking you can get fuel all through the vacuum lines. I have seen it back at the transmission vacuum modulator. So, if you seem to see fuel bubbling out of multiple regulators/dampers go through them multiple times, each time removing any fuel from your test line. Last time i did this, I initially had fuel bubbling up from the dampers and the regulator, as well as back at the transmission. After a few cycles of going around the parts and clearing out the hose I could see that it was only the regulator. Might be wise to do the test with the fuel pump jumpered on, but I usually don't.

Simon928

I did that test with the FPR and rear FPD but I didn't see any fuel in my MityVac hose. Would it be better if I do this test with the car off and a jumpered fuel pump, or would pressurizing the system not make a difference for the purposes of this diagnosis?

karl ruiter

As far as the dampers are concerned you can do it with the car running or not running and the pump jumpered. I usually just do it with the car off, and have been able to see problems that way. Even if your check valve is bad and the system in not holding pressure when off, it will still be full of fuel, not air. So a leak to the fuel side should pull fuel. If you try to do it with the regulator and the car running you will make it run rich, but I don't know if it will die or not. Also, if running, you would want to plug the removed vacuum line.
My 944 has a leaking damper at the moment and since the part is NLA I am waiting and hoping a used part will do the trick. In the mean time, if a video of a failing leak test would be useful to you I think I can make that happen.

Simon928

Thanks for the advice Karl! I can jumper the fuel pump without much difficulty, so I will do a test with the engine off and see if I can make the MityVac pull fuel under those circumstances. That being said (and I don't mean this flippantly at all) if the FPR and FPD can't pull vacuum with the engine off does further testing even matter? Is there any scenario where either the FPR or FPD can't hold vacuum but they are still usable, or are they pretty much a potential fire hazard at this point?

Billu

If it won’t hold vacuum, it is shot.

Brett Jenkins

I vacuum tested my FPR and 2 dampers. None of them smelled of fuel or had any fuel that I could tell in the vacuum line. I drew 15 vacuum on the FPR and it bled off in about 60 seconds. The front damper did the same. The rear damper held 15 vacuum indefinitely.

Speedtoys

Leaking vacuum or fuel, its shot.

If its >10yrs old, it's likely also shot..because it's frozen solid...and not functioning.

GregBBRD

If the dampers and the regulators never dry out...as in constantly have fresh fuel running through them, the diaphragms last a bit longer.
Taking them off an engine and letting them sit around or allowing the fuel system to dry out/allow the fuel to become "stinky" is "diaphragm death".

Brett Jenkins

Considering that the car has been sitting for a minimum of 3 years, I suspect the diaphragms are dried up, so I will be replacing them. Is the FPR the same for 86 and 86.5? My parts diagram doesn't differentiate.