manifold pressure

[qwerty247]

Just been checking the readings on scantool from my 97 4.0.

looking for any leaks in system...

i got the manifold pressure reading as 40 kpa when idle, how does that compare to you guys?

does that sound right ?

thanks

[old_man]

Speak English dammit:jester: Over here we talk in inches.

Ah....two countries seperated by a common language.

[qwerty247]

ohh ok, so that reading dosent make sense to you?

[old_man]

I need to do some conversion.

[8Mud]

1 kPa = .295 inches of Hg

[qwerty247]

i know in bar it is 0.4

not sure what you normally measure it in?

[qwerty247]

ok so in hg it would be 11.8, how does that sound?

[Dr. Dyno]

ok so in hg it would be 11.8, how does that sound?

101.3KPa = 1 atmosphere = 29.92"Hg so 40KPa does indeed = 11.8"Hg, and that's definitely too low so you've got a vacuum leak somewhere. A stock 4.0 should have an idle vacuum of about 17"Hg.

[qwerty247]

thanks for confirming that.

so is that likely to be caused by a crack in manifold because i cant see any problems with the vacuum lines.
heard/read alot about cracks in manifold (but cant notice any)

or is there something else i should check?

i am new to jeeps and its real hard finding anyone that knows what there doing with them here in UK, so thanks for any info you can give its very helpful !

[Matthew Currie]

I think more likely than a cracked manifold would be a bad manifold gasket. But before going into that, I suggest that you plug or crimp off vacuum lines one by one, to see if there is any change.

If you are getting this reading from a scan tool, presumably it is interpreting the MAP sensor. Before worrying too much about engine vacuum I would get a second opinion from an actual vacuum gauge, because the MAP sensor might be wrong.

[langer1]

A bad Power brake booster could also cause it.

[qwerty247]

thats interesting you mention that, because when starting in mornings or first start the brakes are very sharp and then they get 'weaker' got to push the pedal harder.

but when first starting a slight touch and the car STOPS.

as much as i like to hear possible causes, it then leads me onto more complicated issues that i probably cant manage and more questions like how can i know if its the brake boster you mention?

is what i mentioned about the brakes a sympton for the brake booster fault?

thanks again

[90xj06]

i have that same problem i would love to here what you guys have to say about it?

[qwerty247]

just did a search and found more about the booster and from what i read dont think my brake problem is the booster.

from what i understand about the booster is if its very hard to press the brake then its a booster fault.

you have the same brake affect at first start 90xj06 or a compression fault?

yes would like to hear any more ideas about the brakes

[qwerty247]

could it be the abs module?

[qwerty247]

had a good look today and cant see any breaks or split vacuum pipes and all look ok.
also checked the brake booster and works ok as well as the pipe going from that.

so that leaves the map sensor or could it be a crack in the exhaust manifold casuing the low vacuum reading?

i can hear a slight hissing but with all the other noise not sure if that maybe normal

[Matthew Currie]

had a good look today and cant see any breaks or split vacuum pipes and all look ok.
also checked the brake booster and works ok as well as the pipe going from that.

so that leaves the map sensor or could it be a crack in the exhaust manifold casuing the low vacuum reading?

i can hear a slight hissing but with all the other noise not sure if that maybe normal

I don't think you should hear it hissing, so the next step is to try to track it down. You might try a piece of hose to your ear as a primitive stethoscope, and probe around the suspected areas. Manifold and gasket leaks can be detected using propane gas (when it approaches the leak the engine speeds up) or sometimes with something like WD-40 or other thin oil (when it hits the leak you get smoke out the exhaust).

A marginally bad brake booster can still give adequate boost, but lower your vacuum, so don't eliminate it from the hiss hunt. And I would not just visually inspect the vacuum lines. Clamp them off one by one, or cap them.

[qwerty247]

thanks for the tip, but you say clamp them off, can i just pull them off when the engine is running?

so do i pull one off and then block the end? what should i be looking out for when i do that.

also most of the hoses are hard plastic or are they the ones for the ac and cc etc?

[langer1]

As said earlier the only way is to systematical block all vacuum lines. Vacuum runs from the tank in the front bumper and "Depending on the year" all the way to the fuel tank not to mention the heater and A/C controls.
Unplug them and cap them off accept for the MAP sensor.