My vacuum fluctuates and my exhaust “poots”

Question:

I’ve had an idle aggravation and a slight vacuum fluctuation for quite a while. The vacuum gauge fluctuates approximately 1 ⁄2 inch of vacuum but there is a distinct “poot” from the exhaust pipe. The car is a 1980 Pontiac with a Pontiac engine which doesn’t make any difference, but the “one in, two out” Camaro muffler probably makes the poot a little more noticeable. I won’t go into all of the things I have done but the compression readings are within specification

The question is this: We check the compression for each cylinder, but the vacuum gauge reading is for all of the cylinders. Would it make any sense to check the vacuum for each cylinder? An adapter would have to be fabricated but that isn’t a problem.

We measure the “push” (positive pressure) of the piston in terms of compression as the piston goes up, but why not measure the “pull” (negative pressure) of the piston as it goes down? I don’t see any reason why a vacuum gauge couldn’t measure this number.

Hopefully someone can enlighten me.

Answer:

To begin with, did you warm the engine up thoroughly before you did the compression test? It sounds like you have an exhaust valve that isn’t closing all the way once the engine is warmed up. When the engine is cold, clearances are looser, so the valve may be closing completely in that situation, accounting for the good compression. But when the engine is warmed to operating temperature, it is possible that an exhaust valve is either not closing because it is adjusted too tight, or it is hanging up in the valve guide due to carbon or corrosion.

If the car has mechanical lifters that were installed to go with a hot cam, most likely a valve adjustment is in order. Clearances on exhaust valves can tighten over time, and intake valves often loosen, cutting down on performance. If your engine has mechanical lifters, adjust the valves according to the instructions that came with the cam, or are in the shop manual, and do another compression test with the engine thoroughly warmed. (Your temp gauge will come up to normal quickly, but that only measures head temperature, so be patient and let the engine run for at least 20 minutes).

If the engine has hydraulic lifters, look for sticky valve guides or dirty hydraulic lifters. There are auto supply store concoctions that you can put in your oil or fuel that are supposed to free up sticking valves and lifters that you might try, but I have not had any luck with them. Also, there is a complete chart of vacuum test indications in the September issue that will tell you about a host of possible problems and remedies that you should consider before you start taking things apart. But if all else fails, in the end, you may have to pull the heads and determine the problem.

As for checking vacuum in an individual cylinder, you would need to place the probe in the intake port runner which would be hard to do, and since internal combustion engines don’t hold a sustained vacuum, the reading would be transient at best. Such a probe might determine what the suction was briefly on the intake stroke, but since there are no specs to tell you what is normal, I am not sure how useful that information would be.

Putting the probe in the sparkplug hole wouldn’t tell you anything either because what you need to know is whether air is coming in from the intake manifold past the open intake valve as it should, or leaking out from around a faulty exhaust valve after combustion. The poot you hear is most likely the sound of gasses leaking out around a burned, warped or out of adjustment exhaust valve but could also be because of sticky valve guides or dirty lifters.