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  • HPL failure

    Well, I finally got hit by the dreaded HPL failure. Went to start the '03 Cavalier and when I turned on the key, I heard three strange "clunks" from the HPL. I started the car and watched the fuel gauge race to empty followed by a changeover to gasoline. I also got a check engine light with a P0005 code (HPL circuit). Drove to the shop, checked the fuses (all good). Disconnected the power connector to the HPL and applied 12 volts to the valve. It was reading a fairly high current of about 7 amps but the valve would not operate. I guess it is time to change the valve. I am lucky that I have the vent kit with the "let's destroy wha'ts left of the valve" special tool to get rid of the CNG in the tank. I am also lucky that I have a spare valve to install.

  • #2
    Re: HPL failure

    I may have missed what is going on with the HPL. Knowing that the HPL is a pilot operated valve, I thought that it may have been stuck--the clunks I heard sounded a lot different then the normal "tink". To make a long story short, I put a clamp on ammeter on the wire to the valve and hooked the car to a compressor to try to balance the pressure on the valve. The car would run with the compressor attached and when I shut off the compressor the fuel level would fall until the car changed over to gasoline. The current to the valve was running around .5 amps. After a couple of tries, I started to hear the valve operating and the strange "clunk" sounds. Finally, I could get the car to run without the compressor attached but when I tried to drive it, the pressure would fall and you could hear a sound of the CNG running through a restriction until you slowed back down. I let the car sit outside to warm up and when I returned the valve appeared to operate normally with the normal "tink". Sounds like something is sticking---hopefully I can keep the valve working so that I can electrically defuel the tank without destroying the valve and maybe be able to save it.

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    • #3
      Re: HPL failure

      Hope this is the last of this tread. The valve was sticking. We have had some really cold weather here in Florida (I know, the 30's isn't cold for the rest of you). However, the problem was temperature related and if I pulled down the rear seatback and put a heater in the car, the valve would work every time. I figured the valve either had something like dirt in it or possibly carryover oil from a CNG compressor that had dried in the valve. In any event I made one last attempt to fix the problem. The only thing I could think of short of removing the valve was to try to lubricate it. I took the fuel line off at the check valve on the HPL and filled it with PB Blaster (a penatrating oil). I then reconnected the line and hooked up the fuelmaker and ran it until I could hear flow through the line. I then shut down the fuelmaker and cycled the HPL several times in the hope of getting some oil through the entire valve. This morning, 36 degrees outside, and the valve operated normally. It might be on borrowed time but it is working. If it keeps working for a while I will use it until I can get around to changing it---then maybe I can see what is going on in the valve.

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      • #4
        Re: HPL failure

        Siai47--

        I always enjoy reading your well informed posts, but you have me a bit confused. Are you in Florida or Michigan?
        _____________________________________
        '12 Blue Mist Metallic Civic Natural Gas; '03 Galapagos Green Civic GX; '07 Alabaster White Civic GX

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        • #5
          Re: HPL failure

          I try to spend the winters in Deland, Florida (my actual residence) . I also have a place in Cadillac, Michigan where I spend Florida's hurricane season hence the Michigan location in my profile. I have a Cavalier and a GX down here and have two other Cavaliers in Michigan. I have a G3500 CNG van that I take back and forth. As there is no public CNG available at either location, I have FM2Q36's at both places and a backup FM4 that I throw in the van when I travel.

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          • #6
            Re: HPL failure

            I may have the dreaded HPL failure as well: Need help troubleshooting my 2006 chevrolet silverado 4x4 Bi-fuel 6.0l engine (factory CNG)
            Recently purchased vehicle so amount of pressure in tank is unknown. When filling at quick fill station I hear a quick pssst and then nothing. The check engine light is now on. I had Auto Zone read the engine codes P0005 HPL failure and P1433 fuel pressure sensor circuit high input.(ECM sees more than 4.70 volts). With the ignition key on there is 12 volts at the HPL connector. I then placed amp meter in series with a 12 volt source and operated the HPL and it drew about .6 amps and I could hear a mild "tink" each time voltage was applied, but not as loud a "tink" as my cavalier.
            The local GM dealer here quotes a 2006 silverado HPL unit at $1900 (after discount), so I am trying to make sure of exact cause. Could code p1433 failed sensor cause p0005 or p0005 cause p1433? Anyone know of less expensive HPL for this vehicle or site to find a used HPL. Not sure if 2000 thru 2006 GM trucks use this same HPL. I dont have the training to relieve the tank pressure so I would have a certified CNG shop install the unit or at least vent it so I could install it.
            Thanks, Location Cedar City, UT

