Flat spot between 3400 and 3800 RPM revisited

[River City Water Boy]

I originally posted this topic back on March 15th, 2010. With everyone's help I thought I had the problem solved. No such luck. Please take a look at the following paragraph which is the original post and I will explain what happened.

March 15th - "I have a 99 Sunsetter LXi with a 325 Monsoon that runs great except that it has a "flat spot" between 3400 and 3800 RPM. As I accelerate up to top speed, the engine RPM will rise smoothly until it hits 3400 RPM then will stop rising even though I am still moving the throttle forward, then all of a sudden, when I move the throttle forward far enough, it will jump to 3800 RPM. After this happens the engine will continue to rev smoothly all the way up to WOT. It does the same thing as I decelerate. The revs will drop to 3800 and stop until I move the throttle far enough then all of a sudden the revs will drop to 3400 RPM. No matter how I manipulate the throttle the engine will not operate between 3400 and 3800 RPM but will function perfectly on either side of this RPM range. To me, the obvious problem was the throttle position sensor. So I replaced it with a new one. No effect whatsoever. Same flat spot at the same RPM level. Just for the heck of it, since the problem seemed so obvious I went out and bought a second brand new TPS on the off hand chance the first one was defective. The second one had one no effect either. Like I said, the engine runs great, starts great, very smooth with good power, it just has this big hiccup between 3400 and 3800 RPM. The engine is well maintained with fresh plugs and all the appropriate maintenance. Just to make sure there was nothing serious cropping up I checked the compression on all eight cylinders. No problem there either, compression was good (177 to 181 psi) and well balanced across all eight cylinders. The only thing I can figure is one of the other sensors may be bad but I can't imagine which one. Mass air flow? Could the ECM have a problem?

Has anyone out there run into a problem similar to this? Any suggestions? Thanks ahead of time for any ideas you can throw out there."

I received a suggestion that I replace the solid steel fuel injection crossover lines with the newer rubber crossover lines with the logic that the rubber fuel lines would help moderate fuel pressure fluctuations/hammering and solve the problem. I replaced the fuel lines and reported back on March 23rd that the problem was solved. Turns out I was wrong. It seems the problem "went away" on that particular test run for some unknown reason. Since then the flat spot has returned only the RPM range has changed slightly since adding the rubber fuel lines. Now the flat spot is between 3100 and 3500 RPM instead of 3400 and 3800.

Since my last post and since I switched to the rubber fuel lines I have also replaced the knock sensor, throttle position sensor (again), the knock module, the ignition coil, distributer cap and rotor cap. I have experimented with the boat heavily loaded and completely empty except for me at the wheel. I have had the boat at different altitudes from 25 to 3500 feet above sea level. I have stuck a 2X4 under the front edge of the engine cover while running to make sure the engine has plenty of cool fresh air. I have run the engine with the spark arrestor removed. I have tried regular gas and premium gas. None of these has had any effect. The big problem is that 3100 to 3500 RPM is prime skiing speed. This flat spot means you either have to ski at 30 MPH or 36 MPH with no option in between these speeds. Anyway, you get my point. Very frustrating.

Does anyone have any new ideas or suggestions before I pull the rest of my hair out?

Edited July 24, 2010 by River City Water Boy

[Lance B. Johnson]

River- Have you had a technician listen to your boat? The reason I say is because this type of problem can be difficult to diagnose over the internet.....leading to expensive parts replacement as you know.

I guess what I am saying is that a tech can hear things that an average person can't. ex. I have had customers bring me cars for brake work that are running on 3 of 4 cylinders and they never knew. It might be worth swallowing the pride and take it in. Heck if I were closer I would look at it for you.

Before you take it in you could do a compression test.

[Levi900RR]

River- Have you had a technician listen to your boat? The reason I say is because this type of problem can be difficult to diagnose over the internet.....leading to expensive parts replacement as you know.

I guess what I am saying is that a tech can hear things that an average person can't. ex. I have had customers bring me cars for brake work that are running on 3 of 4 cylinders and they never knew. It might be worth swallowing the pride and take it in. Heck if I were closer I would look at it for you.

Before you take it in you could do a compression test.

HAHAHAHAHAHAHAHAHAHAHAHAHHAHAAHAHA

Thanks, I needed that this morning!

[CoMtnBu]

I have something similar going on too. Except my flat spot begins at 3600 RPM. Interesting about it moving down in RPM.

I haven't had a chance to replace my fuel crossovers yet. Hoping I can get my local shop to make them as 170 bucks seems a bit steep for two hoses.

