I'm curious as to what I can tell about how the car is running with the vac readings I get on my boost gauge. I see about 20-22vac at idle, 22-25 vac during decel, and obviously it decreases as I'm accelerating. I'm not sure if these are normal values, so if someone could explain a bit more if there is anything worth mentioning, or point me in the right direction. I'm quite aware of what vac/boost pressures are, but I'm looking more for 'rules of thumb' when it comes vac pressure in engines.
" To study and not think is a waste. To think and not study is dangerous. "
Okay thats cool, just figured I could get something out of it.
" To study and not think is a waste. To think and not study is dangerous. "
low vacuum = crappy milegage or power
high vacuum = idle , or decel
10-20 = cruise rpm , a port fuel injected motors the vacuum reading is usually lower at cruise , than a carb or tb injected
needle fluctuations at idle can show signs of a weak or dead cylinder or other internal damage , like a burn valve or bent valve
Shifted wrote:^ A leakdown and compression test are far better indicators of that. Fluctuating idle vacuum can mean a bad gasket, faulty IAC, etc.
Yes by far they are, but to say a vacuum gauge is useless, is just dumb. Yeah if it was between buyin and installing a vac gauge or doing a compression and leak down test the last 2 would win. If you already have a vac/boost gauge and you see something on it it can help lead your next steps.
FU Tuning
Shifted wrote:Never said it was useless... just said you can't use it to do diagnostics...
Yes, it will tell you "something is wrong"
No, it won't tell you "what is wrong"
Thats what I meant...
Not what you first said, and to say you can't diagnos with it is not so. Part of doing that is finding out if you really have a problem. Vac gauge can do that, but you are right it will not tell you what is wrong, same as a compression check it will not tell you what is wrong, but lead you on another path.
FU Tuning
From the Haynes manual for 1995-2000 Cavalier/Sunfire:
An average healthy engine should normally produce about 17 to 22 inches of vacuum with a fairly steady needle.
Possible readings and causes:
1. A low, steady reading (around 11-12 inches) usually indicates a leaking gasket between the intake manifold and throttle body, a leaky vacuum hose, late ignition timing or incorrect camshaft timing. Eliminate all other possible causes before you remove the timing chain cover to check the timing marks.
2. If the reading is 3 to 8 inches below normal and it fluctuates at that low reading, suspect an intake manifold gasket leak at an intake port.
3. If the needle has regular drops of about 2 to 4 inches from normal at a steady rate, the valves are probably leaking. Perform a compression or leak-down test to confirm.
4. An irregular drop or down-flick of the needle from normal can be caused by a sticking valve or an ignition misfire. Perform a compression or leak-down test and read the spark plugs.
5. A rapid vibration of about 4 inches vibration at idle combined with exhaust smoke indicates worn valve guides. Perform a leak-down test to confirm. If the rapid vibration occurs with an increase in engine speed, check for a leaking intake manifold gasket or head gasket, weak valve springs, burned valves or ignition misfire.
6. A slight fluctuation, 1 inch up and down, may mean ignition problems. Check all the usual tune-up items and, if necessary, run the engine on an ignition analyzer.
7. If there is a large fluctuation, perform a compression or leak-down test to look for a weak or dead cylinder or a blown head gasket.
8. If the needle moves slowly through a wide range, check for a clogged PCV system, incorrect idle fuel mixture, throttle body or intake manifold gasket leaks.
9. Check for a slow return after revving the engine by quickly snapping the throttle open until the engine reaches about 2,500 rpm and let it shut. Normally the reading should drop to near zero, rise above normal idle reading (about 5 inches over) and then return to the previous idle reading. If the vacuum returns slowly and doesn't peak when the throttle is snapped shut, the rings may be worn. If there is a long delay, look for a restricted exhaust system (often the muffler or catalytic converter). An easy way to check this is to temporarily disconnect the exhaust ahead of the suspected part and retest.
Interesting I was wondering this myself. I have the m45 sc and I only get about 15 at idle. I just installed my built engine and do not have any problems with a fluctuating needle.
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Cool, thanks guys
I figured there had to be some little things that you could figure out from it. And yeah I realize I made a typo in the title...
" To study and not think is a waste. To think and not study is dangerous. "
