andydoody

I’m Andy and have a Canadian ‘94 S4 with MTM+1, 3 Bar MAP, 034 High Output coils, Magnecor racing ignition wires, RS2 Exhaust manifold, 3” downpipe, ABT Exhaust, Bosch 040 FP, and FP relay kit. Cats were removed.

Car has low 12 PSI boost; normal boost was 21 then decreases as revs increase. After a few days CEL (code 00537 maximum limit exceeded) displayed @ idle although idling was stable. CEL would shutoff with throttle but returned within 30 seconds. Checked the O2/plugs; both are covered with soot.

Idle now fluctuates, fuel is very rich, and black smoke from tail pipe. Replaced O2, plugs, fuel/air filter, FPR, and MAF but nothing worked. Intake/vacuum hoses checked. Hooked up pressure gauge and got 94 PSI. Removed vacuum to the FPR; pressure increased to about 97/98 PSI. FPR is good; injectors checked/cleaned. Checked return fuel line and exhaust for blockage; nothing found.

Installed old stock pump; pressure became 58 PSI @ idle, understand target pressure @ idle should be 51 PSI. Old stock pump initially stable @ idle but the CEL was on all the time. Reinstalled 040 pump; pressure was 94 PSI; bad idle;black smoke. Returned old stock pump; pressure was @ 58 PSI; this time idle just as bad as with the 040 pump.

Any advice is greatly appreciated.

UrS4boy

Something is definitely wrong. Start with replacing the fuel pressure regulator PN 078133534C(4 Bar Bosch 0280160517):

http://genuineaudiparts.com/parts/in...&siteid=214407

andydoody

Hi Dave. Yes I purchased an FPR with the same part numbers you quoted but fuel pressure still registering 94 PSI. Btw, the older FPR is only a year old or so. The air filter and fuel filter were replaced only 4 months ago when the oil was changed but nonetheless I bought another set to try out. Unfortunately the pressure was still high.

Thinking aloud I'm wondering if the pump somehow moved and is now sitting too low inside the tank. If so, do you think this would cause high fuel pressure? The old stock pump sits in the original basket so I'm pretty sure it's not sitting too low in the tank. I will pull out the 040 pump and check the height of the adapter relative to the old fuel pump basket. Will let you know what I find.

Thank you!

andydoody

Hello again Dave. I was mistaken about the part number of the FPR. the one I have is 0280160575. This is the same part number on my older FPR. Checked a few websites and my part number is listed as compatible with the S4. It is also rated at 4 BAR.

UrS4boy

Looks like we are both right. These are also supposed to be interchangeable 4 Bar Fuel Pressure regulators:

PIERBURG 72160501
PIERBURG 72160551
BOSCH 0280160516
BOSCH 0280160517
BOSCH 0280160575
VW / AUDI 078133534
VW / AUDI 078133534A
VW / AUDI 078133534C

Reference: http://www.ebay.co.uk/itm/Benzin-Dru...item3f252ba837

So there is a flow issue. Pumps, including fuel pumps, can either put out a lot of pressure pumping against an obstruction, i.e. closed valve, or a lot of flow at a much lower (e.g. totally open valve and just line losses) or something in between, creating a pump curve. Your 94 psi is very likely measured at the fuel rail (with an added aftermarket fuel gauge??) and also very likely represents the shut off pressure for the pump (the max it can do at no or very little flow). The general shape of a pump curve is the black line in this diagram. For some reason, you have an obstruction in the fuel line between the pump and the return line dumping into the tank.

<img src="http://www.jensenengineeredsystems.com/wp-content/uploads/2010/10/Single-Line-Pump-Curve.bmp"><img src="http://12v.org/urs/FuelSystemWithFPRinfo_1.jpg">

andydoody

The fuel pressure gauge is indeed running off the fuel line that supplies the fuel rail. Gauge is from 034. I agree that the pump is probably operating at its limit. It's morning at my location so I will be able to look for an obstruction along the feeder line and all the way to the fuel rail. Get back to you with what find.

UrS4boy

I posted your problem on another forum and one of the forum members there remembered a post about this issue by a fellow named Mance E. (screen name "VAP") who investigated the "A" vs "C" versions of the Audi/Bosch fuel pressure regulator.

This is what Mance said:

"A common problem that occurs when using a fixed port rising rate FPR such as the "plug-in" Bosch we use is that fuel pressure cannot be effectively/efficiently controlled when the fuel pump volume being delivered is different or higher than came on the car as a "mated pair" or when a different FPR, even with identical pressure but with a smaller/larger return line orifice diameter is used other than what came on the car. The pump's volume as well as the FPR's return line metering orifice are specifically engineered to work with each other. Important to note at this point there is clear evidence to suggest ALL major manufacturers build fuel pumps that actually deliver 15-20% more fuel when new than they are rated to deliver. The reason for this is simple, ie; to compensate for pump deterioration over the pump's life as well as to compensate for things like dirty/clogged injectors et al.

When/if a different configurtion FPR or fuel pump than came on the car is used, the original balance is forfeited unless balance is restored via matching the components to each other. When an FPR is used that has the correct pressure rating but a smaller return line outlet orifice diameter than was originally designed, engineered and specified to work with a given fuel pump's volume delivery specifications the balancing act between pump and regulator are compromised. The same thing occurs when a larger output pump is used with an FPR that was originally designed to work with a smaller displacement pump. Depending on the dynamics involved the result of this can run from mild/barely noticeable to extremely bad and/or anywhere in between. An FPR with too small an outlet diameter metering orifice with a stock/OEM pump or a higher-flowing pump with an OEM FPR create the same/identical issues as those relate to fuel return.

