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View Full Version : Slight Stutter in my LT-1


Sodar
03-28-2006, 09:40 PM
OK, I have mentioned this problem before in previous posts, but I am in search of an answer.

When I am idling back to get my skier (~1000rpm) and pull back the throttle to either idle or neutral, my motor stutters down, almost dies and then idles normally again(~700). I have had the boat to the dealership, but I could not reproduce the problem. It does not happen everytime, but often enough that I would like to find the cause of this. I have heard terms such as the IAC and PCV Valve as being the cause of this problem. I pulled what I thought was the PCV Valve out of the valve cover, but it appears that it is just a 90º Elbow with no valve in it. Do LT-1's have a PCV Valve?

Any ideas, insight or possible solutions would be great! Thanks!

SKI*MC
03-28-2006, 09:56 PM
bad fuel injectors perhaps..?

LakePirate
03-28-2006, 09:58 PM
Just a shot in the dark here, but have you checked/replaced the fuel filter?

I could be way off here but with a sudden decrease in fuel flow it could choke down.

6ballsisall
03-28-2006, 10:00 PM
Just a shot in the dark here, but have you checked/replaced the fuel filter?

I could be way off here but with a sudden decrease in fuel flow it could choke down.

Tru Dat. I'd also be checking out the fuel pump

Andyg
03-28-2006, 10:36 PM
I had the same problem om my 97 LT-1 and it turned out to be a bad IAC. The IAC I believe stands for Idle Air Control. It basically adjusts the amount of air that it lets into the engine when at idle. I can't remember how much it cost, but I don't remember it being overly outrageous. I had replaced the fuel pump twice and the fuel filter twice before determining it was the IAC.

Sodar
03-28-2006, 10:55 PM
Fuel Pump, Injectors and Filters are fine. This is the only problem I have, if it were the fuel pump I would get hesitation at throttle up. It also does not happen when I pull from high rpm to low, just in the throttle ranges I said before.

Sodar
03-28-2006, 10:56 PM
I had the same problem om my 97 LT-1 and it turned out to be a bad IAC. The IAC I believe stands for Idle Air Control. It basically adjusts the amount of air that it lets into the engine when at idle. I can't remember how much it cost, but I don't remember it being overly outrageous. I had replaced the fuel pump twice and the fuel filter twice before determining it was the IAC.

Andy, you would not happen to have a part number would you?

JimN
03-28-2006, 11:29 PM
The part number may be on it.

Sodar
03-29-2006, 12:24 AM
Where is it located?

JimN
03-29-2006, 12:44 AM
Look at the throttle body for a cylinder with a four wire harness plugged into it. The wires are green/white, green/black, light blue/white and light blue/black. If you remove it (two small bolts) and look at it, you'll see a conical tip and when you turn the key on, it should pull back and when you turn the key off, it extends and then backs off again. Don't start the motor with this out- it'll rev way too high.

If the IAC doesn't move at all, the wires will need to be checked, referenced to the ECM harness and then checked for voltage and ground with the key on. This should be done with the IAC reinstalled, but unplugged.

Don't just replace parts based on suggestions. List what has been done to diagnose the issues you're having with the motor and what progress has been made. Diagnosis needs to be done correctly or you'll just spend a lot lf money with a slight chance of finding the problem.

Sodar
03-29-2006, 01:29 AM
OK, I will try that this weekend. Does the problem sound like an IAC issue? Or would this be the wrong direction to go? As I said before, the dealer said they could not reproduce the problem.

Andyg
03-29-2006, 11:29 AM
I completely agree with what Jim said. In my case I had replaced all the other items due to having two fuel pumps fail, one the first year and one the second year so I new I could eliminate them from my troubleshooting. I just lived with the issue of the engine stuttering coming back down to idle for two years and then finally took it to the dealer and they diagnosed the IAC as the problem. After they replaced the IAC I never had it happen again.

JimN
03-29-2006, 11:29 AM
It could be the IAC, but from where I am, it's hard to come up with a solution since the boat isn't here. If you can reproduce the problem multiple times, bring the throttle back to neutral and then slightly open it again. If it solves it, you could have what's called IAC overshoot. Ask the dealer if they connected the diagnostic computer and if they say no, you should know that they should have. There's a part of the data that shows "IAC follower" and indicates Y/N. If N, the IAC may be stuck. If Y, the ECM is functioning it but the IAC may still be sticky when it tries to open after hard deceleration. Have them repark the IAC and see if the problem goes away. They should know how to diagnose this. Find out exactly what they saw when they checked the fuel pressure.

Has your flame arrestor been cleaned?

