I work at the local school district and maintain 170 school buses, 130 of which are dedicated CNG, plus 40 CNG truck and maintenance vans.
80 Thomas built HDX buses with the 8.9L CNG
30 Thomas Built C-2 buses with the 6.7L CNG
20 Chevrolet G4500 with 6.0L CNG
40 Ford vans and trucks with 6.2L and small V6's CNG
40 Thomas built C-2 diesel, DEF issues but no real engine troubles.
2013 8.9L is garbage, melted pistons, bad sensors, bad computers, bad fuel systems. most over 90000 miles
2015 and up 6.7L is OK, runs Ok, not much trouble. 60000 and up on miles
2014 6.0L Chevrolet OK other then valve seats and O2s going out, most over 100000
2013 and up Fords don't run may miles, but no real problems with CNG.
We have changed out 6 sets of the Chevy heads due to valve seat failures, these were conversions done to standard engines. But we also have changed out as many camshafts due to lifter wear, so the engines problems are not all CNG related.
The district is shopping for new buses due to fleet age and miles, so we are going back to diesels. Thomas Bus, our main suppliers hasn't sold a CNG bus is three years in our area, the demand is just not there. CNG parts are expensive and hard to come buy and getting them worked on under warranty is tough, no one wants them in their garage for fear of a leak. We have two or more a day with check engine lights and / or service calls for low or no power. Spark plugs are $110 each, every 15000 miles times 110 buses, the Chevrolets use standard off the shelf parts. Computers, $2000, Ignition control Module, $1700, 18" long flexible fuel line $800 ( five per bus ) 10' long flex line $1000, fuel tanks, $15000 to $18000 each.
Lets not forget about the mileage, 300 miles for the 8.9L if you run it till it quits, 6.7 will go about 500 miles, the Chevrolet, about 5 miles per gallon equivalent to gasoline.
Our white fleet of maintenance trucks are bi-fuel or full CNG, all Ford 6.2L engines and all getting about 5-6 miles per gallon.
The district started this in 2013, so for 10 years I have had to deal with this stuff. I am a certified tank inspector along with ASE certified alternatives fuel mechanic. I hope you guys running Propane have better luck then we have had. I'am sticking with gasoline!
