Hi all! I am very new to auto mechanics, but has been an interest of mine for some time, so I finally bit the bullet about a 6 months ago and bought a 1977 Monte Carlo. It has been a fantastic car thus far, and has required very minimal work to run like a charm.

That being said, about a week ago, my engine was stalling out every mile or so. RPMs would drop quickly, sometimes I'd be able to save it if I shifted to neutral and revved it up a bit, but most times the engine would stall and power steering/braking would fail.

I was looking at it with a buddy of mine who is a bit more versed than I am from working on his own car, and we noticed that the PCV valve was completely hanging loose. I replaced the PCV valve with a new one, fitted it down the best I could, but we noticed that there are a ton of uncapped vacuum ports that we cannot make heads or tails of. I'm not sure where, if anywhere, they're supposed to connect. I can cap them off so there isn't excess air intake, but I get the feeling some of these are supposed to be connected back somewhere. We have looked up multiple diagrams but cannot seem to find a solution.

Pictures are attached to this thread with the ports circled. Please feel free to respond with any questions or advice for a beginner to all of this (very addictive) madness.

Thank you!

 

Oh boy, where do I start…
You should consult the vacuum diagram(s) in your 1977 Chevrolet service manual. If you don't have one, get one. I'm going to give you some basics but you really should consult the service manual.
Looking at the photos you posted, it looks like someone rebuilt/replaced the carburetor; they obviously didn't do a proper job of hooking everything up.
Starting with the thermostat housing, that's a temperature actuated vacuum valve. This is normally used with the distributor vacuum advance to switch the vacuum source from ported vacuum to manifold vacuum. I would need to review the service manual but I also think it's integrated with the air cleaner in some way. The seemingly random ports on your carb are part of the vac adv., heat riser valve, fuel vapor canister system. I think the E.G.R. valve connects to manifold vacuum.
Before we go deeper into this, it would help to know what your goal is with this car. Do you plan on returning the engine to its original operating condition or are you looking to do a reso-mod?

 

Thanks so much for the detailed reply and information.

My end goal is to do a resto-mod. It's definitely going to take time and money getting it to where I want to be, so just having it drivable in the meantime is ideal. It wouldn't surprise me at all if the carb was rebuilt or replaced, the original 350 was replaced with a 305 by the original owner as well.

One of the first things I had thought about doing was replacing the carburetor with a Holley Sniper EFI kit. Like I mentioned, I'm very new to auto mechanics outside of YouTube and consulting with friends and other enthusiasts, so this is a learning process for me. I'm not sure if running fuel lines and installing the Sniper EFI kit is a little too advanced given my lack of hands-on experience.

I currently do not have a 77 service manual. I am looking into getting one now. I'll dig around, there were quite a few spare parts/extras thrown in when I bought the car, so I will see if there's one collecting dust with those parts. Otherwise, I will look online for one.

I really appreciate your detailed information - hopefully it can help me start making heads and tails of all the uncapped ports.

 

You're the second person I know who's keen on doing that on a 2nd gen Monte Carlo. Holley must have have a top tier marking department.

The Sniper EFI is a throttle body injection system. These types of systems have been around for decades. Fuel injection is undeniably more fuel efficient than carburation and I personally prefer the throttle body type vs the individual injectors. They do not function with the stock fuel pump so a high pressure fuel pump with a gauge will need to be installed which adds more "stuff" in the engine bay – at least the throttle body has a regulator built in. The Sniper system adds more wiring in the engine bay which I find unsightly. Not to mention that every additional electrical doodad is another circuit that needs to be mapped out as well as another drain on the electrical system you have to account for. If you're going to do that, it's recommended to install an auxiliary fuse panel and you really should upgrade your alternator from the stock 10-SI to a 12-SI and upgrade the charging wire from the factory 10ga to 8ga. By the way, electrical issues are a whole other topic. Anyway, each additional circuit is another potential point of failure at some point down the road. I don't disagree that the benefits of the system are appealing but I also think that Holley's marketing is a bit deceitful by saying there are only four wires (Battery +, Battery - , Switched Ignition & RPM) to hook it up… but then the included oxygen sensor is easy to install; doesn't that need a wire too? Then there's the dedicated fuel pump wire; I don't know if that wire has it's own fuse – I kinda doubt it does – so, it would be in your best interest to have that wire go to a fuse panel then from there go to a power relay that feeds more current to the pump. There go a couple more wires. Since this system is computer controlled it's also going to need its own temperature sensor – yet another wire… Like I said, electrical issues are another topic entirely.