Have you tried running some of the better (ie. more expensive)injector cleaner through it?

Is your thottle body clean?

Replace BOTH fuel filters and clean screen on pump?

If the rubber fuel hoses don't fix my problem I'm considering pulling the injectors and having them cleaned.

I know first hand how frustrating chasing it is.

[scfdfireman]

If the motor has a throttle body remove the spark arrester and with a timing light watch the atomized spray emitting from the bottom of the two injectors. The spray should both be equal, cone shaped and small droplets. If they're unequal, misshaped or dripping you have a plugged injector. The timing light is usually bright enough to see the spray, but I never tried it on a lake in bright daylight...

[River City Water Boy]

River- Have you had a technician listen to your boat? The reason I say is because this type of problem can be difficult to diagnose over the internet.....leading to expensive parts replacement as you know.

I guess what I am saying is that a tech can hear things that an average person can't. ex. I have had customers bring me cars for brake work that are running on 3 of 4 cylinders and they never knew. It might be worth swallowing the pride and take it in. Heck if I were closer I would look at it for you.

Before you take it in you could do a compression test.

[River City Water Boy]

Ruffdog - Yes I had a certified Indmar technician out on the boat with me back in March. He had his laptop and fuel pressure sensor all hooked up as we ran the boat on Folsom Lake. Unfortunately and for some unknown reason the flat spot would not happen the day we went out even though we spent a half an hour on the water. This makes me suspicious that when his test equipment is hooked up it is changing one or more of the engine parameters. Not sure if that is even possible but it is curious never the less. At the time his computer showed no error codes and the fuel pressure was spot on. Anyway, I will talk with him this week and we will make another run out on the lake hopefully within a week or so. Also, I have already done a compression test which shows all eight cylinders running 177 to 181 psi. You're right though, a problem like this requires you to swallow your pride and bring in one of the experts. We'll see what happens.

Levi - ?

CoMtnBU - Yes I have run numerous bottles of fuel injector cleaner over the last couple of years. No help there. Yes I have replaced both fuel filters. I would be more concerned about fuel supply/starvation issues and clogged injectors if the engine had problems or seemed to be starved for fuel at higher or lower RPMs or at WOT, but it runs strongly and smoothly all the way up 4500-4600 RPM. It also runs great in the 2000 RPM range when wakeboarding and starts quickly and idles very smoothly. Seems pretty weird to me that an engine can run so well at all RPMs except this one band between 3100 and 3500. Go figure!

Fireman - The boat has MPI instead of a throttle body.

Thanks guys for your input. If anyone else has any ideas I'm all ears. Also, I will keep you informed of what happens when the Indmar Tech and I run our next test.

[REW]

Could there be a problem on the throttle qurdrent away from the motor?

REW

[River City Water Boy]

Could there be a problem on the throttle qurdrent away from the motor?

REW

[River City Water Boy]

I'm not sure. Tell me what a throttle qurdrent is and I'll check it out.

[REW]

I'm not sure. Tell me what a throttle qurdrent is and I'll check it out.

Under your right arm while operating the boat. Not sure if a cable could be sticking or sliding out of the actual throttle arm or something eles. From what you have posted it sounds like the motor is doing what it is being told by the man machine interface. Check to see if something is loose, binding, rubbing or just needs to be lubbed.

Good Luck

If that don't work it is always good to bang your head on the underside of the steering wheel.

REW

[tvano]

If that don't work it is always good to bang your head on the underside of the steering wheel.

REW

i've done that more than once. just about every time i have to climb under there.

[Lance B. Johnson]

Ruffdog - Yes I had a certified Indmar technician out on the boat with me back in March. He had his laptop and fuel pressure sensor all hooked up as we ran the boat on Folsom Lake. Unfortunately and for some unknown reason the flat spot would not happen the day we went out even though we spent a half an hour on the water. This makes me suspicious that when his test equipment is hooked up it is changing one or more of the engine parameters. Not sure if that is even possible but it is curious never the less. At the time his computer showed no error codes and the fuel pressure was spot on. Anyway, I will talk with him this week and we will make another run out on the lake hopefully within a week or so. Also, I have already done a compression test which shows all eight cylinders running 177 to 181 psi. You're right though, a problem like this requires you to swallow your pride and bring in one of the experts. We'll see what happens.

Levi - ?

