Erratic idle question 🤔
November 20th, 2021, 04:38 PM
#81
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Thanks for following along on this, Kenneth. Yes, you're correct.
November 20th, 2021, 04:43 PM
#82
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Kenneth - Honestly, I was hoping we wouldn't go where you're going at this point in time; but, at this point I really was only hoping to get the vacuum hose configurations at least correct - regardless of ported vs. manifold vacuum. We can test that later, I know where you're going and I'm aware of the history of the two (ported vs manifold) relative to cam sizes; but again, just hoping to get the hoses configured correctly for the Xmas delete scenario and IMO, I think that ATM MODULATOR TEE has to go. I'm not "all-in" on why that TEE is in there. Any ideas? I think maybe a PO thought they were replicating a non-AC OEM build of vacuum hoses or did a PO elect to just move the TEE which was already there for the Xmas tree off the ATM MODULATOR line? And, who knows when the Holley was put on - before or after the Xmas tree delete? The mysteries of unraveling a puzzle.
November 20th, 2021, 04:47 PM
#83
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Ah, maybe I'm thinking now (a novelty). A PO knew the firewall AC vacuum tank was direct to intake manifold and they installed the ATM MODULATOR into that AC vacuum tank vacuum line; because why they had no other source?
Dave, I didn't look up in the thread, but is the firewall AC vacuum tank line connected to a full intake manifold vacuum source? Either on the intake manifold or the carburetor (under the throttle plates at the carburetor base plate)?
EDIT: OK, got it. Dave said:
I'm assuming the hose coming up from underneath is the ATM MODULATOR vacuum hose, correct?
EDIT OF EDITS: Yeah, that's what he means, the ATM MODULATOR vacuum hose.
Dave, I didn't look up in the thread, but is the firewall AC vacuum tank line connected to a full intake manifold vacuum source? Either on the intake manifold or the carburetor (under the throttle plates at the carburetor base plate)?
EDIT: OK, got it. Dave said:
Here's my setup Norm...notice the hose coming up from underneath goes into a tee which then splits off to the AC canister and the other hose goes to the intake.
EDIT OF EDITS: Yeah, that's what he means, the ATM MODULATOR vacuum hose.
Last edited by Vintage Chief; November 20th, 2021 at 04:52 PM.
November 20th, 2021, 04:59 PM
#84
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I don't know the answer to this question but I'm just questioning the integrity of the AC vacuum tank (and the ATM MODULATOR vacuum hose TEE) - is there any influence whatsoever regarding vacuum when an AC compressor has been removed from a car equipped with AC? I guess I'm not aware of any, but I still remain suspect of that erratic swinging vacuum gauge at idle - that isn't normal nor is it correct. That needle should be steady - why isn't it? I think it's related to vacuum but where. What about the other side of the firewall AC vacuum tank to the control? I don't think that hose requires integrity relative to engine vacuum since I believe the firewall AC vacuum tank is separated by a diaphragm isn't it? Can a bad firewall AC vacuum tank (cracked/chipped/broken diaphragm) yield unstable (erratic) vacuum at idle?
November 20th, 2021, 05:02 PM
#85
Oldsdruid
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OP- can you post a photo of all vacuum fittings or threaded plugs on the intake, behind and in front of the carburetor and in the coolant crossover passage? Also what brand of intake manifold you have. It will help us figure out where to make vacuum connections.
Last edited by rocketraider; November 20th, 2021 at 05:05 PM.
November 20th, 2021, 05:04 PM
#86
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Previous Owner's Stamp. A very simple explanation.
OP- can you post a photo of all vacuum fittings or plugs on the intake, behind and in front of the carburetor and in the coolant crossover passage? Also what brand of intake manifold you have. It will help us figure out where to make connections.
OP- can you post a photo of all vacuum fittings or plugs on the intake, behind and in front of the carburetor and in the coolant crossover passage? Also what brand of intake manifold you have. It will help us figure out where to make connections.
November 20th, 2021, 05:15 PM
#87
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I’ll say it again since I think my suggestion was lost in countless posts.
Remove ALL vacuum lines from the carburetor and cap off the carb ports.
Doing this will show if there is any difference in operation (there shouldn’t be on a correctly operating engine).
If there is a difference, it is due to one of the external vacuum accessories.
If there is no difference, then there is an internal issue (engine, carburetor, or ignition - and we can sort that out later).
Remove ALL vacuum lines from the carburetor and cap off the carb ports.
Doing this will show if there is any difference in operation (there shouldn’t be on a correctly operating engine).
If there is a difference, it is due to one of the external vacuum accessories.
If there is no difference, then there is an internal issue (engine, carburetor, or ignition - and we can sort that out later).
November 20th, 2021, 05:22 PM
#88
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I’ll say it again since I think my suggestion was lost in countless posts.
