[RS Barn]

[ATTACHMENT NOT FOUND][ATTACHMENT NOT FOUND]In my quest to get the 968RS running properly many weird issues have cropped up.

A first the car was running lean. When it was tuned on the dyno to fix that it would run well for about three laps and then start breaking up. It seemed electrical as it only happened when car was warmed up and I started to drive hard. On the dyno it would do several pulls without any problem. Hum!!! I finally found a one way valve on the fuel cell that was restricted and reducing the fuel flow and eventually starving it enough to cause the misfires. after fixing that and rewiring the fuel pumps the fuel mixture is way too rich. Like 9.8 to 1, and it still runs ok just not a ton of power. i had it back to the dyne to lean out and finally make some real power. At least thats what I hoped. As we started to do dyne pulls at leaner fuel mixtures and getting more power it started to cut out again. and I know we are getting plenty of fuel and the venting is fine. new problem!!!!

We believe the fuel is airating when it is getting returned to the holding tank.

I though I would show you a picture to show you what I'm dealing with.

I may take some of the stuff off and simplify but that's a lot of plumbing I don't really want to do.

In the picture:

Pump 1 (under the two gold transfer pumps) and pump 2 visible

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[Eric_K]

My head hurts. Didn't you mention that there are 5 pumps total, what are all these for? Are there multiple fuel cells?



Eric

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[RS Barn]

Actually there are four pumps and a filter. My Head really hurts!!!!!!!!

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[Jason Judd]

Man,



There's a LOT of braided lines running around in there!



Jason

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[Brett968]

[quote name='RS Barn' date='Oct 6 2005, 08:02 PM']Actually there are four pumps and a filter.[right][post="10804"]<{POST_SNAPBACK}>[/post][/right][/quote]



Um, why??



Brett

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[S_Cal968]

Just curious, was the pressure monitored through the entire rpm range? After looking at your pic I noticed there's a heck of a lot of fittings which reduce flow - I wonder if you're getting the necessary volume[?]. And obviously larger fittings = more volume

Just a thought...

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[RS Barn]

Good thought. These pumps will each run around 600 hp so there shouldn't be a problem with volume even with restrictions.

I am going to run a fuel pressure guage to the cockpit (temporary) and drive the car. If I have any fuel starvation I will see a drop in fuel pressure. That will either show the problem or tell me flow and volume are ok.

Pete

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[Aus968]

Pete,

That has to be the most elaborate fuel system I have ever seen, i see the two bosch pumps but cannot see what type of pumps the other two are, It looks to me like the fuel comes out of the bottom of surge tank into a bosch pump through a filter into another bosch pump then back into the surge tank, or are my eye's deceiving me, man if i were you i would pull all of that out and start again use the same components but just not as many, I just cannot see what its all for, I mean talk about redundency, they look like pretty big pumps surely just one of those bosch pumps should be enough to supply the car with ample fuel and the second one for a back up,

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[flash]

i agree with pete - take a look at the volume and not the pressure - you could easily have 110psi and only 1 gpm - i remember on a supercharged mustang we did, the regulator we first used would dial in the pressure fine, but he volume went in the dumper - had to open it up a lot with a bigger orifice regulator and bigger fuel lines from the tank to the pump

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[Brett968]

With that many pumps I would be concerned that the fuel pressure regulator would not be capable of bleeding off enough volume, especially at idle and lower loads. Reading the fuel pressure should tell the story. Care to explain why you are using so many pumps?



Brett

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[flash]

as an explanation of how pressure and volume interact, take a large diameter garden hose, put a nozzle on it, turn it on and fill a bucket - track how long it takes to fill the bucket, and feel the force of the water



then connect a smaller diameter one - do the same testing



you'll quickly find that decreasing size increases pressure but decreases volume



reading pressure on a fuel system won't tell you what you really need to know, but is a common mistake people make in diagnosis - it will not tell you how much fuel is being delivered - you need to measure volume for that - the increased volume demands of the turbo most likely will mean a need for larger than stock fuel lines at a bare minimum



pete, i don't know what size you have in there now, but i would conduct a volume test, see what you are getting, and go from there - this sounds very much like the problem we had

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[S_Cal968]

[quote name='flash' date='Oct 7 2005, 03:07 PM']i would conduct a volume test, see what you are getting, and go from there

[right][post="10890"]<{POST_SNAPBACK}>[/post][/right][/quote]

yes..

