3.0 merc i/o with bad spark knock

[mikebc]

I have a 1989 merc 3.0 and under a load it makes alot of spark knock, and if under a steady heavy load it will loose power gradually. New plugs and wires, new gas, added a can of carb cleaner in the gas, runs perfect otherwise. Help???

 

[Bondo]

Re: 3.0 merc i/o with bad spark knock

The 2 Main Causes of Detonation are Timing,..??,+ Lack of Fuel,... Or a Lean Condition........<br /><br />Did you Check the Timing????<br /><br />Detonation will Destroy a motor in a matter of Minutes......<br />Don't Run it' till it's Fixed...... You May have Already done Damage.....

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

I have not checked the timing or adjusted the carb. What should the timing be set at? I would not think my motor has any advance that needs to be disabled to check timing, but I've been wrong before! Thanks for the post, Mike.

 

[Don S]

Re: 3.0 merc i/o with bad spark knock

If you plan on working on your own engine and drive, you MUST have a service manual or you are just looking to mess things up even worse.<br />The timing is 6° BTDC at 650-700 rpm. AFTER you set the dwell at 28° to 32°.<br />Make sure your advance timing also works. You should have around 30° total advance at 3000 rpm.

 

[trog100]

Re: 3.0 merc i/o with bad spark knock

lots of things can cause engine knock under load.. <br /><br />knock is when the mixture in the cylinder detonates or explodes instead of burning progressively and slowly as it should.. <br /><br />its a heat thing really..<br /><br />too lean a mixtures makes things in the cylinder hotter than they should be and adds to knock..<br /><br />fireing the mixture too soon adds to heat and knock..<br /><br />little bits of carbon glowing white hot adds to heats and causes knock.. this fires the mixture before the plug and kinda acts like over advanced timeing or too low an octane fuel.. the gas engine become a diesel engine in essence..<br /><br />sparkplugs running too hot and glowing can cause knock.. same as above..<br /><br />too low an octane fuel causes knock.. this goes off on its own too easily and explodes instead of burning..<br /><br />engine generally overheating can cause knock by heating up the fuel air mixture too much..<br /><br />the mixture in an engine isnt meant to explode.. its all worked out assuming the correct octane fuel is being used not to.. the octane rating of a fuel is how much it can be compressed and/or heated up without exploding.. use too low an octane fuel and the engine will knock.. <br /><br />use the correct octane fuel but heat the mixture up to much and it will still knock..<br /><br />the mixture should be fired by the spark.. it should then expand and burn slowly and progressively.. knocking is simply when it "explodes" as opposed to "burns".. <br /><br />knocking is basically a "heat" thing..<br /><br />okay in a perfect world.. everything about an engine should be to spec and assumimg the correct fuel is used it wont knock under load..<br /><br />but.. in a less than perfect world.. carboned up engine.. too low an octane fuel.. retarding the ignition a few degrees is a sure-fire cure for it..<br /><br />not perfect.. but far better than letting the engine knock..<br /><br />why do i have to feel guilty about trying to explain why something happens on this damn forum.. he he he <br /><br /><br />trog100

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Hey,<br /> Thanks for the expination, it actually makes sense to me. I have tried to find a book on this locally, but I guess I'll just have to order one online. I've worked on many a auto, but thats been many years ago, before fuel injection and computers have made our motors almost maintaince free! I have a 2002 LHS with 135,000 miles on it and have never had to do a thing except oil, filters and one belt change! I will try retarding the timing a touch and see what happens. Do I remember correct, clock wise to retard?? The fuel is always a mid grade and fresh. If the timing doesn't help, I'll have to ask someone what screw to turn to richen up the mix, either that or wait for the book to arrive!

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Oh yeah, the temp gage is perfect and once up to temp never moves. Thanks, Mike.

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Ok, I couldn't wait one week for the book, so out I went and I retarded the timming my 2-3 degrees and wow, no more spark knock! But,,, now the idle is so low it stalls after I let it down off of fast idle, but the knock is gone, nuch better! Should I wait for the book, no fun, or adjust up the idle now? If so, anyone know what scre it is? Thanks, Mike.

