Alright dude i have a few things to add because you may be leading some people astray. Like me. So first of all your vacuum reading depends entirely on your Barometric Pressure/Elevation. At or close to sea level you should see a vacuum of 17-22 in/Hg. That number decreases as you go up in altitude (down in barometeric pressure). So at 3-4 thousand feet you should see a vacuum of 15-19 in/Hg. I am at 7,000 ft above sea level, my Vacuum should read 12-16 in/Hg. Keep in mind that Idle vacuum for most engines is about 18-22 in/Hg AT SEA LEVEL but some may produce only 15-17 inches idle depending on your motor. you need to refer to your cars repair manual for vacuum specs. If your valve timing is retarded, the needle on the guage will bounce due to blow by on the intake valves back into the manifold........ not a steady low pressure reading on the gauge. Under load your Vacuum should be zero at cruising speed thats how you know you have a healthy motor.
Great point Josh! What is the formula for determining vacuum at various elevations? I'm at 5K ft. here in Colorado maybe it should be between 13-18 In/Hg?
Nope, at full throttle on a gasoline engine the vacuum will be at or near (filters have some restriction) 0. any time you're not flooring it, that throttle valve is closing in order to create a partial vacuum in the intake manifold (this is how your car restricts the amount of air it ingests).
Good trivial point, if using an actual mercury gauge (which almost no one owns) which measures pressure differential. But not at all related to this regardless of gauge. Please note that there is an acceptable range. This covers pressure differences. Please also consider that the external pressure will have no effect on fluctuations or trends, which are what we look for in diagnostics.
Just purchased this gauge to check vacuum and fuel pressure wow so helpful it all makes sense but you don't think about it until someone tells you, great video mate
Superb explanation I just learned a lot from this vedio.can you please do a vedio on Dry and Wet Compression test and cylinder leakage testing.if you have time.
You need to attach it to something with direct engine vacuum, anything off the cylinder head probably wont be direct engine vacuum. Anything off the intake manifold is a good place to connect to. I like to hook it up to the brake booster line.
hi, i was hoping you could help me with the readings i got from my little mazda truck 2bbl. the needle from my vac gauge reads in between 15 and 18 unsteady. i could never get it to hit 20 in of mercury. when i rev it up, it would go back to 0 and then goes to 20 and then back to 15-18.
Big thanks for the explanation! On my 350 sbc I read at idle a flickering needle on 16hg at 600rpm. Fast idle 1200rpm and the needle is steady at 19hg. Timing is late 8 degree. I think it‘s a mix from bad valve stem seals, that also indicat the blueish puffs after a shifts and a bit more timing mabye 12 degree? Thanks
I am fully trained auto and diesel mechanic, since I started repairing cars and trucks especially gasoline engines I use vacuum gauge for diagnosing engine troubles.I appreciate your excellent demonstration.
This is by far the most simple to the point video that was so helpful and educational! Thank u for making this video and hope more people pay attention to your open your mind video.. thanks
forgot most important step - what cam do you have? stock/stage 1 this would apply, but id drop 2-3 inches per stage of cam, around stage 3 the readings get more whacky from a bad vac signal. My 288HR comp cam idles 7 inches and very few of these principles apply with radical cams. Id say around 230-240 duration and up things start falling apart.
17 - 20 Hg's for an engine with a stock OEM cam. If you have a more radical cam vacuum will be lower. In my case with a .610 lift my vacuum reads around 13 Hg's at idle then drops to 0 at high RPM.
Good video. I have the symptoms of an exhaust obstruction. Strong 20 inches, then as I throttle up (in park, no load) it starts to sputter and loses vacuum. It a simple v8, Oldsmobile 307 but it been troublesome. I’ve been through multiple sets of plug/ wires and distributors. New fuel pump filter even overhauled the motor. I guess I’m gonna check the fuel pressure tomorrow . Oh and I have no exhaust restriction. But maybe I have carburetor problems. I don’t know what else it could be.
Hi, thanks for the video. I bought this equipment to diognose potential problems on my Passat B5 TDİ 1.9 AFN engine. My problem is I want to learn the right spot to connect the equipment on my engine. Can you help me ? By the way, your videos are super🙏
How on earth did you get a vacuum reading from that hose after popping it OFF of the intake manifold??? Isn't that it's vacuum source!?! I'm generally confused now...