I've seen a similar test done to qualify fuel injectors. The injectors are placed in separate graduated cylinders and they had to flow an equal volume within a given amount of time.

I suppose in your situation, [if your injectors are fine] you could just test the main line. The $64 question is what is the optimal flow rate [gph] for your set up[??].

Also I've heard it's better to use 90's only as a last resort - they have the highest restriction.

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[Brett968]

[quote name='flash' date='Oct 7 2005, 05:07 PM']reading pressure on a fuel system won't tell you what you really need to know, but is a common mistake people make in diagnosis[right][post="10890"]<{POST_SNAPBACK}>[/post][/right][/quote]



I disagree entirely. Fuel pressure will tell you what you need to know provided you are testing all running conditions. I don't see any reason at all for checking volume at this stage. The only time I ever test fuel flow is when baselining a fuel pump (or injectors of course) or when checking the integrity of a pump itself. Turning the pumps on and running them with the engine not running while checking the pressure has limited use but checking the pressure while the engine is running and most importantly at full load is the first step in diagnosing the problem. I am quite confident that I would be able to diagnose the problem with fuel pressures, if not just by looking at the configuration itself.



Pressure and volume are of course interrelated and if the volume cannot be met for a given pressure than the pressure will drop. If the fuel lines are too small (and the pump(s) can't make up the difference) then the pressure at the fuel rail will be low when engine fuel demand is high. Even if the fuel lines are smaller than ideal, I wouldn't suspect the feedline to be a huge problem since with 4 fuel pumps there should be more than adequate capacity to make up the difference. I would almost be more concerned with the fuel pressure regulator and return line in this situation. Four fuel pumps means a heck of alot of fuel flow. The pressure regulator will bleed off volume to regulate the pressure. If the pressure regulator is not up to the task (IOW, cannot handle the volume) then the pressure will go up especially when the engines fuel requirements are lower (idle, etc.) Additionally too much flow in the return line will cause a pressure drop across the this line that is proportional to fuel flow and hence will be unpredictable during engine operation as the engine fuel requirements are varying. Since the fuel pressure regulation is now outside the control of the fuel pressure regulator other variables can't start to rear their ugly head. Fuel pump voltage can start to alter the actual rail pressure, as can the pressure drop across the fuel filters which can change with time. None of this is any fun and the result is a very inconsistant and unpredictable fuel mixture.



Specing a proper fuel system is very simple and straightforward. First you need to know the engine fuel requirements. This is basically the maximum fuel flow required. Now take this number and add in a safety factory, say 30-40%. The next step is where you actually measure volume (assuming you don't have definitive numbers from the manufacture.) Make sure that the pump you are using can supply the required fuel pressure (include your safety factor) at the pressure that you plan to run. (When calculating pressure be sure to include a few PSI for the filter, fittings, etc.) OK, that was easy. Now if you are racing buy two of the pumps. You will parallel the pumps. Make sure that you have a one-way valve on the exit of each pump. Now make sure your fuel lines are big enough. This is super easy also. There are guidelines available elsewhere on the net so I won't bother with that. Commonly overlooked is the pressure regulator. If you are using multiple pumps make sure the regulator can bleed the full flow of all pumps at the required fuel pressure. With four fuel pumps I can see this being a likely problem although it is certainly more than possible that something else is wrong in this particular case. Also make sure that the return line is large enough so that pressure differentials are negligible in all conditions.



I am still trying to figure out why one would use four fuel pumps?



Brett

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[flash]

do you have an air compressor? ever try to run an air wrench on a puny one? the volume is a big issue - the thing may put out 150psi, but can't keep it up for long - ever try one of those dumb little 5/16" plastic coil type air hoses you get at home depot? not enough air getting through, even though it's full pressure



it doesn't matter how may pumps you hook up, or how big they are - if the line is too small you will not get any more fuel through it - there is a maximum amount of fuel a line can push - a turbocharged fuel injected 3 liter motor will probably need every bit of a 5/16" line to maintain volume, and may need to go up to 3/8"



i don't know how to explain this any better, it's a pretty basic plumbing thing, but you could have all the pressure in the world and still have very little volume



i'm not necessarily saying the lines ARE the problem, but rather that volume delivery could be - the regulator could certainly be the problem, as could all the connections - they could be limiting the volume of fuel being delivered



the only way to see is to find out how much fuel it flows - the engine requires a specific number of gallons per hour to run, not just pressure

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[S_Cal968]

@Bret

I can understand your point, at a given volume the pressure may drop when the injectors open and the press reg. is not bypassing excess fuel. Although, pump(s) can only produce so much pressure.



