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Old 12-04-2019, 11:26 AM   #1
alexiskai
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Default Vacuum testing to diagnose fast idle

OK, we are back to discuss my fast idle problem in a clean new thread. Previous thread is here. At the end of that thread, it was recommended that I attach a vacuum gauge to help diagnose the problem.

I'm going to start by describing how I set up the vacuum test, because it took me about a week to get all the parts together. The pictures attached here show the individual parts and then what they look like fully assembled.

The vacuum port in most intake manifolds uses a 3/8-fine thread. A vacuum gauge that you can buy in an auto parts store uses a barbed fitting. The challenge is to mate the two. I was able to find the brass fittings I needed in the plumbing section of Home Depot, but it was a challenge because the measuring convention is different (plumbing measures the internal diameter, automotive measures the external) and they didn't have exactly the right adapter.

What you see here is:
  • Vacuum gauge from O'Reilly Auto Parts
  • 1/8" brass hex nipple - this fits the intake manifold
  • 1/4" female to 1/8" female coupling
  • 3/16" barb to 1/4" male adapter

If you can find a 3/16" barb to 1/8" male adapter, that would also work, but I couldn't find that so I had to use an intermediate coupling to reduce the size.

Second picture shows what it all looks like fitted together and ready to go on the car.
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File Type: jpg IMG_2609.jpg (56.8 KB, 54 views)
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Old 12-04-2019, 11:35 AM   #2
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Default Re: Vacuum testing to diagnose fast idle

When I got the gauge plugged into the intake manifold, I started the car up and went through the same calibration procedure as before: reduce throttle, fiddle with the idle mixture screw, fiddle with the spark timing, reduce it a little more, etc.

The good news: I was able to get the RPMs down into the idle circuit, which I could tell because adjusting the idle screw caused it to stall and adjusting the GAV did nothing.

The bad news: Vacuum gauge showed continuous oscillation between 15-19. This is with the spark fully advanced and the engine warm.

Based on the guide posted in the earlier thread, I'm thinking this indicates a burnt valve or loose valve guides something with a valve. Gauge also showed oscillation when I ran the choke test (engine off, turn over with starter motor). I tried spraying PB blaster into the air intake to try to release a sticky valve, but nothing changed.

Am I correct that we seem to have ruled out everything but the valves?
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Old 12-04-2019, 11:40 AM   #3
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Default Re: Vacuum testing to diagnose fast idle

Here's the compression test result from the previous thread:

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All cylinders showed 60-65 psi except for cylinder 2, which showed 40 psi on two separate tests.
So maybe I should focus on cylinder 2's valves?
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Old 12-04-2019, 12:06 PM   #4
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Default Re: Vacuum testing to diagnose fast idle

Got a DM asking about a wet compression test on cylinder 2 – wet test showed 45 psi, not much different.
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Old 12-04-2019, 12:20 PM   #5
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Default Re: Vacuum testing to diagnose fast idle

I would check the tappet clearance on the number two cylinder.
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Old 12-04-2019, 12:22 PM   #6
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Default Re: Vacuum testing to diagnose fast idle

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I would check the tappet clearance on the number two cylinder.
Is that something I check by taking off the valve cover or by taking off the head? I've never worked on valves before, I have no idea what I'm doing.

Edit: Did some googling, looks like it's the valve cover. I found this page that describes a trick for saving time adjusting the valves.

Edit 2: Ah, it's only adjustable via the valve cover if the engine already has adjustable lifters. Not sure about mine, probably not. If not, checking the clearance would simply allow me to rule incorrect clearance in or out as a root cause; fixing it would require removing the head to get the valve out.

Last edited by alexiskai; 12-04-2019 at 12:46 PM.
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Old 12-04-2019, 01:03 PM   #7
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Default Re: Vacuum testing to diagnose fast idle

So, if I remember correctly, your original problems are fixed now? The last was the high idle right? Sounds like you got that done.



Not a huge job to pull the valve cover and take a look. Whatever the fix is, be it new valves or an adjustment, you're going to need to pull the valve cover eventually.


That fordgarage page is a good one. Nothing too difficult about it.
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Old 12-04-2019, 01:09 PM   #8
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Default Re: Vacuum testing to diagnose fast idle

There are numerous charts available to decipher your vacuum gauge readings.
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Old 12-04-2019, 01:12 PM   #9
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Default Re: Vacuum testing to diagnose fast idle

The idle is higher and less stable than it was when I first got the car. It really used to have a nice idle. Somehow I screwed it up or a problem developed (like valves). I had to advance the throttle screw more than I'd like. I was just hoping I could get closer to where it started.

I have to run it in the Xmas parade on Friday and then after that I'll have a few weeks to get the valve cover off and check those valves.

