GM Corvette and F-Body Tuned Port Injection Vacuum Circuit Explained 

Great, glad to hear👍.

Hey buddy, glad to hear that you have been making progress and it's been helpful. Let me know if I can do anything else👍

Excellent, man! Glad I could be of assistance👍. Let me if you need anything else.

No problem, Brian. Glad to help.

Thanks Paul, I appreciate the clarification. I'm glad the video was helpful for you👍

Thanks for the feedback, 87-92 Formulas are one of my favorite cars outside of Corvettes. The look just screams muscle, and they sound great rolling down the highway blasting some Master of Puppets👍 For those that haven't seen the 88 Yellow Gold here is a playlist with details of the car - ru-vid.com/group/PLAM-V1w3acphkZJyLroCKcyLO9GzqR6q4

Mark, glad I was able help you out. Sounds like a great car you have, I'm sure it's a fun cruiser.

No problem👍

Thanks for the feedback, glad you enjoyed it.👍

No problem, glad to do it👍

No problem, glad to do it. Those lines are easy to miss until they don't want to function correctly. Thanks for watching and have a good weekend👍

@@VetteSource After watching I went through mine for shiggles and found 3 open vaccum lines, one to cruise was just sucking wind, one to that t junction for cannister was trash and one to intake itself.. Gracias for your detailed explanations again.

No problem, my friend. Glad to do it👍.

Yep, that's a hard one to locate on the f-bodies. It'll drive you crazy looking for it.

Thanks, will do. I am starting a TPI tear down/rebuild shortly. Keep an eye out for those videos starting soon. I also have two other TPI videos if you haven't seen those, here are the links - ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-JLpVjkR-26E.html ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-TjkGLb_DYPk.html Thanks for watching👍

You're welcome. I'm glad to hear it was helpful Clarence👍.

yea, those intros are definitely annoying. Reminds me of the carnival barkers trying to draw you in to their attraction so they can separate you from your $$$. In answer to your question, yes I have been fortunate enough to own a 1990 and a 1991 ZR-1 in the past. They are incredible cars, extremely fun to drive, with one of the coolest looking engines ever built. As a matter of fact, if you watch the end of my Corvette Spring Cleaning video - ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-YAj18yfXfyU.html you will catch a glimpse of a ZR-1 LT5 engine I have sitting in the corner of the garage. TTYS

@@VetteSource wow the LT5 looks beautiful, I saw it in your spring cleaning video. Are parts getting hard to find for the LT5?

David, thanks for the feedback, I appreciate it👍. The best resource for detailed information and diagrams for your car is still the 1986 factory service manual. They are a wealth of information, and can be found on ebay and Amazon quite frequently. Let me know if you need anything else.

@@VetteSource thanks for the quick response! Ok, good to know. I already ordered the 86 factory service manual. I haven’t gone through the entire manual yet, but I did look for something covering the vacuum system and hoping to find mapping or part identification of sorts. Any direction in that very beefy wealth of information?

I believe it's in the 8A Emissions and Driveabilty section and should have some diagrams showing component location. Let me know if you don't see and I will check out my manual later.

@@VetteSource so I’m still working this never-ending project. I currently have the plenum, runners, valve covers and fuel manifold removed. I do have the factory service manuals and there are a lot of individual vacuum sections in 8A, but unless I’m overlooking something, there is nothing that maps out the system. Even what there is really not detailed enough to even show the back of the plenum like you’re detailing in your video. There is a whole lot of deciphering needed with the diagrams. To make it substantially more difficult (not that it wasn’t already difficult), someone has done a number on mine before I got it. The front left wheel well area has been gutted, lines being left open. The air pump is in place but disconnected; the EGR is in place but disconnected:left open. The 2 switch valves near AC compressor have been removed. The heater core switching valve disconnected. I’m trying to make heads or tails of what’s left and what needs to be replaced. I found a website that has colored diagram mapping bro corvette vacuum systems, but they don’t have the C4. This has been an incredibly frustrating process. I’ve been working so many systems and not at a work stoppage. I avoid this vacuum system because of the overwhelming mess but it’s getting to the point where I won’t be able to move forward without figuring it out.

I have a 1986 C4 Corvette and I'm hoping everything on mine is still in place. Lol. Let me know if any pictures or video of mine would help you out any? Stock 350 TPI Base model. I haven't changed much on it since I got it.

S.B., thanks for the feedback, I appreciate it. The MAP system is a little better than the MAF system at regulating engine airflow and spark when you have high performance pieces like cam/headers, heads, or intake installed. You will see these conversions installed when the owner has made performance mods to the engine so they aren't riding around with the Check Engine light blinking at them all the time.

@@VetteSource Ty for replying. After staring at it all day it looks to me like it still has maf system,not sure why chip was sitting in car, maybe someone had to replace it. Again your videos are awesome at explaining every detail and the 7.75 rebuild cant come soon enough enough lol.

Thanks, appreciate the feedback. Let me check with some of my connections and see if there are any cars floating around out there. Do you have a preference of year or color?

