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1Back to top Go down   Fuel leak pressure regulator... Empty Fuel leak pressure regulator... on Thu Dec 26, 2019 11:11 pm

slowandlow

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Hello!
Ive been doing a complete overhaul of my gas tank, did a test start tonight, everything that I had repaired and replaced inside the tank worked fine after a few moments of hesitation (engine has not run for a year)!

But... Behind the plenum chamber I saw a nasty fuel leak, in the first few minutes of engine running at idle, I saw a jet of fuel splashing (probably) out from the fuel return hose, close to the FPR.

I think the fule return hose is broken (and was the initial problem, not the indeed very poor condition of the parts inside the fuel tank) and I will replace both of the hoses to and from the FPR.

Would it make sense that the FPR at the beginning of engine startup will receive a fuel overpressure, and let it off through the fuel return hose? Or would it be more probable that the FPR itself could be broken or leaking?

Else everything looks and sounds ok, fuel return check valve is cleaned and is in good working order as far as I can determine.

If I replace the fuel hoses and the fuel leak is gone and engine is working ok, can I presume that the FPR is ok, or what do you think?

Thanx!

/ Örjan.

    

2Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 1:09 am

Point-Seven-five

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If it doesn't leak and the engine runs well, I would tend to believe the fuel pressure regulator is okay.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

3Back to top Go down   Fuel leak pressure regulator... Empty Fpr on Fri Dec 27, 2019 1:43 am

daveyson

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If the fpr diaphragm is leaking, it will probably be an internal leak that can't be seen.  It is connected to the rear cylinder by a vacuum hose at the throttle body area. If there are signs of fuel or dampness there when removing the vacuum hose,  that would point to a leak,  especially if the cylinder was on the intake stroke at shutdown.


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11/1985 bmw k100rt (late model)  Vin. 0090567
    

4Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 3:07 am

MartinW

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If leak in the diaphragm is large enough, fuel can be sucked into No 4 cylinder. This can result in a hydraulic lock and a bent conrod.
regards Martin.


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K75s Hybrid
    

5Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 7:16 am

Arlina

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@MartinW wrote:If leak in the diaphragm is large enough, fuel can be sucked into No 4 cylinder. This can result in a hydraulic lock and a bent conrod.
regards Martin.

Got me there.... Just replacing an engine, because the cylinder was damaged too...

Fuel leak pressure regulator... 75264840_548673125951376_4425792214584524800_o.jpg?_nc_cat=101&_nc_ohc=fKxz1FjOMPgAQlAXIAytscNICPOuPz6tpeRcUlCmKw509XiuaqpwPwz4A&_nc_ht=scontent-amt2-1


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Fuel leak pressure regulator... Eu-log10  K1100RS/LT - R1200RT - R1100RS (RIP) - Cagiva SST 350 Ala Verde - K75LT project
    

6Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 8:07 am

slowandlow

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Hello, thanx for answers!

I will check (and replace) the hoses and hope for the best.

How does the FPR work? In what phase of engine operation does it let fuel through the return hose? Should I be surprised to see fuel (leakage) at the return hose at all, or does the fuel (at high pressure) pass through this hose on a (very) regular basis?

BRG/ Örjan.

    

7Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 8:24 am

Arlina

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The return hose should not have any pressure.
It's just a hose with open ending dumping the fuel back into the tank.

The pump pumps at around 6 bar of pressure the fuel to the injectors and the FPR.
The FPR is set to 2,5 bar, any more will pass.
The injectors will receive 2,5 bar.


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Fuel leak pressure regulator... Eu-log10  K1100RS/LT - R1200RT - R1100RS (RIP) - Cagiva SST 350 Ala Verde - K75LT project
    

8Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 8:31 am

MartinW

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The FPR has a internal diaphragm and a spring with a vacuum inlet on the upper side of the diaphragm this counters the spring pressure when the need for high fuel flow dictates high fuel pressure is needed.
Fuel leak pressure regulator... Fuelpr11


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K75s Hybrid
    

9Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 1:48 pm

Point-Seven-five

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@MartinW wrote:The FPR has a internal diaphragm and a spring with a vacuum inlet on the upper side of the diaphragm this counters the spring pressure when the need for high fuel flow dictates high fuel pressure is needed.
Fuel leak pressure regulator... Fuelpr11
Actually, the vacuum, in working against the spring lowers the fuel pressure when the throttle is closed.  Since this is usually done when decelerating, this reduction in fuel pressure reduces the amount of fuel injected into a coasting engine and helps to prevent backfiring.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

10Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 1:56 pm

Arlina

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The K75 I'm working on has a leaking FPR, sending the fuel through the vacuum hose to cylinder 3.
Unfortunally, injector at cylinder 3 did also leak.
Double the fun Laughing