When using a larger displacement pump or an FPR with too small of a discharge/return line port, there is a large pressure spike at idle and with rapid deceleration/high intake vacuum because the smaller FPR return port orifice diameter cannot evacuate enough fuel through its metering orifice when the diaphragm is fully deflected off of its valve seat to the open position. This causes a continuous/ongoing rich condition at idle as well as a momentary rich condition during/after post-decelleration, which may lead to a rough idle quality for a few seconds until the rail pressure stabilizes (but of course it cannot stabilize at idle the way its supposed to because of either 1; the increased volume due to a larger displacement pump or 2: the reduced fuel return orifice diameter which compounds the issue further). Conversely, if the discharge/return port orifice is too large the rising rate/self-adjusting feature struggles due to diaphram "flutter" making precise responsive adjustments for the regulator almost impossible because the fuel flow out of the orifice is too rapid and the diaphram diameter and internal spring of the OEM FPR is too small and subsequently not sensitive or responsive enough to respond quickly or efficiently. The FPR beats itself to death trying to keep up.

Larger, precision stand-alone FPRs get around this by offering replaceable return line metering orifice ports from say .040" to .200" diameters in .010"-.020" incremental diameters to allow the FPR to be tailored to respond precisely to the pump's volume output as well as other variables inherent in any/all fuel delivery systems. And those existing metering orifice diameters can be tailored more precisely with the use of twist drills for even smaller diameter increases. The system as-is is very good offering long life and relatively good when used with OEM pump that closely matches to return line metering orifice. But that is lost if/when the C-series FPR is installed in place of the A series FPR that came on the car. And when higher output pumps are used everything changes again. Not so much with an A series FPR but DRAMATICALLY with a C series FPR. Add a 5 bar FPR and guess what? Yep, even bigger changes! The thing to remember here is that the FPR and fuel pump are the symbiotic relationship in the fuel delivery system. Pressure-wise, any 4bar FPR would work. But volume-wise things can NEVER be the same once a C series FPR is swapped into the system that originally came with a .140" diameter return outlet that now has to "make-do" with an .086" diameter outlet. Something it struggles with given its 45.4% smaller! And that goes double if say an 044 pump is used with a newer C-series FPR!! The fuel pump and FPR were sized for each other originally and without much regard to anything else in the fuel delivery system.

Yes, there does seem to be latitude in what is acceptable and/or tolerable but a 45.4% reduction in fuel return design with the accompanying idle, off-idle and decelleration issues is simply too much for the C-series FPR to swallow (pun intended)."

So the bottomline is the "C" version might be compatible with the stock 091G fuel pump but the return opening is too small when the the supply pump is something like a Bosch 040 or 044. Drilling the return orifice out requires a jig that Mance developed. Otherwise you need non-Bosch fuel pressure regulator that has the capability of swapping in different sized return orifices that will match your 040/044 better.

andydoody

Hi Dave. The information you dug up sounds spot on to me. I will try to find the A series FPR over here in Manila but failing that I will try source it in the U.S.

This morning I hooked up a Porsche 997 GT3 fuel pump and an outboard adjustable FPR to see if there would be a change of pressure. On first test the FPR was set so that the fuel pressure sat at 51 PSI while idling. It idled very badly with a good amount of black smoke when given a throttle up. FPR was adjusted until the idling smoothed out and came out with 61 PSI @ idle. Even at this reduced pressure the CEL would come on. Tried out the GT3 pump with the C series FPR and the fuel pressure increased to 69 PSI. Idle was still good but CEL remained.

I also decided to purchase an original VW/Audi fuel pump just to see what happens. At idle the fuel pressure dropped from 94 PSI to 69 PSI with the C series FPR. Although the idling is smoother the CEL still comes on and only goes away (for a short while) when increasing the revs. Black smoke is also less.

I'm still running rich but that may be because fuel pressure isn't in the 51 PSI range as it should be. My next move is to find and try the A series FPR. I shall keep you posted.

Meantime allow me to extend my sincere thanks to you, the forum member who referred Mance, and Mance for even thinking of comparing the original A and C series FPRs.

andydoody

Hi Dave. It seems finding an Audi original A series FPR is proving to be tough. Everyone is saying it has been superseded by the C series. Would you happen to know where I can purchase an A?

The other alternative as mentioned by Mance is an adjustable FPR with replaceable return line orifice. I found an AEM brand universal FPR with replaceable return line orifice. The orifice sizes offered are 0.100, 0.150, and 0.200. Mance says the original A series FPR is 0.140. Do you think the AEM universal adjustable FPR with a 0.150 orifice be ok for our cars?

Also, what other hardware would I need to install the AEM or similar FPR. It looks like the AEM doesn't attach to the fuel rail.

Thank you!

UrS4boy

I've put the word out on adjustable FPRs with variable size return orifices that fit the OE fuel rail FPR receiver. I'll let you know what turns up.

However, are you sure that you don't have an air leak in your MAF to Throttle body intake system?

Boost leak testing:

https://www.audiworld.com/forums/sho...php?p=24413028