Odd ignition problem..only three cylinders firing

[bobuk]

Hi Guys,

1988 V6 4.3 L/x thunderbolt ignition

I changed my oil today and checked my spark plug gaps. I thought i would make sure all the cylinders were firing by taking the HT lead of each plug one at a time. I noticed that cylinders 1 3 and 5 didnt make any difference! 2 4 and 6 stopped the engine. So basically, only half the engine seems to be working! I thought i would check the other engine and found the same problem but on cylinerd 2 4 and 6. Whats happening!!?

Thanks in advance Bob

 

[bowman316]

Re: Odd ignition problem..only three cylinders firing

are cyl 1,2 and 3 on one side and 4,5,6 are on the other side of the block?
ae all the dead cyl on one side?
try switching around ignition cables, and seeing if it is the power pack.
switch a cable from a good cylinder to a bad one, and see if the bad one now works.
if it does, the problem is in the coils

 

[bomar76]

Re: Odd ignition problem..only three cylinders firing

ae all the dead cyl on one side?
try switching around ignition cables, and seeing if it is the power pack.

if it does, the problem is in the coils

Power Pack????
Coil(S)???
There is only one coil on his boat, and no power pack.

He needs to start with a full and complete tune up with a compression check, then go from there....

 

[chiefalen]

Re: Odd ignition problem..only three cylinders firing

You have 2 motors ?

One bank of cylinders is dead on one motor, and 1 bank of cylinders on the other motor, is dead ?

 

[bomar76]

Re: Odd ignition problem..only three cylinders firing

xxz

 

[Don S]

Re: Odd ignition problem..only three cylinders firing

You have 2 motors ?

One bank of cylinders is dead on one motor, and 1 bank of cylinders on the other motor, is dead ?

Actually, that is probably the answer to the question.

If you have a twin engine boat with a counter rotating engine, you have the firing order wrong on both of them.

 

[bobuk]

Re: Odd ignition problem..only three cylinders firing

Thanks guys, yes twin engines. 1/3/5 onn one side 2,4 6 on the other.
Both engines not firing on one side. I thought this would be an easy one as clearly it would be an ignition problem and surely unlikely to be a mechanical problem. The firing order seems fine checked the rotation of the distributor (direction) and then confirmed the correct firing order / sequence as per the sticker i found on one of the engines today. The fact it is occuring on both engines indicates firing sequnence, and i'm possibly wondering now if the distributors have been inserted incorrectly on both engines in the past?

Thanks Guys

 

[chiefalen]

Re: Odd ignition problem..only three cylinders firing

Bomar posted , xxz what that means is a mystery.

And hes a expert. Has a really , really nice setup.

What was done to the boat?

What i mean is, did you just buy the boat with this problem.

Or you installed something.

What is different?

Thats why i asked the original question.

Don s and i, believe the same thing, firing order was changed.

And it was done to both motors at the same time !

Why would both dizzys be changed at the same time?

And who did the work?

Don't run these motors sir till you sort it all out.

Heres a link to the manuals.
http://forums.iboats.com/showthread.php?t=251571&highlight=4.3+merc+service+manual+05'

 

[abj87]

Re: Odd ignition problem..only three cylinders firing

Wow i'm surprised it runs!
Could the distributor be 180 deg out?
Try this
Remove all the wires.
Put the engine at tdc (compression stroke on 1).
Pull the cap off the distributor and see which way the rotor is pointing, put the cap back on and rotate the body of the distributor it till one of the posts on cap lines up with the rotor. Tighten the clamp.
The post you lined it up is now the #1 cylinder.
Now go around the distributor in the direction it turns (bump the starter to find out beforehand) putting plug wires on from the cylinders in the firing order.
You should be all set and the timing should be close enough to start the motor to time it.

 

[a70eliminator]

Re: Odd ignition problem..only three cylinders firing

Bomar posted , xxz what that means is a mystery.
Another way saying "Daaa" I wasn't thinking clearly must have been half asleep.

