Q45: What is normal manifold vacuum for cruise at 40 - 60 mph?

[93 Q-SHIP]

My '93 Q45t has lost power. What is the normal manifold vacuum when cruising at 40 - 50 mph?

The transmission will not shift under full throttle acceleration. Transmission shifts normally under normal driving conditions. I was wondering if the vacuum is used to control the transmission shifts?

[qsiguy]

about 15" on my '94...

I believe it's all electronic for the trans. The diagnostic display should say Trans malfunction if it detects a problem. These cars have many built in diagnostics features.

[93 Q-SHIP]

Thanks for the info. Mine is only about 10". I think the pre-cats and or converters are plugged up. I've dropped the exhaust system and will test it this weekend. It's not to loud with the pre-cats. No obvious damage to the pre-cats looking at the exit side of them.

[Q45tech]

Cat failure on 90-93Q is a very rare event.

[qsiguy]

I agree, I doubt that's the problem. It doesn't take much to go from 15" to 10". A moderate headwind, low tire pressure, or very slight grade could change it by that much or more. I'd check out other things before coming to that verdict. Fuel filter, pump, FPCU, Spark plugs, etc. There are many things that could effect power. Unless you, or someone else, has been using leaded fuel or there has been another serious issue, I doubt the cats are bad.

[Q45tech]

Using inches of mercury is so confusing as it prolongs the misconception that there is some kind of vacuum instead of just a lower atmospheric pressure in the plenum.

The lower the " of HG ...............the higher the plenum pressure in psi.