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Vacuum gauge reading low possibly? Help
I installed a vacuum gauge in my car last night. I didn't use the tube that came with it because it did not seem very durable. I bought a tube from Napa and used that.
Here's where i have it connected on the vacuum line: http://i115.photobucket.com/albums/n...2/IMG_3457.jpg http://i115.photobucket.com/albums/n...2/IMG_3459.jpg This is the gauge: http://i115.photobucket.com/albums/n...IMG_3460-1.jpg Today i was able to see how it was acting and i think it maybe reading a little low from the research that i've done :confused: At idle it is at 17-18, which i think is normal. When i was accelerating slowly from a stop, it would usually be at 5. Once i was up to speed cruising around town at speeds between 30-45 MPH on level roads, it would be 15-18. On the highway going 55-60 MPH, on level ground it was at about 12-15, going up a hill it would drop to 5-10, and going down a hill it would be at 15-20. When i had my foot off the gas coasting to a stop, it would be at 20-23, usually with the needle shaking at whatever number it was at. If my car was in fuel cutoff mode, slowing down or coming to a stop (wideband read ---- and SG2 read 9999), it would be at a steady 23. Never the whole trip, to and from my destination, did it ever go above 23 :(. Are these reading usual or are they low? If they are low, what should i do? |
I think the readings seem slightly low.
My Echo reads about 21 at idle and it reads slightly higher under similar circumstances. It's a manual and has a very low idle speed, it will be higher for an auto in gear, or any car with a higher idle speed. The fluctuating gauge needle is an indicator of one of two possible problems. It could be an engine problem, but I would not assume that to be correct yet. Its more likely that you have tapped the vacuum near one of the individual runners in the intake manifold. I can't really tell from the photos. You need to tap the vacuum from the intake plenum, where all the manifold runners meet. My preference is the hose to the brake booster, which should connect to a steady vacuum source. If you try that and it still fluctuates it might indicate there is a problem with the engine, induction, or ignition systems. Idle vacuum is also a function of idle speed, and engine compression. My Echo is 10.5 to one so it reads fairly high idle vacuum. Also understand that your altitude makes an difference as well as atmospheric pressure. Higher altitudes and lower atmospheric pressures will reduce idle vacuum readings. Otherwise your ranges seem to closely match mine, just slightly lower. My 76 Z car would do about 18 at idle, but it's compression ratio was 8.5 to 1. regards Mech |
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This is where i have it connected to currently (sry the arrows are kinda small): http://i115.photobucket.com/albums/n...2/IMG_3466.jpg http://i115.photobucket.com/albums/n...2/IMG_3468.jpg |
I have one on a 93 Chevy Van and a 2000 Ford Mercury with the 4.6 motor and your readings sound normal to me.
And the bouching pointer may just be the egr system, I have seen the very same thing on my Ford once it warms up and the egr starts to cut in... I am a old time machinc and had a panic reaction to seeing the pointer bounch around at cruse untill I read it is more or less normal on some new cars... A reading ot 10 to 12 at 70 MPH is what I see and I am getting 29 MPG at that.... Rich |
if you are "Teed" into the Purge line , when the purge valve is duty cycled by the ECM / PCM
the vacuum gauge will pulse with the duty cycle . 17 to 18" at hot idle is too low at sea level but more important is that at hot idle there should be ZERO flutter in the needle the brake booster vacuum line has a check valve in it , if you "T" into the line after the check valve the vacuum value you measure will be incorrect. |
I noticed today that whenever i was stopped with the car in drive and had my foot on the brake, the needle was between 17 and 18 and also was shaking a little. When i put the car in park and released the brake pedel, the needle went up to 20 and was steady.
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That is what I get as well, in gear 17/18 out of gear 19/20.
