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ahhudgins
06-10-2010, 08:45 PM
95 Maristar 200
5.7L
TBI

Before I de-winterized the boat this spring, I installed new plugs, wires, rotor and cap. Iíve been using the boat the last month and I really hadnít ran it at WOT until today. I noticed that something didnít feel exactly right, like there was a slight miss in the engine.
It was a little choppy and the wind was blowing so I stopped the boat and slowly ran the RPMs up in neutral. When I reached 2500 RPMs the engine started to miss and dropped back down to 2000 even though I was still increasing the throttle. Eventually it picked back up and started going up to over 4000 but it was sputtering.

I found a smooth cove and did the same thing with the engine in gear. The boat accelerated smoothly up to about 4200 RPMs, which is about 43 MPH with my 4 blade prop. I started to feel the slight miss and my RPMs were bounching between 3900 and 4200 but my speed stayed at 43 MPH. To be honest, Iíve never really paid any attention to my tach until now.

Took the boat back to the dock and disconnected the throttle cable at the throttle body. Moved the throttle by hand and it hesitated and dropped at 2500 for a few seconds and picked back up (in neutral). I have never revved the engine up in neutral since Iíve owned it so I donít know how long this has been going on. If I press the throttle pretty quick it will rev up to 5000, but doing it slowly it messes up at 2500 every time. Does pretty well under load except for the little miss at WOT, in neutral it sounds pretty bad.
I double-checked all of my wires, plugs, and cap.
Any ideas before I take it to a mechanic?

JLeuck64
06-10-2010, 09:00 PM
Hope ya didn't put those crappy Bosch platinum plugs in... hate them dang things!

Maple Leaf
06-10-2010, 09:17 PM
Do you still have the old rotor and cap? Might be worth a try switching back to eliminate the chance you have a cracked new one...

Is there a chance the timing could have been changed? The distributor normally wouldn't get rotated unless there was some reason to unbolt the clamping bolt.

JimN
06-10-2010, 09:58 PM
Pull the fuel pump and clean the screen at the inlet. This is the usual symptom when that's clogged. Revving well in neutral means nothing- power under load is the only thing that matters.

ahhudgins
06-10-2010, 10:20 PM
I got my plugs and wires from skidim, I had a bad episode with the Bosch plugs on one of my cars and I won't use them again. I threw the old wires and cap out a long time ago because I've been using the boat for over a month with no problems, but this was the first time I was going to barefoot and run it close to WOT.

Jim, it's just the opposite of what you said. I can barely notice the miss when it's in gear and the boat is under way. The engine is smooth pulling up skiers and I can only feel a slight miss over 4000 rpm. It's when I put it in neutral and slowly increase the RPMs when it starts to miss (badly). RPMs come up to 2500 (in neutral) and then it drops off to 2000. I keep increasing the throttle and it picks back up but has a very bad sputter in it. I put a new fuel filter on it last year and it was running great when I put it away for the winter. We've been skiing and wakeboarding for a month and I didn't notice the miss until I got to barefoot speed.

I will pull the fuel pump and check the screen. I'm on my 3rd tank of gas so far and I did switch from 87 octane to 89 this year. If the screen looks ok I may run out all of the gas and check my tank too. Thanks for the input.

JimN
06-10-2010, 10:40 PM
I got my plugs and wires from skidim, I had a bad episode with the Bosch plugs on one of my cars and I won't use them again. I threw the old wires and cap out a long time ago because I've been using the boat for over a month with no problems, but this was the first time I was going to barefoot and run it close to WOT.

Jim, it's just the opposite of what you said. I can barely notice the miss when it's in gear and the boat is under way. The engine is smooth pulling up skiers and I can only feel a slight miss over 4000 rpm. It's when I put it in neutral and slowly increase the RPMs when it starts to miss (badly). RPMs come up to 2500 (in neutral) and then it drops off to 2000. I keep increasing the throttle and it picks back up but has a very bad sputter in it. I put a new fuel filter on it last year and it was running great when I put it away for the winter. We've been skiing and wakeboarding for a month and I didn't notice the miss until I got to barefoot speed.