If we question what is the optimal pressure - it's a product of the size the injectors and spray duration. The problem can be corrected by running more pressure or volume. It's easy to add a little more pressure, but it's safer to increase volume. Also I would guess the pressure would go up if the regulator wasn't able to bypass enough volume and would appear faulty[?].



The point is there's several things to consider when troubleshooting a system and this is why I mentioned earlier about checking the fuel delivery during a dyno run and not just at idle. If they're running more than one pump at a time, perhaps they were having a flow issue, [perhaps fixing the wrong end of the horse...(per se)][?]



Also I can't tell from the pic, but is the fuel filter downstream of all the pumps..?

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[rustech]

Flash, it's quite basic really... <img src="/forum/images/smilies/968/dry.gif" class="smilie" alt="" />

...Since the fuel pressure regulator is responsible for holding pressure in the rail and is situated "post rail", ...Since fuel pressure is taken "at the rail", ...And since the fuel lines in question are situated "pre-rail", though the pressure would be misleading "Pre-fuel lines in question" due to possible restrictions...If the pressure is maintained consistently at all engine loads at the rail "Post-Lines in Question", then there "IS" enough flow to keep the rail pressurized properly.



The injector flow may indeed be the issue, however is independent of the "line restriction" stated.

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[Brett968]

Sorry, Flash, but you aren't quite correct here.



I am quite versed in the relationship between pressure and volume. In this case it could not be much simpler. In your analogy you talk about the pressure being 150psi yet not enough air going through the lines. That is because the pressure at the air wrench has droppped. It has dropped because the pressure drop across the line is higher than a larger diameter hose at the same line entry pressure and flowrate. The 150psi is at the compressor. If you were to measure the pressure at the air wrench you would find it was much lower with the narrower hose. There is no need to measure the volume of air to determine that the flow rate is lower. You can determine this by the pressure alone since the characteristics of the wrench are the same. If you don't believe me try it yourself.



In the case of an engine, the pump is the compressor and the engine is the air wrench. If the pressure at the fuel rail drops (assuming a given injector duty cycle, etc.) then the volume of fuel entering the engine drops. There is no way that the pressure at the rail will be the same and the volume of fuel delivered to the engine will change (assuming the exact same engine operating condition.) This defies physics. The volume will only change if the pressure does. However, this does not mean that the volume will be constant if the pressure at the pump is constant. (Just as in the case of the air wrench where the pressure at the compressor was constant but the volume and pressure at the air wrench varied because of the pressure drop across the lines.) If the feed line is too small and the pumps cannot make up for the extra pressure drop then the pressure at the fuel rail will tell the story.



And the number of pumps being hooked up does certainly matter and needs to be considered. Please reread my previous post. I am 100% sure that I am correct.





Brett

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[Brett968]

Here's what I would do if I were stumped on the issue:



Connect three fuel pressure gauges. Connect one at the fuel pumps, connect the second one at the fuel rail, and connect the third gauge just after the fuel pressure regulator on the return line. Now operate the engine under all conditions and record the readings. To diagnose:



Constant or Intermittent Low pressure at fuel pumps - inadequate fuel delivery meaning fuel pumps are too small or restriction or other problem pre-pumps



Constant or Intermittant Low pressure at fuel rail (but good pressure at pumps) - feedline overly restrictive (IOW, too small) or problem with fuel filter (if between pump and rail)



Anything other than a very low pressure after regulator - return line too small



Intermittent High pressure at fuel pump and fuel rail (but low pressure after regulator) - regulator not capable of flowing pump requirements. IOW, regulator too small. Problem more likely to occur at lower engine loads. Could also conversely appear as intermittently low pressure at fuel pump and fuel rail during high engine loads. Either way it is the same thing.



Keep in mind that there could be more than one problem. For instance, one could have an undersized return line and an undersized regulator. In this case, I would fix the return line first and retest.



Brett

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[Greimann]

Brett, that analysis is right on. I think that is the last word on that.

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[flash]

i'm clearly sucking at explaining what i am trying to say here online, and it reads like it's spinning into hostile, and i know nobody wants or intends that - brett you have a PM with my phone number - give me a buzz

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