 

[trog100]

Re: 3.0 merc i/o with bad spark knock

mikebc.. the downside of retarding the ignition a little is a slight loss of power.. slight loss of gas mileage and slightly slower idle.. just adjust up the idle now and enjoy your boat while reading the new book.. he he<br /><br />trog100

 

[Dunaruna]

Re: 3.0 merc i/o with bad spark knock

Wait for the manual. Adjusting the timing without carrying out other tuneup checks may be just masking another problem. Read Don's post again.<br /><br />Do it once, do it right.<br /><br />Aldo

 

[trog100]

Re: 3.0 merc i/o with bad spark knock

Durana.. rubbish.. he has already cured his knocking.. now all he needs to do is to give his throttle stop screw a quarter turn to cure his slow idle..<br /><br />your do it once do it right stuff can all come later.. <br /><br />course he could have kept useing his knocking engine and wrecked it during the time its takes for that manual to arrive and him to read and fully understand it and apply the do it once do it right nonesense.. would that have been a better option..<br /><br />trog100

 

[Dunaruna]

Re: 3.0 merc i/o with bad spark knock

Trog, for the record it's dunaruna, but I prefer Aldo.<br /><br />You shouldn't feel guilty for putting across your views. That's what a forum is all about.<br /><br />As you pointed out, many things can cause knocking. Mike fixed the symptoms, not the cause.<br />If it was me, I'd want to know why it started knocking in the first place. Did the distributor bolt come loose? Did the timing change for some reason? Is the fuel below par? Are the plugs correct? Is the dwell set correctly, if not, why?<br /><br />That's my opinion, maybe I ask myself the wrong questions when I'm looking at a problem.<br /><br />Aldo

 

[rodbolt]

Re: 3.0 merc i/o with bad spark knock

by just retarding the timing he has no notion of what the ell is wrong, if it was retarded from the correct timing it may just be masking a problem elsewhere. could have been the last guy set it by ear. I run into techs all the time that tell me they can set timing by ear and hold plus or minus 2 degrees and can tell by the sound of the engine what the max advance is. my ears are not that good so I have a timing light with an advance and tach function and a set of service specs. I can always tell on the rebuild if someone was monkeying about cause all the carb settings are wrong where someone was a monkeying. if the engine is correct and the fuel is good the package require no futher engineering and will work at the factory specs. if the specs change you wil find a tech bulliten about it. so without a dwell meter a timing light and the service specs coupled with the commonsense to use them its a guessing game. guess wrong and the tech that overhauls it hopefully will find the mistake.

 

[Don S]

Re: 3.0 merc i/o with bad spark knock

Originally posted by trog100:<br /> Durana.. rubbish.. he has already cured his knocking.. now all he needs to do is to give his throttle stop screw a quarter turn to cure his slow idle..<br />

Nothing was "CURED", he only stopped the noise.<br />Like rodbolt said, he has now made it stop knocking, how he knows 2 or 3 degrees without a timing light is beyond me.<br />The next step (after a few more hours of running that way) will be to find out why the head gasket blew out. And that is a common problem with the 3.0L engines if the timing is off.

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Actually, I borrowed a timming light. I know it's a few degrees retarded now, and can very easily return it to that position when I have better instructions from, say the book I ordered. I do appriciated everyones advice. Once the book arives, in a week or so, I will figure out what the real problem is. I do have a question for you all, what is dwell? I've worked on cars back in the day, set alot of cars timming, but again, never heard of dwell, or a dwell meter. Fill me in, I love to learn. Thanks, Mike

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Oops, hit the wrong button twice

 

[ziggy]

Re: 3.0 merc i/o with bad spark knock

definition, cam angle or dwell(ign.) Number of degrees breaker cam rotates from time breaker points close until they open again. or ya can set point gap with a feeler gauge. best with a dwell meter that will tell the degrees. also, dwell affects timeing. but timeing don't affect dwell. in other words, set dwell 1st, then timeing.

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

Ok, now you have me interest up, I know I could wait for my book to arrive, but I am very interested, how do you check dwell? With either a feeler gage, I have, or a dwell guage, don't have. I like working on motors, just not in middle of the lake! Thanks for feeding my interest! Mike.

 

[Dunaruna]

Re: 3.0 merc i/o with bad spark knock

You need a dwell meter. Most multi-function digital meters (average run of the mill auto test meter) have a dwell function. Connect to coil negative and start the motor - wah-la, dwell. Open the points to lower dwell, close the points to raise dwell. It is a very accurate way to set your points. It can also alert you to a distrubutor shaft/lob problem.<br /><br />Aldo

 

[mikebc]

Re: 3.0 merc i/o with bad spark knock

I have a digital multi meter, is it like measuring resistance or something? I can pick one up at the marina, I just didn't want to sound like I don't know anything about dwell, kinda like I don't! Anywhoo, the more info I have, the better. How about one of "trog100's" explainations! Thanks, Mike.