K75 Flooding

[daveson]

Doh, I was editing this post, not realising there was a reply on the next page, so I deleted it and copied it downstream:

Fuel should also be returning to the tank when it's cranking. If fuel isn't retuning while cranking, that would explain why the pressure is lower at revs.

One of the other basics when you're in the tank, is to see if the fuel filter is installed the correct way around, and that it's not blocked.

Was the reading on the return hose, upstream or downstream of the regulator? Because downstream of the regulator the pressure should be about nix pounds per square inch. The fuel retuning to the tank should be dribbling down, not spraying in

[haristocrat]

I have tested the "inlet" pressure (hose from the tank) and the "return" hose  - both show 36 PSI.
Pete

By ‘“return” hose’ do you mean the hose curling back around from the fuel rail to the fuel pressure regulator OR do you mean the hose from the FPR back to the tank? If you mean the former, then 36 psi sounds correct as the pressure should be the same everywhere upstream of the FPR, and the FPR is probably functioning correctly.

I disagree with Daveson on the FPR behaviour. It does in fact make sense that the fuel pressure (relative to ambient atmosepheric pressure) drops at high revs as compared to idle. The reason for this is that there is a vacuum hose between the throttle body and the fuel pressure regulator diaphragm that supplies the FPR with the actual (negative) pressure that is present at the point of injection. This allows the FPR to compensate in real time for the greater vacuum present when the engine revs up. This keeps the pressure differential at the injector relative to the intake air stream constant. Higher revs means lower intake air pressure hence the fuel pressure is decreased to maintain the same injector duration parameters.

[daveson]

I disagree with Daveson on the FPR behaviour. It does in fact make sense that the fuel pressure (relative to ambient atmosepheric pressure) drops at high revs as compared to idle. The reason for this is that there is a vacuum hose between the throttle body and the fuel pressure regulator diaphragm that supplies the FPR with the actual (negative) pressure that is present at the point of injection. This allows the FPR to compensate in real time for the greater vacuum present when the engine revs up. This keeps the pressure differential at the injector relative to the intake air stream constant. Higher revs means lower intake air pressure hence the fuel pressure is decreased to maintain the same injector duration parameters.

Yes I see your point, but at high revs the vacuum could be high or low, depending on the situation. I haven't done any tests to verify my point, but according to the LE Jetronic diagnosis manual, the pressure should be under 30 psi at idle,  and 32 to 36 psi at 3,000 rpm.

I agree with your distinction about the return hose pressure being lower after the regulator, than before it.

[Laitch]

The only fuel "return line" is attached to the top of the regulator and connects to the fuel tank return port.

[haristocrat]

Yes I see your point, but at high revs the vacuum could be high or low, depending on the situation. I haven't done any tests to verify my point, but according to the LE Jetronic diagnosis manual, the pressure should be under 30 psi at idle,  and 32 to 36 psi at 3,000 rpm.

I agree with your distinction about the return hose pressure being lower after the regulator, than before it.

Agree with you. I don’t know the exact behavior but based on your quote it sounds like under steady state the high RPM throttle pressure is higher than idle.

 I guess my point was that the throttle vacuum relative to idle may decrease the fuel pressure under some conditions (e.g rolling off throttle at high revs) so it’s not necessarily a fault condition.

[Soggz]

I had this. A new temp sensor fixed it. Also, I sent the injectors off for testing. They came back ok.

[NMPete]

Hi  -  Maybe that is my problem: I thought the return pressure should be 36 PSI.  From the Troubleshooting Guide:
"Fuel pressure is sent from the pump to the Fuel injector rail. The fuel pressure
regulator will keep a constant pressure into the fuel injector rail[/u]. The fuel pressure
regulator is located on the back of the fuel injection rail (not easy to access) and its
depression is collected at the intake manifold for cylinder #4. The fuel pump is
delivering 65 PSI of pressure, and the pressure regulator drops it to 36PSI."

I assumed the pressure regulator was working correctly. 

Should the return pressure be less than 36 PSI?

Thanks
Pete

[NMPete]

Soggs - Which temp sensor did you replace (water or air)?

[Laitch]

Should the return pressure be less than 36 PSI?

The line from the rail to the pressure regulator is used to control and adjust fuel pressure and fuel delivery from the pump; it is not the return line. The return line is attached to the port at the top of the regulator and then to a port in the floor of the tank. Return flow should be a thin stream at the outlet within the tank (10psi to 0psi). If the return line were clogged, psi from the tank to the rail would spike well above 36 psi.

[daveson]

Should the return pressure be less than 36 PSI?

Yes.

As Laitch says, the fuel pressure (downstream of the regulator) should be close to nothing.

If it's 36, that's a problem. that would mean there's a blockage downstream of where you connected the gauge, either the hose or the tank. It should be easy to blow air through it.

If the blockage results in 36 psi, that's an unfortunate coincidence, that should not result in flooding.

That guide is a really good one, but there are a few small mistakes in it, you've just found one. The regulator doesn't reduce the fuel pressure, it increases it from close to nothing to 36 psi. If there's a total blockage for example in the return line, the pressure would build until something failed, for example a hose might burst. To prevent damage, the pump has a pressure relief valve, it opens at about 60 psi.

Is fuel also returning to the tank while cranking?

[Laitch]

I've edited my previous pos (post #33) which was a product of sleep deprivation and general impaired consciousness.  Maybe it will make more sense now.

[The Mighty Gryphon]

To continue the merciless flogging of this deceased horse and distilling the knowledge dispensed above..

The pressure at both ends of the fuel rail should be the same and approximately 36psi or 2.5bar.  At the return spigot on the fuel tank it should only be enough to open the check valve which should be very close to 0psi.

More than 10psi at the return connection on the tank probably indicates a tank problem. 

High pressure on the rail but low at the tank indicates a problem with the pressure regulator.  Low pressure on the rail could be caused by a bad regulator, weak pump or a plugged fuel filter.

[daveson]

I've edited my previous pos (post #33) which was a product of sleep deprivation...

Good morning Laitch. Did you post an acronym? I think so not. You didn't, didn't you? I've only had one coffee yet but.

[Laitch]

IDKIIDON
 

[NMPete]

Hi  -  Hooked up my fuel gauge again and I get 35 PSI from the tank to the injector rail.  Gas is returning to the tank O.K.

So  - This Is amazing -  It started while testing fuel pressure and lo and behold it stayed running!!!!!!  Warmed it up and it revs to 7K smoothly - sounds like a nice bike!

Not sure what changed to stop the flooding, but think it might be a connector to the Fuel Injection Module.  I unplugged and replugged several time so maybe that was it.

See my new post.

Thanks
Pete

[NMPete]

Hi  -  Hallelujah!  I finally got it running and it sounds great.  Revs smoothly from idle to 7K.  Must have been a bad connection because all I finally did was clean up connectors so not sure what the flooding problem was. Now I feel it is worthy of a nice renovation.

I want to say a special thanks to everyone that took time to give advice - it was a great help!

Pete

[natalena]

Great to hear your brick is running. Cleaning up connections and grounds is always a sound idea. Cheers

[daveson]

...Ok, Thanks for the update. Threads where the problem is fixed are the best threads.