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Old 08-18-2009, 02:11 PM   #1
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Cracked exhaust manifold on my bus

My bus is a 99 P30 with the Chevy 350 and TBI intake. From what I can gather, 95 was the last year of TBI for the regular cars and trucks, and that Chevy probably just used up it's older stock on stuff like the large vans and buses.

My driver's side exhaust manifold is cracked and needs to be replaced. I was given an exhaust manifold that looks almost identical to it, but it doesn't have the fitting on the back of it for the tube that runs up to the intake. I'm assuming that this is an EGR tube. Does this tube actually do anything other than look pretty? There are no sensors on it that I can see.

I was looking around on Ebay and ran across some shorty headers for 88-95 Chevy trucks, and they list them as having an O2 sensor hole. My O2 sensor isn't on the manifold - it's down on the pipe just in front of the muffler. I'm wondering if this O2 hole is the same thread and diameter as the EGR tube that's on mine. Would anyone know about this?

I hate to buy a used exhaust manifold if I don't have to. I would rather buy a set of shorty headers if I'm going to have to put out money on something.
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Old 08-18-2009, 08:36 PM   #2
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Re: Cracked exhaust manifold on my bus

its time for the experience to step in...he will know.however, i have heard of this before and the non o2 manifold has been installed-i think it was a 92-3 era,but no remember.suks when they crack or break studs timbuk
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Old 08-18-2009, 09:19 PM   #3
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Re: Cracked exhaust manifold on my bus

Originally Posted by timbuk
its time for the experience to step in...he will know.

EGR tubes...a nemesis of all though Ford owners with Windsor small blocks feel the pain the most (especially when you factor in the AIR injection tubes).

Does EGR do something besides look pretty? It sure does! It reduces NOx emissions. It's a tough line for vehicle manufacturers to walk. There are three kinds of emissions (generally speaking) that they concern themselves with. Those are hydrocarbons (HCO) which are unburnt fuel essentially, carbon monoxide, and oxides of nitrogen (NOx). The problem is that as you reduce HCO and CO more and more your NOx emissions go up. NOx emissions are the product of high combustions temperatures and pressures, due mostly to high load, lean mixtures, and highly advanced timing. Three way catalytic converters help reduce this, but so does the introduction of exhaust gas via the EGR system. Simply put, EGR partially fills the combustion chamber with inert exhaust gases. With the space taken up less fuel and air can fill the cylinder and NOx emissions are thereby reduced.

Ok, enough with the theory stuff. Can you eliminate the EGR system on your bus? Absolutely, provided it is a vacuum type and not a digital linear type EGR system. I've never seen a TBI vehicle with the digital EGR. On a vacuum unit there will be a vacuum diaphragm on the valve somewhere in the plumbing. Block off the inlet to the manifold, the vacuum line, and eliminate the tubing. I have heard rumors of people having problems with engine ping following removal of the system, but this is a problem exclusive to carbs. Your knock sensor should be more than able to take care of adjusting timing, if needed at all.

There are certain legal issues with modifying the pollution control systems on a vehicle, but I'm not here to lecture you on that at all. Just understand that it may or may not prove to be a problem for you.

So...headers. Why do they put the oxygen sensor bung in the header tube? There are two big reasons I can think of. First is that some vehicles DID have the oxygen sensor in the manifold though this is not terribly common. A much more plausible reason is that oxygen sensors all require one common thing to work and that's heat. I would think your bus most likely has a 3 or 4 wire oxygen sensor which has a heater circuit in it to get it up to temperature and get the vehicle into closed loop fueling (meaning if can adjust itself as needed and is not on base fueling) faster. However, many older (and still some modern vehicles) use 1 wire sensors. These sensors are unable to provide heat for themselves and rely on the exhaust gas heat to get them warm enough to switch into closed loop.

Headers are made of thin steel and multiple tubes. The dissipate heat much much better. The truth of the matter is that an oxygen sensor downstream of the headers might just not ever get warm enough to function correctly! If your sensor has 3 or 4 wires then you can just plug the hole off in the headers and run as is. If you have a 1 wire sensor you will need to relocate it to the headers and plug the hole off further down. I believe, but am not positive, that the thread on the bungs is 14mm by 1.25 pitch. I DO know that Motormite spark plug antifoulers can be cut off and make great weld in bungs.

Where exactly is your manifold cracked, anyway? Usually I see them go at the flanges, not elsewhere, in which case there are 2 piece repair flanges that can be used.
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