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            • #7
              Re: HPL failure

              WOW a 2006 Silverado---nice even if it doesn't run on CNG ! As to the codes you are seeing, neither one of them would set the other one. However, first guess (from the P1433 code) would be that you have a bad ground from the electronics to the tank. The reason for thinking this is that the GM system provides 12 volts operating voltage or 5 volt reference voltage to all of the components. The components are switched on or off (or in the case of a sensor operated) by grounding the circuit. In the case of the tank pressure sensor, loss of ground to the sensor would send the 5 volt fuel pressure reference voltage instead of the 4.1 volt maximum voltage to the FICM which could be the cause of the code. P0005 monitors the operation of the HPL by assuring it is reacting to the FICM signal. A bad ground here (circuits use the same ground path) could also set that code. The current you measured for the solenoid and the "tink" you heard seem to suggest that the HPL is operational. I would probe the gounds from the tank through the 3 connectors in the circuit back to the FICM and make sure there is no corrosion in the connectors or none of the pins are pushed back in the connectors. Also check for any corrosion at the grounds for the FICM. The wire harness in question runs from the tank then under the drivers side of the truck, up into the engine compartment, across the firewall and over to the FICM on the right fender. I know I harp on this all the time, but if you had a Tech2 GM scantool you could look at the reported tank temperature and see if it made any sense. This should be close to outside temperature---if it is wrong that would also indicate a TPS ground problem as both sensors share the same ground. At least it doesn't cost anything except time to diagnose a bad ground. Finally, as to the $1900 (with a discount) for the HPL, if you really,really need one, have your dealers parts guy check to see if any other dealers have one. I thought I really, really needed one (the reason for this thread in the first place) but was able to get the one in my car running again. I did buy one for under $700 from a dealer who had one on the shelf that was ordered by accident. Just about any GM HPL (Silverado, Express van, Cavalier) will work.
              Last edited by siai47; 03-26-2010, 04:03 PM. Reason: typo

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              • #8
                Re: HPL failure

                Thanks for the reply: I will check continuity on the sensor ground/low reference wires when weather dries up a little, we had 5" new snow last night and still snowing. I did not realize that GM used the same HPL on other models. When older vehicles tanks start to expire there may be used GM tank valves (HPL's) available. It sounds like my 2001 cavalier has the same HPL as my 2006 Silverado.

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                • #9
                  Re: HPL failure

                  I checked out the sensor ground/low reference wires the best I could with my meter and they checked out ok. I also cycled the HPL solenoid about 10 more times and listened carefully to a louder thunk on power up and a quieter thunk on release. After pulling the negative battery terminal I unplugged the two long connectors on the power control module PCM and reseated them into their sockets, then put everything back together--Presto!
                  The readout on the instrument cluster shows "CNG Level Shown" . The check engine light was still on but went away the next day after a bit of city driving on CNG. What worries me though is I have put on about 50 miles of mostly city driving and the CNG level still shows clear full and has not budged off the full mark. This is the first time I have had the CNG system working since purchase. Maybe the fleet shop I bought vehicle from filled it to 3600 psi +.
                  My Cavalier always comes off of full after about 10 or 20 miles of city driving. If the pressure sensor circuit has too high input I believe the check engine lite would be on and reading the P1433 code. I will put on more miles and see what happens. I am just excited to be running on CNG. Thanks.

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                  • #10
                    Re: HPL failure

                    Congratulations! You most likely found it. A lot bigger tank in the Silverado then the Cavalier. Depending on the fill my Cavaliers come off somewhere between 15 and 30 miles with a slow fill and a top off after the tank cools down, your vehicle should be coming off full soon. The grounds are a bugger on these vehicles so checking and cleaning them doesn't hurt. I use a product called Corrosion block or Corrosion X (marine supplies sell it) on all the connectors (including the PCM). Around the external grounds I use Boeshield T9--which forms a thicker film. Enjoy your Silverado!

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