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Engine will only run 4150 rpm at wide open throttle


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2000 Malibu Response LX ski boat with MPI - 

I have a 2000 Malibu LX ski boat with an Inmar MPI 5.7 liter engine.  The specs on this engine are WOT at 4600-5200 rpm, and historically this boat has run about 4800 rpm and about 48 mph WOT. This season, I find that the engine will not go over about 4150 rpm which is about 41-42 mph.  I’m running a factory 13”x13” prop.   I’ve done the following to try and get my high end back, but have not been successful:

First, this is a garage kept boat, so the hull is waxed and clean.  All tests were run with 2-3 total passengers in the boat i.e. not overloaded.

Boat has fresh premium fuel from a marina (100% gas i.e. no ethanol).  Others at my marina have purchased fuel here and have had no fuel issues.

Swapped the prop with a brand new lower pitch prop I had as a spare, 13”x12”.  With this prop, I can get 4350 rpm, but top speed remains 41-42 mph.

Changed the fuel filter

Ran a fuel pressure test at the fuel rail.  I measured 45 PSI at key-up, and the specs are >40 PSI at key-up.

Removed and cleaned the flame arrestor.  It had a little dirt on it, but not much.

While the flame arrestor was off, I physically inspected the throttle linkage as I pushed the throttle forward.  The throttle kept advancing as I pushed forward and was not binding.

Did a minor tune-up consisting of cap, rotor, and plugs.  The cap and rotor had minimal wear.  Some, but not all of the plugs were pretty black.

While I was swapping the plugs, I ran a compression test on each cylinder with each cylinder measuring between 150-170 PSI.  Specs are >100 PSI with no more than 30% delta between high and low cylinder.

I suspected a bad throttle positioning sensor (TPS), so I had a mechanic test the TPS sensor on his EFI engine tester.  The TPS accelerated smoothly from 0-94% which was 4.32 volts at wide open.

I will note that I reach 4150 or 4350 rpm at about 2/3 throttle.  I can keep advancing the throttle, but get no more power; it’s like the last third of the throttle range is dead.  I will also note that right when the throttle/engine reaches 4150 or 4350 rpm, I get a tiny surge where then engine goes to say 4170 or 4370 rpm for a split second, then drops back to 4150 or 4350.  It seems like something is limiting my rpms.

Last, but not least, the engine revs up fine in neutral.  I can get 4600+ rpm in neutral.  It’s under load when I’m struggling to get high rpms.

I’m running out of ideas.  Please help.

Terri

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What is the fuel pressure at key on, idle, and wide open?

if fuel pressure is spec, then it sounds like time to have the fuel injectors rebuilt and mapped.  

Edited by Bozboat
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did you check the screen at the bottom of the fuel pump - mine was dirty and I picked up about 200 rpm.   Also you might have 2 fuel filters, did you change the one in the trunk too ?

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Fuel pressure spec is at least 40.  Mine reported 45.

One fuel filter was changed.  Need to do the other and check the screen at the fuel pump.  I am also planning to change the spark plug wires.

Fuel injector rebuilt sounds like a big job.  Yes?

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In addition to the inline fuel filters, injectors themselves also have a screen filter at the inlet.  Those can clog as well.  There are numerous YouTube videos on how to "rebuild" you injectors.  Not a complicated process.  

Also, if someone could share the injector part number off this motor, there are places like THIS that sell rebuilt/flow matched injector sets for a decent price.

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18 hours ago, Terri said:

Did a minor tune-up consisting of cap, rotor, and plugs.  The cap and rotor had minimal wear.  Some, but not all of the plugs were pretty black.

 

3 hours ago, Terri said:

I am also planning to change the spark plug wires.

Coil for that year is pretty cheap...since they're probably due anyhow, I'd definitely change the wires and coil before diving into injectors.

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On 10/5/2020 at 7:10 AM, Terri said:

Fuel pressure spec is at least 40.  Mine reported 45.

You reported 45 at key up.  What is the pressure at wide open throttle, as @Bozboat asked?

