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VAC gauge diagnosis

aeaglenfitch

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'98 Mounty 4.0 SOHC 4x4
i have a vac gauge installed on my mounty, usually it reads like 18 hg but today i noticed it reading 20 at idol. i know it's within spec but it seems odd. so i came in and looked up some vacuum gauge diagnostics articles on google and did the 2,000 rpm test where you hold it at 2,000 rpm and check vac. it says it's supposed to be at 5 hg well i go do it and i get like 25 hg. if i gas it quick and hold it the needle goes way up to around 5 then slowly goes back up to 24 ish. whats going on? the idol has been feeling rough.

earlier today my belt came off and i was almost to where i had to be so i just kept driving. i went into job training and came out knowing i'd have to put the belt back on 2 hours later. i poped the hood and the radiator hoses were all suctioned flat and i had to remove the radiator cap and the hoses went back to normal. im assuming this is because the water pump wasnt going. but could driving without the belt on for about 2 miles in the city could have caused any problems?
 
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manaen

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dirty air filter maybe :dunno:

that shouldn't make a difference at idle though
 
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aeaglenfitch

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any other suggestions? what about possible problems from driving without the belt?
 
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aeaglenfitch

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i really need to get an answer if possible?
 
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manaen

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Driving without the belt can cause all sorts of havoc with over heating. without the water circulation the heads will heat rapidly. I don't think that is your problem because you would be seeing less vacum with a head problem not more. where are you connecting your vacum gauge?
 
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manaen

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I'll go out and run the same test on my truck and see what happens and let you know. That will give you a good baseline to see where the SOHC's tend to fall.
 
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manaen

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OK I ran the test, I thought something didn't sound right with numbers you read about, where did you get the 5hg figure for 2k rpms? After looking in my books, the numbers your getting are normal. I get 19hg at idle and 21 holding at 2000.


Here are the procedures for vacuum testing
Engine Testing With A Vacuum Gauge
Intake System Leakage, Valve Timing, Or Low Compression
Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, manifold-to-carburetor gaskets, vacuum brakes or the vacuum modulator. Low readings could also be very late valve timing or worn piston rings. See figure 3.

Exhaust Back Pressure
Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present.

Cylinder Head Gasket Leakage
With the engine Idling, the vacuum gauge pointer will drop sharply, every time the leak occurs. The drop will be from the steady reading shown by the pointer to a reading of 10" to 12" Hg or less. If the leak Is between two cylinders, the drop will be much greater. You can determine the location of the leak by compression tests.
 
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