Captain Neon Posted July 8 Report Share Posted July 8 I replaced a fuel pump on my 1946 Plymouth more door this weekend. It had "vapour lock." Car would stall while idling and not restart. It runs great now. The ethanol-resistant pump that came off was installed new just 4 years ago. Thinking I was being proactive, I regularly dosed my gasolene with Marvel Mystery Oil at the prescribed 1 oz./2.5 gallons petrol. Did I just have a bad pump or does Marvel Mystery Oil do bad things to fuel pumps? Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted July 8 Report Share Posted July 8 not enough info.....what failed in the pump that left you high and dry.....Murphy is omnipresent seems more at play here....could be you just fixed a suction leak hose/fitting when replacing and the pump is still a valid item. Quote Link to comment Share on other sites More sharing options...
Captain Neon Posted July 8 Author Report Share Posted July 8 In a prior attempt to eliminate the "vapour locking," I disconnected the lines from my old pump, and wrapped the threads with Teflon tape before tightening them back up. The "vapour lock" persisted after this. I ordered a new fuel pump after that. The problem was corrected after replacing the fuel pump with new Teflon tape applied to the fitting threads. I've only ever read that Marvel Mystery Oil added to fuel would lubricate a fuel pump and help prevent what occurred. Any detractors to using Marvel Mystery Oil stated that it did nothing and was just a waste of money. I thought I was doing a good thing adding the Marvel Mystery Oil to my fuel. Maybe I wasn't? Quote Link to comment Share on other sites More sharing options...
greg g Posted July 8 Report Share Posted July 8 Is it vapor lock? These vehicles are not prone to actual vapor lock. When does it happen? Teflon tape should not be used for fuel connections as it can partially dissolve and cause blockages to occur. Have you ruled out air leaks on the suction side of the pump back to the tank? Have you cleared the lines to the tank with compressed air? Is your rubber line between the pump and frame in good condition. Have you done the fuel volume test outlined in the service manual? Have you ruled out perculation? Have you checked the debris screen inside the pump? MMO is a blend of light petroleum distillate (machine oil), Stoddard solvent (an industrial cleaning fluid), and red coloring. What are your expectations for it to do? Quote Link to comment Share on other sites More sharing options...
Captain Neon Posted July 8 Author Report Share Posted July 8 New pump installed yesterday. Old pump was stalling at idle and not restarting. Car works great now. MMO good, or MMO bad? I probably just had a bad fuel pump. I was wondering if anyone else ever had issues using MMO. I'll go back to using MMO the next time I fuel up. Quote Link to comment Share on other sites More sharing options...
sidevalvepete Posted July 9 Report Share Posted July 9 Bit hard to go anywhere with this... 2 Quote Link to comment Share on other sites More sharing options...
Sniper Posted July 11 Report Share Posted July 11 On 7/8/2024 at 10:09 AM, Captain Neon said: I've only ever read that Marvel Mystery Oil added to fuel would lubricate a fuel pump and help prevent what occurred. Not sure where you read that, but there is nothing wetted by gasoline in our pumps that needs lubricated. On 7/8/2024 at 8:18 AM, Captain Neon said: Did I just have a bad pump or does Marvel Mystery Oil do bad things to fuel pumps? Since you haven't said what failed in the pump no one can make a cogent argument one way or the other. Not sure why you keep harping on MMO though. I used it all the time in older engines to clean them up internally. Never had a problem and whomever these detractors are you mention, well everything out there someone will complain about, rightly or not. Quote Link to comment Share on other sites More sharing options...
DeS-11 Posted July 11 Report Share Posted July 11 I used Marvel on nearly every fill-up on another old car (also with a mechanical fuel pump) that I daily drove a few years ago, and never had an issue with the fuel pump in the entire time I drove it, which was seven or eight years. I think it's a great product and will continue to use it in the same way. Quote Link to comment Share on other sites More sharing options...
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