FOUND SOMETHING! CYLINDER NOT FIRING!

[FreeBeeTony]

I have been thinking of performing this test for sometime and finally did it.<br /><br />Using a jumper with aligator clips on either end I shorted out each plug wire (with a probe on one end of the jumper wire) at the dist cap. I did this test at different throttle positions while under way. And to my surprise when the "bog" occurred one of the cylinders (#3) was not firing. It was defintly firing at a lower RPM. I did this test a few times to make sure!<br /><br />By the time I got back to the dock it was getting dark and the manifold was too hot to take out the plug.<br /><br />This is the first conclusive test I have run thus far!!<br /><br />Now the questions are:<br /> <br />1) Why is this cylinder not firing at higher RPM's? Guess I need to determine if it is getting spark. Why wouldn't it not get spark at higher RPM's and get spark at lower RPM'? The wires and plugs are new! Could I pull the wire of this plug and put it on a "test plug", bring the boat up to speed and check for spark?<br /><br />2) If it is getting spark, could this be an indication of a worn cam or possibly even sticky lifters? I would then pull the valve cover and measure the lift of the cam. Or is there something else I could/should do? <br /><br />If the cam was worn this bad causing the cylinder not to fire, would I see evidence of this in the vaccum test?.........<br /><br />I know I need to run more test but I am very excited because I feel I am finally on to something!<br /><br />Am I causing any damage running the boat like this?<br /><br />Any other suggestions with this new information?

 

[lowshovel1]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

tony,you can barrow or buy an in line spark tester,I payed about 15 bucks for mine off tool truck.it has a spark plug boot on one end that you put on plug--on the other end is a male tip that plugs into your plug wire.In between the two ends is a clear plastic cylender that you can see spark when engine is cranking and running.

 

[Buttanic]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

You didn't prove it wasn't getting a spark by shorting the plug to ground you only showed that there wasn't combustion taking place in the cylinder. To check for a spark you need to pull the wire off the plug and hold it near the engine to see if a spark is being produced. If it sparks than the problem is not the ignition system.

 

[FreeBeeTony]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

Agreed buttanic, The first thing I am going to do tonight is pull the plug for inspection, then take another test run with a "test plug" to see if there is spark at the higher RPM's.

 

[imported_scott_m]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

Now we're getting somewhere! Although you can get bad (new) parts out of the box, I'm going to guess that you'll find it's getting spark - there's no reason for one plug to not spark at higher RPMs. I could see all 8 going out at high RPMs (weak coil, battery, alternator, or ignition box problem), but not one. Probably what you're going to find is a wiped cam lobe.

 

[rodbolt]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

why not just buy the tester. they also make an inductive unit that you lay on the wire any place along it and it flashes if spark is occuring. costs 10 or 15 dollars and is a good tool. the snap on Kv tester is even better but its about 140 or so. to test the plug swap it with another cyl and see if the condition moves. if it does not change theories. be aware that a "shorted" plug may still flash the timing light or tester. however it can be rapidly found with a Kv tester. an indication of loss of compression or a shorted plug will show up as something less than 2 Kv. normal for that motor is between 6 and 10 Kv. an open gap or excessive combustion pressure will spike it to above 15Kv. much above that and the voltage will attempt to find an alternate path to ground on that ign system.

 

[imported_scott_m]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

Sorry for the double post - previously you mentioned compression numbers - between 140 and 170. This would be outside the allowable 10 percent variation between cylnders. If you've got one at 170, you want your lowest one to be no lower than say 153. If you've got one in the 140's this is too low. Any chance you have the details from the test? My money is that #3 is running 140psi due to the aforementioned cam (or other valvetrain problem).

