John your point is valid, this is and has also been an issue in the proper performance of the forklift. One of the steps to properly maintaining an Impco LPG fuel system is to be sure that an LPG supplier is providing the proper grade of LPG. I was suprised at the response when I made several phone inquires to area suppliers inquiring as to what grade of LPG they supplied. Unfortunately, to my surprise, most of them did not know. Just as gasoline has different grades, so does LPG. Typically, in the US there are two grades, HD-5 and Commercial. While in most instances, the Commercial grade does meet HD-5 standards, this isn't always the case. We all have seen many times failure to use the correct one, can lead to negative effects on the overall life of the fuel system components of the forklift. This is why I always asked the end user of the problem plagged truck to check with his supplier to see what they are selling them and have them ask for verification of this.
Yes John, contamination is one of the largest causes of failure of a LPG fuel system. This is largely due to little monitoring of suppliers by the government, many of them don't maintain their storage tanks properly. This is why it's so important to perform regular maintenance of an LPG system. This needs to be done in order for the power unit to operate effectively and efficiently. I would suggest to an end LPG end user checking with their supplier and see how often they hav their storage tanks purged. If they can't tell them, then it may be in their best interest to check for an alternative sources.