[Tab]

Would a sizeable crack in a stock exhaust manifold (4 feet before the o2 sensors) cause the computer to make it run rich enough to make the intake ports gummy with crap? When I took the old stuff apart I think it looks like there is way too much gunk in the openings of the intake. Farther in looks ok and the valves I can see look great but the ports look real shitty. Oh and I don't have a cat either but havent for a long time and never really noticed anything odd after taking it off. But I could put it a new, smaller one if it would help as I kinda need to reroute my exhaust anyway.

 

[junk90xj]

the crack shouldnt matter enough to cause any of your problems.

missing a cat if you have pre and post cat o2 sensors can cause fuel trims to be off and it will change the fuel delivery map the computer uses.

not sure if its your problem though, really the only way to find out is to hook a quality scanner up and look at fuel trims and other specific info to see how its running.

 

[Tizken]

I had bad rings causing blowby leading to oil getting blown into the airbox and gunked up everything.

 

[junk90xj]

blocked pcv lines can cause the same symptoms too

 

[Tab]

I'm probably making it sound worse than it is I suppose. Theres just a layer of gunk going back into the port about and inch and a half but it kinda wipes out ok and I'll just do that. I just wasnt sure what it should look like. It has 80k on it and the manifold has been cracked for a long time. I'll get it going and have someone scan it and see I suppose. Thanks

 

[Yota Bill]

I have seen cracked/leaking manifolds cause the O2 sensor to read lean, forcing the computer to dump in excess fuel. As the exhaust pulse goes past the crack or leak, and the valve closes, it causes a vacuum to build between it and the next exhaust pulse, which will draw in air and cause the O2 to read it as high oxygen content (lean)...it pretty much will only happen if the crack or leak is at a point where it only affects one cylinder, before the different cyls merge into a single exhaust collector, because after they merge, the pulses are much closer together so the time that air can be pulled in is reduced to a point of being negligible

 

[Tab]

I have seen cracked/leaking manifolds cause the O2 sensor to read lean, forcing the computer to dump in excess fuel. As the exhaust pulse goes past the crack or leak, and the valve closes, it causes a vacuum to build between it and the next exhaust pulse, which will draw in air and cause the O2 to read it as high oxygen content (lean)...it pretty much will only happen if the crack or leak is at a point where it only affects one cylinder, before the different cyls merge into a single exhaust collector, because after they merge, the pulses are much closer together so the time that air can be pulled in is reduced to a point of being negligible

This is kinda the senario I was thinking could happen but wasn't sure all the ports would have buildup in them. I had one tube (up near the head) that was cracked 80% around but it wasnt opened up much. I think it would have more to do with not running a cat anymore. Thanks