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2014 Massimo MCU500 Put Diesel in Fuel Tank

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I recently inherited a 2014 MCU 500. A nephew accidentally put diesel in the tank, proceeded to run it until it stalled out. We proceeded to empty the tank and put fresh gas in it. It ran maybe for 10 minutes then shut off. Haven’t been able to get it started since. That’s when I started to mess with it. This is what I’ve done so far. I might add I know the very basics of on engines so I thought I would come here and get some advice. 

- emptied tank and lines leading up to fuel. Filled and primed with new gas.
-cleaned air in-take, it was pretty dirty. 
-did a full oil change. I felt like there was definitely diesel mixed in. 
-changed spark plug. Tested and working appropriately
- tested fuel injector and it appears to be working 

That’s where I’m at so far and still nothing. At this point looking to see what the next steps would be in trying to diagnose what’s going on. Appreciate the help in advance. 




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any progress?  have you tested fuel pressure/ The diesel may have damaged pump seal or filter material...

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No progress yet. I'll take a look at the fuel pressure. I watched it once I removed the fuel line from the injector. It looked good, but not a quantitative measurement. I borrowed a compression test gauge from a buddy and it was 25-30. I went ahead and adjusted the valves last night, but no luck.  I'll definitely look at the fuel pressure and keep y'all update with any progress, or lack there of. 

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An update. The fuel pressure was good Was good but I went ahead. So I dived into taking the engine apart. 

This is what I found. This doesn’t look good to me so I’ll be changing the piston.  Surprising, the cylinder looked alright, but I’ll have it honed at a shop. I’m going to look at the pistons too. I think I’m on the right track, hopefully. 


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wow... looks like the diesel washed the oil off and thats the result?   I am shocked the cylinder walls dont look like that too.. 

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So I changed out the piston and went ahead and got a new cylinder. There were some vertical marks and I was advised since I had it all apart to go ahead and replace the worn/damaged ones. I went ahead and replaced the camshaft too. There was quite a bit of wear on the exhaust lobe. I went ahead and looLed at valves and everything was great. Put everything back together and cranked it this afternoon and sounds better then it did when it was running. 

I’m a veterinarian and this is really the first time really diving deep into something mechanical. There were some points when I thought to myself, there are some surgeries that I do that are easier then this.

All in all, its always a good feeling making something run when they were just going to get rid of it. Plus it was just given to you. 


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Great job.. Hope it runs  well,  for a long time.... I wish I could work on mine, but 3 back surgeries prevent me from doing mechanical work anymore.. It kills me to have to wait weeks while the only shop in the state that will work on Massimo machines, slowly does the work... Still waiting on mine now.. 

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