OMC Cobra 4.3L Exhaust Manifold Overheating

[billywhizz26]

Hi Everyone,

I have an OMC Cobra 4.3L V6 with an port exhaust manifold overheating problem.
The manifold got too hot and make the rubber exhaust hose go brittle and crack therefore expelling exhaust gasses and water.

I have a few worries and would like to air them and ask for advice.

I have removed the water hose from the thermostat housing and the water floods in when the engine is running therefore I assume the new impellor, new water circulating pump and thermostat are ok.

I then removed the rubber exhaust hoses and joiner and put a water hose down it and turned on the tap. At the same time I blocked the Starboard side with a rag to stop it coming up that side. The water flooded out of the exhaust in the leg. I assumed this was all clear too. I noticed the flap in the exhaust Y-Pipe on both sides was missing but can't remember if this was not there beforehand. I know OMC recommend removing them anyway. They couldn't have fallen down and blocked the exit as the was passes out ok.

So that leaves either the elbow into the manifold or the manifold itself.

Can anyone tell me what is in the manifold? Is the water separate from the exhaust till it exits? like a jacket? or is it all one chamber? might explain why the exhaust doesn't pressurise the water system backwards. Before I taake it off, any info would be nice.

Does the Elbow on the front of the manifold screw in?

Any light on this would be great!

Thanks in advance chaps!

Paul.

 

[Randy J]

Re: OMC Cobra 4.3L Exhaust Manifold Overheating

the flappers probably are in the bottom hose at the foot. melted. water can still exhaust there as the diamond pattern slits under side let the water go out there if it cant go out hub. you need the flappers in the top part of Y pipe they keep water out of engine. but stopping fast , launching boat off trailer fast will force water past the flappers even if new and properly installed.

 

[HT32BSX115]

Re: OMC Cobra 4.3L Exhaust Manifold Overheating

Howdy Paul,


Welcome aboard!


I guess a few questions are in order.....


What kind of manifolds/risers do you have now? How old are they, do you operate in salt water?

In general, A manifold/riser is too hot because there's not enough water flowing through it when running. They typically should be a tad too hot to hold your hand on very long. If it's too hot to touch, it's too hot.

Not enough water flow can be due to a bad impeller and blockage between the impeller and the rest of the system, or clogged (with rust) manifold and/or riser or both.

If you have the old style "bat-wing" manifold/riser, you should replace them (with 2 piece units)

Can anyone tell me what is in the manifold? Is the water separate from the exhaust till it exits? like a jacket? or is it all one chamber? might explain why the exhaust doesn't pressurise the water system backwards. Before I taake it off, any info would be nice.

They are jacketed. and separate until at the exit point so there's NO water that can get back into the exhaust (pretty much the entire purpose of the riser/elbow)
regards,

You didn't say what year or model you have....but the exhaust "seal" (#22 below) is what you say is missing. Volvo did remove them from certain years but I don't know any OMC info deleting them. Yours may be in the bottom of the Y-tube(#1)

The picture below is a 1989 4.3L, yours may be different.

Rick

 

[bruceb58]

hi Rick, i realize this post is 6 years old, but are the manifolds in the above schematic the "bat wing" style? i have these on a recently acquired 88 Reinell 173E with the 4.3l OMC Cobra... and i like the way they look and would hate to get rid of them...

i purchased the boat with the disclosed knowledge of an overheating problem and a seized motor... completely rebuilt the motor with a new block because the original was cracked and went through the outdrive to inspect and replace the seals in the water pickup tube and so on... upon inspection i found a lot of melted plastic which im guessing were the flaps you were referring to (#22)?

anyhow, just fired it up yesterday afternoon and ran it for a good 15-20 minutes with the ear muffs on and all seems to be functioning fine.... looks like 160-170 degrees with tap water running through it?

You should not be posting on an old thread. That being said, get rid of those batwing manifolds if you boat in salt water. Ticking time bomb that will take out your motor.