Super high fuel pressure, at a total loss

**kompressorgolf** · 03-02-2014 11:58 AM

done a ton of searching only to find 1 thread on all the sites about people having trouble with too high of fuel pressure, and it was a b6 returnless system.

Heres the back story. car started hard starting warm or cold, but seemed to start fine after warm and you restarted after only a few minutes. long story short after replacing wear items such as plugs with ngk bkr7e gapped at .24, fuel filter, engine speed sensor, coolant temp sensor. then i find the factory fuel pressure regulator blew out, fuel in the vac line. i figured its what took out my oem pump as well causing it to overwork. instead of replacing it with an oem i mounted a walbro 255 for future stage 3 aspirations. i replaced the FPR with one from an allroad same 4 bar but it was used. and then all was well, then i had a maf code, so i sourced an MTM stage one mbox ecu and converted the car to hitachi maf 2000 S4 manual BTW. All was fine. a week later im having hard start problems and notice the car has misfires only at idle, mainly cyl 4. I did the whole swap coils and plugs, change out my ICMs with known good backups, even swapped out a spare injector, still cyl4 mis, only at idle, runs awesome up top, 15 psi. So i decided to purchase a fuel pressure test kit, and ran the car, and this bitch is running 85 to 90 psi, so this time i bought a new bosch 4 bar fpr and popped it in, restarts great again but still had this misfire, checked fuel pressure again, still 85 90 psi at idle, even the bad fpr with fuel puking from the vac line was still pushing 80 psi. ive pulled the rail out and made sure nothing was plugged, blown through the fuel return line to the tank, ran a line from the return port on the rail to a bucket, all with no change. the only way i can get the fuel to a proper 58 psi at idle is by pinching the feed line almost closed. completely bewildered! Cant find anything about too much pressure! I run a walbro on my 400hp turbo vw rabbit, with the early style 3.5 bar digigant style fpr and its regulated perfect for years. please anyone have any insight or ideas>>???? Thanks for any help on this!

Also tried removing gas cap with 0 change

**kompressorgolf** · 03-02-2014 12:02 PM

only thing i can think of is the higher pressure pump is overwhelming the factory FPR i.e. the return port is too small like the guy with the b6 returnless system was having, but so many people including friends with stage 3 s4 have no problem with the bigger pumps.

**zillarob** · 03-02-2014 12:09 PM

Your hardware should work just fine together.
I know you already looked at it, but I would be triple checking the return side, fpr to tank.

**kompressorgolf** · 03-02-2014 12:25 PM

even with the return line off the rail and ran to a outside container, gets no drop in pressure, i will check it again, both methods and see what i get, thanks for the reply!

**kompressorgolf** · 03-02-2014 12:30 PM

another added not, may be important, is i never get air pressure from the gas cap no hiss or whoosh when i fuel up.

**zillarob** · 03-02-2014 12:34 PM

Check from fpr to the return nipple on the rail, maybe something in there from the failed fpr.

**kompressorgolf** · 03-02-2014 03:05 PM

had it all apart already, clean as can be inside the rail, took it off pulled all injectors and blasted 120 psi through all of it, every port and hole. didnt get any debris at all.

**zillarob** · 03-02-2014 03:38 PM

Thats pretty much all I got. Many people run that same setup with no probs. You got the right fpr?
Never swapped lines so not sure what that would do.

**kompressorgolf** · 03-02-2014 04:31 PM

i know right! doesnt make any sense whatsoever, checked absolutely everything, guess that walbro gonna have to come out! run RS4 pump or something!

was a 4 bar fpr, both times.

**zillarob** · 03-02-2014 04:53 PM

You take the cap off it before you put it in? lol, jk
Maybe look in the socket where the nose of the fpr goes in, maybe jacked up before it gets to the fpr?

**airjawed** · 03-02-2014 05:05 PM

Does the fuel pressure change when you take off the FPR vacuum line? With that line on and off you should see a change in fuel pressure. Also make sure that line is airtight...otherwise FPR will think manifold pressure is whatever atmosphere is.

What are your fuel trims? VCDS block 032?

**julex** · 03-03-2014 06:36 AM

It must be something past fuel regulator. The walbro 255 doesn't have enough power (especially on stock wiring!) to overcome stock FPR.

**S4James** · 03-03-2014 08:23 AM

check the two return lines in the tank. the ones that are long and coiled around. My mechanic bent them in half and kinked them closed, causing all sorts of messed up behavior.

