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73-87 Chevy _ GMC Trucks => 73-87 Chevy & GMC Trucks => Topic started by: Deathscythe on August 23, 2018, 06:40:33 pm
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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.
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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.
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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
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See, this is why I love this forum. Thank you!
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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
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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.
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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.
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That is why i said it doesn't sound like your issue. Glad you got it fixed.
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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.
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Do you have emissions inspections where you live?
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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.
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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
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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.
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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
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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.
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I'm still not 100% done figuring out exactly everything I need to do to make sure everything will work when I'm done.
As far as I can tell so far, is I'll basically pull the EGR, get a block off for it, pull the heat riser valve, get a spacer for it, remove the thermostat housing and replace with an older model one without the vacuum ports, pull the air pump and lines, replace the bolt holes with something that matches the thread and pitch of the line bolts.
Once all that is done, I'll run constant vacuum from the intake manifold to the brake booster and transmission, one from the PCV port on the carb to the PCV, one line from the distributor port to the vacuum advance and figure lines for the vacuum reservoir, hvac and charcoal canister. Then cap whatever is still open on the charcoal canister.
Sound about right?
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Hi DS:
PCV: just make sure you still keep a vent on the pass side valve cover for the PCV system if you dont run it back to the air cleaner with a hose.
Evap cannister: as time passed, these cans got more sophisticated in their designs in that there was a valve introduced into the can that timed when venting occured. Early cans just had 2 hoses (one from tanks and one to carb or intake) and later cans like I am sure you have had many more hoses and maybe even a electric solenoid. I am sorry but I am unfamiliar how yours works...but I am sure that if you disable it, just make sure the hose from your tanks stays vented to atmosphere. You probably have a vent line from the float bowl of the carb to the cannister as well and I dont think this line needs to be vented to atmosphere...it can be plugged at carb...I think the Q-jet carb has other vents for the float bowl.
Other than that, the rest of your mods sound ok to me.
Regards,
Henry
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I have been reading up on Qjet carbs and associated lines, I will probably be replacing the lines instead of getting rid of them, just for peace of mind.
The spacer for the heat riser is easy to find online but pretty pricey for what it is, I'll probably just pull the old one, pull the guts and just weld any holes shut and put it back in. That'll save me around $60.
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Hi DS:
Evap cannister: I misspoke about how the can functions...there is a timed "purge" and not a timed "venting" (purge is usually at a certain engine temp). The can always allows fuel tank venting. Yeah, it is conservative to leave it in there for now...it doesnt hurt anything, even when it does not function right. The evap cans do not rob performance or hurt fuel economy. They have a replaceable filter in the bottom and other than that there is little to go wrong except for the hoses degrading.
Regards,
Henry
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sounds like we're at similar places Deathscythe; I'm attempting to resolve oil leaks, going to a Z28 aluminum IM, and cleaning up needless emissions hoses.
here's my diagram from under the hood- 1986 C10 305ci.
My ESC has not caused problems in the past, so I'm leaving it for now as well as my original distributor (5 pin ignition module) previously there was no vac assistance connected to the distributor, but I plan to connect that.
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The spacer for the heat riser is easy to find online but pretty pricey for what it is, I'll probably just pull the old one, pull the guts and just weld any holes shut and put it back in. That'll save me around $60.
why does the heat riser need to be altered?
thx
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The spacer for the heat riser is easy to find online but pretty pricey for what it is, I'll probably just pull the old one, pull the guts and just weld any holes shut and put it back in. That'll save me around $60.
why does the heat riser need to be altered?
thx
The heat riser diaphragm is controlled by vacuum switches and lines that have been removed. In order to keep it from being in the closed position I removed the diaphragm and wired it to the open position.
In an effort to simplify everything and remove any potential problem in the future I will be removing the valve completely. Instead if buying a spacer, I will just take the valve out and weld the hole shut where the shaft goes into the valve and will reinstall it.