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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 14299
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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My name is robbie, I'm working on a mercruiser 350 mag mpi

Customer Question

Hi, my name is ***** ***** working on a mercruiser 350 mag mpi horizon. I installed a new long block in the boat over the winter, it ran great after the install. The fuel filter and raw water pump were changed last week, now the engine will not run unless at half throttle, and it sounds way out of time. I plugged in a mercruiser techmate diagnostic scan tool, the only code that came up was for a throttle position sensor. Fuel pressure is 43psi, engine has mefi3 computer, no serial number on the engine. Is it possible the tps changes timing and will cause this issue?
Submitted: 5 months ago.
Category: Boat
Expert:  Jason replied 5 months ago.

Hi Robbie, my name is Jason

On an MPI engine the timing will change very very quickly for a number of reasons. The engines computer is deciding what the optimal timing should be hundreds of times per minute.

If you are getting a code for the TPS, you would replace the TPS and re-test. A bad TPS will cause the symptoms you are describing. With the engine, the ECM is always going to do what it can to try to keep the engine running. When the TPS goes bad, the engines ECM simply doesn't know how much throttle you are giving the engine, and that causes the ECM to get confused, and although it's going to try to keep the engine running, the engine is not going to run the correct way if it doesn't know how much throttle you are trying to apply. Does that all make sense to you?

Expert:  Jason replied 5 months ago.

I think that should do it. But I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.

Thank you

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