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Normal Vacuum specs under load

9

962drrhino

NAXJA Forum User
Location
saratoga, ny
Symptoms:
Underload, (defined as uphill at low RPMs say 1k-2k,) engine has NO POWER. Vent moves to defrost. Inline vacuum filter replaced, checked again, checks out. Vac lines triple checked.

Question:
on the Vacuum gauge (haven't hooked it up yet), what reading is spec for and uphill load?

Details:
Recent tune up, (cap+rotor, plugs) after issue, no change. After cresting hill, no issues, normal power.

Wishlist:
smoke machine. Yes I've tried carb cleaner, no change.


TIA!!
 
I was going to ask the elevation where you live, but lugging up a hill with engine speed below 2300 rpm is not the thing to do. Next hill, drop down a gear and let the RPMs go up.
 
You want 15-20 hg (mercury) inches at warm idle.

Vacuum pressure will fluctuate a lot during driving. Vacuum is created when pistons pull air into the combustion chamber, so pressure in the manifold will depend on factors such as RPM (suction) and throttle angle (resistance)

Vacuum should be about the same everywhere however the accessory harness that controls the A/C and cruise control has a check valve and the reservoir so it will read different there.
 
Hypoid said:
I was going to ask the elevation where you live, but lugging up a hill with engine speed below 2300 rpm is not the thing to do. Next hill, drop down a gear and let the RPMs go up.
Click to expand...


Yeah, I usually have to drop it into drive. Problem is its any SLIGHT incline, say 10 degrees. This is also a new symptom. It was a torque monster a month ago.

I really miss my manual trans. Auto was not my choice.
 
ehall said:
You want 15-20 hg (mercury) inches at warm idle.

Vacuum pressure will fluctuate a lot during driving. Vacuum is created when pistons pull air into the combustion chamber, so pressure in the manifold will depend on factors such as RPM (suction) and throttle angle (resistance)

Vacuum should be about the same everywhere however the accessory harness that controls the A/C and cruise control has a check valve and the reservoir so it will read different there.
Click to expand...


Yeah I got all that, I'm just wondering what type of readings people are observing uphill. Thanks for the reply though!!
 
WISHLIST: BASICS--year/engine/etc.

You have a vacuum system problem for sure.

The vents default to defrost when the vacuum level drops. Check the condition of your vacuum reservoir and the lines running to it, particularly around the battery.

87-90 4.0 could have an issue with the EGR and a low/lack of vacuum.
 
Ok, 96 no EGR so start with fixing the known problem in the vacuum system and post back up what you find.

Good luck.
 
Vacuum with the throttle wide open under heavy load will be quite low, maybe 3-5 inches. Vacuum is caused by the cylinders sucking the air in against the restriction of the throttle body. If the throttle body is wide open there is little restriction and the vacuum drops.
 
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