I'm having this done today on.my 98 dodge ram 5.9l w/ 187,000 "ticks on the clock." This explains alot for me to know if I need a rebuild or not, thanks!
Everything is new in my 68 Mustang, with a crate stock 302, runs well no issues. But the vacuum gauge shows 15. I managed to get 17 after advancing the ign. timing a little. What do you think? Should I advance it further to achieve 18 or more?
I have a 1970 Boss 429 and at 800 RPM idle the vacuum is steady 3 inch and at about 3000 RPM it has a steady 17 inch. I have a high performance Comp cam in the engine. Engine run great and doesn't have 100 miles sense rebuild. Is this normal?
Heres a scenario, my engine is at normal in the green area.....but when I rev it really quick to about 3000 rpms the gauge goes up to 25 hg's when i let go of the accelerater, the gauge slowly drops back to normal green area...but it takes at least a minute or 2 in order for it to get there.
Sounds like its starving for air when you open the throttle it allows air in wich is why I drops to zero. If it's pulling higher vaccum at higher rpm would be due to lack of air in the intake.
i have a 1600 british pinto engine am getting a vibrating reading between 13 to 15 which i believe is advanced timing ?? the car runs rich mixture but runs really smooth ( it was set up on a dyno test and it was the only way it would run smooth ( car is now 41years old ) can anyone tell me me can i fit a 7" dual booster -- what i have now is a 8" single and it works fine but the master cylinder has a problem and the parts are non existent so can i fit the 7" with an upgraded master cylinder of 15/16" original is 3/4" but running on single pot calipers but now have 4 pot calipers each side can any of you guys help me solve this problem
Hi my trucks been sitting for a year untouched ,diesel Hilux Rebuilt engine ,brake boosters not working but I have a new vacuum pump and it registers at 25-6 very steady inches of mercury (300m above sea level ) I do remember the first day I installed the engine I had brakes only ran it for a couple of minutes next morning nothing ,pedal doesn't move at all when turned on ,is it possible the diaphragm weakened over that time
Thanks for the great video! I have a 351M in a 78' F250, that starts to have white smoke after warming up, but not on first starting. After watching your video I bought a gauge and the reading on start up is a vibration between 15-16 and then after warming up, it fluctuates between 6-20. Do you think I have a head gasket leak?
steady vibration 1/2 inch between 21 and 21.5...worn valve guide? Its only 1/2 " but the head is very old, new valve seals new head gasket, all valves adjusted.
4:51 ...okay wait, so the declining gauge pressure is a sign of both an exhaust obstruction AND an air leak in the in the intake manifold? also, if you disconnected one of the vacuum hoses, what is plugging up the port that you disconnected it from?
Dude, I do love your explanations here on what can be seen with such a guage when you know how to read it and what the reading means. It's crazy how very few mechanic just don't the easy steps to properly diagnostise an engine with the proper tools to fix the right thing once and for all. Same goes with multimeters and voltage drops across the starter, block or alternator cables. I've seen stupid mechanic screaming after the parts guy that his last 3 starters were worn and a bad connection that couldn't be seen bared eyes was the problem of slow cranking... again, i can't wait to get a hand on a vacuu, gauge and fix my engines myself better. thanks a lot for this helpful video
Let's see if I can get a response okay I have a Chevy Cruze and the p0011 code has to do with my intake cam problem which the diagnostic said it was the actuator I have changed both intake and exhaust did not fix the problem check engine light still on when I do the vacuum test my engine seems to rev Indy healthy area about 19 2 20 inches of mercury while in idle the needle has a fast vibration and it moves up and down from 18 inches of mercury 2 about 20 and a half inches of mercury so it stays steady in the green healthy area but it is moving back and forward a little bit I'm trying to figure out is something here the cause of my check engine light remain on because the diagnostic mentioned something about timing being off a little I can't really tell everything with the timing chain that I can see lines up to top dead center could this be anything dealing with some valves or Springs
Great video. There's a lot to remember- there was a mistake. This it critical so perhaps have the info written down to refer to in the future. Good though.
On a fully warmed engine and when I blip the throttle,. my needle drops to zero but then goes straight to 20 and stays steady. It doesn't go to 25 and then slowly settles back down. Do I have an issue?
