1992 K75 fuel pressure control

Hi. I'm still trying to get my K75S running and have a question about pressure in the fuel rail. I'm taking the previous owners word that the fuel pressure was tested and found to be within specifications. Even after sitting for 2 days, when I remove the fuel line there is enough residual pressure to blow a stream of gasoline out. I know I'm missing something that should be obvious but I don't understand how the fuel pressure is controlled since the fuel return line is unrestricted in the '92 because there is no in-line regulator. It appears that when fuel enters the rail there is nothing to impede its direct return to the tank. So how is the rail pressurized? BTW, the only time it runs at all is when it's running out of fuel or the fuel line is disconnected ( for a few seconds).

Unless you relieve the pressure in the line before removing the tank you WILL have gas squirt out for a few seconds; simply run the engine, disconnect the fuel pump voltage connector on the back lower right side of the tank and the engine will stop running a few seconds later.

THEN you can disconnect the fuel lines or remove the tank without gas shooting out.

The fuel comes out of the pump (capable of 100psi) in the tank, through the filter (also in the tank),and out the rear nipple on the left front lower edge of the tank.

It goes through a hose (about 8" long) to the front of the fuel rail, where the injectors draw the fuel from.

At the rear end of the fuel rail is a hose that goes to the fuel pressure regulator. The fuel pressure regulator is hidden behind the throttle bodies between the throttle bodies and the air filter box.

The fuel pressure regulator bleeds off any pressure above 38psi. That bled off pressure goes forward through a hose back to the tank through the nipple just forward of where it came out of the tank.

So the fuel rail pressure should stay at about 38psi unless the fuel pressure regulator is defective. It can fail either too high or too low.


Don't EVER rely on previous owners troubleshooting when trying to get a bike running. VERIFY!!! Remember, he couldn't get it running before he sold it (thereby losing at least $1000 in the sale), so he wasn't a very good troubleshooter.

The best way to test the pressure is to TEE a gauge between the rear of the fuel rail and the hose that goes to the regulator.

You have already pulled and inspected the pump,damper,pickup sock and inspected the bottom of the tank where the fuel is picked up and CHANGED the filter, right?

IF those are OK, the other fuel issues might be clogged injectors, failed regulator (TEST pressure!), or a sticking air flow meter.



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O.K., here's the latest and, keep in mind that this bike sat unused outside with a tarp over it for over two years. I couldn't find the fuel pressure regulator but I tried blowing and sucking air through the return line but could move no air through it. So I disconnected the regulator air hose from the manifold in an attempt to open the regulator by using my mouth to create a vacuum. That didn't work so I tried putting a better vacuum on the line using a vacuum brake bleeder. I pumped it to -25psi and still could not blow through through the return line. From this I assume that the regulator is frozen closed or that the hose is seriously kinked in some location not visible from either side. The frozen closed regulator seems the best conclusion because in inspecting the inside of the tank some of the steel parts such as fuel pump retaining nuts are rusty. I'm going to wait for your ideas before trying to get at the regulator which looks like it will be a major undertaking for a novice.

That is not a valid assumption! The regulator only passes pressure IN EXCESS of 38psi.. There is no way you are creating that kind of pressure with your lungs.

Quit messing around and do a pressure test WITH a GAUGE!

Odds are that's not the problem, but until you know FOR SURE what the fuel pressure is all your other troubleshooting will be based on an assumption. Getting a bike that hasn't run for years and you have no information about takes slow METHODICAL checking to establish what is right and what is not.





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Accessing the regulator is not too hard. Just remove the air box.

[LIST][*]Unscrew and remove the plastic intake manifold. you can wiggle it out of the air box.[*]Loosen the hose clamp on the hose on the front of the air box. (#1)[*]Unlock the three clips (#3) (and maybe remove them) and remove the air filter[*]Gently pull out the top of the air box, remembering that it is connected with a wire to the air flow meter. Let it hang.[*]Remove the two allen screws that connect the air box bottom to the engine block.[/LIST]

You'll probably find a bunch of bugs and stuff you'll want to clean out of the lower airbox. I also found 20 years of baked-in dirt and pebbles on the engine top under the airbox. Had to scrape and vacuum that out.

[IMG]http://www.ascycles.com/Illustrated_catalog2/MicroImage.aspx?ImageFileName=\ETK_Grafik\dbvis5105996784846613299.bin&width=550&height=400[/IMG]

While removing the airbox is a necessary evil to gain access to the FPR it is not really all that easy as Tim claims above---in my opinion and experience. I just finished installing a new aftermarket regulator which ended up requiring me to remove/reinstall the airbox twice in a week's time.

First of all, you WILL need to remove the tank just to access the retaining clamps screw on the FAR SIDE of the large intake tube. Once the tank is off you will then be able to poke a long flat blade screwdriver (or really extra long nut driver) down through the nest of wiring to reach the screw. Be careful not to loosen it too much as then the retaining clamp will slide out of range---and if you forget to set into position when reinstalling things you won't be able to tighten it back up.

DO NOT try to remove the tube from the airbox side as it will not come out and off until you can swing the airbox out of the frame. Believe me, I learned all of this the hard way.

