I drive a rusty trusty '88 Grand Caravan because it has all the toys like power windows and seat but only one very reliable Engine Computer. This truck will be the same in that it won't have all the unnecessary computers the insane engineers are hanging on every system. Some of the newer stuff is nice and beneficial like anti-lock brakes and air bags, but given the reliability of electronics and the environment they live in, you know there's going to be failures.
Most of the computers can detect problems and set diagnostic fault codes which can make starting the diagnosis easier, but your truck is relatively basic and easy to repair, especially for a do-it-yourselfer who doesn't understand how all the computer sensor circuits work. In this case you have a misfire that you may not be able to feel at idle. On '96 and newer vehicles the Engine Computer can detect and tell you exactly which cylinder is misfiring, but on yours the place to start is with the basics. Spark plugs and wires are the first suspects. If that results in an improvement but not a complete fix, or if the characteristics of the rough running changes, the ignition coil may be incapable of producing high enough voltage for all running conditions. That can be masked for a while by new spark plugs that have the correct gap making the spark need less voltage to jump. As older spark plug electrodes burn away, it takes more voltage to fire them and the point can be reached where the coil can't develop that voltage.
If the problem persists, which you should expect because the current owner would have fixed the misfire if it was easy to do, is ignition timing or fuel supply issues. If fuel pressure is lower than specs, not enough will spray from the injectors. This is a bigger problem when you have six or eight individual injectors instead of two. You could also have a partially plugged injector that can't flow enough fuel at higher speeds. The way to identify that is to unplug one injector at a time, then watch how much the engine speed drops. If a cylinder isn't developing full power, the engine speed won't drop much when you disable that cylinder. The complication here though is the Engine Computer will maintain the idle speed. What works best is to connect a scanner that displays live data, then look at the percentage of throttle opening the computer is setting the idle air control valve to. Not much change in that setting will take place when you disable the weak cylinder.
Look at the ignition timing as you change engine speed. You can also move the timing light's pickup to each of the other cylinders. If you're losing spark intermittently to one of them, the timing light will stop firing, and you can see that. If the timing is bouncing around, suspect a worn bushing in the distributor or a stretched timing chain on a high-mileage engine. To check for a stretched timing chain, you may have a mechanical fuel pump block-off plate on the engine that you can remove, then reach in with a coat hanger wire made into a hook on the end, and see how much you can push and pull on the chain. You might have to turn the crankshaft pulley backward a little to get all the chain's slack on that side. When it's loose, it will flop around and cause the camshaft timing to vary. The distributor runs off the camshaft so its timing changes too.
With a worn bushing in the distributor, the shaft can wobble around. That will not affect the spark jumping the gap between the rotor tip and distributor cap terminal but it will change the position of the reluctor. That's the toothed wheel that triggers the pickup coil, similar to breaker points on older cars.
What you must resign yourself to is less than ideal front tire wear, particularly on the right front. Ford hasn't had a vehicle with a suspension system that can deliver good tire wear in decades. Other than the Escort / Tempo disasters, the twin I-beam that only Ford used is the worst design for tire wear. It's strong and simple but the much better designs used by all other manufacturers are more than strong enough. If you do notice abnormal or accelerated tire wear, if it's mainly on the right front, install a heavy duty shock absorber and a stiffer strut rod bushing. Those both should be replaced in matched pairs, one on each side, for even handling and balanced braking, but the tire wear benefit will be seen just on the right side where it is needed.
If both front tires are wearing faster on the inner edges, the coil springs are sagged from age and must be replaced. Changing them is pretty easy to do but it's usually overlooked when the truck is being aligned. Experienced alignment mechanics will measure ride height, and they won't align the truck if it's low. Both tires will be tipped in on top, and with this design, parts and a lot of muscle and time are needed to make the changes. That will get the wheels standing straight up and down again but it doesn't address the wrong angle the I-beams are sitting at. The wheels and tires already make a horrendous arc as the truck bounces up and down on the road. With the wrong suspension geometry that arc gets exaggerated even worse causing accelerated tire wear. When you're told parts are needed to make alignment corrections, look at ride height first. New springs will restore that height and very often eliminate the need for changes on that part of the alignment.
Tuesday, October 22nd, 2013 AT 11:52 PM