Remove ALL vacuum lines from the carburetor and cap off the carb ports.
Doing this will show if there is any difference in operation (there shouldn’t be on a correctly operating engine).
If there is a difference, it is due to one of the external vacuum accessories.
If there is no difference, then there is an internal issue (engine, carburetor, or ignition - and we can sort that out later).
Remove ALL vacuum lines from the carburetor and cap off the carb ports.
Doing this will show if there is any difference in operation (there shouldn’t be on a correctly operating engine).
If there is a difference, it is due to one of the external vacuum accessories.
If there is no difference, then there is an internal issue (engine, carburetor, or ignition - and we can sort that out later).
November 20th, 2021, 05:50 PM
#89
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Wow...all good suggestions from you guys. I will have to go back and respond accordingly, but let's put this to rest for tonight and get back at it tomorrow...Reveille for me is 3 15 am so I will be hitting the hay soon😀
November 21st, 2021, 12:46 AM
#90
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So here's something I thought about last night as I was drifting off to dreamland...
Suppose we get this figured out and are able to find the gremlin that's causing the issue...what will I stand to gain other than the satisfaction of figuring it out? Will I gain performance and/or fuel mileage? Because as I've stated already, it runs fine and the erratic idle doesn't happen all the time. Matter of fact, here's a video I shot last night after getting home from work, and you can see the tach is pretty steady. Not saying I'm ready to close this chapter, but wondering if all the guessing and head scratching is gonna be worth it.
Suppose we get this figured out and are able to find the gremlin that's causing the issue...what will I stand to gain other than the satisfaction of figuring it out? Will I gain performance and/or fuel mileage? Because as I've stated already, it runs fine and the erratic idle doesn't happen all the time. Matter of fact, here's a video I shot last night after getting home from work, and you can see the tach is pretty steady. Not saying I'm ready to close this chapter, but wondering if all the guessing and head scratching is gonna be worth it.
November 21st, 2021, 02:26 AM
#91
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Since I do not know the availability of your ported vacuum sources and your full intake manifold vacuum sources, you'll have to guide the next part of the discussion with your knowledge of those ports and their availability. I suggest you review your Holley carburetor ports, but if you tell me the model number of your Holley, I can find a diagram to help with the routing, as well.
So, in essence you'll need a full vacuum source for your ATM MODULATOR. The ATM MODULATOR vacuum hose is connected to a full intake manifold vacuum source; and, the distributor vacuum advance vacuum hose is connected to a Holley carburetor ported vacuum source and the vacuum tank on the firewall should be connected to a full intake manifold vacuum source. NOTE: Full intake manifold vacuum sources can be located anywhere below the throttle plates of the Holley carburetor. The reason they are located there is because the throttle plates provide no interference\influence with full intake manifold vacuum source because the port is located below the carburetor throttle plates & directly over the intake manifold. Anything above the carburetor throttle plates is considered a ported vacuum source.
So, Dave at this point I have my suspicion the TEE you see in the vacuum tank mounted on the firewall is in appropriate for your engine vacuum hose routing. That TEE should be removed and the firewall vacuum tank should be sourced directly to an intake manifold full vacuum source with the ATM MODULATOR connected directly to a full intake manifold vacuum source. If I think about this a second, I don't know if it's even needed at this point since you have no running AC. None-the-less, if you're going to have a vacuum hose connected to the firewall mounted vacuum tank it needs to be a dedicated vacuum hose connected directly to a full vacuum source.
So, in essence you'll need a full vacuum source for your ATM MODULATOR. The ATM MODULATOR vacuum hose is connected to a full intake manifold vacuum source; and, the distributor vacuum advance vacuum hose is connected to a Holley carburetor ported vacuum source and the vacuum tank on the firewall should be connected to a full intake manifold vacuum source. NOTE: Full intake manifold vacuum sources can be located anywhere below the throttle plates of the Holley carburetor. The reason they are located there is because the throttle plates provide no interference\influence with full intake manifold vacuum source because the port is located below the carburetor throttle plates & directly over the intake manifold. Anything above the carburetor throttle plates is considered a ported vacuum source.
So, Dave at this point I have my suspicion the TEE you see in the vacuum tank mounted on the firewall is in appropriate for your engine vacuum hose routing. That TEE should be removed and the firewall vacuum tank should be sourced directly to an intake manifold full vacuum source with the ATM MODULATOR connected directly to a full intake manifold vacuum source. If I think about this a second, I don't know if it's even needed at this point since you have no running AC. None-the-less, if you're going to have a vacuum hose connected to the firewall mounted vacuum tank it needs to be a dedicated vacuum hose connected directly to a full vacuum source.
November 21st, 2021, 02:27 AM
#92
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Previous Owner's Stamp. A very simple explanation.
