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Putting an '86 TPI into my 78C3. Almost ready to fire it up and now concentrating on the little things. I did a search for this question and found I had asked it in 2015 ( has it really taken that long to get to this point?). Anyway, using the original canister on the 78, can I just hook up a vacuum line to the canister top port, the PCV valve and the fuel tank and plug off the second valve arrangement ( going to the carb bowl)? It seems too simple and I must be missing something. Any help would be welcome. Thanks.
On my '72 the canister has 3 lines to the top, one from the float valve at the tank, the other small one is going to manifold vacuum, and the fumes on the other large line shuld go to a spot above the throttle blades, vacuum is varied by throttle position....
on MY setup, some hears ago I found my valve stuck open...so from the tank to the canister, other large line went to before throttle blades, works well....
TPI should have a Purge Valve connected to the canister. ImI guessing but convinced both the C3 and C4 canisters have 3 ports. But I'm guessing the C4 cannister will have an upstream control valve between canister and tank so it doesn't continuously draw from the tank.
I'm sure you can find emissions diagrams on line for both cars to help you sort things out. Auto Zone and others have diagrams in their TecnTechn Assistance sections for emisions.
Hope this helps.
BTW you can buy nylon fuel line in bulk (Dorman) it you need fast and easy fuel/vapor lines for this.
Last edited by cardo0; 09-15-2018 at 04:34 PM.
Reason: BTW
Mrvette, on my 78 cannister there are 4 ports on top plus the line going to the tank. One marked CONTROL VALVE which I guess at one time was the EGR valve, no longer on the car. Below that is PVC and that is connected with a "T" fitting to the other port that says MANIFOLD VAC. The last one says CARB BOWL. No electric purge valve on the vehicle, which is the way I would like to keep it. Your decription sounds like what I think will work. Small line to manifold vac, line from tank as marked and the one marked CARB BOWL to the larger line coming off the throttle body. The other 2 ports would get blocked off. That would give me the total 3 ports that you have on the 72. Just not sure about when you say ABOVE throttle blades or BEFORE blades. Does that mean the same thing ( the lower port coming off the TB?) Thanks.
The early canisters had 3 connections: timed/ported vacuum to signal the purge valve (so it would NOT purge during idle and screw up idle settings); line from fuel tank to canister; and line from canister to carb base (tied with PCV valve sometimes) for dumping the contents of the canister. Later canisters had up to 5 hoses; one may have been manifold vacuum, but not sure how it was used.
I know of no reason why you could not use the vapor canister system with the TPI induction. You would just need to figure out how to dump the vapor purge contents into the TPI system without messing up idle quality.
Yes, the large hose goes to the point that is similar to the air cleaner, above/in front of the throttle blades, so the vacuum is not super high which will suck air from the tank and render the entire system worse than useless, because the tank cold well be put at manifold vacuum, and THAT would collapse the tank.....OUCH!!!!!
Thanks Mr. Lionel. I can't use the stock TPI set up since I have a custom harness and prom with no emission capability. On some of these L98 engines they had the canister hooked up in line with the PCV valve. Either that or the port on the side of the throttle body. Funny that with all these swaps there is not a lot of info on this problem. Thanks to you all.
MrVette, thanks for the PM. My throttle body has a large fitting that sends fresh air to the passenger valve cover. Would I tap into that on for the line going to the cannister? Or just tap into the pcv valve line which I have seen in some diagrams. I also have another fitting in the middle that comes in after the butterflies and, according to what I have found, goes to some pollution tubes above the exhaust manifilds. Then there is the small fitting under the TB which would go to the top of the canister control fitting. Do you see any flaws in my thinking? Don't hesitate to tell me, my wife never does!!! Thanks to all for your help.
MrVette, thanks for the PM. My throttle body has a large fitting that sends fresh air to the passenger valve cover. Airflow is just the opposite, applying a very small vacuum to the valve cover/engine oil/crank/sump picking up that blowby that always exists.....the vacuum is very minor and most of it is picked up by the PCV valve, the rubber hose to eh PCV valve is to full manifold vacuum
Would I tap into that on for the line going to the cannister? NO, that like said above is full manifold vac....which is AFTER the throttle blades.....now at IDLE that vac is much higher but when throttle is opened under load, depending on how much load/vs how much throttle, the pressure difference drops like mad....
