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About Bob_fromLA

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  1. I read this thread with much interest, as I got into this issue after a P1781 code on my 2001 8.1L Sierra. As usual, Murphy strikes and the code showed up on the day I intended to take the truck in for Ca SMOG. After a bit of research, I found the P1781 code was related to an older, low voltage battery issue with the Allison transmission and that there had been a ECM update described in a TSB. A new battery fixed the base problem, but I was told that I could have no record of a code to pass the new (post mid-2015) CA rules and to clear the code and drive for a couple of days to set the tests. Only then did I find that everything went complete, except the secondary air. I have the original service manual and found the attached drive cycle. I also have inop AC due to probable leak in evaporator area, which is very expensive / difficult to fix. After about 10 drive cycles, turning everything I could on, turning the wheel back & forth to load the engine at cold start, etc. I still had no luck getting the monitor to pass. On investigating the secondary air system, the pump was like a shop vac, so there was no lack of air going in. However, the rubber hose part of the vac line going to the valve on the pump output was in bad shape (working, but degraded after 15 years) and easily replaced with much better 5/32 vac line. I then dismantled the rest of the system and found the check valve on one side was fine, but the other side had a slight "back crack" where it did not totally seal pressure coming from the exhaust manifold. I found Standard Brand AV56 check valves online (Rock & others) and Standard Brand VG222 gaskets, noting that nearly all parts for the 8.1L are no longer available from GM. The AV56 check valves are physically the same size, but (possibly because they are new) have a slight restriction on air flow from pump to manifold. Therefore, I replaced both OEM valves with some difficulty. As the pair of pipes to the left and right manifolds were not available, I had to cut the old valves off. I started by removing the valve assembly (bell shape) from the threaded section and then carefully cutting the threaded section attachment along the pipe until I could break it off the pipe without damage to the threads on the pipe. After cleaning as much crap on / in the pipes as I could, the new AV56 check valves were screwed on with some old aircraft anti-seize compound that I had. I also had to manually trim the VG222 gaskets with angle snips as they were larger than OEM and the bolt holes were not in the right place. After all that work, additional drive cycles still did not get the monitor to pass. At this point, into the local dealer for at least the ECM update for the original problem and hope that it might help with the monitor. The initial report from the dealer, actually very honest, was that there was nothing actually wrong with the mechanics and that they would perform some drive cycles with whatever they found from GM. In the end, they got the monitor to pass, without needing an AC fix, for $360 (reprogramming and the drive cycles). I did note that they used a half tank of gas and nearly 100 miles during their drive cycles. The 9/2/2016 CA Smog Check OBD Reference describes all the standard exceptions to the "all monitors must pass" requirement, with the Instruction to the Inspector of "Likely broken vehicle; Repair and Retest." All I could think of was welcome to crap from the Peoples Republic of California!!

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