 

[newport dave]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

The "allowable" compression difference between cylnders, according to Mercruiser, is the lowest cylinder should not be less than 70% of the highest cylinder.<br /><br />Highest cylinder = 170<br />70% of 170 = 119<br />140 = ok<br /><br />Using a plug (not in the cylinder) to check for spark under load is not a valid test. The plug must be in the combustion chamber and tested with an inductive KV tester and the results compared to the other cylinders to be a useful test. <br /><br />Dave

 

[rbezdon]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

As for the spark test, I bet you could just hook the timing light to that cylinder and see if the thing flashes, obviously you cant check timing that way but if the light flashes you got spark, especially if you have the inductive kind like I do.<br /><br />In the miss, my bet is on the cam lobe or a bad/bent valve. It is really tough for ignition to miss at high RPM and not low and darn near impossible for it to happen on one cylinder where as the symptom is exactly what you find for a valve not working properly.

 

[FreeBeeTony]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

Thanks everyone for your ideas.<br /><br />I have an inductive plug tester I will try tonight.<br /><br />If the problem is a bad valve or cam wouldn't it have showed up during the vaccum test?

 

[Olds Eddie]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

A weak valve spring will cause what you have described as well. It will close the valve at low rpm but allow the valve to be opened when it should be closed at higher revs causing the compression to leak down before the spark occurs.

 

[Buttanic]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

I had said before and agree with olds eddie that it could be a weak or broken valve spring allowing the valve to float at higher RPM. The noise you said you hear at the time of the bog could be valve float.

 

[Scaaty]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

I'm leaning to valve spring problems too. You are too hung up on spark theory

 

[FreeBeeTony]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

I'm not hung up on spark theory...just something else to eliminate.<br /><br />If I remove the valve cover is there a special tool I would require to test the spring(s)?<br /><br />How about running the boat like this? Or is my season over?

 

[Haut Medoc]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

Yes. But unless I'm wrong, the spring must be removed to check pressure at a certain length(closed) and pressure at a certain length (open)and it's free length as well....Don't know what this will do in the long run, (this season), but I myself would probably not keep running it. If you really think you have isolated the right cylinder, it shouldn't be that hard to check with the proper tools & manual.......JK

 

[Buttanic]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

If it is a valve spring there is a chance of the spring retainer coming loose and the valve falling into the cylinder. As you can imagine this would not be good and would cost a lot more than a valve spring or two to fix.

 

[FreeBeeTony]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

I rechecked the test tonight....#3 is not making power. <br /><br />Checked spark best I could with a test plug and the inductive tester.......spark appears to be ok.<br /><br />Tomorrow I plan on checking the cam lift and and inspecting the valve springs, rocker arms and push rods.<br /><br />I may be able to borrow a valve spring compressor to maybe remove the springs and maybe I can find someone to test/check them. If I put this piston at TDC, will this hold the valves up keeping them from falling into the cylinder?

 

[rbezdon]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

no, the best and almost the only way to remove the valve springs is to pull the heads. If you have a reasonably tight set of rings and valve seats, I have heard of guys putting like 100 lbs of air pressure into the spark plug hole and the air pressure holds the valves up but I dont know of any one who actually does it this way. Those springs are strong and really tough to compress. You cant do it by hand and the standard spring compressors require the heads off.

 

[Trent]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

Dont pull the heads..... !<br />Put the piston at top dead center on that cylinder. Then you can either buy the fitting to screw into the cylinder and add air pressure to hold the valve up and remove the spring. <br /><br />Or you can do it another way.. Put it at top dead center and get some rope. Insert it into the cylinder (sparkplug hole)... Keep feeding all the rope you can into the cylinder till you cant get anymore in their. THis will keep the valve from falling out into the cylinder. Remove the spring and check.

 

[Buttanic]

Re: FOUND SOMETHING! CYLINDER NOT FIRING!

You can get a cheap valve spring compressor at the auto parts store that can be used with the head on. It fits on the rocker arm stud. Also Auto Zone will usually lend specialized tools, all they require is a deposit.<br />The rope trick works well, rotate the engine so the rope is pressed against the valves by the piston. Any automotive machine shop should be able to check the springs.