The good news is that once you learn the technique it becomes almost easy as described above. The airbox is really wedged into the frame/engine area tightly and there's precious little room to do things. Unless you have really small hands (I enlisted my wife's help) it is an absolute PAIN to get the 3rd spring clip back on the back side of the airbox upon reassembly. It is also imperative that the air filter goes in the one, correct way otherwise you will struggle to no end.

If you apply a tiny bit of Armour All or Back To Black lubricant along the top and bottom edges of the air filter it will snap back into place a whole lot easier, too.

I would also advise using a long pair of forceps to position the 3rd spring clip as I did the last time I put mine back together the other day---it worked so well I'll never try doing so without one (they're available at Harbor Freight for a few bucks).

All of what I've written may be in vain, though---[I]as I agree with 98Lee that you should go through the bike methodically and check other things first[/I]. Especially true since your bike sat for so long; all kinds of gremlins are bound to appear.

I'm almost in the same spot as you with problems that I thought were one thing but may turn out to be something else completely (see my Chirping Noise While Hot thread if interested; my current nightmare of sorts!).

Good luck and do heed Lee's and other's advice here; they really know how to help you with your Brick. :thumb

[QUOTE=Zagando;829175]...you WILL need to remove the tank just to access the retaining clamps screw on the FAR SIDE of the large intake tube. Once the tank is off you will then be able to poke a long flat blade screwdriver (or really extra long nut driver) down through the nest of wiring to reach the screw....[/QUOTE]

I forgot about that :) I flipped that clamp over when I reassembled so that I can loosen it with just a long screwdriver from the bottom/front. I don't have the RT fairing installed anymore either, so that makes things easier.

I may have missed your response so let me ask, What parts have you already inspected cleaned and/or replaced?
Has the air filter been changed? The air box cleaned out? I found a mouse nest, nuts, rocks and who knows how much mouse poop in a bile that sat for a couple years.
Do you have good spark?
Can you hear the pump run when the ignition is turned on and the starter button depressed?
Have you pulled the fuel rail and checked that the injectors are squirting?

FWIW - I've managed to access the FPR and the throttle cable on both K100's and K75's by removing just the bottom of the filter housing and air filter itself, and using some blocks to hold the top and AFM up. You won't do it if you have really big mitts, but with normal size hands - you can reach in and get to most any of the mountings and hose clamps needed to R&R the FPR.

Thanks for the detailed and knowledgeable responses. In my Clymer manual it says that an alternative way to relieve fuel pressure before disconnecting the hoses is to suck really hard on the vacuum hose end at the throole body to open the valve. What I failed to make clear was that I used a vacuum brake bleeder to try to open the regulator at -35 psi, not to force fuel through the line. I feel certain that vacuum pressure at the throttle body is not greater than that. The plugs, injectors and filters all were replaced or tested. The only obvious test that was not performed, and I know this is basic to trouble shooting, is the fuel pressure from the pump. Although not measured, we know at least that there is plenty of pressure (perhaps too much) being supplied to the rail. Because the exhaust fills with raw fuel so it seems that too much fuel is being injected but not burned. Another indicator of too much fuel is that it only runs well when it's running out of gas or the fuel line is disconnected. Of course it only runs for about 10 seconds with the fuel supply interrupted. I will get a fuel pressure meter and follow up on this thread just to eliminate that factor from the mix. Thanks again.

Thanks for the pictures, I will be in touch. You may be getting a continuous trigger from your fuel injection ECU causing the injectors to stay open or open to often. I'm a carburetor and drum brake guy myself so my FI knowledge is very limited.

From my days as an MB VW tech I remember the old EFI cars and some of the odd ball problems they would be dragged in with.

Another simply check is to make darn sure all of your grounds are clean, tight and all the wires are attached.

Good luck and don't let it get the best of ya its only a machine!!!

AFTER you verify the correct fuel pressure, if it is correct, a definite possibility is a stuck air meter door.

This would be telling the computer that there is WAY more air coming in than really is. That would cause the computer to pour way more fuel in than needed. As has been stated elsewhere, critters like to build nests inside the airbox. This has been the cause of many stored bike issues.

But verify the fuel pressure first!!!!!




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[QUOTE=broadstone;829194] In my Clymer manual it says that an alternative way to relieve fuel pressure before disconnecting the hoses is to suck really hard on the vacuum hose end at the throole body to open the valve. [/QUOTE]

What page was that on?

That sounds strange since, as far as I knew, the regulator normally operates in a very narrow range (about 36-44psi) and the vacuum just slightly bumps the pressure under hard acceleration (low vacuum). I was under the impression that the MINIMUM pressure was set at 36psi, so any manipulating of the vacuum would still produce at least 36psi of backpressure.

I'd like to read that section. I could always learn something new.:thumb



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I found it and tried it on a spare regulator that I had. It did not work.

I would try it on my bike, but I'm leaving on a six day trip tomorrow morning and I don't want to do anything that could screw up the bike (like blow out the diaphragm in the regulator).

Clymers has some things in it that are not correct.

Has anyone used this method of releasing the pressure on a 2 valve K-bike??? Please respond with year and model if you have.


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