**spennerus** · 03-03-2014 08:32 AM

Are you sure your pressure gauge is good? Might test it with a couple to see if your fuel pressure is actually high

**airjawed** · 03-03-2014 08:48 AM

Originally Posted by spennerus

Are you sure your pressure gauge is good? Might test it with a couple to see if your fuel pressure is actually high

My thoughts as well...that is why I asked about fuel trims.

**kompressorgolf** · 03-03-2014 09:06 AM

wow thanks for all the replies, i have completely bypassed all the intank stuff by running a hose from rail return to a bucket with no change, i just spent 550 bucks on this MAC tools fuel pressure tester, the first car ive even used it on! that bastard better be reading right! The vacuum does indeed change the fuel pressure, with it plugged in its reading 80-85 psi, unplugged i go to 90-95, with the long hose on and taped to the window, under boost it gets up to 110 or so. So its acting completely normal, but with 20 excess PSI. My next step is to pull it back out the tank and check that all again, if the strainer fell off or something , would that cause the spike in pressure>?

**julex** · 03-03-2014 09:46 AM

NOw I know that your pressure gauge is completely off mate. Walbro 255 has internal bypass valve set to ~80psi and at anything above 90psi it practically stops flowing as all the fuel goes via bypass not into the fuel line. It would never reach 110psi, let alone flow anything close to what's needed for WOT.

**airjawed** · 03-03-2014 10:48 AM

Originally Posted by julex

NOw I know that your pressure gauge is completely off mate. Walbro 255 has internal bypass valve set to ~80psi and at anything above 90psi it practically stops flowing as all the fuel goes via bypass not into the fuel line. It would never reach 110psi, let alone flow anything close to what's needed for WOT.

Maybe...maybe not. Depends on the valve design...it might be set to shut at 80psi if there is not back pressure. I think it is just a safety valve in case a fuel line ruptures. But if the fuel line is pressurized then the valve isn't seeing a 80psi gradient?

Rent a fuel pressure tester from autozone or whatever and see if numbers are right.

**julex** · 03-03-2014 11:14 AM

Walbro 255 flow chart. Obviously it is not possible to run 2.7 with this pump at 110psi... The gauge is off.

OP. Do you have any codes? Running so much pressure at idle would result in CEL code for fuel trims.

Originally Posted by airjawed

Maybe...maybe not. Depends on the valve design...it might be set to shut at 80psi if there is not back pressure. I think it is just a safety valve in case a fuel line ruptures. But if the fuel line is pressurized then the valve isn't seeing a 80psi gradient?

Rent a fuel pressure tester from autozone or whatever and see if numbers are right.

**kompressorgolf** · 03-03-2014 11:53 AM

Its the only logical thing, have cheap VCDS full version en route. so no logging fuel trims yet.

In theory, with the car running fuel pressure reads 80+ psi, with a clamp on the fuel line i can pull the guage back to 58 psi, so if the pressure is actually correct, with the clamp
im dropping it to around 38, would the car hesitate or stumble on that low of pressure>? because it was still smooth at idle with the clamp, ill find another way to get another guage, you fellas have been a great help, i plan to return as many favors as i can to be a real help to the forum. best audi forum ive found, my threads on audiworld stand idle after over a week haha. thanks guys!

**julex** · 03-03-2014 12:35 PM

Originally Posted by kompressorgolf

Its the only logical thing, have cheap VCDS full version en route. so no logging fuel trims yet.

In theory, with the car running fuel pressure reads 80+ psi, with a clamp on the fuel line i can pull the guage back to 58 psi, so if the pressure is actually correct, with the clamp
im dropping it to around 38, would the car hesitate or stumble on that low of pressure>? because it was still smooth at idle with the clamp, ill find another way to get another guage, you fellas have been a great help, i plan to return as many favors as i can to be a real help to the forum. best audi forum ive found, my threads on audiworld stand idle after over a week haha. thanks guys!

ECU dynamically adjusts short fuel trims (+-25%) and then stores long term ones in memory. Once you run around with too much or too little pressure and these stored fuel trims exceed 10%, you get CEL with a respective code indicating that condition.

Pinching supply line (to a degree) will not cause the engine to stumble if done progressively. As is, it is probably running maxxed out negative trims (injects less fuel than normally trying to reach stoich) so if you pinch the line, it can swing to the other end of scale by more than 50% (-25% to +25% and then long term trims from -10% to +10%, both are multiplicative) without a sweat.

