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    Doubledown and hair pulling fuel pressure puzzles
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    The issue:
    We wrapped up the buggy wiring, fuel lines, chain tensioner and other odds and ends on Thursday night. Took the little beast for a spin and it was running horrible. The engine would not rev past 5k RPM without bogging down. So we pulled her back into the shop and closed down for the night figuring we would troubleshoot the next day.

    The next day the ECU started throwing a c29, Secondary Throttle Control Valve (STCV) FI error. The buggy would not even start which, was strange since it started the night before just fine. The STCV was replaced first with the same results. Next we stripped the wiring thinking there had to be a short somewhere. We pulled the cable sheath back to the ECU and all the way up to the fuse block and found nothing. Stumped we decided to call out for support and since Rich (Doubledown) was just up the road we called him. He said to bring the buggy up to his shop and he would have a look. Wow! OK we're on the way.

    Diagnosis:

    So after a pleasant drive through the burnt out San Diego back country we arrived at the Doubledown shop where we met up with Rich and Clint. Looking over the buggy the first thing they spotted was the fuel pump and fuel regulator. After talking it over, Rich and Clint suggested we ignore the c29 error and attempt to get the buggy started. Apparently the STCV is not even required to run the engine, more on the STCV later. The first step was to check the fuel pressure to the fuel rail with a flow meter inline after the regulator. Sure enough the pressure was off the scale pegging the flow meter well past 100 PSI. Obviously this was the root cause of the buggy not starting and even when it would start it would not rev past 5k. Stock pressure should be 43 PSI so 100+ PSI was just too much for the injectors to handle.

    Plan of action:

    Rich and Clint suggested we rig a stock GSXR fuel pump inline and try starting the buggy with the proper fuel pressure. These guys went out of their way for us at this point. Rich pulled out a stock harness so we could use the fuel pump leads. Clint pulled a battery charger out so we could recharge the battery while we waited. We pulled the new plugs to check them and found they were all dark powdery black, carbon build up. Clint cleaned them to beyond new condition while Rich rigged up the fuel lines to use the GSXR fuel pump. The last part was to find a fuel tank to house the stock pump. Coffee can to the rescue. WARNING: I DO NOT SUGGEST EVER DOING THIS FOR OBVIOUS REASONS. So we filled the coffee can until the pump intake was submerged, making sure the wiring was well away from the fuel source in case there was a short.

    The results:

    Rich gave the go ahead and we pulled the trigger (key switch), the little beast fired right up and purred like a 1000 should. The little beast finally would rev past 5k and sounded strong the whole way up to redline. The c29 error also disappeared although we did still have a c13 and c21 since the air box sensors were disconnected. Not a big deal, we know now that the fuel pump, the regulator or both is to blame for the whole mess.

    Further troubleshooting at home:

    After looking at the specs of the fuel pump it turns out that this unit adjusts output pressure by voltage. I had used the stock harness fuel connector off the stock relay to power the fuel pump. This way the pump would turn on/off by control of the ECU, just like the stock pump. However, the problem is that the stock harness puts out 12.5vdc to the stock fuel pump. This fuel pump is rated for 10 - 12vdc depending on the pressure required. Even .5vdc will result in a significant increase in fuel pressure output from the pump. This was why we were seeing more then 100+ PSI for a pump rated at only 80 PSI. The Bosch fuel regulator was not able to handle the pressure and as a result the internal plunger mechanism was being forced wide open. Basically the fuel regulator was not regulating fuel at all. Hind sight is golden, the first pressure regulator was assumed to be bad. As soon as the fuel pump was engaged fuel would spew from the set screw which, is used to control pressure from 35 45 PSI. Again, the abnormally high pressure was causing the fuel pressure regulator to malfunction.

    Note on the above: I'm still a little sketchy on on the relation between voltage and pump pressure. The vendor sites are not the most helpful when it comes to researching this issue. They like to throw out lots of specs with little to no practical application. If anyone has experience with aquiring the proper fuel pressure from an after market external fuel pump I would appreciate seeing how it was acheived.

    The solution:

    There are a couple of routes to go here to solve the issue. One would be to use the stock fuel pump, buy a square fuel safe and mount the pump to that. Could even buy an adapter from Protodie and have it welded to the existing spun aluminum tank. Another option would be to lower the input voltage down. Not hard, could wire resistance inline or even use a POT so the voltage would be adjustable. This method would even return the usefulness to the fuel regulator. The last option would be to buy a 43 PSI fuel pump and lower the voltage by .5VDC or as required. There are various GSXR turbo kit suppliers that have working external fuel pumps rated at 43 PSI.

    c29 error and the STCV:

    The STCV it turns out is nothing more then an emissions, sound level and low end throttle control device. The easiest way to bypass the STCV is to remove the top butterfly valves to eliminate the effect of the STCV. The STCV is still there and could eventually fail. So this is not the proper approach in my mind. Eliminating the STCV and fooling the ECU into thinking it is still there working properly would be the proper approach to take.