I will pull the fuel pump and check the screen. I'm on my 3rd tank of gas so far and I did switch from 87 octane to 89 this year. If the screen looks ok I may run out all of the gas and check my tank too. Thanks for the input.

In that case, re-check the plug wires vs the firing order. It happens.

1redTA
06-10-2010, 10:43 PM
When trouble shooting start with the cheap things, I would check the throttle position sensor with a volt meter for smoothness and then get the ignition contol module tested. What about the coil?

JimN
06-10-2010, 10:57 PM
When trouble shooting start with the cheap things, I would check the throttle position sensor with a volt meter for smoothness and then get the ignition contol module tested. What about the coil?

The TPS won't cause a miss.

One thing that makes a difference on my truck (which has the same motor) is the coil tower- if it's crusty, it runs like crap but it doesn't get that way unless I do nothing to it for too long. Since the wires were recently replaced, I would check to make sure they're snapped on snugly.

ahhudgins
06-11-2010, 07:52 AM
I thought about the TPS since the RPMs dropped down at the same position each time (2500), but ONLY in neutral. Pulling up a skier it runs smooth as silk until barefoot speed so I didn't think it was the sensor. Since I already threw away my old parts, I may have to drop another $100 to eliminate my tune up. I have a solid fuel line between the fuel filter and the throttle body, is there any way to check fuel pressure at the throttle body? My local engine builder said he could make me a fuel line with a gauge in it.

TMCNo1
06-11-2010, 08:16 AM
Check for a broken spark plug.

JimN
06-11-2010, 08:38 AM
I thought about the TPS since the RPMs dropped down at the same position each time (2500), but ONLY in neutral. Pulling up a skier it runs smooth as silk until barefoot speed so I didn't think it was the sensor. Since I already threw away my old parts, I may have to drop another $100 to eliminate my tune up. I have a solid fuel line between the fuel filter and the throttle body, is there any way to check fuel pressure at the throttle body? My local engine builder said he could make me a fuel line with a gauge in it.


Is your ECM at the side of the motor, below the head? If it is, remove the plastic hose attached to the MAP sensor (it goes to the base of the throttle body) and check for liquid in it. Check for any codes, too. If you see 34 or 35, you may have issues with the MAP sensor or this hose.

Look on the hard fuel line for a black plastic cap. That would be the Shrader valve and that's where you attach a fuel pressure gauge.

Re: a bad TPS- if it goes bad, the return voltage will be low, or high. If it's far enough out of range, the ECM defaults to 12% throttle position and the throttle response will be sluggish. If the MAP sensor goes bad, it will usually go about as fast as a stove.

ahhudgins
06-11-2010, 02:32 PM
Is your ECM at the side of the motor, below the head? If it is, remove the plastic hose attached to the MAP sensor (it goes to the base of the throttle body) and check for liquid in it. Check for any codes, too. If you see 34 or 35, you may have issues with the MAP sensor or this hose.

Look on the hard fuel line for a black plastic cap. That would be the Shrader valve and that's where you attach a fuel pressure gauge.

Re: a bad TPS- if it goes bad, the return voltage will be low, or high. If it's far enough out of range, the ECM defaults to 12% throttle position and the throttle response will be sluggish. If the MAP sensor goes bad, it will usually go about as fast as a stove.


The boat is at the lake but I'm pretty sure my ECM is mounted on top of the intake manifold. Except for all of my sensors, most of my system is packed together between the throttle body and the thermostat housing on top of the motor. I had the fuel line off last year when I replaced my filter and there is no Shrader valve in the line, solid metal. My job is taking me out of state this weekend so I won't get much of a chance to trouble shoot the problem. What little time I do have, I will recheck my wiring, check the fuel pump screen, check the TPS, and go ahead and put new plugs in. I've seen plugs do some weird things.

ahhudgins
06-21-2010, 05:46 PM
I went to Auto Zone and get a set of Autolite plugs and took out the NGKBR6FS that I got from Skidim. The slight miss at barefoot speed went away but it still does the weird RPM drop when I rev it up in neutral. The fuel lines all check good and I will have time to check more this weekend. Going to put my plugs back in one at a time and find the bad one.