**Scotty@Advanced** · 03-03-2014 12:57 PM

change your FPR, about the only thing that can cause too high of fuel pressure is a restriction in the return line either at the FPR or the line itself.

**kompressorgolf** · 03-03-2014 01:26 PM

Thanks, ill try that if the guage proves accurate, FWIW scotty, your trans rebuild kit is in this car, great stuff. first thing i did to it

**kompressorgolf** · 03-04-2014 09:09 PM

Just verified, my guage is 100% accurate, checked and rechecked everything, now im pulling it out the tank to look for trouble. still runs at 80 psi with vacuum on, neutral reving the engine, it builds almost 7 psi and the guage rises to 89 psi. there is a valve on my pressure guage and it almost has to be closed off to get it to 58 PSI at idle, i cant log fuel trims yet but you can hear the engine runs smoother and revs crisper with the correct 58 psi. It also backfires unrestricted and blows fire between shifts. Kinda cool but i dont prefer it haha. gonna get a new bosch regulator in later this week and try it again, even though it will be the third one that shows this high psi. i cant wait to find the shit causing this!

**kompressorgolf** · 03-04-2014 11:39 PM

So the mac guage is accurate, and i put the ECS tuning inline fuel pressure guage from my rabbit on it, with identical results, Also pulled the whole assembly out of the tank, and checked for anything that looked miffed, all was totally fine, no kinks in lines etc. checked the return while there, and was as previously tested, totally clear. Guess it may be time to change to a different fuel pump. im beat from this one.

**airjawed** · 03-05-2014 03:42 AM

Make sure your 3 lines to engine are correct. On occasion two lines have been switched causing weird things to happen. People have also wired the pumps wrong.

Get a hold of the Bentley manual if you don't have it. Can be found online for free.

I have seen some talk of return lines being insufficient for pumps but I don't have any first hand experience to say if that is really possible on your setup.

**rnagy86** · 03-05-2014 03:57 AM

It's more than super duper enough for a walbro 255. Did you check the EVAP system? N85? vacuum lines to it? one way vales to it?

**julex** · 03-05-2014 06:01 AM

He is saying that straight fuel rail dumped to a bucket causes this high pressure... I would still look at rail closely - the return side. Something clogged there.

As to return line being too small, that's how my car is now. I have walbro 416 E85 pump though with direct battery wiring to it so I know it can pump volumes (almost 2x more than Walbro 255 on stock wiring). But even then I don't get 80psi at idle, I get slightly less than 4bar (should be close to 3bar on 4bar FPR).

**rnagy86** · 03-05-2014 06:09 AM

Originally Posted by julex

He is saying that straight fuel rail dumped to a bucket causes this high pressure... I would still look at rail closely - the return side. Something clogged there.

As to return line being too small, that's how my car is now. I have walbro 416 E85 pump though with direct battery wiring to it so I know it can pump volumes (almost 2x more than Walbro 255 on stock wiring). But even then I don't get 80psi at idle, I get slightly less than 4bar (should be close to 3bar on 4bar FPR).

What? No. It should be around 3.9 bar with the engine idling and the vacuum line attached to the FPR, not 3 bar.

**julex** · 03-05-2014 07:36 AM

Originally Posted by rnagy86

What? No. It should be around 3.9 bar with the engine idling and the vacuum line attached to the FPR, not 3 bar.

And you know that how? 4bar fpr maintains...wait for it.... 4bar pressure difference between one side (rail) and the other side (manifold) of fuel injector. If idle is around -20inhg in manifold, that's 2/3 of 1bar (as 29.97 inhg, or soemthing close to it, is an absolute vacuum and 1bar is normal atmospheric pressure) . 4bar - 0.66 = 3.33 bar (closer to 3 than 4). So I give it to you, it is no "near 3bar" as I said, but 3.33bar, or 3.33 * 14.5psi = 47.85psi on gauge that shows pressure difference between ambient and line.

If you yank vacuum line out of FPR, you should see 4bar (58psi) on pressure gauge.

**airjawed** · 03-05-2014 07:45 AM

With vacuum line disconnected and engine at idle fuel pressure is 3.8-4.2 bar

With vacuum line attached a drop of 0.5 bar........ according to Bentley manual.