Sounds like you have a bad connection (corrosion) wire insulation rubbing through to ground somewhere. I'm not knowlegable enough to say anything else about coil sequence of operation.

 

[newport dave]

Re: Odd ignition problem..only three cylinders firing

Those test results are suspect. I'm not sure what you're doing wrong, but I get the feeling you may be making a problem where there isn't one. You have not mentioned that the engines are running poorly, and if they were actually running on three cylinders they would be running terrible, they wouldn't idle, and would produce very little power.

Test results aside, how do the engines seem to be running?

 

[chiefalen]

Re: Odd ignition problem..only three cylinders firing

How about we find out if there is a problem.

Perform a vacuum test on both motors.

If there is a problem has to show up.

And heres the link to how to do it.

http://www.secondchancegarage.com/public/186.cfm

 

[abj87]

Re: Odd ignition problem..only three cylinders firing

How about we find out if there is a problem.

Perform a vacuum test on both motors.

If there is a problem has to show up.

And heres the link to how to do it.

http://www.secondchancegarage.com/public/186.cfm

If his timing is out of order there is no way that test is going to tell you anything. I don't think it will run enough to do it anyway!
Are you even sure there is a problem? how do the engines run?

 

[bomar76]

Re: Odd ignition problem..only three cylinders firing

Or he is pulling wires off the wrong engine...

 

[chiefalen]

Re: Odd ignition problem..only three cylinders firing

He said it was running pulling wires nothing happened.

You mean the vacuum test won't show if a cylinder is not firing?

 

[abj87]

Re: Odd ignition problem..only three cylinders firing

He said it was running pulling wires nothing happened.

You mean the vacuum test won't show if a cylinder is not firing?

If saying if the firing order is messed up it ( ie firing with the exhaust/intake valves open) its not going to do any good. You will get readings but they wont be any good.

 

[bobuk]

Re: Odd ignition problem..only three cylinders firing

Thanks for the interest guys, I do understand the feeling there may not actually be a problem. I am used to car engines and i did feel electrically with the thunderbolt ignition could there be circumstances where you could pull the HT leads down one side of the engine and not notice any difference? but i cant possibly see that ( i think i will try pulling all three at the same time). When i pull a HT lead on the "healthy cylinder" side the engine dies straight away. I am going for the basic idea of finding TDC cylinder 1, then check the firing order is correct from there. Just got to beg, borrow, or buy a compression tester!
Will keep you all informed how i get on, thanks again.

Cheers Bob

 

[Silvertip]

Re: Odd ignition problem..only three cylinders firing

This is simply rediculous. Under the circumstances described, this is a boat with two very poorly running engines and to have both of them exhibiting the same exact issue is simply unreasonable. I personally doubt either engine would actually run with half its cylinders disabled. And I find it odd that one could not determine that the engines are or are not running properly since loss of three cylinders would produce very poor idle quality. Further, either engine should run on five cylinders so pulling one plug at a time should not kill the engine. In fact, one can actually cross two plug wires (plug wires on wrong cylinders) and the engine may idle reasonably well depending on which two wires were crossed. So either we have two engines that are running normally and the testing is flawed, or we have two engines that have absolutely zero compression or spark on the 2-4-6 side of both motors in which case both should either not run at all or run like an old John Deere. Connect a timing light to each plug wire to verify spark on every cylinder. A compression test is in order after that. It is certainly possible that both engines injested water but to have it impact 2-4-6 on both engines identically means you should probably not go to Vegas and bet a load.

 

[chiefalen]

Re: Odd ignition problem..only three cylinders firing

Silvertip, thats why i asked the first question. Seems highly unlikely.

But maybe someone was fiddling with the motors who doesn't know a monkey wrench from a screw driver.

What was done to the motors?

Someone had to be fiddling with it, at some point.

Who know?

I'm not gonna imply that maybe a zoo lost a bunch of monkeys that got into his engine bay.

 

[Bondo]

Re: Odd ignition problem..only three cylinders firing

This is simply rediculous. Under the circumstances described,

Ayuh,.... I can Totally Agree with that....