I took my MEFI service manual with me on the plane trip this weekend. I've decided to purchase a MDTC so I can check any codes, and I might look into a scan tool. I want add a "check engine" light to my dash, and if I understand the manual correctly, I can add a bulb or led across E and F of the data link connector. I guess JimN can answer this one? Is this date link connector a standard GM connector or is marine specific? I want to make up a cable with a plug on one end and a led on the other end.

ahhudgins
06-21-2010, 07:01 PM
Check for a broken spark plug.

Didn't see any cracks on the new plugs and they only have about 10 hours on them, but one of them is definitely causing a problem. Going to check the MAP sensor next as JimN suggested. The service manual talks about the rev limiter kicking in but it's only related to the engine temperature. After checking it again this weekend, my RPMs will increase up to exactly 3000 RPMs (in neutral) and then start to drop and sputter if I keep increasing the throttle. The engine runs great in gear and under load, so the MAP sensor or hose is the most likely culprit.

Still working on it!! Thanks for everyone's help.

JimN
06-21-2010, 10:03 PM
I went to Auto Zone and get a set of Autolite plugs and took out the NGKBR6FS that I got from Skidim. The slight miss at barefoot speed went away but it still does the weird RPM drop when I rev it up in neutral. The fuel lines all check good and I will have time to check more this weekend. Going to put my plugs back in one at a time and find the bad one.

I took my MEFI service manual with me on the plane trip this weekend. I've decided to purchase a MDTC so I can check any codes, and I might look into a scan tool. I want add a "check engine" light to my dash, and if I understand the manual correctly, I can add a bulb or led across E and F of the data link connector. I guess JimN can answer this one? Is this date link connector a standard GM connector or is marine specific? I want to make up a cable with a plug on one end and a led on the other end.

Why are you not using AC plugs? I have never had much luck with Autolite or Champion plugs.

You should be able to do the LED indicator easily enough. Make sure to observe the correct polarity.

JimN
06-21-2010, 10:07 PM
Didn't see any cracks on the new plugs and they only have about 10 hours on them, but one of them is definitely causing a problem. Going to check the MAP sensor next as JimN suggested. The service manual talks about the rev limiter kicking in but it's only related to the engine temperature. After checking it again this weekend, my RPMs will increase up to exactly 3000 RPMs (in neutral) and then start to drop and sputter if I keep increasing the throttle. The engine runs great in gear and under load, so the MAP sensor or hose is the most likely culprit.

Still working on it!! Thanks for everyone's help.

How old are your plug wires?

Your issue starting at 3000 RPM sounds exactly like a clogged pump inlet screen.

Before you replace the MAP sensor (or any other), make sure it's actually bad. Check for leaks along the intake manifold and throttle body. Watch the fuel spray cone and look for gaps- this can be caused by a clogged injector.

ahhudgins
06-21-2010, 10:48 PM
Just installed plugs, wires, cap and rotor this Spring. Got all parts from Skidim.com. I just stopped by Auto Zone on the way to the lake and picked up another set of plugs to see if the new NGK plugs were causing my problem. I didn't feel any "crack" when I installed the new NGK plugs in March, but when I took them out this weekend the miss at 40 MPH went away. I'm going to put the NGK plugs back in next weekend to make sure it wasn't just a fluke.

It's just weird that I can pull up skiers and barefoot at 40 MPH and it runs like a top. Put it in neutral, slowly rev the engine and it bogs down at 3000 RPMs every time. I'll let you know what I find the next weekend.

ahhudgins
06-21-2010, 10:54 PM
How old are your plug wires?

Your issue starting at 3000 RPM sounds exactly like a clogged pump inlet screen.

Before you replace the MAP sensor (or any other), make sure it's actually bad. Check for leaks along the intake manifold and throttle body. Watch the fuel spray cone and look for gaps- this can be caused by a clogged injector.

I have looked at the spray at the injectors. They are both coned shaped, but they do have what appears to be gaps in the cones. I've owned the boat for about 4 years and it has always looked like that. I know the injectors are "pulsed" so I wasn't sure how the cone should look. I've always felt that it idled a little ruff, even more around 800 to 1000 RPM and I can always smell gas in the exhaust.