2000 LSV Hesitation/Backfire - Need Suggestions

[MillersMarineOtis]

Hello everyone, I have a 2000 Malibu Luxury Sport V with I believe an indmar monsoon 320hp (5.7 mpi) which has a problem on hard acceleration. If you put in gear and slam it to full throttle it just bogs out and backfires. If you ease into it a little bit its fine with no issues. Ive checked fuel pressure, changed both fuel filters, changed temp sensor for ECM (was running very rich all the time and this sensor has solved an almost identical problem in 2 chevy trucks that ive fixed), new cap/rotor/plugs/wires. Im just looking for other suggestions on things to check as its almost impossible to pull the boat and bring it to a shop until after October 1st when they allow people to use the local beach as a boat ramp again. Any input would be greatly appreciated. Thanks everyone!

[MadMan]

The throttle position sensor (TPS) is used by the MEFI to detect a rapidly changing throttle and compensate.  You could try replacing it.

[MillersMarineOtis]

I just looked it up on Bakes. $125... Im assuming its the same sensor as a 1999ish chevy 5.7 MPI in a pickup truck? $40 at the local autozone. Pictures look exactly the same. Thanks for the suggestion, I thought the ecm temp sensor fixed the rich condition but it did not. would the TPS also cause a really rich condition?

[BlackBluMalibu]

On 8/12/2017 at 0:04 AM, MillersMarineOtis said:

Hello everyone, I have a 2000 Malibu Luxury Sport V with I believe an indmar monsoon 320hp (5.7 mpi) which has a problem on hard acceleration. If you put in gear and slam it to full throttle it just bogs out and backfires. If you ease into it a little bit its fine with no issues. Ive checked fuel pressure, changed both fuel filters, changed temp sensor for ECM (was running very rich all the time and this sensor has solved an almost identical problem in 2 chevy trucks that ive fixed), new cap/rotor/plugs/wires. Im just looking for other suggestions on things to check as its almost impossible to pull the boat and bring it to a shop until after October 1st when they allow people to use the local beach as a boat ramp again. Any input would be greatly appreciated. Thanks everyone!

Did you use marine rotor, cap and wires? If automotive was used your spark could be breaking down just enough to give these symptoms. If you used OEM stuff, it could be fuel pressure regulator.   Pressure regulator will also give this symptom.  Sometimes just a good wrap on it will loosen/free the diaphragm enough to allow it to function.  Put a fuel pressure Guam on the Schrader valve should be 46-48 at key on and idle and 50-52 at full throttle. Also check injectors if the the prior 2 suggestions don't work.  Good luck, safe and fun boating!

[BlindSquirrel]

I would change the fuel line(s). Does it do what ours did in the video? We would get a backfire too at times.

 

Edited August 21, 2017 by BlindSquirrel

[MillersMarineOtis]

BlackBluMalibu.. Yes I used a Sierra Cap/Rotor/Wires I've always had very good luck with them and have installed dozens of sets over the years. No change between the old and new parts. Also I put a fuel pressure gauge on it and lake tested it and the pressure is right where it should be.. No drop upon hesitation/backfire. I was thinking maybe injectors but why would it only do it on a very hard acceleration from idle to 3/4-full throttle? I think the TPS recommendation might be the best shot at the moment. Ive been away and haven't had a chance to get a new one.

 

BlindSquirrel.. No its not similar to that problem.. When you shift from neutral to forward and stay at idle speed and then hammer it to start a skier or something it pops out the throttle body and bogs it wont pick up at all. Ill try to get a video of it soon and post so you can see for yourself what its doing. I know its hard to visualize a problem without seeing it first hand.

[BlackBluMalibu]

17 minutes ago, MillersMarineOtis said:

BlackBluMalibu.. Yes I used a Sierra Cap/Rotor/Wires I've always had very good luck with them and have installed dozens of sets over the years. No change between the old and new parts. Also I put a fuel pressure gauge on it and lake tested it and the pressure is right where it should be.. No drop upon hesitation/backfire. I was thinking maybe injectors but why would it only do it on a very hard acceleration from idle to 3/4-full throttle? I think the TPS recommendation might be the best shot at the moment. Ive been away and haven't had a chance to get a new one.

 

BlindSquirrel.. No its not similar to that problem.. When you shift from neutral to forward and stay at idle speed and then hammer it to start a skier or something it pops out the throttle body and bogs it wont pick up at all. Ill try to get a video of it soon and post so you can see for yourself what its doing. I know its hard to visualize a problem without seeing it first hand.

TPS could be the problem. Is there a way to test it before you buy one?  Keep us posted on progress.  All the best and hope that you get if fixed soon!

Edited September 18, 2017 by BlackBluMalibu