Sodar
03-29-2006, 12:06 PM
It could be the IAC, but from where I am, it's hard to come up with a solution since the boat isn't here. If you can reproduce the problem multiple times, bring the throttle back to neutral and then slightly open it again. If it solves it, you could have what's called IAC overshoot. Ask the dealer if they connected the diagnostic computer and if they say no, you should know that they should have. There's a part of the data that shows "IAC follower" and indicates Y/N. If N, the IAC may be stuck. If Y, the ECM is functioning it but the IAC may still be sticky when it tries to open after hard deceleration. Have them repark the IAC and see if the problem goes away. They should know how to diagnose this. Find out exactly what they saw when they checked the fuel pressure.

Has your flame arrestor been cleaned?

Yes, I clean my flame arrestor/K&N Filter every time I change the oil. I am looking at my dealership paper work and it says. "Fuel Filter Replaced, Fuel Pressure 43 psi, Could not reproduce problem, boat checks out fine."

Sodar
04-07-2006, 07:19 PM
Look at the throttle body for a cylinder with a four wire harness plugged into it. The wires are green/white, green/black, light blue/white and light blue/black. If you remove it (two small bolts) and look at it, you'll see a conical tip and when you turn the key on, it should pull back and when you turn the key off, it extends and then backs off again. Don't start the motor with this out- it'll rev way too high.

OK! I just pulled the IAC out and tested it. It extends when the key is turned off, but it does nothing when the key is turned on. I can feel the unit kind of try to do something when the key is turned on, but the cone does not move. Does this show a problem with the ECM or the IAC?

JimN
04-07-2006, 07:44 PM
This isn't a device that gives feedback to the ECM, so it will never throw a code but could still be bad. Reinstall it and look for the conical tip (called a pintel) in the throttle body. Plug the harness into it and start the motor, looking for any movement by the tip. You don't need to have your face right next to it, but good lighting will help. When you start it, the tip should back off about 1/4" and open more when you open the throttle, and if you keep the throttle in the same place, it should go back roughly to the idle position. It's not a very fast-acting device, but it should move in and out when you open or close the throttle. If it does this, you should be OK.

IIRC, you checked the fuel filter(s), did you check the screen at the fuel pump inlet? This can get clogged and cause the same kind of issue. I also mentioned the fuel pressure regulator- it's on the fuel rail and has a vacuum line going to it. If this line has a leak or the regulator is bad, you can have some problems.

André
04-07-2006, 09:59 PM
I think you can test it by getting the engine at NOT,note rpms,then turn it off for at least 10 seconds.Unplug IAC,restart engine and it should revs 200 rpms higher i think.Well let me go get my 96 Mastercraft EFI service manual...
OK I'm back...
Engine at NOT.
Idle stable
Record RPMs
Ignition off for 10 seconds
Unplug IAC
Restart engine
Is RPMs higher by 200 rpms?
Yes,replug IAC,idle should go back within 75 rpms of the first reading whithin 30 seconds? If so it's working good!
No,Ignition off 10 seconds,unplug IAC,Restart,Use a test light connected to ground,probe each each one of the four IAC terminals.
Does the light blink on the four terminals?Yes?Check for poor IAC connections or replace the faulty IAC valve.
No?Locate and repair poor connection,open,or short to ground in the IAC circuit that did not blink.If a problem was found repair as necessary.Was a problem found?
NO?Repair faulty ECM connections or replace faulty ECM.(Hope you don't go there)
They say the first real step is to perform OBD(On board diagnostic)system check.Dealer only for this one!
Straight out of the Mastercraft Marine Electronic Fuel Injection Service and Manual for TBI and Port fuel injection with HEI Distributor Type engine,year 96.
Wow ,one of my longer post...

JimN
04-07-2006, 10:18 PM
One thing the manual doesn't include- probe the green pair separately from the blue pair. There are no common wires for the IAC, and while the test light may go on when probing the blue/white and the green/black, the blue pair moves the tip in the opposite direction of the green. The wires with the white tracer denotes IAC coil high and the black tracer denotes low. Using a DVOM is helpful for diagnosing problems with IAC and grounding issues because it shows a value, not just lighting up.

Thrall
04-10-2006, 04:40 PM
Well, JimN and Andre are on it. Sodar, these guys will help you out more than the dealer (IMO). BTW guys, 2 years on the new ECM now, no issues, if you remember.
I'm following this because my boat does the same thing, sometimes. When you come off the throttle rapidly to idle or neutral, it seems to stutter a little like it's going to die, only for a second and then idles fine. Never saw it as much of a problem, but very interested to hear what you find out.
Andre, where did you get the MC engine manual?

Sodar
04-10-2006, 04:47 PM
Never saw it as much of a problem, but very interested to hear what you find out.

I totally agree about Jim and Andre, they are awesome. My dealer charged me an hours work to change a fuel filter, check the fuel pressure and tell me everything was OK! I don't see it as a huge problem either, it only happens 2/15 times, but it is enough to bug me. Anyways, I am gunna get back to tinkering this weekend and see what I can find... Ill keep you posted.