CoMtnBU - Yes I have run numerous bottles of fuel injector cleaner over the last couple of years. No help there. Yes I have replaced both fuel filters. I would be more concerned about fuel supply/starvation issues and clogged injectors if the engine had problems or seemed to be starved for fuel at higher or lower RPMs or at WOT, but it runs strongly and smoothly all the way up 4500-4600 RPM. It also runs great in the 2000 RPM range when wakeboarding and starts quickly and idles very smoothly. Seems pretty weird to me that an engine can run so well at all RPMs except this one band between 3100 and 3500. Go figure!

Fireman - The boat has MPI instead of a throttle body.

Thanks guys for your input. If anyone else has any ideas I'm all ears. Also, I will keep you informed of what happens when the Indmar Tech and I run our next test.

Hmmm, did the tech have a mechanical fuel pressure gauge hooked up then? Try running it with/ and without the gauge. If you remember the "fuel hammer" thread the gauge works as a dampener. It should have been fixed when you replace the lines but who knows?

[BreretonBu]

Ineresting - to find reference to the exact same engine performance problem as that which we've recently discovered with our 1998 Sunsetter (equipped with 325 Monsoon)! The difference being that the flat spot we're dealing with is between 2600 and 3000 RPM. Also a 400 RPM span. Also right in the thick of prime skiing speeds for the kids. Also experienced during acceleration and deceleration. Also that the engine runs perfectly in all other regards above and below this flat spot range.

Hmmmm! Odd that an answer is not found in a forum such as this. Actually, with all of the work that you've done on the engine with no change in performance, the issue borders on worrisome. Is it a solveable matter? Or is it a performance issue we've got to deal with through the remaining tenure with this boat? I plan on taking it to the shop this coming weekend to hopefully sort out - but after reading your postings, I'm beginning to doubt a quick turnaround in the mechanic's hands.

[River City Water Boy]

Ineresting - to find reference to the exact same engine performance problem as that which we've recently discovered with our 1998 Sunsetter (equipped with 325 Monsoon)! The difference being that the flat spot we're dealing with is between 2600 and 3000 RPM. Also a 400 RPM span. Also right in the thick of prime skiing speeds for the kids. Also experienced during acceleration and deceleration. Also that the engine runs perfectly in all other regards above and below this flat spot range.

Hmmmm! Odd that an answer is not found in a forum such as this. Actually, with all of the work that you've done on the engine with no change in performance, the issue borders on worrisome. Is it a solveable matter? Or is it a performance issue we've got to deal with through the remaining tenure with this boat? I plan on taking it to the shop this coming weekend to hopefully sort out - but after reading your postings, I'm beginning to doubt a quick turnaround in the mechanic's hands.

[River City Water Boy]

To give you a little hope for an easy resolution to your problem - When I first started trying to solve this problem more than a year ago my mechanic and I spoke with an Indmar Tech who was positive the problem was a faulty throttle position sensor. He said TPSs are graduated in 400 RPM increments and when one of the windings fail you get a 400 RPM flat spot just like I had. Unfortunately this was not the case for my boat but you may very well get lucky. Don't give up hope just yet. As a matter of fact they are easy to replace yourself by unplugging the wire, removing two Torx head screws, removing the sensor, installing the new one and hooking it back up. You can buy the new sensor at the auto parts store for about forty bucks or pay Indmar $125 (I have purchased both) for one of theirs. This might be good way for you to start.

It has been an interesting journey trying to solve this problem. I am confident that it can be solved though. To make a long story short (assuming you have not read the thread I originally started back in March) I first initiated this discussion on this website back in March. One reader referred me to a previous post from another member who had the same problem. While trying to troubleshoot the problem he found that when he hooked up a fuel pressure gauge to the fuel system while the engine was running he found that the problem went away. The theory was the fuel pressure gauge itself solved the problem simply by absorbing some of the pressure fluctuations in the fuel system caused by the opening and closing fuel injectors. When he removed the fuel pressure gauge the problem returned. In order to permanently solve the problem he installed rubber (as opposed to steel tube)fuel crossover lines at the front and back of both fuel injector rails. These rubber fuel lines apparently accomplished the same thing as the gauge and absorbed some of these fluctuations/pulses. The difference is that these new fuel lines solved the problem for his boat but not for mine. I am in the process of making an appointment with my mechanic to take another look and will hopefully report back the solution. My mechanic is mobile boat mechanic which I like better than a dealership. He comes to me instead of me dumping my boat to rot in the sun for a week or two at a dealership. Also, this problem has to be analyzed while the boat is in the water and under a load so instead of having to pay a dealership to haul the boat to a waterway I just meet him at a nearby lake. He hooks up his computer and gauges and away we go. Works pretty well. Anyway, until I can post back a definitive answer I will pose my own theory (it's only a theory so I may very well be wrong) is that the problem may lie in the fuel pump despite the fact the fuel pressure gauge shows it maintains appropriate fuel pressure. It would seem to me that a properly working fuel pump should be able to deal with these pressure fluctuations. If that does not solve the problem the only other thing left for me to replace on the entire engine is the ECM.

