I recently installed an under dash vacuum gage in my Country Sedan. The reason I did is because the car is really sluggish when I hit the gas so I wanted to check the vacuum readings to see if there was a leak (finding the leak would be the next step). The vacuum reading is around 20 inches which is perfect. However, the reading drops to about 10 or 12 when I hit the gas. It goes back to 20 when I reach the speed I want to cruise at. Is the drop when accelerating normal, or is that indicative of a leak?
It's normal for vacuum to drop under acceleration. Perhaps hook up a timing light and see where that's at. Sometimes bumping up the timing higher than spec can help improve performance in these older cars.
Yes it is normal for engine vacuum drops when the throttle plates open. The way to check for a vacuum leak is probe around gaskets and hoses with a flammable material while the engine is running. (I use propane) RPM's will increase when the fuel finds the leak. Make sure the line to the modulator on the transmission hasn't come off. You should also look into the timing. It may be too retarded. The vacuum advance may not be working. Ford's of that era had temperature controlled vacuum manifolds, usually on the thermostat housing.
That's normal; vacuum levels drop (manifold pressure gets closer to atmospheric pressure) when you open the throttle plates. At wide open throttle (WOT), the vacuum level will approach 0" HG, or atmospheric pressure. Does it sag or bog when you begin to apply the throttle, or does the problem last longer than just the initial push on the accelerator?
Yes I will check the timing next. No it doesn't bog down when accelerating, it just doesn't want to get up to speed. I know, being a large wagon, it's not going to be a speed demon or go 0-60 in milliseconds but it should get up to speed quicker than it does. The tech guy (Ray) at Hemmings suggested the timing chain could be stretched or a cog missing a tooth. That will be checked too though it will have to wait until the weather gets a little warmer here.
That's quite a leap when your symptoms are simply sluggish acceleration. The 20" reading at idle is indicative of a healthy engine. Check the basic tune up, be sure that your throttle is opening all the way and if an automatic transmission that it downshifts when commanded. That car is no ball of fire but even with a small V8 should move out just fine.
I agree with ross's comments above. If you're seeing 20" at idle and it's not popping or back-firing, the timing chain is probably not the problem. As already mentioned, confirm that the base timing is correctly set AND that the vacuum advance is working. The advance plates can stick and wreak havoc with performance and fuel economy.
My first thought is to take a look at the muffler. If you bumped bottom, it could be partially collapsed, and that will cause performance problems under wider throttle openings, similar to a clogged cat in a newer car.
I would start with a complete tune up on the engine, and if it has the original ignition coil, I would replace that also. There are lots of problems that will have similar symptoms as yours, so the only thing to do is to start a systematic approach to resolving them. The engine sounds healthy from the vacuum reading, but something is amiss, and you are just going to muddle through it till you find the problem.
Is there a shop in your area that still has an old Allen or Sun scope with a 4 gas analyzer on it. It can check your problem in real time and save you money by pinpointing the problem, instead of just replacing parts that you don't know if they are good or not. In lieu of that, do you have access to an ohm meter. You can ohm the coil and wires to see if they are up to spec and those specs are available in most old tuneup manuals. I totally agree with this statement. Do yourself a favor and buy an inexpensive multimeter and learn how to use it. It's one of the best diagnostic tools you can have in your box.
Our OP is well equipped to make the test for this. Holding the engine revs STEADY, throttle position cannot change, at perhaps 2.5-3k watch your vacuum gauge, it should also remain steady, the reading not so important, if the vacuum is dropping off while the revs are steady this is indicative of restricted exhaust.
...like already said, 20" of vac is very good. I think it just needs a tune up. The fuel filter can be near plugged , the air filter can be plugged, exhaust problem like suggested.....I seen a exhaust pipe inner layer collapse restricting flow yet on the outside the pipe looked perfect, tranny slipping ( auto ) , ignition out of specs.
