Mercruiser 5.0LX problems

I am having some continuing problems with my 1989 Rinker Captiva with a Mercruiser 5.0LX Alpha 1. It is a 230 hp model with a Quadrajet carb and electronic ignition. Due to earlier problems, I’ve done the following: rebuilt the carb and replaced the float, replaced the fuel filters, plugs, wires, cap and rotor, checked the timing. The boat ran very well initially, but after about ½ hour at a 3500 rpm cruise, it started missing and could no longer maintain a plane. I was able to make it back to the launch ramp at 1200 rpm. I was able to maintain this rpm without problems. After getting back to the ramp an hour later, I tried giving it full throttle, and it seemed to run normally again. According to the gauge, the temp never exceeded 160 deg. I’m stumped, and could use some suggestions as to what to look for next. Thanks in advance for the help.

Re: Mercruiser 5.0LX problems

Check that Quadrajet. I don't know about on the boats, and a Quadrajet is a great carb, but when it comes time to rebuild, you are usually better off putting it to the side. I have known few people who have sucessfully rebuilt these things. I don't know the exact prob with it, but once you take it apart, it is usually over. You might wanna check the bolts on the manifold, they might have vibrated lose, and would give you a lean miss. If the spark plugs are looking good, the carb is most likely at fault. Good Luck

Re: Mercruiser 5.0LX problems

Sweet Carbs... Quads... I don't believe that is your problem... How many have I rebuilt... hundreds... how many 2GC/2bbl... about the same.<br /><br />Sounds like an ignition problem... breaking down!!! After it cools down (module) it runs fine... welcome to the world of electronics!!!<br /><br />I will check svc bulletins to see for you...<br /><br /><br /><br />------------------<br />"Keep It Between The Bouys" & "Good Luck"!!!<br /><br />fone_man

Re: Mercruiser 5.0LX problems

You must know your carbs fone_man. I have only had experience with the car one's. Don't get me wrong, I love the one on my 77 trans am, but I had 2 carbs shops attempt to rebuild it before I bought a reman. Nobody in AR seems to know how to work on em.

Re: Mercruiser 5.0LX problems

Thanks fone_man. What you said seems to make sense. While I idled back to the ramp, I had the sunpad open. This would have brought the engine compartment temp way down. It was 95 deg out that day. Would the module that you are referring to be the one inside the distributor or the one bolted to the exhaust elbow?

Re: Mercruiser 5.0LX problems

I am running thru svc bulletins right now... let me ask a question... 95 degrees outside probably 200 plus inside engine compartment???<br /><br />There is a possiblity that this is a "vapor lock" that is identified by Merc. What happens is mfg do not want you to hear engine noise... plus want you to have all the room in the world. So they put more insulation in engine cover & make it smaller for room for occupants!!! So this cause an excessive heat build up... BINGO, vapor lock... engine don't want to run or runs terrible, hard starts, etc..<br /><br />How was it on cool days???? Try again with blower running to keep engine compartment cooler plus ventilated... see if that helps!!!<br /><br />Need email address & i will forward that svc bulletin to you.<br /><br />c-ya for now... got to go to work<br /><br /> <br /><br />------------------<br />"Keep It Between The Bouys" & "Good Luck"!!!<br /><br />fone_man

Re: Mercruiser 5.0LX problems

Fone_man my address is jraciti@dynacast.com. I thought about vapor lock, but I've owned the boat since it was new and never had this problem before. Could the fuel pump be getting weak? I did a flow test and it seemed okay. Of course the engine was cold. Also, the anti-siphon device was removed by the dealer when he delivered it to me in '89. Thanks again.<br />JJR

Re: Mercruiser 5.0LX problems

Quadajets are great carbs, but they are prong to a few quirks....the main one being that in service they wear the primary thottle shafts and once these are loose develop a poor idle quality....several places offer a teflon bushing kit for this.....they are also harder than some to adjust to your application few wrenches understand the meter rod and main jet relationship. they are very good and really good design and should be rebuildable just pay attention to the details.

Re: Mercruiser 5.0LX problems

I used to be a mechanic and still am on the side, every single time i had a high load, operating temp miss, it has been the ignition coil. And i didn't see it listed in the parts you've replaced. The coil works off of resistance/inductance, i don't know what internally happens when they go bad, but once extra heat is applied, the resistance goes way up, and the spark breaks down. Check it for cracks, and better yet, keep your fingers away from it, but start it in the dark, focusing on the coil ( i'm not for sure, are these 5.0l mercs fitted with the GM style HEI?) You may see blue arcs traveling across the cap from tiny cracks in the top of the coil. Good luck!

Re: Mercruiser 5.0LX problems

This thread should really be in the MerCruiser I/O forum instead of the Mercury/Mariner outboard forum.

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