View Full Version : Bucking LT-1 in 1995 ProStar 190

04-23-2005, 06:35 PM
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.

Tom Jones
04-23-2005, 06:48 PM
Try checking inside your gas tank, yu might have crud in it.

04-23-2005, 07:04 PM
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.

04-23-2005, 08:06 PM
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.

04-24-2005, 09:08 AM
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.

04-24-2005, 09:58 AM
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!!

04-24-2005, 11:52 AM
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.

04-24-2005, 12:33 PM
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.

04-24-2005, 12:53 PM
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.

04-24-2005, 02:32 PM
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.

04-24-2005, 03:24 PM
Brilliance, pure brilliance. Who is this JimN guy anyway!

Sorry to interrupt. :(

04-24-2005, 05:18 PM

Obviously, you post with some knowledge; however, I don't profess to be an expert and I 'm seeking as much information as I can seeing as the nearest Indmar service shop is 2 hours from my residence and I'd like to have some idea what I may be getting into seeing as my relationship with this service shop is limited.

With the boat being stored in my garage attached to my house, I use fuel stabilizer yearly and fog the engine every other year. I drain all water by opening petcock on engine block, removing temperature sensor, and manifold plugs followed by a quick engine turnover. I use no antifreeze given my current boat storage situation.

As mentioned in my very FIRST POST, all spark plugs were changed. Besides carbon deposits and slight fuel residue, no indication of moisture (water) was apparent. All spark plug wires appeared in good physical condition.

The wires were colored black and white with female slot connectors on each end. The black wires is connected to the male slot pin on the driver's side with white connected to other male slot pin.

I guess I should take exception that because I appear to be relatively unskilled in engine mechanics that I'm moron enough not to be aware of local pollution laws, can't read use instructions on antifreeze and pull off spark plug wires from a running engine. If only MasterCraft provided as much detail in its owner's materials as antifreeze manufacturer's have to list on their products...

TeamMC -- What a misnomer!

04-24-2005, 05:55 PM
The problem with only being able to read something is that it's impossible to hear the way something would have been said. If what I posted made it seem that I was dumping on you, I'm sorry. It wasn't meant that way at all.

I didn't remember the first post, so I should have re-read what you said you had done to it. You used the AC MR43LTS plugs?

Is there a way for you to post a photo of the wires and the connector?

I never assumed anything about your abilities. Because I don't know everything you have tried and exactly what you know, I thought I would state some things that aren't obvious to everyone. There have been enough posts in the past by people who used regular anti-freeze and just dumped into the water they use, along with the ones from those who used RV anti-freeze on their LT-1.

Pulling the plug wire off of a running motor is OK if it's done with an insulated puller, from the ignition end, not the plug. However, this ignition's output being as high as it is, I didn't want to hear that there had been a problem form doing this.

Since we're talking about plug wires, do you know how old yours are? 3 years is about the normal life expectancy for these and if they're much older, they get leaky and can crossfire. I would try to find Taylor wires, the same as original.

Again, I didn't want to give you the impression that I thought anything bad of you, in any way.

If you don't want to buy a lot of test gauges, you may check around your area for Advance Auto Parts or some other place that loans test equipment. That way, you can check the fuel pressure and compression without shelling out too much money(unless you just want to have the tools handy for another time or vehicle). The fuel pressure and compression gauges I have are available at Sears and NAPA, as well as quite a few other places. The Fuel pressure gauge will/should have an adaptor to fit the two generally used valves. The compression gauge has a hose with a threaded end and a quick release coupler(like an air line for a compressor) on the other. The rubber tipped kind is pretty useless for this.

Keep us posted and if you don't have much luck with it, I'll dig out my LT-1 manual to look for other possibilities.

Was there any other odd sounds or smells while it was running this way?

04-24-2005, 06:05 PM
jimN - dumb question on my part, but why do you not check for spark at the plug end with motor running assuming of course that the device used to remove the plug wire is insulated? i hate getting shocked and usually use a timimg light to check spark.

04-24-2005, 06:16 PM
It's not a dumb question, but with 80KV coming out of the coil pack, it's possible that the insulation is OK for a normal system and would leak on this one. The timing light shows that the voltage is/is not getting to the plug, pulling the wire tells if it's actually getting to the plug's electrode. Getting a handful of voltage is no fun, but it really $ucks when it's on one of these, when it's not expected and the plug wires are leaky.

04-24-2005, 06:40 PM
jimN -- i use an extension plug wire that connects the plug electrode to the end of the end of the plug wire and attach the light to a metal contact point on the extension wire. light is a real cheapo and uses 110v to boost light strobe or it can't be seen.

04-24-2005, 06:45 PM
It's not a dumb question...
Well,I've got one.
What's a misnomer ? :confused:

04-24-2005, 10:53 PM

As someone who has been on this board a while I can definitely tell you that JimN is giving you the straight info and definitely didn't assume you were a complete novice. Some people have more knowledge than others and Jim generally tries to be very thorough so everyone can learn from his posts. I can't add anything to his post, except to tell you that he's helped more than one of us get our boat running again and if you report back on your progress, good or bad, you'll get more help.

It really is a great group and I'm sure you'll get your boat running in short order.

04-25-2005, 07:10 AM

As someone who has been on this board a while I can definitely tell you that JimN is giving you the straight info and definitely didn't assume you were a complete novice. Some people have more knowledge than others and Jim generally tries to be very thorough so everyone can learn from his posts. I can't add anything to his post, except to tell you that he's helped more than one of us get our boat running again and if you report back on your progress, good or bad, you'll get more help.

It really is a great group and I'm sure you'll get your boat running in short order.

3 chears for JimN :toast:

In addition to what JimN said - I'd check the fuel screen on your fuel pump (I believe you have an in-line pump at the back of your motor, near the stringers). I had a similiar problem and it ended up being junk in the screen. But - of course, I didn't realize it until after I had a new $250.00 pump installed.