JimN
04-10-2006, 04:47 PM
It's still working because the other guy doesn't own the boat. Thrall- your RPM sag is normal and caused by the IAC follower which means that the IAC pintle closes because on deceleration, it doesn't need any fuel, air or spark. It's called IAC overshoot and is basically starving the motor for air, momentarily.

Actually SodarSki- this could be your issue, too. If it's just overshoot, I wouldn't worry about it. For some reason I though it was stumbling on acceleration, not deceleration.

Sodar- look at the mototr and see if there is a black hose going to the #8 runner on the intake manifold. If there is one or if there's one going to the base of the throttle body, the other end is where the PCV valve should go. Look on the other valve cover for the PCV.

Sodar
04-10-2006, 04:49 PM
It's still working because the other guy doesn't own the boat. Thrall- your RPM sag is normal and caused by the IAC follower which means that the IAC pintle closes because on deceleration, it doesn't need any fuel, air or spark. It's called IAC overshoot and is basically starving the motor for air, momentarily.

Wait, this is exactly what happens to my motor.... this is normal?

JimN
04-10-2006, 05:00 PM
Did the dealer document the fuel pressure numbers? They need to check it at key on, idle, 2000 RPM and WOT. This is about the best way to see what it's doing. Just checking it at idle is useless. They should have a diagnostics worksheet which they got when they went to technical training. If they went. I would ask about this if I was taking my boat in- they need to keep up on their diagnostic skills. It really shows up when they have some of the odd problems like 'cranks but doesn't fire' or 'I ran it for awhile and when I restarted, it ran like crap and wouldn't go over 2000 RPM'. There are some marine-specific issues that can pop up and a car guy may not know about them.

Sodar
04-10-2006, 05:02 PM
Did the dealer document the fuel pressure numbers? They need to check it at key on, idle, 2000 RPM and WOT. This is about the best way to see what it's doing. Just checking it at idle is useless. They should have a diagnostics worksheet which they got when they went to technical training. If they went. I would ask about this if I was taking my boat in- they need to keep up on their diagnostic skills. It really shows up when they have some of the odd problems like 'cranks but doesn't fire' or 'I ran it for awhile and when I restarted, it ran like crap and wouldn't go over 2000 RPM'. There are some marine-specific issues that can pop up and a car guy may not know about them.

I see no fuel pressure numbers. But is this the issue normal and just something to be lived with? I think I might have overstated the problem, because Thrall stated the issue PERFECTLY!

JimN
04-10-2006, 05:06 PM
As long as it doesn't kill, it won't hurt anything. I don't know why I spazzed on this- I honestly though it was on acceleration. If you ease the throttle down (you don't need to do it really slowly, just a little slower than dumping it into neutral), it probably doesn't do it at all.

Sodar
04-10-2006, 05:13 PM
So now that we are on the same page, should I continue with the IAC and ECM, or should I just take it as a common problem that is not correctable? The thing that strikes me as odd is that I can dump a skier, pull the throttle back quickly and the motor does not do it, but if I throttle back slowly it happens.

JimN
04-10-2006, 05:24 PM
Has anyone commented on a strong gas smell from the rear of the boat (especially if they're in the water) when this happens? It could be a leaky injector (or more than one).

Check the hose from the MAP sensor to the intake- if there are any cracks or leaks, throttle reponse can be variable or sluggish. Also, a glitchy TPS can cause this if the bad spot is near idle. throttling down very slowly would make it more moticeable than dumping it to idle because when the ECM sees a -20 degree delta in throttle position, it goes into deceleration enleanment but recovers pretty quickly.

Did you check for codes yet? Look for 21 or 22 for the TPS and 33 or 34 for the MAP sensor. The IAC won't throw a code but if it recovers before killing, I doubt that it's bad.

Thrall
04-10-2006, 05:27 PM
It's still working because the other guy doesn't own the boat. Thrall- .
Yeah, that may have something to do with it!


Sodar, if your boat is doing the same thing, I wouldn't get too worried, although it is a little annoying (only to me, noone else that drives it seems to notice).

JimN
04-10-2006, 07:33 PM
If anyone with the LTR can chime in here- does your motor do this too?

Sodar
04-10-2006, 08:47 PM
I think I am going to just take this thread into the dealership with me and see if I can get them to run these tests. I can not see it being a huge issue, since the boat has done it since the day I brought it home and has not gotten any worse or better. Thanks for your help Jim!

André
04-10-2006, 09:03 PM
Andre, where did you get the MC engine manual?
Thrall
Glad to read that you new ECM is working fine.You don't want to replace that one too often!
Got the manual from a guy who is the skiboat specialist in my area.He used to do service for the only MC dealer at the time to remote customers with problems so you didn't have to trailer your boat 6 hours to the dealer.Took his manual to a copy center and they copied it for me.
Very useful.EFI is so simple and easy to troobleshoot sometimes. :steering: