99, 5.0L Mercruiser, Carbed- misfiring and backfiring

[ss3964spd]

Andrew, not sure if you've seen this PDF but it details both the TB IV and V ignition systems. http://www.boatfix.com/merc/Servmanl/16/16B4R2.PDF

Scroll about halfway down the doc - TB V info starts on page 16.

According to this document the knock sensor is down low on the starboard side of the engine, just before the starter motor.

Dan

[johno270583]

Thanks for that Dan.

I have seen similar instructions in my manual, but not with a picture of where the Knock sensor is located.

I had assumed it would be on the port side, near the oil sensors, and you would think I would have noticed it on starboard side, but have not. The other thing is that I did not think it had a knock module, as it is not attached with the ICM, so will have to check all this again, but could well be a source of the timing not advancing.

Unfortunately I am away until Monday, so will have to check it then and advise back.

Another thing is that I have tried previously to diagnose the problem using the flow chart for the TBV ignition. I have spark, so probably doesnt apply. Got to the 4th point, unplugging red/white wire and checking for 12 volts. Mine read only 10V. Is this a problem, or am I barking up the wrong tree, because I do have spark.


Thanks again for the help.

Andrew

[ss3964spd]

No problem Andrew.

10V instead of 12V certainly indicate there's an issue but I have no idea how tolerant the ICM/KCM are of under-voltage. Certainly warrents investigation though. Based on this statement " The 12 volt signal from the ignition module to the distributor is carried thru the WHITE/RED wire “8”, to the distributor sensor and back to the ignition module thru the WHITE/GREEN wire “7” " I'd think if you were seeing only 10 on the W/R then you'd also see only 10 on the W/G. Regardless, if you are seeing full battery voltage at the 12V supply to the ICM and KCM on the Purple wires, and you've verified/cleaned the grounding contacts, then your ICM is, at least, suspect.

Dan

[johno270583]

That was the answer I expected, but didnt want to hear!!

Ill update when I am near the boat again.

[johno270583]

So I have talked to another mechanic about this, since I have come to the conclusion my other mechanic has no idea what he is on about.

He said possibly the ignition sensor, as they can go. He also mentioned that it could be caused by the Alternator. If unhooking battery overloaded Alternator, it could have caused some damage, and will cause ignition to not function properly. He said would have to try another alternator to see if this is the case.

Anyone else heard of this happening?

[oneradride]

I would confirm that possibility..... There is either an internal or external regulator in this system and it's quite possible you "made it angry" by disconnecting the battery while it was running. It's plausible.
With a bad regulator you can get 20+ volts and melt a battery or the opposing 10 or less.....and the entire system will be jacked especially if you don't have a good battery to compensate.

PS even if you did have a good battery... and the regulator is jacked.. you probably don't anymore....

[ss3964spd]

It's an easy test Andrew.

Engine off: Check battery voltage across + and - batt terminals. Should read just a bit more than 12V.
Engine at idle: Same test as above but reading should be above 13V but below 14.2 (ish).
Engine at 2000: Same as with engine at idle.

If your readings are as above the battery is probably fine and the regulator is fine too. The Alt has an internal regulator which is designed to produce a more or less constant output voltage at all RPM's. I suspect that specification can be found somewhere. So as long as the regulator is producing voltage within its specified range it's fine. In my experience that range is above 13V and below 14.2.

He's right in that disconnecting the battery with the engine running can cause the regulator to go nuts but doing so usually kills the regulator, but it can certainly damage other electronics. However, if that were the case then replacing the alt/regulator isn't going to fix any damaged components.

[johno270583]

Thanks for the advice.

So if engine off/ running battery test checks out ok, should there be any need to try another alternator? If I can get my hands on one I will probably try anyway, but if I have to buy one in, then I will be a little hesitant to spend money on something that may not be a problem (if it tests ok).

Will try this today if I get the chance.

[ss3964spd]

I see absolutely no need to replace it if it tests out ok, but perhaps you have a place you can take it (after removing it of course) that can test it.

Dan

[johno270583]

UPDATE

Ran Dan's test today
Engine off- 12.2V
Engine Idling- 13.9V
2000RPM- 14.2V

Above readings would suggest to me that the alternator should be ok, but let me know if you think otherwise.

