Hey everyone. I got a Toyota 7FGCU25 with a 4Y LPG engine with about 25500 hrs on it. The problem I have is under constant high RPM load the engine will start to shudder and backfire, let go of the throttle and after about 45 second it clears up. If it does shut down and u go to start it, it sounds like no compression. However if you hold down the ignition it will eventually start and run great until u give it a constant high RPM run. The tune up has been completely done. The oil is new and even a Lucus Oil treatment is in. (Customers choice, machine run almost 24 hrs a day). Does anyone have any ideas. I have tried to check compression but by the time I get the gauge on its ok. Same with trying to inspect the lifters. I am ready to blow this thing up and tell them to buy a new unit.
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Try mrfixit's idea. Had the same problem with an old Clark CGP25. Turned down the max RPMs and put in 5W20 synthetic oil (Royal Purple to be exact). Now it runs fine but the oil needs to be changed more often.
How about trying some 5-30 oil and turning the governor down to reduce the top rpm?
mrfixit is right. the valve seats have recessed into the head and as strange as it may seem the lifters pump up after extended runs and the valves wont close completely. this is a fairly common problem on high hour units and a valve job will take care of the problem.
It sure seems like the lifters are "pumping up" and causing the valves not to close all the way. I never have seen that happen but I've heard of it. Did you check for broken valve springs? I know when you overheat aluminum heads they can warp bad and if there has been head gasket work on that truck and had the head milled more than once, the rocker arm may be sitting too low and the lifters are near their max compression.
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