'99 MerCruiser MPI surging question

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BillWilliams

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May 23, 2021
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We have a 1999 MerCruiser 310 with a Bravo 3 serial number OL342220.

The engine has suddenly started "surging" with fluctuations of 100 to 200 RPM. It's worst when cold and seems to settle down at higher RPM (over 3000) once the engine warms up; however, it will still do it at lower RPM and especially during a hole shot. It's definitely hurting the acceleration.

I will admit we purchased this boat last summer and I have yet to do a tune up on it and plan to at least put some spark plugs in it to see if that helps.

I also plan on running some fuel injector cleaner through it.

Any other ideas I should be considering?

Thank you
 
alldodge

alldodge

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Being a 310 its a 7.4 MPI, thanks for the serial number

Need to get a scanner on the motor to see what the IAC, TPS and MAP show during the surging. Check fuel pressure to see if its a 43 psi running. Then remove the vacuum line and see if it goes up a tad
 
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BillWilliams

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I've never dealt with a boat with fuel injection. Does getting a scanner on the motor involve getting a marine mechanic or is this something that can be purchased reasonably?

Also, what are the chances of doing a tune up will take care of this issue given I have no idea how old the cap, rotor and wires are?
 
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BillWilliams

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Thanks for the great detail.

Forgive me, but can the fuel pressure be checked just running on ear muffs at the house or does it need to be under load on the water?
 
tpenfield

tpenfield

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Not sure a tune-up or injector cleaner will fix the issue. Keep in mind that the engine is run by a computer and the computer is responding to what the various sensors are telling it.

So, it is a fair bet that there is one sensor that is starting to act up and producing 'false' or erratic values.

I had a situation where my MPI engine would surge . . . it turned out that the TPS was getting worn and the voltage output from the sensor was fluctuating . . . therefore the engine was surging. Replacing the sensor fixed the problem. Your situation may vary.
 
alldodge

alldodge

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The TPS and IAC work off of 12V. Can get to the contacts like Ted was talking about you could watch and see if they are changing
 
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BillWilliams

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May 23, 2021
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Here's an update.

I did find some information on testing a TPS with a multimeter. I did that and everything looked good. No dead spots or voltage spikes.

Fortunately, our neighbors' (they're farmers) mechanic works on boats so he stopped by and took a look.

He put a fuel pressure gauge on and it was running around 37 PSI (obviously below 43 PSI) and dropped as the throttle was advanced. He then removed the spark plugs and one of them had broken porcelain and all of them showed they were running lean which makes sense given the lower fuel pressure.

We decided to start by replacing the fuel filter and the spark plugs.

Once that was done, he put the gauge back on and the pressure was showing higher but the gauge was fluctuating a lot so it was difficult to determine the pressure. He removed the vacuum from the regulator and the pressure steadied at 45 PSI.

At this point, we decided my wife and I would take the boat out and see how it ran without the vacuum on the regulator and then if it's running well, hook the vacuum back up on the regulator to see how things went.

The boat ended up running great both with and without the regulator so at this point, we're chalking it up to either the fuel filter or spark plugs (I'm voting for the filter).

I'm going to finish the tune up with a new distributor cap, rotor and wires.
 
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