Vacuum Gauge/Exhaust Value Problem ?

marc hildebrant

marc hildebrant

Well-known member
Group,

I think that I have an engine with a defective valve.

I'm at 5000 ft, so that a vacuum reading should be lower...about 5 inches.

With the car at idle, I hooked up a vacuum gage to the windshield connection. The vacuum gauge moved between 12.5 to 16 inches in a regular manner.

I also saw the engine speed increase a small amount with the vacuum gauge disconnected ?

Do I have a burnt valve ?

Marc
 
Patrick,

I haven't measured the compression yet as I don't have a gauge at this time. Should buy one though.

With regard to the engine speed going up with a vacuum leak...why would that happen ?

Marc
 
Dear Marc, I think it could be a leaky valve or intake manifold , or leaky wiper gasket ( if you a vacuum wiper )
But with a leak tester or compression tester you wil get the right answer.

Gerard
 
" What made you hook up the gauge to begin with? Does the engine have a miss in it or run rough? "

Larry,

Good question. The idle is O.K. and it seems to have good power (although it's my first model a and I'm not sure what "good" is ).

I found the vacuum gauge in some car stuff from my father in-laws estate. So, being curious I hooked it up to the vacuum line coming from the intake manifold going to the windshield motor. I disconnected the line at the firewall and slipped the hose onto the line/fitting.

Oh, one more thing...when I rev the motor, the vacuum is steady.


Marc
 
marc hildebrant said:
Patrick,

I haven't measured the compression yet as I don't have a gauge at this time. Should buy one though.

With regard to the engine speed going up with a vacuum leak...why would that happen ?

Marc
Click to expand...

The engine can suck more air, thus more speed if the carb is slightly rich. If the carb is slighly lean, then any added air should make the engine speed drop.

The guage needle will flucuate due the the Model A only having 4 cylinders, so it opens an intake valve each half turn of the crankshaft. A V8 has more valve overlap between the number of intake vlaves open at any one time, so you will have a much smoother reading of the vaccum guage needle. What you are looking for is a sudden drop of the needle that seems out of place with the regular flucuations. This would indicate a slow to close valve, or a stuck valve, or a burned valve.

It's best to do a compression test and a vaccum check. A slow to close valve can give a good compression reading, but at speed higher than cranking, the vaccum guage will pick up a slow to close valve by the sudden drop of the needle each time cylinder with the slow to close valve comes around.
 
marc hildebrant said:
" What made you hook up the gauge to begin with? Does the engine have a miss in it or run rough? "

Larry,

Good question. The idle is O.K. and it seems to have good power (although it's my first model a and I'm not sure what "good" is ).

Oh, one more thing...when I rev the motor, the vacuum is steady.


Marc
Click to expand...


You are probably looking for a problem that does not exist.

But, to get an idea on expected power run a compression test and also to see IF a condition exists that might become a problem.

Then you can isolate the cylinder or cylinders if a lower than normal reading shows up and perform the oil test and pressure leak test.

Perform the test as per my instructions in the thread link below. Post #4

Compression check procedure:

Address:http://fordbarn.com/forum/showthread.php?t=4960&highlight=compression+test

Also, be advised that lower compression readings across the board doesn't necessarily mean that the engine is worn out. I've seen many engines show compression in the 50's but still run well and have plenty of power. Of course the readings were uniform or within a few pounds of each other and that is what you are looking for.
 
Last edited:
Thanks for the comments. I'll pick up a compression gauge and adapter for the Spark Plug hole.

I'll post the results when done.

Marc
 
Problem Solved.

Today, I added some Marvel Mystery Oil and the problem went away !!!!

Well, not quite.

While I did add some MMO today to the gas, I also repeated the test with a new auto Vacuum Guage. The old guage was an unknown model many years old.

At idle, I have a reading of 16 inches which is O.K. at my elevation (5000 ft).
The needle "flutters" at a very rapid rate and doesn't really move much from the steady value. I believe that Tom W. was correct in that a 4 cylinder motor is just "coarse" , in contrast to an 8 and that what I'm seeing is intake valves working and causing a small motion.

This guage has a much larger needle and most likely the larger needle "damps" the pulses better than the older smaller one.

I'll still use the compression test later.

Marc
 
marc hildebrant said:
Problem Solved.

Today, I added some Marvel Mystery Oil and the problem went away !!!!

Well, not quite.

While I did add some MMO today to the gas, I also repeated the test with a new auto Vacuum Guage. The old guage was an unknown model many years old.

At idle, I have a reading of 16 inches which is O.K. at my elevation (5000 ft).
The needle "flutters" at a very rapid rate and doesn't really move much from the steady value. I believe that Tom W. was correct in that a 4 cylinder motor is just "coarse" , in contrast to an 8 and that what I'm seeing is intake valves working and causing a small motion.

This guage has a much larger needle and most likely the larger needle "damps" the pulses better than the older smaller one.

I'll still use the compression test later.

Marc
Click to expand...


I never thought about it but there must be some differences in the gauges among manufacturers because mine does not flutter when hooked to my Model A.
 
My modern vacuum gauge also "bounces" when hooked to my A at idle (300 rpm, probably a bit slow) ... it smooths-out when the engine is speeded-up to about 600 rpm...

I wonder if some vacuum gauges have a restriction orifice to prevent sudden extremes from wrecking the gauge; have seen oil-pressure and steam gauges so-equipped...

I'll have to find my 1930's Stromberg "Engine Analyzer" , which has a vacuum gauge, and see how that behaves on the Ford...
 
Special Coupe Frank said:
My modern vacuum gauge also "bounces" when hooked to my A at idle (300 rpm, probably a bit slow) ... it smooths-out when the engine is speeded-up to about 600 rpm...

I wonder if some vacuum gauges have a restriction orifice to prevent sudden extremes from wrecking the gauge; have seen oil-pressure and steam gauges so-equipped...

I'll have to find my 1930's Stromberg "Engine Analyzer" , which has a vacuum gauge, and see how that behaves on the Ford...
Click to expand...

Yes, I've seen some vaccum guages with a very small restriction hole to reduce pulses.
 
Group,

Follow up to my note.

I measured the compression on the cylinders. 1,2,4 were 70 lbs. 3 was 65 lbs.

Looks like things are O.K.

Marc
 
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