I've herd people using vacuum gauges to correct idle mixture but I can screw my idle mixture screws in all the way until the engine dies and it doesn't effect the vacuum gauge
When your doing these readings. Is it good to turn all Accessories off? Soon as my compressor comes on my readings drop alot? Which seems normal. Am I wrong?
My small block chevy shows normal on the gage. But the needle is jerking back and forth a tiny tiny bit. It's a brand new crate engine. Is something wrong?
Nice video about probably the most forgot and underrated cheap tool to have in your tool bag. Most nowadays think it's not worth because it lacks blinking lights or a PC attached. LOL ;-) Thumbs up
Great video Mat Motorz, thanks. Josh Arbour, I presume pressure gauge shown measures relative pressure(1), therefore altitude of engine is irrelevant for this analysis. (1) pressure gauge indicates zero when not connected at all altitudes
Lets do some math on this. At sea level atmosferic pressure will be 100 kpa or 29.9 in Hg. We can asume we live in a city at an altitude of 4000mts Over sea level. Over there armosferic pressure will be little more than 60k Kpa equivalent to 18 in Hg. With 18 in of armosferic pressure how can you get 22 in of vacumm. I believe Josh is right.
Hi my friend. Is this tool suitable for turbo diesel engine cars? I have a Passat B5 1.9 TDI AFN engine. If so, would you please show me on which pipe to make a T connection on my car? Thanks
awesome awesome awesome video !!! Ignore any naysayers. Anyone can be a critic (really? I should account for barometric pressure? How about humidity? How about durometer rating of the line? Sheesh) I have a 1966 Mustang 289 4 barrel and I get good vacuum but have that very fast small fluctuation. SO not sure if its valve seals or as you pointed out to someone else, new, so don't worry...BUT ...changing to aluminum heads soon so I will worry then...thanks for a great video
I have a car that's got a new engine rebuild a year ago and at idle the needle on the boost/vaccun gauge flutters very slighty. Is this normal? sr20det engine
That sound like providing food, when your're thirsty! LOL 😂 All in all very nice but don't fulfill the necessary requirements. Compression test, leak down test, vacuum test. They all have a place and time to be done and work as complements rather than substitutions of each other! Cheers
This is why they made vacuum pumps so you retain power brakes and anything else you use vacuum for. Like my '68 'Bird in my profile picture... If I put a massive lumpy cam in the 429, I'd have to use a vacuum pump to keep the power brakes but also to keep every vacuum operated item on it (too many on this car). If you aren't using anything vacuum powered, the procedure is still the same, just be sure to keep in mind you're going to need more initial advance to run the engine with a cam like that.
Ok so, I have a vacuum vibrating rapidly between 18 - 15 in/Hg, at idle. After me doing a wet compression test, I just happened to start the engine with the vac. gauge hooked on and to my surprise, the vacuum reading was stable, just for around 5 seconds at idle, then it went back to the vibrating reading. So now I think my vibrating vacuum is caused by a worn piston ring, what do you guys think? Btw for the dry/wet compression test result, and my cylinder 3 is around 30psi lower compression than the rest at dry and all normal at wet test. Its just that after a lot of searching online, I still have not seen anywhere that worn piston rings can cause the vibrating vacuum symptom.
Me es de mucha utilidad éste video, gracias por compartir tu conocimiento. Recibe fuerte abrazo y saludos desde la ciudad de Tampico Tamaulipas México. Gracias.
Nice video thanks. I'm weighing a valve adjustment on my Sentra B15 because of low acceleration chatter - I'll see if I can get any clues w/ this Harbor Freight tool tomorrow. By the way, I've been able to thread the hose under the hood and place the gauge on the windshield so that I can view it when performing snap-throttle. If it''s too short for your location just go to the auto parts store and buy a 2' section of comparable vacuum line (take the original for comparison).
thinking about getting a vac tester, question, when yo connected it another port was open? does it matter if it's t fitting for a complete air tight fit or in this case it didn't matter one port was open?
MattsMotorz ok...it seemed in the video you only connected one hose, though it was kinda obscure/hard to see in video...I presumed it needed full seal..thank you
No it doesn't matter, what usually hooked on the intake manifold is: brake booster line, head Breather lines, EVAP purge valve and in some cases an A/C vacuum solenoid, all of them are receiving Vacuumed Air from the engine and disconnecting any one of them to hook a vac gauge should seal back the system
