I am helping a friend with his 1979 40 and I can't figure out where this capped off port goes. The diagrams sort disappear into a sea black ink. Any ideas?
Mystery port, RTH. (1 Viewer)
- Thread starter elripster
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- #2
Looking at this schematic, that port doesn't show a connection. Also interesting, the schematic doesn't show this port on the air cleaner housing that points to the driver's side above the carb. Not sure what to do with these.
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- #3
That must be one mysterious port. Is there not someone in the mud community that can post a picture of where it goes and maybe a general picture of that side of the engine bay on a CA 2F smog legal truck?
Frank
Frank
I have a fully smogged 79 and can get you an answer later this evening. Interesting bit of clear tubing coming off that gas filter. Might want to change that out with something a little more heat resistant.
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- #5
Thanks! Agreed the clear stuff needs to go.
fjc-man
SILVER Star
It appears that your EGR modulator valve is maybe newer? Your capped line goes to another port on the modulator I have. I took a couple of pictures of the mystery part. The consensus is it’s function is a controlled vacuum leak during cold start, then tapering off as the engine warms up.
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- #7
Very interesting indeed. I will see what I can learn. My friend had been building this truck for years and when it couldn't pass smog it sat for almost 10 years. It was garaged and is in pristine condition, but if the wrong part was swapped in it's likely that memory was lost to history. This might just be a case of getting the right part.
I am painfully familiar with the vacuum routing on my 60 but this is the first time I have dug into a 40 so am learning the differences. The help is appreciated.
Frank
I am painfully familiar with the vacuum routing on my 60 but this is the first time I have dug into a 40 so am learning the differences. The help is appreciated.
Frank
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- #8
I am a dunce. I see the lower left part of that diagram shows that port going to the EGR modulator.
Frank
Frank
My FedSpec 79 actually has a factory cap on that port. One of the differences between the Fed and Cali trucks in 79 and 80 is that vacuum modulator. The Cali one is taller with an additional vacuum port at the top so if your replacement modulator was a Fed Spec one, then capping that port made sense to the PO. I would have probably done the same thing
fjc-man
SILVER Star
Yup, that’s the difference. Figure 4-2 is federal. Figure 4-4 is California.
Slight Correction to my previous post. My lighting was poor and I also just checked it against the emissions manual. The Federal version actually has two ports plugged right there at the spaghetti tubing. That’s because the Federal vacuum modulator has 2 upper vacuum ports and the Cali modulator has 4!
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- #12
Been shopping but having trouble finding a 4 port. Hmmmmmm
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- #13
Actually, a lot of them, like on my 60 have that larger port on the bottom of the modulator. Does that one in the picture have all 4 on the side or does it also have one on the bottom?
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- #14
I see now, in the schematic, the modulator has an extra port "S" where most only have P, Q, and R.. His only has P, Q, and R so it looks like I need to find one with a total of 5 ports.
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- #15
And.... This port seems to supply vacuum ultimately to the air injection pump which is critical to passing smog. So, we need to figure it out. Either we need the right modulator or I need to see where else we can T in to get vacuum to that line...
Other than the fact that both AI and EGR use manifold vacuum to operate there really is no link between the two systems. If you study how the EGR works in the manual I bet you can confirm that yours is probably working just fine. The modulator just opens the EGR valve by supplying vacuum to it under certain conditions. Is the visual inspection for smog in Cali really that good that they would spy a slightly different modulator?? Even if it’s all working correctly?
Now I see what you did on the tracing of the schematic. The BVSV for AI is pink. The BVSV for EGR is light blue. Gas Filter is just a common manifold vacuum source for both systems. I don’t think your plugged port is affecting AI.
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- #18
I agree, I think I might need to just T it into vacuum so it's not staring the smog tech in the face but it does appear the AI is fine.
Frank
Frank
