proper vacuum pressure at idle? (4.0 HO)

hey guys

I have a vacuum gauge installed in my 1996 XJ, and the engine hasnt been starting so well, and idles a little poor when cold (initial startup).

i seem to remember running about 15 in Hg of vacuum at idle, andf i think im down to around 11 or 12 now


what should proper vacuum be at idle??

thanks!!!

CaptTrev

Is the vacuum needle steady, or is it moving? Does it move slowly or does it drop at a rythmic or consistent rate? Proper vacuum for a correctly timed and smooth running engine should be 18-22"hg. I would say that 15" should be OK at idle, but anything lower would indicate either incorrect timing or a plugged exhaust system. Other things may contribute to low vacuum, worn rings, worn camshaft lobes, or retarded timing. Also a bad valve or low compression in one cylinder could cause the needle to jump around too. I would start checking the basics and go from there.

Bryan

vacuum is rock solid steady


ive never seen it about 15 though, ever since i installed the gauge a year or so ago anyway

needle drops exactly along with throttle (gas pedal) response

i dont think its a plugged exhaust cuz ive got as relatively new stainless carsound cat and new borla cat back

i use techron every 3k miles and always take care of tune up items on time

could it be (some sort of a )bad valve in the vacuum bottle tank thingy?
(i read this in an earlier post somewhere)

help!

CaptTrev

CaptTrev said:

hey guys

I have a vacuum gauge installed in my 1996 XJ, and the engine hasnt been starting so well, and idles a little poor when cold (initial startup).

i seem to remember running about 15 in Hg of vacuum at idle, andf i think im down to around 11 or 12 now


what should proper vacuum be at idle??

thanks!!!

CaptTrev

Click to expand...

The correct vacuum at idle should be 16-20"Hg.
Check all the vacuum lines going into your intake manifold and make sure none of them is either disconnected/cracked/broken. Also check the tube going from manifold to MAP sensor.

On my 95 there's a capped tee in the line that runs along the right side of the engine compartment to the vacuum reservoir. A vacuum gauge on that might let you know if there's a problem in the reservoir or its lines. A common failure point is the line under the battery, where acid eats it up.

thanks guys!

im going to look into this....

CaptTrev said:

thanks guys!

im going to look into this....

Click to expand...

You can also try to vary the way you hook up the vacuum gauge. If I hook up my test gauge directly to the manifold(where the small CCV line goes) it will read 18 to 21 in/Hg. My dash gauge is hooked up to a tee fitting on the same line, but closer to the valve cover and reads 11 to 12 in/Hg at idle, but seems to respond as expected. Could be one of the two gauges is reading wrong, but I'm more inclined to think it matters where you tap in and if it's a tee vs dead-end connection. BTW, this is on a '90 with 235K miles, so it's a bit tired.

MaXJohnson said:

You can also try to vary the way you hook up the vacuum gauge. If I hook up my test gauge directly to the manifold(where the small CCV line goes) it will read 18 to 21 in/Hg. My dash gauge is hooked up to a tee fitting on the same line, but closer to the valve cover and reads 11 to 12 in/Hg at idle, but seems to respond as expected. Could be one of the two gauges is reading wrong, but I'm more inclined to think it matters where you tap in and if it's a tee vs dead-end connection. BTW, this is on a '90 with 235K miles, so it's a bit tired.

Click to expand...

hmm...

THATS very interesting. . .

I installed the vacuum gauge "T" about 6" off of the intake manifold (in between the intake manifold and the valve cover vacuum line)

so, maybe my vacuum pressure is fine, after all?

any way i can test the vacuum canister????

tomorrow im going to remove my battery and check the line that run behind/under it.

i HOPE i find something ( i could REALLY use a a cheap/easy increase in gas mileage!!!)

CaptTrev