Remove the flame arrestor again and look into the throttle body as you open the throttle.  Does the butterfly open all the way at WOT, so that it is parallel to the throat?  If not, you may have worn out cable nuts.  If they are not too bad, you can adjust the linkage so that you get full range on the butterfly.

And I'll second the plug wire change.  They do age and change resistance, weakening the spark.

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In addition to the suggestions above, check knock sensors.  Sounds a bit like timing may be not advancing to design range.  You note boat peaks then seems to lose a bit of power at the early segment of your WOT test.  That could indicate either a knock sensed and thus some timing is being pulled out.  Root cause could very well be in the fuel system.  May be worth having the injectors tested and cleaned.

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On 10/5/2020 at 7:10 AM, Terri said:

Fuel pressure spec is at least 40.  Mine reported 45.

One fuel filter was changed.  Need to do the other and check the screen at the fuel pump.  I am also planning to change the spark plug wires.

Fuel injector rebuilt sounds like a big job.  Yes?

I agree that you should change the wires and coil, then test again.  Also, as JustGary mentioned, verify that the buttterly on the intake manifold is opening all the way when the throttle lever is moved to wide open. You can check this with the engine off.  Here is a thread where I went thru similar issues and had my fuel injectors rebuilt.  Pretty simple and not all that expensive.

 

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Terri, I started having this issue as well (2001 Response LX w/5.7).  If I advance to full throttle is bogs down and sputters trying to get up to speed.  In neutral it runs like a scalded dog straight to ~5k RPM.  With mine the condition gets worse over time.  When we first noticed it, we could only get to ~3900 RPM (under load) after making some slow speed runs.  We had a long ride back to the ramp and the max RPM was about 2900 by the time we got there, but still 5k if not in gear.

Keep us posted and I'll chime in if I get mine fixed.  I noticed that the hose going to the PCV is crushed.  I'm going to replace it regardless, but could that cause or exacerbate this issue?  From other threads with similar issues, it seemed fuel flow was restricted somewhere, so that's what I'm looking at first.  At least it waited till end of my season to crap out...  +1 winter project.

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i would be very hesitant of suspecting the PVC system of causing such a malfunction.  Typically, the PVC system doesn't genereate very much positive pressure (unless you have REALLY bad piston rings).  The hoses usually dont even have clamps on them.  If the pressure was that bad, you're more likely to get blow by around the valve covers gaskets. 

As @justgary and @Bozboat said, wires degrade over time.  They may still be "ok", but replacing them is cheap insurance that they arent the culprit.  And, coils weaken over time.  Again, not expensive to swap out a new one. 

Edited by Texan32
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8 hours ago, FlyinBrian said:

Terri, I started having this issue as well (2001 Response LX w/5.7).  If I advance to full throttle is bogs down and sputters trying to get up to speed.  In neutral it runs like a scalded dog straight to ~5k RPM.  With mine the condition gets worse over time.  When we first noticed it, we could only get to ~3900 RPM (under load) after making some slow speed runs.  We had a long ride back to the ramp and the max RPM was about 2900 by the time we got there, but still 5k if not in gear.

Keep us posted and I'll chime in if I get mine fixed.  I noticed that the hose going to the PCV is crushed.  I'm going to replace it regardless, but could that cause or exacerbate this issue?  From other threads with similar issues, it seemed fuel flow was restricted somewhere, so that's what I'm looking at first.  At least it waited till end of my season to crap out...  +1 winter project.

Check your fuel pressure at key on/engine off, idle, and wide open throttle.  That means under load, not just revving it up.  Revving it in neutral only tests the rev limiter, not anything important at all since it has no load.

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Thanks for all the helpful tips.  I will be able to do a couple of these things this weekend and will update what if anything changes.  I'm running out of weekends as I live in Ohio.

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2 hours ago, Terri said:

Thanks for all the helpful tips.  I will be able to do a couple of these things this weekend and will update what if anything changes.  I'm running out of weekends as I live in Ohio.

Gonna be 80 in Ohio this weekend ! You aren't in the Akron area are you?

Steve B.

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Hey Steve, my boat is on the Sandusky River at Riverfront Marina in Fremont.  I  live in Westlake.  Right you are on the weather.  77 in Westlake and 82 in Fremont.