I see the bounchy pointer at cruse....right where the EGR should be working... Not totatly happy with a shaky pointer at idle in gear but is it a 4 cylinder...? My Toyota shakes the whole car at idle in gear.... Rich |
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vacuum values
operating temperature
UN boosted engines . typical "normal" at sea level vacuum values are ; 21 to 21" at hot idle ,no load, the needle must be steady smooth , flutter is NOT allowed no load, at 2k rpm 2" more than at idle what ever the idle value is UNless the EGR is opening like on some subaru WOT snap values should be 0" when the throttle is opened wide and then as the engine decelerates about 24 to 25" , no load . WOT snap means ; as quickly as you are able ... snap the throttle to MAX opening and the millisecond it is open , let the return spring snap it closed , DO NOT DO this in gear . flutter at hot idle with no load = valve train problem no load low vacuum at hot idle almost always = incorrect camshaft timing no load , low vacuum at 2k rpm hot usually means restricted exhaust ............................... these are typical values ; there are exceptions ; they do not apply to some engines , like 2.0 9A VW dohc engines values will be lower at high altitude |
MWebb; in case there is an issue with incorrect cam timing, like if there's timing advance or retard, how would that effect fuel economy?
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mwebb has it right.
An automatic in gear will have a slightly lower reading than in neutral. The flutter does not exist in my echo with the source in the booster line. Most boosters have the check valve in the booster where the hose connects. If you connected it after the check valve the vacuum would not drop after you shut off the engine. Typically a fluctuation in the needle indicates a weak cylinder, not producing the same power as the other cylinders. If your idle is rough then that would be another indicator of a mechanical problem. Most problems would give you a MIL malfunction indicator light, unless they were very minor, below the threshold of the point where the mil limap would come on. There are many different reasons for a fluctuating vacuum guage, if it is because something is not right with the engine. weak valve spring valve not sealing properly low compression on one or more cylinders head gasket intake manifold gasket plug misfiring bad ignition signal injector not spraying properly thats just some of the problems that could give you a fluctuation. the purge line for the fuel tank will give you a fluctuation that would probably change somewhat depending on the fuel level in the tank, since the evaporative emissions control system maintains a slight vacuum in the tank. full vacuum would collapse the tank so its a regulated vacuum that is very small compared to the highest manifold vacuum you would see. regards Mech |
I am a old machinic as well and I would aggree if it was not the fact that a bouching pointer is not all that unlikely with todays motors.
So watch and see it the pointer bouches on a cold motor...egr does not normally cut in until the motor is warmed up. Second I would shut down the egr to test the gauge before I tore into the motor. Last is of course a dry and wet compression test. My 2 cents worth. After all I have tons of power and get a nice clean 29 MPG stock with a bouching pointer... And have done so for the past couple of months. Rich |
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imho no one should ever tamper with cam timing . but , there are always exceptions ... to every rule . your 2005 corolla has variable cam timing , you should not frak with it. in my experience whenever i encounter a cam timing error , performance is way off , the people are not concerned with fuel economy as much as they are concerned with poor driveability and the CHECK MONEY light glowing orange in their instruments cluster . http://farm4.static.flickr.com/3488/...4eaa05ab_b.jpg having said that i am tampering with cam timing on a geo metro , so far , my test results are not what i was hoping for - the screen cap above looks neat , but it is flawed ,amplitude values are low , the connector to the pressure transducer was leaking , however , it does show EVO occurs at 126 degrees after TDC and EVC occurs at 366 degrees after TDC . with a stock OEM cam and stock OEM timing values . that is already very advanced for a stock EVO value typical values are closer to 150 degrees after TDC. i have installed a 3tech economy cam and a +10 degree cam wheel , now that i have a new connector for the pressure transducer ... i just need a spare hour or so to connect up ... |
http://farm4.static.flickr.com/3585/...1d943991_b.jpg
since most people are not familiar with running compression waveforms ... there is three stacked up the bottom one is "good" relative to the upper two , it shows 720 degrees of engine rotation and what pressure or vacuum is present in the combustion chamber at what degree of engine rotation 720 degrees is two complete turns of the crankshaft there is no spark , there is a pressure transducer in the spark plug thread connected to a digital storage oscilloscope . below zero is vacuum , above zero is pressure , in the combustion chamber that fluke pv350 pressure transducer gets zero set to local ambient barometric pressure prior to capturing a waveform which is zero on the screen s |
^ Some pretty interesting stuff, mwebb.
Thanks for all the input guys. Last night, i found the original line that came with the gauge and replaced it with the other line that i had on it. The needle still shakes, but has decreased and doesn't do it as often now. |
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