Emissions control device

[Deathscythe]

I'm going to try and explain this as best I can so it wont be as clear as mud. Here goes....

I have an 86 GMC Sierra Classic with a 305/350 turbo transmission with a 4 barrel QuadraJet carb.

The issue is this; the truck has been running rough and I replaced the plugs, wires, cap and rotor and got good results and it has run fine for a couple weeks. Today after work it started running rough again, hesitating at low rpm and idling rough and running rough through all rpms.
I got under the hood to see if I may have a vacuum leak of some sort and found a broken vacuum attachment on the top of the thermostat housing. This emissions device has 4 vacuum attachments that route through the engine compartment, the best way to explain where is this;

Top center of the device runs to a T on the back of the intake manifold (connects to passenger side leg)
       - The right leg runs to T to the vacuum reservoir and firewall by the heater core.
         - The top leg on the manifold to a vacuum diaphragm on the transmission.

The 2nd to the top on the device runs to the heat riser valve.

On the device the bottom drivers side vacuum runs to the vacuum advance. This is the one that's broken.

Passenger side bottom of the device runs to the vacuum advance, carb, and to multiple diaphragms on the front of the motor.

I wouldn't even know what to call this vacuum attachment or if they are very expensive, I'm assuming it will be. I know it controls vacuum until the engine is warm, but would that specific leg cause the issues I'm having or could there be bigger problems?

I'm starting to think that the truck was sitting for awhile, a few vacuum lines have been replaced due to them being completely deteriorated and there's old mud dobber nests on the water pump housing and intake manifold.

Emissions and vacuum are two things I don't know much at all about, and need to learn. Hopefully someone here has an idea what the part is, if its needed and if not, how to bypass or eliminate it while rerouting vacuum to keep everything working. As always, any help would be greatly appreciated.

[VileZambonie]

Look on here for these keywords: TVSV, TVS, or thermostatic vacuum switching valve and you will find plenty of posts. Nevertheless, plug the vacuum leak and see if it makes a difference but it doesn't sound like that is your issue.

[VileZambonie]

https://www.partsgeek.com/3bs3724-chevrolet-c10-ported-vacuum-switch.html?utm_source=shopzilla&utm_medium=pf&utm_content=dcs&utm_campaign=PartsGeek+ShopZilla&fp=pp&utm_term=Chevrolet+Ported+Vacuum+Switch

[Deathscythe]

See, this is why I love this forum. Thank you!

[Henry]

Hi DS:
Yup, that little device is the TVS valve. See the Autozone website images 117-126 or so to see what your vacuum hose routing should be for the switch: https://www.autozone.com/repairinfo/repairguide/repairGuideContent.jsp?pageId=0900c1528004c3cb#

Lots of possible leak points in the "engine spaghetti" days!

Regards,
Henry

[Deathscythe]

I've been looking at and downloaded a diagram last night, it does seem rather complicated.
I read on another forum of someone who eliminated that vacuum valve by switching the Thermostat Housing to one out of a '75 305 and rerouted a bunch of his vacuum hoses. I'm contemplating doing the same.
On the other hand, I do want to swap out the 305/350T for a 350/700r4, so maybe I'll wait until then to eliminate things and just repair what I can now. The part isn't that expensive and it's easy to get to, so it would be an easy fix.

[Deathscythe]

Update:
I actually feel pretty stupid. The wiring restraint clips for the drivers side plug wires came loose and two of them ended up against the exhaust manifold. They didn't burn through completely, but enough to cause it to run horribly.

On the upside, easy enough to fix and got some good information along the way.

[VileZambonie]

That is why i said it doesn't sound like your issue. Glad you got it fixed.

[Deathscythe]

I am as well, I had a feeling I was missing something simple. It figures I'd dismiss work I had just done without considering it as the problem.
But, after spending that much time under the hood trying to figure it out shows a big issue I have with the truck, all of that vacuum line mess. I'm probably going to take most of the emissions stuff out and simplify the vacuum system.
Now I will be looking into what everything does, if its needed, and what to do to eliminate almost everything if possible.
If there are any how to threads on here to eliminate the emissions system, I would be happy to learn what I can from those who have already done it.

[VileZambonie]

Do you have emissions inspections where you live?

[Deathscythe]

No, I do not. I've been doing some looking into it and figured I'd just need a few things.
Various hoses to replace and return vacuum lines.
EGR block off plate.
Heat riser delete fitting.
Thermostat Housing from a non emissions 305.
Smaller alternator belt.
Bolts to put in place of the air pump lines.

Sounds about right?

I would just have to run lines for the PVC, brake booster, transmission, vacuum advance and one for the cruise control if I decide to keep that. Doesnt seem too overly complicated to eliminate the emissions in one of these.

[VileZambonie]

I would advise that you catalog pictures and box everything up after you remove it. If you ever sell or want to revert to original, there's that option. Or someone who lives in an emissions state may want to buy your setup.

Essentially the HEI vacuum advance, HVAC controls, brake booster, PCV

[Deathscythe]

The HVAC is the small line that runs into the firewall near the heater core correct?

I've seen videos of guys just hacking everything off with a saws all, just ripping it apart. I'm not going to do that, who's knows what the future holds, I may have to put it all back on at some point.

[Henry]

Hi DS:
Yeah, a small vacuum line (typically hard plastic) that goes through the firewall near the heater is for the air flow control doors on the HVAC system.

Yeah, you should keep everything in storage just in case of who knows what...if you have storage space. In addition, you should make notes of where those lines go into the carb as I recall the Rochester carb connections being many and confusing as to what goes where.

What about your evap cannister?...if you decide to remove it you should be careful as to what you do...you cant just block off the line from the fuel tanks...you need this vent for the fuel system to work properly...I would recommend you keep it hooked up at least as long as you keep the oem Rochester carb.

Regards,
Henry

[Deathscythe]

As far as all of the vacuum lines and carburetor, it all looks factory, all of the vacuum diagrams I've seen are spot on.
I hadn't really though about the can yet, figured I'd just disconnect whatever runs to it that I'm going to pull. Does the can require a vacuum source to work properly for the fuel line? If so, I'll probs ly just tie it in to the brake booster vacuum line.