Last edited by alexiskai; 12-04-2019 at 06:59 PM.
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Old 12-04-2019, 01:14 PM   #10
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Default Re: Vacuum testing to diagnose fast idle

Let us try that again...

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Old 12-04-2019, 01:14 PM   #11
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Default Re: Vacuum testing to diagnose fast idle

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Originally Posted by Y-Blockhead View Post
There are numerous charts available to decipher your vacuum gauge readings.
OK great, see this right here, that's what it was doing. What does "loose valve guides" mean?
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Old 12-04-2019, 01:48 PM   #12
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Default Re: Vacuum testing to diagnose fast idle

The thread on the intake is "pipe" thread. You did not have to go through great lengths to plumb as you are dealing with vacuum. Easy to fab tubing etc. If I am correct you were running full advance at idle?
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Old 12-04-2019, 01:51 PM   #13
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Default Re: Vacuum testing to diagnose fast idle

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Easy to fab tubing
Says you, with your set of tools. Not easy for me.

Yes, I was running full advance that's what the vacuum test instructions said to do. I realize the RPMs will decrease if spark is retarded.
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Old 12-04-2019, 05:21 PM   #14
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Default Re: Vacuum testing to diagnose fast idle

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Old 12-04-2019, 06:34 PM   #15
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Default Re: Vacuum testing to diagnose fast idle

Your local hardware store should have a 1/8 pipe barb fitting. If the barb is not your the size get creative with vacuum and plastic hoses along with sealant and tape if needed.
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Old 12-04-2019, 11:59 PM   #16
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Default Re: Vacuum testing to diagnose fast idle

You are never going to get a rock steady vacuum reading on a Model A at idle. It should steady up as speed increases. Try the gauge at max retard. Using a tachometer with a vacuum gauge helps with diagnosis. Loose valve guides means an air leak through probably worn valve guides.

That said, the low compression reading on #2 is worthy of investigation. I could be a lot of things but lash is the first and easiest thing to check as noted. I personally doubt that low compression in one cylinder is going to have a major effect on idle, although it could cause a mis-fire.

Sounds too simple, but have you checked manifold torque, and/or gasket leak? Easy to check, and I like to check the easy things first. It is too often that we over-analyze problems.

I personally think it is a carb problem. (Or ignition as second choice.) The best/easiest way to check/eliminate carb is to swap your carb with a pal's carb that runs right, and try your carb on his car and see if you can adjust it.
Troubleshooting is an art. Trace one system from start to finish instead of guessing. Any symptom can have multiple causes which must be eliminated. For example you might even have an obstructed muffler. These things can get wacky. Back in the day, before computers, a good troubleshooter was golden in any shop. But look at it this way--you are going to learn alot about your car and that is a large part of the fun of the hobby.

Last edited by PC/SR; 12-05-2019 at 12:09 AM.
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Old 12-05-2019, 12:04 AM   #17
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Default Re: Vacuum testing to diagnose fast idle

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Originally Posted by PC/SR View Post
Sounds too simple, but have you checked manifold torque, and/or gasket leak? Easy to check, and I like to check the easy things first. It is too often that we over-analyze problems.

I personally think it is a carb problem. (Or ignition as second choice.) The best/easiest way to check/eliminate carb is to swap your carb with a pal's carb that runs right, and try your carb on his car and see if you can adjust it.
Yeah, we went over a lot of this in the previous thread. Manifold and gaskets have been checked. The carb is nearly new, from Bert's. Steve at Bert's thinks it may be the ignition; I'm waiting on a response from him to see what tests he recommends.

I ran the engine speed up to around 2000 and the reading didn't steady out very much.
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Old 12-05-2019, 01:09 AM   #18
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Default Re: Vacuum testing to diagnose fast idle

if you had a intake or carb gasket leak you would have low vacuum. Altho you have a pulsating vacuum reading it is not that low (15-19).

Trust what the vacuum gauge is telling you. With #2 low in compression I would say either leaky valves or guides in #2. And yes I think one low cylinder is going to effect idle.
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Old 12-05-2019, 08:22 AM   #19
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Default Re: Vacuum testing to diagnose fast idle

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Originally Posted by alexiskai View Post
Steve at Bert's thinks it may be the ignition; I'm waiting on a response from him to see what tests he recommends.

That just jogged my memory. My old boat with a Chevy 305 started running terribly so I went through all the checks, timing, fuel pressure, AND vacuum. The gauge read low around 13-17inHg and bounced around just like you described. From the chart I diagnosed just like you, loose valve guides.


Well, turns out it was a bad coil...
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Old 12-05-2019, 10:21 AM   #20
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Default Re: Vacuum testing to diagnose fast idle

I've actually checked out the coil already, way back in the beginning since it's such an easy thing to swap. I find a valve issue much more plausible on this engine, which hasn't been apart in 35 years and for all I know still has the original valve components.
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