Vacuum gauge rapid oscilation

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Dumindu
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Vacuum gauge rapid oscilation

Post by Dumindu » Sat Feb 22, 2020 11:18 am

My car is a Toyota Tercel 1987 DX Wagon with 3A engine.

My car engine sound is smooth but not upto the expected level. So I tested the manifold vacuum.

There is a rapid oscillation of the needle between 17 and 13 inhg.

So I have done a compression test. All the cylinders had 150 psi (desired range was 128-178).

I have tested the spark wires with a tester. All the plugs were working.

Engine timing was set with a torch at 5 BTDS.

What could be the possible cause of that oscillation of the meter?

EDIT :
I have a digital RPM meter bought recently (means that I havent experience with a digital tachometer). it shows 600, 1400, 1200, 700,... such changing values. Not a constant value. (Perhaps there may be a problem in the tachometer) The values are changing rapidly at certain occasions. Certain times it changes slowly.

Engine has a mild tik...tik ..tik... sound.
Last edited by Dumindu on Sun Feb 23, 2020 7:30 am, edited 2 times in total.

john
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Re: Vacuum gauge rapid oscilation

Post by john » Sun Feb 23, 2020 3:56 am

Hi Dumindu, I found this info on-line. "If the needle vibrates rapidly between 14 and 20 inches of mercury, but then steadies as rpm increases, you may be dealing with worn valve guides. How fast the needle vibrates is telling of how many valve guides may be worn."

Did you try your test at different RPM's? I hope it will be something easy. Sincerely, John

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Dumindu
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Re: Vacuum gauge rapid oscilation

Post by Dumindu » Sun Feb 23, 2020 6:19 am

Yes when RPM exceeds vibrating range drops to 18-16 range.

Problem is If the valve guides are worn, compression should be dropped. Isnt it?

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Re: Vacuum gauge rapid oscilation

Post by john » Sun Feb 23, 2020 11:55 am

You are correct Dumindu. I had forgotten that your car has excellent compression, which probably means this will be a simple fix once figured out! I use a vacuum gauge infrequently, and have trouble remembering how to interpret the results. Someone on this site will have better ideas. I did read on-line that slow fluctuation might be a problem with spark plug wires and related stuff, but you have good spark and rapid oscillation so I don't know. Best wishes with this. John

edit: just noted the part about fluctuating rpm, is that at idle? If so, might be a vacuum leak. Mine recently was stumbling and needed PCV replaced and fixed the problem. Mine was hollow, and no longer had a check ball in it.
so maybe check for a vacuum leak? John

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Dumindu
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Re: Vacuum gauge rapid oscilation

Post by Dumindu » Mon Feb 24, 2020 4:54 am

No. Your advice had a critical point. Engine has a mild tickling sound. It is mentioned that tickling sound comes from either by valves or by contacts. Perhaps valves may have a problem.

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Re: Vacuum gauge rapid oscilation

Post by john » Mon Feb 24, 2020 8:15 pm

Hi Dumindu, The valves are easy to adjust. Do you have a workshop manual? They can make a tic tic sound when a little loose. John

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Re: Vacuum gauge rapid oscilation

Post by Petros » Wed Feb 26, 2020 12:14 am

worn valve guides or out of adjustment valves will not affect compression test. neither of these affect the drivablity much, and will not harm it to run that way. if it is out of adjustment, that is an easy fix.

usually an oscillating vac gauge indicates vacuum leak. you might try a power drop down test, with the engine at a fast idle, pull each spark plug in turn. the rpm should drop the same amount (this assumes spark plugs and wires are all good). if one drops less, or not at all, that one cylinder is running lean, so look for a vac leak around that intake runner.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

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Dumindu
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Re: Vacuum gauge rapid oscilation

Post by Dumindu » Wed Feb 26, 2020 5:20 am

Again thank you Mr. Petros for your valuable advice.

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