I'll be sure to report back when I know more.

Edited August 4, 2010 by River City Water Boy

[River City Water Boy]

Under your right arm while operating the boat. Not sure if a cable could be sticking or sliding out of the actual throttle arm or something eles. From what you have posted it sounds like the motor is doing what it is being told by the man machine interface. Check to see if something is loose, binding, rubbing or just needs to be lubbed.

Good Luck

If that don't work it is always good to bang your head on the underside of the steering wheel.

REW

[River City Water Boy]

Banging my head on the underside of the steering wheel is always the first thing I do when there is a problem. It usually works.

I am confident the throttle system is working properly. I have actually run the boat with the engine cover open and the spark arrestor removed and watched the throttle plate/butterfly while the engine surges to 3800 RPM or falls back to 3400 and there is no movement of the throttle assembly or throttle plate when this happens.

Edited August 4, 2010 by River City Water Boy

[SmoothWaterMan]

I believe you are still experiencing the fuel hammer issue. I just worked on a '99 Monsoon with the hammer issue, and found that the replacement cross over lines did change the rpm hammer spot, but did not eliminate it. I have replaced them with longer cross over lines that did solve the issue on this engine.

They are not mfg approved lines, but they work if anyone wants a pair. Just email me -

Thanks,

Peter

[River City Water Boy]

I believe you are still experiencing the fuel hammer issue. I just worked on a '99 Monsoon with the hammer issue, and found that the replacement cross over lines did change the rpm hammer spot, but did not eliminate it. I have replaced them with longer cross over lines that did solve the issue on this engine.

They are not mfg approved lines, but they work if anyone wants a pair. Just email me -

Thanks,

Peter

[River City Water Boy]

Peter thanks for the information. I may very well be interested in a set of your fuel crossover lines since changing the fuel lines on my boat changed the location of the flat spot as well. Before I do that though I would like to know more about them, who makes them, are they made from marine grade rubber fuel line, what type of fittings do they have, what do they cost, etc. As I am sure you can imagine, a fuel leak in a small engine compartment like the one a direct drive has could create a pretty bad situation. Not a place where I want to take any shortcuts. Beyond that I would be curious if anyone has figured out why this is happening to some boats and not others. I feel like replacing the fuel crossovers with rubber ones is really just a bandaid without figuring out what the true underlying problem is. Maybe I'm wrong but I would be interested to hear your thoughts or theories.

Thanks,

Grant

[jhardwood]

What a great thread to find this morning. I have a 1998 Response LX w/Monsoon 325, 635 hours, maintained very well, runs great...that is, except for the dreaded flat spot. One other thing that I have noticed lately. If I am on the upper edge of the flat spot with a skiier in tow, then he pulls very hard or I go into a teardrop turn to put a load on, the RPMs drop through the flat zone to bottom side, resulting in an obvious speed change. If I nudge the throttle a little when this happens, RPMs jump back up over the flat spot.

I am just starting to think about how to troubleshoot/correct this problem. Unfortunately, the nearest Malibu dealer from me is 4 hours away, so I would like to be able to fix this one myself. I appreciate all of the good input.

Peter: I would also be VERY interested in your crossover lines. Can you please provide more information? Thanks!

[Big Tuna]

I just purchased an 02 sportster LX with the 310 hp indmar carbd engine, which has this exact same flatspot issue. Still trying to self diagnose before i break down and take it to the mechanic. I wonder if the TPS theory is the same with carb'd engines.

[River City Water Boy]

I just purchased an 02 sportster LX with the 310 hp indmar carbd engine, which has this exact same flatspot issue. Still trying to self diagnose before i break down and take it to the mechanic. I wonder if the TPS theory is the same with carb'd engines.

[River City Water Boy]

If you have a carbureted engine then I would be pretty positive you do not have a throttle position sensor. Totally different fuel delivery system. A carb is pretty simple. it is controlled by the vacuum of the engine whereas a fuel injected engine is electronically controlled by the ECM. On a carb'd engine it sounds more like it is out of adjustment (float bowl level would be the first place I looked) or dirty jets in the carb. Since you just bought the boat it makes it really hard because you don't know its history. How well it has been maintained, winterized, did they use fuel stabilizer in the winter etc., etc. If possible see if you can get any history form the previous owner that may give you a few clues as to where you should start.