back fire

[ASD]

if there might be anything else I could try to fix the back fire on cylinder #6 or could this problem be something else

1994 BAYLINER 2252

4.3 175 HP MERCURY CRUISER 6 CYLINDER

In the fall and early spring no problems. My mechanic informed me that the starboard side manifold needed to be replaced(original and it should be replaced 5-7 years) which I did and once it was replaced, I have been having a popping noise (back fire )from cylinder #6only.Performed pressure test with good results. I can run boat with out the wire on the cylinder and it will not back fire. I have already replaced wires and cap in hopes it might be that.
When running boat in gear at 2800 -3000 rpms popping will happen sporadically. sometimes it stops for about 30 -45 seconds. I have not pushed engine in fear of blowing rod.

Mechanic stated that he believes the rod has rust on it and it is sticking.I have also added two pints of marvel mystery oil to gas tank in hopes it will smooth out rod preformance.


Could this be true? any further info would be great.

Thank you in advance for your help.

 

[JustJason]

Re: back fire

if water was leaking into the cylinders its possible you have a bad valve... Start with a compression test on the cylinder, or just skip to a leakdown test to confirm or deny that diagnosis.

 

[ASD]

Re: back fire

Thanks for the info. I already preformed pressure test and pressure was good. Did notice rust on the manifold once it was removed. It is possible for a peice of rust to fall into rod or cylinder area? if so could that be what it is?

My only problem is, the engine was running fine until I replaced the manifold.
Was told to run the engine with the spark plug wire on and with the marval oil in the gas and it will clear it self out.

The more rpm I put on it, the louder the noise. I dont want to push engine to much.
Dont know if I am getting the correct information.

 

[carcraze]

Re: back fire

The mechanic was probably refering to a stuck valve, I assume as a rod will not stick, that I know or, while running.

Can you detail what pressure test you have done?

 

[ASD]

Re: back fire

Thanks for the response.

NO, Mechainc completed test and told me that I have good pressure in the cylinders.

still dont know why I got this problem after I changed manifold. everything was fine before.

 

[JustJason]

Re: back fire

have you triple checked your firing order? You can also try swapping that cyls spark plug for another cylinder and see if the problem follows the plug or not.

 

[ASD]

Re: back fire

No, I will check firing order. how is that done?
Great idea about spark plug, I will do that today.

 

[carcraze]

Re: back fire

I am not that familiar with your 6 cyl. Go to the adult only section and find the manual for you system. It should detail the firing order or possibly it is on the intake manifold.

Basically the plugs fire in a specific order starting with number 1 and the wires have to be in the proper order coming from the distributor to match the firing order required. Not always is number 1 on the cap in the position in manuals depending on if the distributor has been out so it is important to get number one cylinder up in top dead center during the firing stroke and verify where the rotor points and then check the order of wires to the plugs in the order it should be.

Again I am not familiar with your engine combo so get a manual.

Other here have experiance with this unit though I am sure.