Or just tap into the pcv valve line which I have seen in some diagrams. I also have another fitting in the middle that comes in after the butterflies and, according to what I have found, goes to some pollution tubes above the exhaust manifilds. Then there is the small fitting under the TB which would go to the top of the canister control fitting. Do you see any flaws in my thinking? Don't hesitate to tell me, my wife never does!!! Thanks to all for your help.
That small fitting under the TB goes to the main control valve on the canister, the small fitting on top on MY canister, but I only have 3 fittings on mine, you mentioned 4 on yours, so I have no clue where that 4th one goes......Pollution tubes above the exhaust manifolds.....been too long, that one escapes me, sorry man....I too always ask my wife when I forget to remember, it's a standing joke between she and me....
The port on the throttle body that connects to the valve cover DOES provide fresh FILTERED air to the engine. The PVC valve sucks dirty air out of the other valve cover. Air doesn't go through that hose to the throttle body. You'd get dick-all for vacuum on that port unless you put a really restrictive intake on the engine. All it would do is allow fuel vapors to be pulled into the engine block which isn't a great idea.
The system would have been connected to manifold vacuum originally so why can't it do that any more? If it didn't collapse the tank before then it won't now.
Connect the purge or control valve fitting to ported vacuum. That control valve fitting will turn-on the valve that allows vacuum to suck fumes from the manifold vacuum port. It was likely ported so it only worked at part throttle. It would have been tied to the EGR since that valve is operated at part throttle too.
Connect the T'd PCV and manifold fittings to manifold vacuum. If you want to slow the airflow then put a restriction in this manifold vacuum line. It might have been connected to the PVC valve originally since that was manifold vacuum.
You don't have a carb bowl anymore so you can cap that one.
There must be some kind of valve that doesn't allow too much vacuum on the fuel tank. It might just be a 1-way valve in the gas cap that allows air in but not back out.
Ideally, you would have kept the electric purge solenoid and let the PCM turn on the purging.
Last edited by lionelhutz; 09-19-2018 at 09:49 AM.
I probably would have kept that purge solenoid if I had thought that far ahead. This has been a long term project starting with the body off. Drivetrain, 200R4 trans, computer controlled engine, new custom wiring throughout the car, lots of things to think about.
The carb bowl port will be capped ( along with the small valve fitting on top of it) which puts me at the 3 ports that MrVette has. Small fitting under TB will go to the top small control valve fitting on top of canister which leaves the one marked PCV and the tank. The tops of every canister I have looked at over various configurations all have PCV ports. As I understand things right now, the vent back to the tank is loacted under tha canister and lets air back in it. I will check that out tomorrow with the canister on the bench. Here is a good read on this that I came across. Building the engine was easy, wiring the car is easy, but the fumes, not so much. Thanks again for participating.https://tech.corvettecentral.com/201...orvette-gassy/
TPI vacuum line diagrams are also posted at the above link and show #4 going to the purge control valve.
So, the purge control valve fitting needs to go to the ported vacuum source and not manifold vacuum. The PCV fitting goes to manifold vacuum (can T to the hose going to the PCV valve) and not to a connection before the throttle body in the hope it might have enough vacuum to purge the canister.
In other words, marry the 2 systems together so it's connected as close to original as possible instead of trying to re-engineer it to something else that won't be as effective.
Last edited by lionelhutz; 09-19-2018 at 12:32 PM.
Thank you SIR!!! That is the diagram I needed. Same canister I have with the only difference the carb base, so I can plug up carb bowl vent. Once I look at the diagram and block out all the emission stuff I no longer have it seem not so complicated. I also couldn't figure out the # 4 on the TB. I knew it went behind the butterflies but I was focused on #3 port. but now this all just fits together nicely. I didn't want to mess with the fresh air port #3. I had been to Grumpy's a few times but alsways zoned out after awhile since there is so much info on there. I ended up looking at the TPI diagrams and forgot the carb set ups. Thanks to you all on this. I appreciate your help and input. If any of you ever get to Charleston SC, give me a shout.
09-15-2018, 01:35 PM
TPI and charcoal canister
