An acquaintance of ours named Jim bought himself a 1966 Mustang coupe. He got a good deal on the 6-cylinder, automatic car and was willing to take the chance that its poor running was fixable. Everything else about the car was in very nice condition and the recently-applied paint looked terrific, so Jim figured that in the worst-case he might have to get a new engine. Since the only real problem was the car's driveability, Jim reasoned it was worth the risk.
Specifically, the engine started just fine but its idle was extremely rough and the rpm wandered up and down. There was excessive vibration at low rpm and tremendous hesitation when the accelerator was depressed, almost to the point of stalling. Once moving the car bucked and stumbled badly until it got to 25-30 mph, at which time it smoothed out. Any quick movement of the accelerator would result in more hesitation, however.
He brought it over to the Second Chance Garage and asked us to pronounce it "dead" or, hopefully, fix it. Since we could hear the car coming a block away and its vibration rattled the garage door, we figured the diagnosis wouldn't take too much time away from other projects. So we grabbed the test equipment and directed Jim's car into the bay, confident the process would be over in a few minutes. Hah!
Disregarding the noisy exhaust leak and front-end rattling (a loose bumper bracket, as it turned out), we put a vacuum gauge on the engine while it was still running. The needle jumped wildly between 5 and 10 inches (mercury) of vacuum and wandered lower when the engine's rpm changed (all by itself, by the way).
The vacuum reading was far too low and that made us suspect valve leakage. With the engine turned off we removed the spark plugs and did a cranking compression test on the cylinders. (Well, that's not quite accurate, since the #5 plug wouldn't come out and Jim was afraid it might shear off.) Anyway, the other five cylinders all maintained at least 150 psi, a good indication that the valves in those cylinders were fine.
We then turned to the distributor. A quick hookup of the dwell meter showed us a major problem. The dwell wandered between 25 and 40 degrees, a strong indication of shaft bushing wear. We put a piece of hose on the vacuum diaphragm port and a quick suction revealed no movement of the breaker plate. We told Jim to order a new distributor, since its cost would be far less than that of rebuilding the old one.
We moved the car out of the way, confident that the distributor's faults were contributing to most - if not all - the engine running problems, with the exception of low vacuum. However, it made no sense to troubleshoot further without a properly-functioning ignition.
A few days later the distributor arrived and we put it in, taking the time to crank the engine to TDC before installation. It went in fine and the engine started, but our attempts to time it resulted in widely-fluctuating timing readings. After about a half-hour of readjusting the distributor we came to the conclusion that our timing light was failing.
We were able to get the engine running well enough to troubleshoot the vacuum leak, however, and attributed it to an inability to bring the throttle plate down to its resting position without killing the engine. Therefore, the idle mixture screw had little or no effect on the running of the engine. A few moments of trying adjustments proved the carburetor's idle circuit was clogged or faulty, because the engine wouldn't run with the throttle plate in its correct position. In addition, the choke assembly was rusted completely and frozen in place. We measured the fuel pump pressure just to rule it out as a cause. Once again, the cost of a new carburetor was far less than that of rebuilding, so one was ordered.
A few days later the carburetor arrived and we removed the old one. Before doing anything else we removed the water jacket plate beneath the carburetor to inspect the gasket between it and the intake manifold. The gasket was almost completely disintegrated, so we made a new one out of the appropriate gasket material. Surely the gasket had something to do with the low vacuum, as did the partially-open throttle plate.
We put the plate and carburetor assemblies back together and started the engine. It started easily and ran quite a bit smoother and a vacuum check showed a normal range of about 17 inches of mercury. However, the vacuum gauge needle still fluctuated in time with a noticeable engine miss.
With a proper ignition and fully-adjustable carburetor and no external vacuum leak we could turn our attention to the miss. We turned off the garage lights and looked into the dark engine bay, instantly noticing a succession of arcs from the plug wire to the engine block on that #5 cylinder. The sticking plug had to come out.
With Jim's permission we slowly pulled on the socket with a breaker bar. The plug turned hard, but eventually gave way and came out. Its electrode was heavily caked with carbon and it was obvious it hadn't been firing for quite a while. We sent Jim to buy a new set of plugs since we didn't know how long these had been in the engine.
While he was gone we did a compression check of the #5 cylinder and it only yielded 100 pounds, hot. That wasn't good, we knew, but we wanted to get all cylinders firing to properly check for sticking valves, etc.
With the new plugs in we cranked the engine and it ran smoother than ever. However, there was still a periodic miss and the car drove with some missing under load during a short test drive down the street.
The vacuum gauge was put back on and we could see a 3-inch wobble of the needle, indicating an almost certain problem with one or both valves on #5 cylinder. In all likelihood the intake valve is sticking, since lots of fuel built up during the time the plug wasn't firing. There's also a slight possibility the missing is excessive back-pressure building up in the noisy, corroded muffler.
Good question, and that's what Jim asked. "Drive the car for a while," we replied, "and see if the valve clears itself over time. There is a possibility that the engine will continue to run better. If not, it's going to need a valve job."
Jim drove off, happy that the car was running far better than it had.
Was all that replacement of parts necessary? Yes, it was. Every one of the parts tested bad and needed to be replaced anyway, so even if the car needs cylinder head work the money wasn't spent in vain. Now we know why he got such a good deal on the car: the previous owner believed the engine was "shot" and simply wanted to unload it.
The story isn't over yet, since the engine may need some machine shop work. At this point Jim has spent about $400 beyond the cost of the car, not very much when you consider the purchase price. Even if the cylinder head is rebuilt or replaced, the total investment in the car won't be excessive.