OP- can you post a photo of all vacuum fittings or threaded plugs on the intake, behind and in front of the carburetor and in the coolant crossover passage? Also what brand of intake manifold you have. It will help us figure out where to make vacuum connections.
OP- can you post a photo of all vacuum fittings or threaded plugs on the intake, behind and in front of the carburetor and in the coolant crossover passage? Also what brand of intake manifold you have. It will help us figure out where to make vacuum connections.
November 21st, 2021, 03:51 AM
#93
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So let me re-iterate...my only erratic reading is at the intake port, and this is the port where the ATM modulator hose connects after the tee at the AC vacuum canister. If the modulator hose is disconnected when I run the test at the intake port, then the modulator portion of the vacuum system is taken out of the equation, correct?
November 21st, 2021, 05:33 AM
#94
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So let me re-iterate...my only erratic reading is at the intake port, and this is the port where the ATM modulator hose connects after the tee at the AC vacuum canister. If the modulator hose is disconnected when I run the test at the intake port, then the modulator portion of the vacuum system is taken out of the equation, correct?
Dave - I don't suspect you have a serious issue with your engine's performance. I don't suspect your erratic vacuum at idle is a significant hindrance in either performance or fuel economy. At any time you want to discontinue the process of troubleshooting the erratic vacuum at idle, we can stop - your call. It's good you brought it up, you have an insight into an issue which may or may not effect performance as you move forward in time with your vehicle. Knowledge of the current operation is always helpful to assist in understanding why your car performs the way it does. If you elect to hook up the AC in the future, have an issue with timing, acceleration, or any other perturbation to the system - you've gained some knowledge to assist you at another point in time. Your car came with the OEM Xmas tree, you car came with a OEM Rochester Q-Jet, your car came with a OEM iron manifold. Ascertaining if the vacuum hoses are configured correctly is the first process in determining/troubleshooting if they are suspect regarding your erratic vacuum at idle.
If you want to discontinue the troubleshooting that's fine. You have an anomaly. You'll most likely enjoy replacing your combination valve anyway so maybe you should steer in a different direction.
November 21st, 2021, 06:03 AM
#95
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Yes, most likely. Both the modulator and the AC vacuum tank need an independent full intake manifold vacuum source - IOW, each device needs a separate full intake manifold vacuum source.
Dave - I don't suspect you have a serious issue with your engine's performance. I don't suspect your erratic vacuum at idle is a significant hindrance in either performance or fuel economy. At any time you want to discontinue the process of troubleshooting the erratic vacuum at idle, we can stop - your call. It's good you brought it up, you have an insight into an issue which may or may not effect performance as you move forward in time with your vehicle. Knowledge of the current operation is always helpful to assist in understanding why your car performs the way it does. If you elect to hook up the AC in the future, have an issue with timing, acceleration, or any other perturbation to the system - you've gained some knowledge to assist you at another point in time. Your car came with the OEM Xmas tree, you car came with a OEM Rochester Q-Jet, your car came with a OEM iron manifold. Ascertaining if the vacuum hoses are configured correctly is the first process in determining/troubleshooting if they are suspect regarding your erratic vacuum at idle.
If you want to discontinue the troubleshooting that's fine. You have an anomaly. You'll most likely enjoy replacing your combination valve anyway so maybe you should steer in a different direction.
Dave - I don't suspect you have a serious issue with your engine's performance. I don't suspect your erratic vacuum at idle is a significant hindrance in either performance or fuel economy. At any time you want to discontinue the process of troubleshooting the erratic vacuum at idle, we can stop - your call. It's good you brought it up, you have an insight into an issue which may or may not effect performance as you move forward in time with your vehicle. Knowledge of the current operation is always helpful to assist in understanding why your car performs the way it does. If you elect to hook up the AC in the future, have an issue with timing, acceleration, or any other perturbation to the system - you've gained some knowledge to assist you at another point in time. Your car came with the OEM Xmas tree, you car came with a OEM Rochester Q-Jet, your car came with a OEM iron manifold. Ascertaining if the vacuum hoses are configured correctly is the first process in determining/troubleshooting if they are suspect regarding your erratic vacuum at idle.
If you want to discontinue the troubleshooting that's fine. You have an anomaly. You'll most likely enjoy replacing your combination valve anyway so maybe you should steer in a different direction.
November 21st, 2021, 06:20 AM
#96
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Ya know Norm...as I was driving home last night I thought about it, and came to the conclusion that it's a matter of priorities, and the combo valve is where my thought process led. So, for now, we'll put this on the back burner until I get a chance to revisit the matter. Besides, if you look at the video I posted earlier this morning, you'll see that she holds pretty steady...it's just every once in while I get that 200 rpm swing from 800-600 and back again.
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