I have found a couple more tests that can be done on the alternator in my manual, so will try them in the next couple days, however I would think problem is now isolated to Ignition sensor, ICM, or the below issue that I found. Correct me if Im wrong.

Another few observations I have made today:

Least importantly- Boat does not have a knock sensor. Checked all around both sides of the block, nothing near the starter motor, or on the port side.

More importantly- Pur/Yel wire on starter motor had come off. This wire leads to the oil pressure sensor, electric choke and FUEL PUMP!
I had noticed that I had not been hearing the fuel pump buzzing away after engine had been shut down, but assumed it was ok because the motor would still run. I held the wire against + terminal, and fuel pump immediately started buzzing away.

I have attached wiring diagram- I assume power comes from Purple wire into oil sensor, into Pur/Yel wire.

Now the questions that come to mind are:
1: Was the fuel pump running without this wire hooked up? Why did I not hear it buzzing when shutting engine off?
2: If not, was the carb/motor able to siphon fuel up from the tank?
3: Why was I getting 12V at the electric choke when the engine was running, and why was the choke coming off?

I dont think this is the main cause of my problems, as if the carb was short on fuel, it would have coughed spluttered and died if fuel starved, however it is a problem still that needs to be fixed.

Sorry, another big rant from me, just trying to give all info and thoughts as I find them.

Andrew

[johno270583]

After some further reading, I have found that the Pur/Yel wire from the starter provides power to the fuel pump at cranking, so wouldnt be causing any problems.

Back to the drawing board!

[johno270583]

So the boat ran pretty much like a dream today!!

All that I have done is adjust the timing to approx. 10* BTDC, instead of having it set at zero (idiot!).

What I dont understand, is why up until a couple of weeks ago, it had been running ok, and been able to get onto the plane no troubles?


Anyway, problem solved, unfortunately due to my misinterpretation of information, this could have been solved alot earlier.

Only thing is now still a hesitation from idle to WOT, but doesnt do it too bad if only push throttle to 3/4, so will continue to do this, and tinker away until I can get the hesitation out.


Thanks everyone for all the help and comments.

Andrew

[ss3964spd]

Glad it's closer to being sorted Andrew. Though it has taken some time you've armed yourself with quite a bit of knowledge in the process.

Good thing, that, because you've beconme rather high maintenance.

Now finish with getting that carb sorted out.

Dan

[johno270583]

You would nearly have to say I am somewhat of an expert with my boat now!! Bloody thing!

It has been a great learning experience, though obviously a slow and tedious one, since its not like a car where you can change something, then take it for a quick spin around the block to see if the problem is solved.

Anyway, nearly there, will check the plugs before and while IM out next time, and see how they look, and sort out this carb as well.

Thanks for all your help Dan, and everyone else who contributed.

WIll have to get some photos up soon now its running again and before summer finishes in S Hemisphere.

Andrew

[johno270583]

Hi All

Just a quick update to say that I am pretty close to having this thing purring like a kitten. (FINALLY)

In the past little while, I have pulled the carby again, had to open the secondaries a hair to be able to close the primary throttle plates enough to cover the transfer slots, and so now am able to adjust the A/F mix screws to set the idle.

I bought some timing tape to glue to the Balancer, turns out I had the timing set perfectly at 10 BTDC, so my calculations were spot on!

I have checked spark plugs after cruising, and they seem to read ok, ground terminal a bit white, but ceramic is a tan colour.

I have also replaced the ignition sensor, though I dont think this has made any difference, I didnt check the timing advance before and after, but seemed to run exactly the same before and after, so I am holding the older one for a spare for now.

So, after all this time, finally got it running the way that I want! Must be time to sell it now!

If I were to make any further changes at this stage, I would probably switch props again. Put a 4 blade 19p Solas on recently, max RPM about 4600. Since skiing is the primary use I would probably go down to a 17p, which would probably take me above the max RPM range, but I never use it anyway, and would think would give even better holeshot.


Anyway, I will say it again, thank you to all that have helped on my little adventure, your help with all of my problems has been great, and also a great learning curve for me.

I dont really know what the going rate is for donations for the site, but I intend to donate, since I hopefully wont be spending all my hard earned on the boat now!

photos to come before the end of summer (southern hemisphere)


Cheers

Andrew