my exhaust manifold in my nissan 4cyl 4x4 pickup has a major crack in it. since this truck has an o2 sensor, I would imagine this would effect my gas mileage. is there a way to patch it other than welding it?
You may want to use an epoxy called JB weld. I've used it in all sorts of applications and its never given out on me. I've heard of it being used to fix cracked engine blocks before, not sure that it will work on a high heat manifold, but for just a couple of bucks its probably worth a shot.
Common problem on Nissans, I have replaced hundreds.
It can affect your mileage if the crack is bad enough for air to get near the sensor.
No patch will work, you could try a used one. The 4 cylinder motors in the 87 an up trucks had the manifold on the drivers side, with the intake on the passengers side, so its not a real bad job to do yourself.
Worst problem is broken studs, you want every manifold stud to be there to keep the gasket from leaking. When you remove the nuts from the manifold dont go overboard with the force necessary to get them to break loose. Alternate between loosening and tightening them to get them to break the corrosion bond. Dont just crank up the force trying to break them loose or you could snap off more of them, When they break off flush you have to drill them out and use an easy out.
If you have to do this the secret is to actually try to rotate them clockwise just a little, again to break the corrosion bond.
Then very carefully turn them counterclockwise to back them out of their threads, but if they bind up go back clockwise and then counterclockwise.
Be patient and you can get them out. Some penetrating lubricant also helps, but the real secret is to keep reversing the rotation until they finally come out.
If you see missing nuts and broken studs you will have to do this procedure. It may be better to let a pro handle it if there is someone you trust.
Most of the mounting locatios use a special washer that is designed to let the manifold move slightly in relation to the cylinder head. Make sure you get those washer and put them in the right locations.
Also make absolutely certain you torque them properly. I can't remember the spec but it was only about 14 foot pounds, or close to that. Depends on the year of your engine.
Up to 86 was the L series engine with intake and exhause on the same side. 87 and later was the NAP-Z twin plug engine, then they went to a twin cam engine.
There is a one part putty called "cold steel" or something similar that will.
Also if on a tube section that's regular enough in shape to do this, muffler cement will hold with mechanical support. You'd get a piece of flashing or thin steel as a bandage and some big hose clamps, "butter" it all over with muffler cement and then slap it on top and hose clamp it tight.
Doing either of these methods, first clean off the manifold really well, but don't polish it smooth, leave something for it to grip on. Drilling out the ends of the crack with a 1/8 bit will stop it spreading. Now, run it a little to get it warm, and let it cool off enough that there's some heat there but you're not gonna burn yourself working on it, ideal would be when you can bear to touch it for a whole second. Slap on the repair. Leave it for an hour or so to start curing. Then idle it for a half hour or so to help bake it on. For the first day, do not go WOT, keep the revs low and throttle light, you don't wanna blow it off while it's still curing.
Edit: Woven fibreglass tape might offer enough support for muffler cement also, it's messy but if you completely coat the tape in it, and bandage it round and over in several layers, nice and tight, that might do it. Leave this to dry for 6 hours before running it, and idle it for an hour... when it's dry protect it with high temp paint. (Muffler cement not protected in a joint or covered has a tendency to be affected by moisture)
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I second RoadWarrior's tip on giving muffler cement a try.
I have used it with some success on a tractor exhaust manifold with a rust hole through it. I patch a dime sized hole every 3 years or so (that's how long the patch lasts). I mix the muffler cement with a little fiberglass insulation and then wrap a few layers of thin wire around the manifold and "patch" and then add more patch on top of that.
I would weld. I've always had good luck welding cast iron manifolds. I heat them with until they are pretty hot (not read hot as my MAPP gas doesn't produce enough heat to make it easy). Then weld as usual, no special rod or anything.