I have a 1989 Chevrolet S10 Blazer Truck with 4WD 4.3L TBI 6cyl
I have a crack in the left exhaust manifold rear side (firewall's side), that causes some noise coming out of it (which I kind of like), and I wanted to replace it after I was told that it can't be welded in place, but my friend says that with such a little crack, it's not worth replacing it, as bolts might brake and cause me problems. So do you know if that crack can cause me any problems like bad O2 readings or any other? Knowing that sometimes (only) when I'm going up a hill, the engine's light goes on for few minutes then goes off. Put it on a scanner, and it shows O2 error, knowing that I changed it few months ago.
Thanks in advance for any help

 

A crack can allow outside air into the exhaust throwing off the O2 sensor readings. I would replace it.

 

Thank you for the quick reply.
Well, the carck (pushes) lets air out when the engine is running, so I don't think any air can come in.
I want to replace it, but I'm worried about bolts! Do you think that they can break? Even more than one mechanic told me that it could happen

 

You are partially correct. While the exhaust valve is open and the exhaust pulse is traveling past the crack, your assumption is correct. However, after the pulse passes the crack, there is a low pressure region (caused by scavenging) that can pull air into the exhaust. I have seen a false lean condition caused by a small crack in the manifold or leak at a gasket/donut cause some pretty serious driveability problems.

 

Man! Thank you for the info. I didn't think of the low pressure thing! What makes me assume that you are right about the cause for the O2 error, is that the light goes on few minutes after I start going (home) up a hill slowly on 4th/OD gear (automatic transmission) with low RPM, causing longer intervals between pulses, pulling more air through that crack?! Am I right? I know you gave me enough info to know that I need to replace it, but it would be great to learn new stuff
And What about bolts/screws?

 

*Moved to '1st Gen Tech'*

 

About the bolts... Sure they could break, but your run that risk with just about any fastener in extreme conditions. Just be gentle with them and you should be ok. Make sure you have a good drill bit and some EZ-outs on hand along with the correct tap and some new bolts.

 

I bought some of this stuff that was called something like "Freeze in a can" and you would supercool the exhaust studs with this stuff when the engine is running temp. That would cause the bolts to shrink a little in the hot expanded heads... Should make the likelihood of them breaking on you alot less and make them cold rusty weakened bolts come out alot easier. Its cheap insurance cause easy outing on the side of the heads is a real bugger if your drill wont fit in the space... I used it removing a 2000 durangos exhaust manifold studs, one was broken off but enough was sticking out that I was able to use my stud extractor on it. Let us know how you make out on it. Oh and dont reuse your studs, go get new ones.

 

Isn't there any kind of seal I can use to fix the crack? I'll try to post a photo of it later

 

I've never had good luck with the high temp epoxies that are marketed towards this sort of repair. I have tried a few and they all have failed in what I would call a short period of time. And all of the times I have attempted repairs like this, I had done a lot of prep work. Almost to the point that I had wished I had of just removed the manifold and either welded it up or replaced it instead of wasting my time.