I would appreciate any opinions on this vacuum test. Car is corolla 2001 1.8 engine with the code p0171. While the needle is in the normal green range, it fluctuates rapidly with very small movement. Engine has a very minor barley noticeable vibration while idling but runs fine. Please view the video i have uploaded Youtube video for the Vacuum test

Thank you!

1 Answer 1


The fluctuation means a problem with a valve not seating properly or valve that is burned or warped. I can hear a valve tapping noise in your video (starts about 0:10 sec.). It's time for a compression check to confirm. I just did a vacuum test on my own engine. Here is the page from my shop manual that interprets the result of the test:

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  • Hi Carguy. Thank you again for replying and posting the image. The ticking noise you hear comes from the fuel regulator area and i don't hear it at all with cold starts until the rpm goes down when engine gets warmer then i can hear it. I can see that it might be a valve issue but couldn't it be the spark plugs gap like figure #12? Thanks!
    – Flamenco77
    Commented Dec 9, 2020 at 7:31
  • Sure, check the spark plug gaps. No matter what, however, the tick doesn't sound normal. Hold a long, flat head screwdriver to the fuel regulator (and other parts of the engine) and press the handle to your ear. Hold the screwdriver again each individual exhaust pipe at the head. You might be able find where exactly the tick originates. Check for exhaust leaks (see the ChrisFix video on YouTube). I would resolve the tick first before troubleshooting elsewhere. Given the age of the car, definitely check cylinder compression.
    – Carguy
    Commented Dec 9, 2020 at 21:29
  • Carguy you are a godsend. I don't know how to thank you for taking the time. Ok, so i did the screwdriver test and that clicking is 200% is coming from the Purge valve. The clicking will not happen with cold start until the rpm goes down ( about 5 minutes ) then it will start making the ticking sound. as a side note i added the Lucas upper cylinder lubricant and injector cleaner 2 weeks after i bought the car.
    – Flamenco77
    Commented Dec 9, 2020 at 22:49
  • I have always thought that the ticking sound is too loud so i asked in a different forum and googled and some said it is normal. Now i read that the failure of purge valve can lead to lean condition which is my problem plus it will bring the tank fumes to the engine which is my second problem. Do you think this is the culprit??? That minor vibration on idle will always start when the loud ticking begins.
    – Flamenco77
    Commented Dec 9, 2020 at 23:07
  • Ok. I got a used Purge valve & MAF sensor from junk yard. Replaced purge valve and still no change and guess what? makes the same ticking noise. I replaced the MAF too and didn't make a difference. I got em cheap so i thought i would give it a try.
    – Flamenco77
    Commented Dec 10, 2020 at 1:29

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