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 So we did a few more things this past weekend.  We replaced the ignition control module and the fuel filter at the engine.  We tested the throttle linkage and it advanced properly.  The screen at the high pressure fuel pump was cleaned.  We also tested the timing which was within spec.  We then put on the fuel pressure gauge to test in water.  In neutral we get 45 psi, but when running WOT it is at 15 psi (with the last 1/3 of the throttle essentially dead).  A thought was the fuel pressure regulator, but the dealer said that rarely goes bad.  Should I believe him?

These adjustments produced only 200 more rpm and only 2 mph.  It still only maxes out at 4150 rpm and 42 mph -  but it should go to 4800-5200 rpm and 48 mph.  And I should have acceleration on the throttle all the way down.

Something is causing it to starve for fuel at high speed.  I will also note that I have two fuel pumps.  The one is on the engine which is factory installed.  The second is under the floor between the engine and fuel tank.  This was added due to vapor locking.  That vapor lock issue was corrected by Malibu when they did a redesign in 2003.  Fuel pumps are expensive, so I am trying to rule out everything else before replacing those.

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Terri,

I know of 3 boats on my lake THIS YEAR that have required new FPRs.  I would not say that it is a rare failure.  I can't speak to Malibu's vapor lock fix but in nautiques, the low pressure pump is almost always the one to fail (not the high pressure nearer engine).  Could you install a schrader valve "after" the low pressure pump to see what it is reading?  

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My understanding is that a bad fuel pressure regulator will result in high fuel pressure and that low fuel pressure indicates a bad fuel pump.   If this is the original factory installed fuel pump on your 2000 model boat, that is an amazing run for this era fuel pump.

Edited by Bozboat
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12 minutes ago, Bozboat said:

My understanding is that a bad fuel pressure regulator will result in high fuel pressure and that low fuel pressure indicates a bad fuel pump.   If this is the original factory installed fuel pump on your 2000 model boat, that is an amazing run for this era fuel pump.

boz, I think anecdotally you're right.  I don't really understand the physics of the FPR that well, but in theory, the line to intake manifold could play games with the pressure at injectors.  In fact, after watching some videos on FPR I'm even more confused.  Literally a guy said the FPRs job is to NOT provide a regulated pressure?!  Point for OP, FPR can and do go bad.  

 

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3 hours ago, 85 Barefoot said:

boz, I think anecdotally you're right.  I don't really understand the physics of the FPR that well, but in .theory, the line to intake manifold could play games with the pressure at injectors.  In fact, after watching some videos on FPR I'm even more confused.  Literally a guy said the FPRs job is to NOT provide a regulated pressure?!  Point for OP, FPR can and do go bad.  

 

I think the FPR’s job is to bleed excess fuel pressure off the Rails to the return line. So that the rails have a constant pressure. 🤷🏻‍♂️

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5 hours ago, Terri said:

Something is causing it to starve for fuel at high speed.  I will also note that I have two fuel pumps.  The one is on the engine which is factory installed.  The second is under the floor between the engine and fuel tank.  This was added due to vapor locking.  That vapor lock issue was corrected by Malibu when they did a redesign in 2003.  Fuel pumps are expensive, so I am trying to rule out everything else before replacing those.

Fuel lines are fairly cheap.  Sound crazy, but they've been known to deteriorate and cause issues.  Next, verify ALL fuel filters are changed...you might have 2. 

After that, I'd change the pump.  For my model year, it is a very expensive pump out of production but a few here have done a swap.  See thread below and referenced pump models.  Not a bad price at all...

https://www.summitracing.com/parts/crt-p90043

https://www.summitracing.com/parts/crt-p61171

 

Edited by formulaben
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Only the https://www.summitracing.com/parts/crt-p61171 is marine rated AFAIK.  There's been a lot of talk about it  needing special fittings (made of Unobtanium??) that no one seems to have solved, expect by buying other pumps that have them.  Or https://skiboatpartsonline.com/Indmar-High-Pressure-fuel-pump-kit?search=pump with all the bits.  Although spendy, it is WAY less than what Indmar was charging for the OEM replacmeent when you could get it.  

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