**rnagy86** · 03-05-2014 08:02 AM

Originally Posted by julex

And you know that how? 4bar fpr maintains...wait for it.... 4bar pressure difference between one side (rail) and the other side (manifold) of fuel injector. If idle is around -20inhg in manifold, that's 2/3 of 1bar (as 29.97 inhg, or soemthing close to it, is an absolute vacuum and 1bar is normal atmospheric pressure) . 4bar - 0.66 = 3.33 bar (closer to 3 than 4). So I give it to you, it is no "near 3bar" as I said, but 3.33bar, or 3.33 * 14.5psi = 47.85psi on gauge that shows pressure difference between ambient and line.

If you yank vacuum line out of FPR, you should see 4bar (58psi) on pressure gauge.

I've never seen less than 3.7 bar fuel pressure at idle with the FPR line on, with brand either a brand new FPR or a used one. My gauge might cheat a bit then.

**slow ride** · 03-05-2014 08:35 AM

I'd guess you have to have some sort of restriction after the regulator or the port the joins it to the rail, etc. All the regulators you have tired can't be bad. Just because air can pass doesn't mean the passage is open enough to regulate the 255 down to 4 bar either. I'd look into getting a used rail to try/run.

**kompressorgolf** · 03-05-2014 11:22 AM

Ok, ill try to run something through that passage, to make sure. theres an allroad at the junkyard, if its reasonable ill grab the fuel rail to try that. already taken most everything else. My vacuum is low, had a code once for evap large leak but went away and hasnt returned. ive checked the green one way valves and they are solid and aimed the right way. The N80 clicks like its supposed to every now and again. when i blow through the line that runs to the back of the car from the n80 valve, the air just comes out the filter on the LDP in drivers wheel well, no restriction. Is the LDP blown out then too? my Vacuum is at 15-16 in/hg at idle and it only goes to 20 on decel in gear. been searching for months trying to figure why its so low. Boost pressure tests to 25 psi show no leaks at all. spider hose and F hose are all tidy, no issues there. TBB boot has been removed and thoroughly checked and its fine. Ill post the full car stats so everyone can see the big picture.

2000 S4 6MT
abox to mbox conversion with hitachi maf and recoded
MTM stage 1 chip spikes 17psi holds 15
fully de catted DPs all 4 cats removed, rear 02s spaced out with anti foulers
stock cat back (i know) getting a neuspeed cat back soon
forge 008 DVs blue springs one shim in each
Stern rear diff ring
RS4 clutch and Advanced Automotion trans rebuild kit
KO3s are tight as can possibly be literally o shaft play in and out side to side want (k24s or f21 soon)
and apparently gods personal walbro 255 :/
all recently new (fuel filter, rpm sensor, coolant temp, maf hitachi, walbro pump, switched out known good injector to #4, 2 new fpr,

**slow ride** · 03-05-2014 11:41 AM

Well Vac should be around 20-21in with stock cams with a good engine (sealing/timing) around 800rpm with all load off the engine. Either way, with the engine not running you should be able to run the pump and get 4 bar at the rail with a 255 as the others have said. With the pump running and the engine off, the regulator has to bypass the most fuel it will ever have to for the most part so if it works then it should be fine when running.

**kompressorgolf** · 03-05-2014 10:58 PM

pulled the return line from rail off today, and pulled the regulator, couldnt find anything to feed through the passage, so i used an 18 guage piece of wire and shoved it through the hole, when it came out the other side absolutely nothing came out, so i ran it through a couple times, nothing came out. put it back together, now im running 67 psi at idle, 2 guages verify. not sure what happened, but im pretty happy about it! Guess using alot of air pressure wasnt good enough, not sure what was in there, or why its on par now, but ill keep an eye on the pressures and report back with any changes.

**jibberjive** · 03-06-2014 02:11 AM

Be careful, whatever you dislodged can clog your injectors and burn up a cylinder. Keep a vigilant eye on your knocking. I actually wouldn't put that rail back on until you verified that it's completely clean.

**airjawed** · 03-06-2014 02:33 AM

Originally Posted by jibberjive

Be careful, whatever you dislodged can clog your injectors and burn up a cylinder. Keep a vigilant eye on your knocking. I actually wouldn't put that rail back on until you verified that it's completely clean.

I think the block was on the return side... so if moves it will move back to the tank...or get lodged again and cause high fuel pressure. No need to worry about blocking injectors.