__________________________________________________
Fuel leak pressure regulator... Eu-log10  K1100RS/LT - R1200RT - R1100RS (RIP) - Cagiva SST 350 Ala Verde - K75LT project
    

11Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 6:00 pm

Laitch

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@Point-Seven-five wrote:
@MartinW wrote:The FPR has a internal diaphragm and a spring with a vacuum inlet on the upper side of the diaphragm this counters the spring pressure when the need for high fuel flow dictates high fuel pressure is needed.
Actually, the vacuum, in working against the spring lowers the fuel pressure when the throttle is closed. . . .  
cheers scratch

In the L-Jetronic, the FPR's sole function is to maintain consistent—not fluctuating—fuel pressure. Vacuum connection aids in stabilizing pressure.

High fuel demand and low fuel demand are satisfied at the same rate of fuel rail pressure. Closed-throttle fuel delivery rate is governed by the throttle switch signal's effect on fuel injector firing.


__________________________________________________
1995 K75 77,000 miles
Fuel leak pressure regulator... Usa-lo10
    

12Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Fri Dec 27, 2019 7:27 pm

Point-Seven-five

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From the Bosch LE Jetronic Manual:

"The pressure regulator keeps the pressure differential between the fuel pressure and the manifold pressure constant."  

ie. The pressure in the rail will be reduced by high vacuum.

"The spring chamber is connected by a tube with the intake manifold downstream of the throttle valve.  This has the effect that the pressure in the fuel system is dependent upon the absolute manifold pressure.  Therefore, the pressure drop across the fuel injection valves is the same for any throttle position." 

ie. The vacuum acting on the regulator spring causes the regulator bypass to open at a pressure lower than the spring rate, thereby reducing the actual pressure in the rail as measured by a gauge in atmosphere.

What this means is that the gauge pressure in the rail will vary for different throttle/loads.  It does this to compensate for the changes in intake manifold pressure which will vary from 0psig to fairly high vacuum readings.  This is important in overrun situations with the throttle closed where the high intake vacuum would result in excessive fuel delivery causing increased emissions and decreased fuel efficiency, and in the case of our bikes, backfiring. 

Under heavy load, the intake vacuum goes to near 0psig, and the effect is to raise fuel rail pressure to deliver the proper amount of fuel for combustion.

Operation without the vacuum connection is virtually unaffected at high loads and wider throttle openings.  At smaller throttle openings, the engine will run with a richer mixture, with the amount of enrichment dependent on how high the intake manifold vacuum is.  Maximum enrichment occurs when closed throttle in high vacuum engine braking situations.  Rich mixtures under these conditions leads to backfiring.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

13Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Sat Dec 28, 2019 5:27 am

Laitch

Laitch
Life time member
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@Point-seven-five wrote:From the Bosch LE Jetronic Manual:
Neatly copied!

Our understanding seems divided by a common language questionably translated. Perhaps we can agree that the throttle switch has a greater effect on closed throttle performance than the FPR, based on how irritated some riders get by throttle response at low speed.


__________________________________________________
1995 K75 77,000 miles
Fuel leak pressure regulator... Usa-lo10
    

14Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Sat Dec 28, 2019 2:24 pm

Point-Seven-five

Point-Seven-five
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You are right, we are both talking about the same thing from different points of view.

I agree that the TPS works on a closed throttle, but the vacuum applied to the Fuel Pressure Regulator prevents overfueling in situations where the throttle hasn't been totally closed and when the owner has disconnected the TPS because of notchy throttle response at low speeds.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

15Back to top Go down   Fuel leak pressure regulator... Empty Re: Fuel leak pressure regulator... on Sun Dec 29, 2019 3:40 pm

cycleman

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Replace the hoses going to the Fuel Pressure Regulator (FPR)and see if that fixes your problem.  I'm in the process of doing that to mine and also going to relocate the FPR. The hoses & the FPR are in a very high heat area, so they do fail.

With respect to the vacuum comments on the workings of the FPR.  On our bikes is there an advantage of drawing vacuum from all the cylinders, similar to manifold vacuum in a car/truck engine?  I've read several comments about those who have made stuff that allows the FPR to get vacuum from all 4 cylinders on our K's. It is supposed to be a benefit.  That actually makes sense to me as you wouldn't have the large drop in vacuum at the FPR when under acceleration, when you are drawing vacuum from only 1 cylinder. Drawing from all 4 would smooth out the vacuum drop.

That being said, I doubt the engineers who designed the engine and its components ever felt it was necessary, and the engine ran fine with just getting vacuum to the FPR from just one cylinder.  It also wouldn't matter which cylinder you got the vacuum from, #4 ( K100 ) was easiest to route the vacuum line, but when moving the FPR you can take the vacuum from what ever cylinder is closest.

    

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