@@VetteSource 87 to 90 if that's not too difficult. Back many years ago I test drove a 87 with 305 TPI yellow like yours, 5 Spd, it was fast, it impressed me back then. There must be someone out there that bought one and sat on it. Much appreciated you doing this. Thank you

That is a tough one. I would suggest disassembling the throttle body to check for any internal vacuum leaks around the IAC housing. They have been known to develop cracks in the aluminum with age.

That's it. I knew there had to be one in there, just couldn't remember where. On the Corvettes it's in the driverside inner fenderwell in plain view. Thanks for watching👍

That is interesting, maybe a case of excessive vibration?

Glad you enjoyed it👍.

Paul, thanks for the feedback👍. Currently working on methods to make the videos more stable while maintaining the POV look and feel.

@@VetteSource Have to agree with Paul, love your videos, found you tonight, however, the sudden movements are very difficult to watch, one becomes dizzy from motion sickness.

Yep, that's has been a struggle for me. Trying to figure out how to do the walk-arounds and keep some stability has been a challenge.

No problem, glad to do it. Thanks for watching and have a good weekend👍

No problem, glad it was helpful👍

Thanks, appreciate the feedback👍

No problem. Glad to be of assistance👍

Thanks for the feedback. The bracket you are looking for is installed underneath the back two intake bolts on the passenger side. I point it out in this video here - ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-vZCgHAHTObY.html Let me know if you need any further information

@@VetteSource thank you that was very helpful , regarding ignition coil bracket !

Done👍.

Hey Dustin, the T junction will feed back directly into the engine harness that runs through the firewall to hook directly into the interior HVAC control that distributes the vacuum to the individual blend doors.

Hey Oswald, thanks for the feedback. Glad it was helpful. Dorman makes a GM/Ford vacuum line repair kit with fittings. I believe it should still be available at your local parts store. Let me know if you have trouble locating it and I can point you in the right direction👍.

Have you checked to see if any trouble codes are popping up in the system? An erratic idle can be symptomatic of several different problems, so a trouble code would help to start narrowing it down.

It's possible, but that particular symptom can be caused by one of many things. I would recommend investing in a factory service manual for your car. It has all the information and diagnostic information you need to keep it running properly.

Hey Jeff, the vacuum line should have a T junction and then feeds into the main engine harness that goes through the firewall into the passenger compartment.

@@VetteSource Much appreciated, had the car stored while I was disabled for some time and the mice seemed to really enjoy the vacuum lines. Thank goodness it was just the vacuum lines. Will try to get out to my shop and see if I can hopefully find some part of a line going into the firewall there. My 1989 GTA is slowly returning to it's former glory days and running again.

That's the spirit, slow and steady gets you across the finish line every time👍.

I may be remembering the f-body routing incorrectly, but I think there is a T-junction under the water pump like the Corvette with one side going to the heater core, andd the other to the oil cooler. I would have to look at the diagram again to refamiliairize myself. I would suggest looking for the correct year model GM service manual. I know for sure the diagrams are in there.

I've never tried starting the car with them totally disconnected. There would be enough of a vacuum leak at the plenum to probably cause it to run very poor at the least and maybe not start at the worst.

The vacuum circuit for the MAP sensor connects directly to the rear most vacuum port on the side of the intake plenum, directly in front of where the distributor cover attaches.

Hey David, I don't have a video made for that particular scenario yet, but with the checks you have already performed and with the car firing up with starting fluid, I would look closer at the fuel injector harness connectors to make sure they are pulsing when cranking. Sometimes the ECM injector driver circuit will fail causing a similar situation to what you are describing. Let me know how it turns out👍.

Sure, which areas do you need me to highlight?

Hey Heath, have you pulled the codes from the ECM and checked the basic components yet?

Off the top of my head, it seems like your crankcase is pressurized for some reason. Do you have the PCV system disconnected, or have you checked the condition of the individual components?

Dorman makes a repair kit for these vacuum lines. They are available at most auto parts stocked under hard vacuum line repair kit.

yep, the Corvettes are just a little bit different due to the location of the evap canister and cruise control vacuum module. Instead of having a direct connection at the front under the throttle body it is a T junction that runs off the driver side and feeds into the front wiring harness.

Yep, that should be tye default position with no vacuum circuit hooked up to it.

@@VetteSource Thank you!

I don't have my service manual in front of me, but if I am remembering correctly, you should be able to hook it to the rear vacuum port on the center plenum to get a good reading.

👍

Yep, that is correct. You will lose the accessories and HVAC system which all require vacuum input.

It actually feeds into the main wiring harness that goes through the firewall to the HVAC controls. It's easy to miss when everything is installed still.

Brian, that hooks into the underside of the throttle body, at least on.the corvette. I believe it's the same on the f bodies.

It should be directly connected to the plenum, and doesn't share the vacuum circuit with any other accessories. I don't have my diagram in front of me, but IIRC the rear most vacuum port on the plenum is used for the HVAC/cruise control/vacuum tank.