I would only select the Sniper system if I were going to pull the engine and do a full resto-mod, and in that scenario, I would go for broke and get the in-tank fuel pump retro fit, the synched distributor, etc., so as to take full advantage of every benefit this system can offer.

I can totally dig it.

If you live in a state where your car is subjected to smog inspections, you're probably going to need to replace the missing vacuum hoses, etc. If your state doesn't care, you can bypass/delete the emission systems.

Can you tell me a little more about the car? From the photos, it appears like the AC may have been converted to R134 and it appears you have cruise control?? And what's up with the ball valve on the heater hose?

 

Below is the vacuum diagram from the 1977 service manual for your reference.

 

Apologies for the delayed response! Last few days have been a bit chaotic.

So the ball valve on the heater hose... I couldn't make this up. I thought that my heater was broken because even when I'd turn on the heat full blast, all I would get is cold air. I have absolutely no idea why, but that ball valve was put in place to switch between AC and heat. So depending if I want heat or not, I have to shutoff/open the valve accordingly. It also does have cruise control, but for some reason it is currently disconnected. I have not tried to hook it back up yet as I haven't had much of a need for cruise control (just hopping around town when I drive it).

Reading through your comments about the EFI Sniper kit, I agree with what you said. The more I look in the engine bay right now, honestly, the more overwhelmed I feel. There are loose hoses everywhere, a whole mess of wires that I am trying to make sense of, etc. I don't know that adding in more electrical and complexity to this car (at least right now) is a wise decision. Making heads and tails of everything as a very new DIYer has been incredibly challenging.

Thanks for providing that diagram. That is a humungous help as well. I am going to print that out and put it up on the wall in my garage to reference back to.

I think I am going to get some painters tape and just start labeling all of the hoses and open ports as I start identifying what is what. It's like every time I look right now, I am finding another loose hose or open port. I am honestly shocked that this car was running so reliably up until a week or so ago. It seems like things are a bit of a mess.

What kind of hobby did I get myself into...

 

I was thinking about this and one easy thing to check is your distributor vacuum advance. The quick & dirty way to check it is to remove the vacuum hose that goes to the the distributor vacuum advance while the engine is running then plug the end of the vacuum hose with your thumb. If the RPMs don't change then the vacuum advance may need to be replaced. To be certain, you'll need a vacuum pump like this one:

Hand Held Vacuum Pump Tester Set Vacuum Gauge and Brake Bleeder Kit for Automotive

Hand Held Vacuum Pump Tester Set Vacuum Gauge and Brake Bleeder Kit for Automotive

If you don't have a hand pump, may as well get one. With old cars that have lots of vacuum operated servos, there are lots of places where a vacuum leak can occur. Having one of these hand pumps really comes in handy when tracking down a vacuum leak. By the way, the various air distribution doors in your AC/heater system use vacuum to open and close and there should be a vacuum operated heater control valve on the intake manifold. Considering the chaotic state of the vacuum lines in your car, your heater situation may be solved by putting all the vacuum lines back the way they were from the factory.


One that can be very rewarding. Just yesterday, I was at the supermarket and an older guy parked next to me and said, "Wow, a classic Monte Carlo. I remember I bought one new back in 1976 and me & my buddy used to drive it all over the place… wow… man, I haven't thought about that in years. These are the sweetest rides…" The guy went on for a little more before going into the market, all the while sporting a smile. You can't put a price on evoking fond memories for someone simply by looking at your car.