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Old 04-23-2005, 06:35 PM
brezan
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Question Bucking LT-1 in 1995 ProStar 190

During a recent outing, engine started bucking particularly above 30 mph. Have replaced fuel filters, treated fuel with SeaFoam, run without filter over flame arrester and changed plugs resulting in no improvement. If smash trottle to accelerate from idle speed, bucking will occur. Engine will remain above idle rpm speed for 10-20 seconds after slowing down after causing engine to buck. Any insight? Next logical thought is fuel pump.
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Old 04-23-2005, 06:48 PM
Tom Jones Tom Jones is offline
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Try checking inside your gas tank, yu might have crud in it.
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Old 04-23-2005, 07:04 PM
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bret bret is offline
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You might want to check the fuel pressure through the schrader valve - you should have a port on the back of the fuel rail to monitor pressure. See what the pressure is doing. Just a thought, unplug the battery for two minutes, unplug and reseat the main ECU plug, and then see what happens.
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Old 04-23-2005, 08:06 PM
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If you had to idle out to the water for any more than a few minutes and then hit the throttle, it was probably in an overheat situation. Did you look at the impeller before you ran it? What about the operating temperature? Also, check the oil cooler. Generally, the only reason it should buck is during RPM Reduction, caused by an overheat. Even if you had a bad spark plug or injector, you should normally not notice much more than some hesitation during acceleration and loss of top end.
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Old 04-24-2005, 09:08 AM
brezan
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JimN -- Engine temperature gauge indicating normal operating temperatures and post-use hand check of manifolds confirms. I changed the impeller late last summer. Would you check the oil cooler by hand? I've had engine overheat (clogged transmission cooler and bad impeller) and this situation seems quite different.

bret -- Would an auto parts store carry a fuel pressure gauge for the schrader valve? Where is the main ECU plug located?

Tom Jones -- Where inside the tank would you recommend checking for crud? New fuel filter (transparent plastic housing) remains very clean from change out after problem initially started several weeks ago.
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Old 04-24-2005, 09:58 AM
mtrask mtrask is offline
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This happened to our "97" LT 1.. We found another external filter located under the engine mount almost impossible to see.. you had to run your hand along the line under the mount. We rebuilt our fuel pump, checked pressures, and replaced other filters in line before finding this filter. It is important that you open the fuel tank though because that was the source of the trouble.. We found (4) dead frogs in our fuel tank. (another story for another day)

If you do find out it is the fuel pump through pressure tests--send me a note, we have a fuel pump for that engine. We ordered it before finding that filter and the debris in the tank.

We could not believe that filter was under the engine mount

Final check--When you pull the suction out of the tank--check the angle check valve at the top of the suction.. It is there --tough to see--it is part of the 90 degree fitting. Clean any debris from that as well.

Good Luck!!
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Old 04-24-2005, 11:52 AM
brezan
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mtrask
So there are 3 fuel filters... I read your frog post yesterday and if fuel pump, will keep you in mind. Thanks for posting a response.
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Old 04-24-2005, 12:33 PM
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JimN JimN is offline
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Bucking- is this a quick or slow interval between surges?

You can check the oil cooler by removing the hose on the hull side and looking into the opening. There may be sticks or some other material there. Use a flashlight- there may be stuff in the little tubes that carry the water to the other side. This is best done on the trailer unless you have some way to stop the positive pressure from trying to fill the bilge with water.

Re: the fuel pressure tester- I got mine at Sears. It has a relief valve and a bleeder line so you can get rid of any air at the point where the fitting and Shrader valve come together. You will be looking for 30-40 pounds at idle, depending on the year of your boat. 40 is normal for the newer LT-1 motors(which is what you have). If it's a cople of pounds light under load, that's ok. If it drops considerably under load, look for the fuel pressure regulator on the front of the motor(this has a vacuum line attached to it and this sometimes cracks, causing loss of vacuum and fuel pressure). Check your PCV valve and any lines or hoses going to the throttle body for leaks. Is your idle still at 600RPM and rock solid? If it's higher than this, you have a vacuum leak somewhere.

You have a fuel pickup screen in the tank, a flter on top of the tank and one near the motor. There is also a screen at the inlet to the fuel pump. This one requires removing the pump to get at easily. Check the fuel shutoff vlave on the top of the tank, too. These sometimes work their way toward closed.

What was done to prepare the boat for winter? Fuel stabilizer and fogging oil? You may want to take a fuel sample when you check the pressure. This can easily be done by getting a clean bottle or plastic jar and putting the bleeder line from the fuel pressure gauge into it, pressing the bleeder and letting some of the fuel go into the bottle/jar. If you see anything abnormal(debris of any kind or foggy gas), you know that there is a fuel quality issue. Let the gas sit for about an hour(in the sunlight if possible to speed up the settling) and look for beads of water to settle out.
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  #9  
Old 04-24-2005, 12:53 PM
brezan
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JimN -- I would consider the bucking fairly quick (~every 2-3 seconds). I added Stabil to the fuel prior to laying up for the winter (November-March) in garage attached to house (unheated but monitor temp. and if near freezing, use light bulb in motor box during very cold spells). I will check the fuel but I did add new fuel after the bucking started because I thought I ran out of gas and learned that earlier post on how to hook up leads to new fuel sensor were backwards (so I have another nuance for my boat setup). With regard to fuel filters, the filter above that tank and at the back top of motor have been changed and unable to find any other fuel filter. Idle RPM is good and solid at ~600 rpm. Now that I think about it, the outing that the bucking began started with idle RPM at 1500 rpm; however, I noticed that the two wires going to "plug" at lower back of motor (near propeller shaft) were disconnected. I re-connected and idle rpm returned to ~600 rpm.

Thanks for the insight. I hope to get busy troubleshooting this afternoon.
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  #10  
Old 04-24-2005, 02:32 PM
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JimN JimN is offline
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When you drained the water from the motor, did you use anti-freeze of any kind? RV type is a huge NO-NO on aluminum heads. The car green type is really bad for any life in the water and the DNR will come after you big time if they see a cloud of it around your boat. Better to leave it drained and dry.

What color were the wires that were disconnected? Did you fog the motor when you stored it? That keeps moisture from corroding the valves, cylinders and rings. Do a compression test with the motor warmed up, the spark plugs removed and the fuel pump fuse pulled. You should see about 150# on each cylinder. You can also check the plugs(since you haven't said that you did this yet) for water, gas or whether they're dry. Dry- could be a bad injector, water- could be leaking through the gasket, wet w/ gas- could be a bad plug, plug wire or low compression on that cylinder. Did you test for spark on all cylinders yet? DO NOT JUST PULL PLUG WIRES OFF BY HAND WHILE IT'S RUNNING!!!!! This ignition system puts out between 50,000 and 80,000 Volts. If you have one hand on the motor and one hand pulling a plug wire, expect to have some problems. If you just pull a wire and don't touch anything else, it's still gonna be a good jolt if the coil is good.

There are a lot of things that should be checked that you haven't talked about. This needs to be checked thoroughly before we're asked to guess about the cause of the problem. So, do all of the things mentioned before and the new ones here, then ask what the cause could be after listing the results of the tests.


Have you run it on the trailer, and does it do the same thing at higher RPMs? It may not show up since the motor's load is totally different.
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