@@VetteSource thank you for the reply. Thats what i thought but in putting that circular hose there i'm still trying to figure out where these other 2 vacuum lines go. Thats the confusing part. I replaced my fuel regulator because i was noticng raw fuel coming out of my tailpipes and after i replaced the regulator it still spits out that carbon soot out of the pipes. So ure saying i should place that circular line on that port ?

Try that first, but with raw fuel exiting the exhaust pipe you may have a torn diaphragm within the fuel pressure regulator itself. Something is allowing too much fuel to enter the combustion chambers. Let me know how it goes.

@@VetteSource ok i was thinking it might have been one of the fuel injectors placed in the head wrong because the guy had to remove them to replace the manifold and i don't think they were lubed up before being placed back in the manifold and they look like the factory injectors so i ordered new ones to replace those just in case while i was down in there anyway. But while i'm replacing them i will check the fuel regulator for tears.

Good idea. Let me know if you need anything else.

Hey Ron, lots of possibilites once a crate engine is installed, but first thing I would do is hook it up to a professional scan tool and see what the ECM is doing in real time. That will give you some insight and clues as to what to look for. Off the top of my head I'm wondering if you have a software problem between the fuel injectors on the crate engine and the factory ECM settings. Let me know how the troubleshooting goes👍.

@@VetteSource done that! My issue is not enough vacuum at idle

Ok, sounds like you could have an EGR circuit problem. How much vacuum are you pulling at idle? Does it have a pretty big cam?

You mean under the intake manifold plenum?

@@VetteSource yeah.

Oh, the one at the rear of the plenum in front of the distributor. That's the fuel pressure regulator.

No problem, the MXO simply spells out your car came equipped with an automatic overdrive transmission, and the MD8 identifies it as a TH700R4 transmission

@@VetteSource Thats just your average gm 4 speed automatic? The one thing ive noticed is people dont talk very highly about the 700R4's. Mine shifts like a dream, there is absolutely no play between gears. And of course I wouldnt know if a shift kit was installed but it gets a mean second

Yep, that's the corporate GM unit. I'm not sure why people talk it down. Its basically just a meat and potatoes automatic overdrive unit. They put them in about a billion GM cars, and they are extremely durable.

@@VetteSource Can you point me to a place to get parts for the Borg Warner rear end for this car. Can you get originals or just reproduction parts? I know having the 3.27 does not help my cause as I have heard its about the rarest of the bunch that were available...Of course .. I want to keep it original with aluminum calipers and wut not

You know, quite a few of these items are still available from places like Rock Auto and other similar online vendors. I think another manufacturer took over the Borg Warner tooling for the gearsets and rebuild kits, but I can't remember the name off the top of my head.

Sure, glad to help. You can reach me by phone @832-654-7367 or email vettesource@hotmail.com

I have a 91 with that problem also

Yea, common problem that can be frustrating to run down sometimes. I would suggest starting with hooking a scanner to the car while it's running to check senor outputs to see if they are out of whack. Most common issues with the erratic idle are the MAF, IAC, TPS, or a combination of the three.

It ended up being the computer

Ah, yes. The other problem area I forgot to mention. 😁

That's the vacuum reservoir and it acts as a vacuum storage device while the engine is in operation. They have been using them since the 60's, and have slowly figured out how to make them smaller as the years have progressed.

What feeds off of it?

The EGR valve and solenoid, HVAC, and cruise control.

I still have to reinstall mine , just for hvac only . Carbed

Ok, gotcha. Yep you will need it to activate the blend doors so the A/C and heat will blow from the desired vents.

Hey Noah, thanks for the comment and heads up. Funny how certain items will be in different places than you expect them to be even by the same manufacturer

It’s below the battery tray area on the driver side, globe type canister

Mac, thanks for sharing the info👍. On the Corvettes it's easily viewable from inside the engine compartment, so it's interesting that this one is somewhat hidden between the inner fender and front bumper.

Those will work for sure. The reason they used the hard plastic originally was to protect from the underhood heat. Of course the hard plastic lines also eventually fail. Dorman makes replacements that you can find in the HELP section of the local auto parts store.

Cool, which one?

Yep, the same MAP sensor was used on many of the 80's-90's GM vehicles.

Are you talking about where it feeds back into the wiring harness going into the firewall?

Both

After routing through the check valve it feeds back up towards the driverside front of the car and terminates at the vacuum reservoir.

@@VetteSource thank you very much

NP, glad to help👍

I hear ya. The hoses actually serve different purposes between the Corvettes and F Bodies. On the Corvette there is a T junction that runs to the vacuum reservoir, and on the F bodies that port services the EGR switch? if IIRC. I haven't looked at it in a few months and I'm forgetting off the top of my head.

I'm sure that's a shocker when you pop the hood👍.

Yep, realized that after I made the video. Funny thing is that the routing of that line is different between Corvettes and F Bodies. On the Corvette the line runs out the driverside to the vacuum canister mounted in the inner fenderwell. On the F body it runs straight back to the EGR switch if I am remembering correctly and then tees off to the vacuum canister. Thanks for watching👍