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Sun Mar 21, 2010 7:00 pm

I have an early 76 Bay 2.0 with the 6 pin AFM. I'm running stock heat exchangers with an extractor muffler. This leaves me with no preheat and while I understand what the preheated air does, I don't really understand how the stock preheat pipe worked with the two holes in the rear sheet metal. Was air drawn from the engine bay through the left port via hose down into the preheat pipe and then from the right side of the preheat pipe up to the air intake duct?
Since the bus is nearing the end of extensive repair and restoration, and I've spent tons of time trying to replace everything the POs removed, I could use some advice on the importance of the preheat system and how I could fab one to do the job. Otherwise I will simply seal the holes.
Thanks for the input-

Sun Mar 21, 2010 7:50 pm

Welcome, indeed the 75-76 preheat was a second tube piggybacked on the crossover pipe, but FI really doesn't need preheated air as there is no venturi in the system to ice up. Unless you are willing to fabricate a tube that can heat the air to a temperature that will open the preheat thermostat in a few minutes you're best option is to block the holes and disable the thermostat in the airfilter, it's another potential vacuum leak that L-jet doesn't like much. You can put a dummy hose on if you want it to look stock.

Mon Mar 22, 2010 6:07 am

Thanks Busdaddy- I'll seal it up and disable the thermostat flap.

Mon Mar 22, 2010 9:59 am

Whitley wrote: I have an early 76 Bay 2.0 with the 6 pin AFM. I'm running stock heat exchangers with an extractor muffler. This leaves me with no preheat and while I understand what the preheated air does, I don't really understand how the stock preheat pipe worked with the two holes in the rear sheet metal. Was air drawn from the engine bay through the left port via hose down into the preheat pipe and then from the right side of the preheat pipe up to the air intake duct?
Since the bus is nearing the end of extensive repair and restoration, and I've spent tons of time trying to replace everything the POs removed, I could use some advice on the importance of the preheat system and how I could fab one to do the job. Otherwise I will simply seal the holes.
Thanks for the input-

Are you talking about the vertical metal chimneys coming from the heat exhangers....one on each side.....about 2 inches or so from the joint where the heat exchangers connect to the muffler? If so...those are generally not heat risers for intake. They are to connect to the auxiliary fan that pushes air into the heat exchangers on models with gas furnace. Ray

Mon Mar 22, 2010 10:36 am

I'm pretty sure he's referring to the 75-76 only preheat system, the tube is #24 here:

And the holes in the tin for it are these:

BTW all type 4 powered busses have the booster fan tubes, gas heater or not.

Mon Mar 22, 2010 10:51 am

I'm talking about Part #24 on the diagram- which does not exist when you use the aftermarket muffler/tailpipe combo Bus Depot and others offer. I have the chimney tubes from the heat exchangers to the heater booster fan no problem. I was just concerned that with the 6 pin AFM and no intake air temp sensor, how vital the preheated intake air was for good operation. I'm assuming its not an issue as long as the ambient air temp is above 68 degrees F, and even when its cold outside, the engine bay warms up relatively fast, so the the intake air is warm anyway. It will just require disabling the flap on the air intake duct that was specific to the 75-mid 76 buses. After all my searching, I've never found any info about this issue, so I wanted to throw it out there for those of us who have this specific, overly complicated exhaust/intake set-up.
Thanks-