    The reports I've found on various sites (not the least of which was gixxer.com http://www.gixxer.com/forums/showthread ... val&page=3) is that low end throttle response is much better after the mod. There are even claims that HP gains can be achieved up to 8k RPM by removing the STCV butterfly valves. Ive been unable to source any dyno results to back these claims up though. More often then not the claims come from the old Butt dyno which really proves nothing factually.

    Conclusion and lessons learned:

    The lesson learned is to stop stand back and look at the greater picture before ripping into something attempting to resolve the problem. The c29 error was just a symptom of the existing fuel pressure problem. The c29 error showed up as a result of the fuel regulator failing completely as a result of the enormous pressure from the fuel pump. My blinders were on and I was solely focused on an electrical (harness) issue and never even thought to look at anything else. An extra set of eyes and some experience dealing with the GSXR brand of engines saved the day.

    Side note:

    It was truly amazing to me that the Doubledown team would spend three hours of their time helping someone who was not a customer of theirs. This coupled with the fact that they were hard pressed for time to get their latest creation out the door to a waiting paying customer. Rich and Clint are truly a pair of stand up guys that just happen to be enthusiast within the buggy community. After all the work was done figuring out the issues with the Seuferer car we spent the next 45 minutes drooling over the new Doubledown two seat buggy. The new car is a very nice design and completely diverges from the DesertKarts style of their earlier models. Rich should have the car buttoned up soon with updates on the Doubledown site soon. Im sure he will stop by and let us know when its complete so we can all take a peak at the finished product. Rich, Clint thank you for you help and the advice of a few other mods that will improve performance and reliability. Bravo Zulu!
    Currently on a two year walkabout
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Really do have to thank Double Down a lot. They did go way out of there way for solving all the problems.
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Millenium Member plkracer's Avatar
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    The voltage adjusts how fast the electric motor inside spins. The faster it spins, the more pressure. I've done the same thing, did into something totally unrelated, such as when I cleaned my engine in my truck and the choke light stayed on. I could have swore it was connected after i took the air cleaner off... [smilie=banghead.gif]
    Proud owner of a two seat pucker-mobile. Funco inspired mini buggy powered by a Busa. Giving out free rides to anyone brave enough.
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Millenium Member plkracer's Avatar
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    BTW, did the pump say the amp draw at 12v?
    Proud owner of a two seat pucker-mobile. Funco inspired mini buggy powered by a Busa. Giving out free rides to anyone brave enough.
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    The pump doesn't say much since it is an inanimate object with no vocal chords. *

    Sorry, couldn't help myself.

    I'll pull the specs tonight when I get home. There's a nice little graph that details voltage to pressure.

    Here's the new regulator with a return line that we will be using. http://static.summitracing.com/global/i ... -4305m.pdf
    Currently on a two year walkabout
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Millenium Member plkracer's Avatar
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    Lol, I figured someone would pound me for that. Should have asked if the instructions had anything regarding amp draw written on it. * *That regulator looks like a good unit, and it even has the vaccum pressure boost.
    Proud owner of a two seat pucker-mobile. Funco inspired mini buggy powered by a Busa. Giving out free rides to anyone brave enough.
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Admin Gene's Avatar
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    Rusty,
    Scott Horner suggests: *If you are going with the braided stainless the best pump is the Accel part number 74701 *and use the Aeromotive regulator pn 13301. The regulator is installed after the fuel rail and has a screw to regulate pressure. It comes with low and high pressure springs and you must change to the high pressure spring since it is in the box. I have this setup on my car and it works perfectly.

    Why haven't we seen pics of your car? Kudos to Doubledown!
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Millenium Member plkracer's Avatar
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    There are a few pics of the car here and there. It's probably some special military project..
    Proud owner of a two seat pucker-mobile. Funco inspired mini buggy powered by a Busa. Giving out free rides to anyone brave enough.
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Hey Russ, glad you'all got things figured out, we were glad to help last weekend. As i said, its a bit of schooling for all of us. *I use a aeroquip regulator on the race car, *i still contend that for the money it is hard to beat a $25 ebay stock pump. On the race car i have a $100 in the pump, $165 in the regulator and then at least $ 45 in earls fittings. If i new for sure that methanol wouldnt hurt the stock pump and keep up enough volume, i would probably use one on it as well. The buggies get a stock pump and 2 feet of push-lok hose!. *Again thanks for the kind words, Clint and myself were just happy that we were indeed able to solve the issues.
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    Re: Doubledown and hair pulling fuel pressure puzzles
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    Rich, I agree the stock pump is cheaper and easier to deal with. The DK build is using a stock pump mated to a spun aluminum tank, work done by Bugpack. Kevin had so much dumped into fittings, fuel pump and the like he just decided to buy a regulator that would work. One thing I noticed about this type of setup, the fuel pump makes a lot of noise. Loader then the engine at idle but can't be heard above 2k RPM.
    Currently on a two year walkabout
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