You're right gizmoguy about code 12. I'm accustomed to ignoring it because it is so common. On some scanners the description comes up as "Switched 12 Volts Lost" so I assumed all these years it was from the ignition switch. I just checked my '88 Grand Caravan and indeed there is no code 12 coming up.
Willi1, what I'm referring to about measuring the voltages at the injectors only pertains to a no-start condition and the possibility of corroded terminals in the electrical connector. '88 models were just changing over from the 12 volts coming from the ignition switch to coming from the automatic shutdown (ASD) relay. The ASD relay only turns on during engine rotation, (cranking or running). I don't have the service manual for that year to see which system you have so I'll describe both systems.
This probably doesn't apply to your problem because you're getting fuel on the spark plugs. That suggests the injectors are working. Nevertheless, with the ignition switch in the "run" position, measure the voltage on one of he injectors. You should find full battery voltage on both wires. If you do, that voltage comes from the ignition switch. If you find 0 volts, measure again while a helper is cranking the engine. If you find voltage now, it's coming through the ASD relay. That's how all models worked by around 1988 to '91. Either way, that proves voltage is at the injectors and you know the common feed circuit is okay. If any of the other wires had a bad connection they would affect just that one injector so you'd have a misfire, not a no-start condition.
For this no-start condition to be caused by broken, corroded, or disconnected wires, all six of them would have to be affected which isn't very likely. The Engine Computer can cause no injectors to fire but that isn't common either. For a 6-cylinder engine there are going to be two, three, or six separate drivers, (electronic switches). They rarely fail to begin with and it's not likely they would all fail at the same time, and especially not from just sitting unused for a few years. It's more likely a fuse link would be burned open, but they feed multiple things so you would have different symptoms. You wouldn't be looking in the injector circuits. The computer itself could be the problem, but Chrysler had extremely little trouble with theirs.
The last thing that would cause the injectors to not fire is missing signals from the camshaft position sensor in the distributor, but those signals are needed for the computer to trigger the ignition coil and create the spark. You have spark so you know the sensors are working and the ASD relay is turning on.
All you can check on the second injector wire is for the presence of battery voltage. In the extremely rare event is it missing when you have 12 volts on the other wire, the coil inside the injector is either open, the wire is grounded, or the computer is shorted and keeping that wire grounded. That means that injector would be staying open and filling the cylinders on that side of the engine with raw gas. By now you would be smelling the gas and probably seeing the level go up on the oil dipstick. That is so uncommon there's no need to elaborate on diagnosing it.
The short pulse width gizmoguy referred to is related to when the engine is running. Those pulses occur and are over with so quickly that your voltmeter won't pick them up. Even with the engine running, the meter will display near battery voltage, but it is likely to be bouncing around causing some confusion. To simplify the description, lets say you have 14 volts feeding the injector while the engine is running. (It's higher than battery voltage because the alternator puts out higher voltage). The second injector wire will also have 14 volts, ... Most of the time. The computer will ground that second wire for a few milliseconds to turn the injector on, then it will go back to an open circuit and the injector will turn back off. During that firing time, the voltage on the second wire will be 0 volts. That's what you need the oscilloscope to see.
A digital voltmeter takes a reading, thinks about it, then displays it in the readout. Then it takes another reading, analyzes it, and displays it again. As it takes those readings, most of the time it's going to see 14 volts, but every once in a while it's going to take one when the voltage is at 0 volts. Since it tries to somewhat smooth out the readings, what you will see is not a steady 14 volts, but rather a confusing bouncing voltage. That is to be expected, but you wouldn't be bothering to take those readings if the engine was running. The voltage readings will be much more stable during cranking because there are fewer pulses of 0 volts in a given amount of time. The older analog meters with a pointer smooth the readings out a lot because it takes the pointer time to respond. You won't get near the accuracy of a digital meter, but without straining to do the math in my head, you might expect to see 14.0 volts on one wire and around 13.8 volts on the second one. Again, that pertains to an engine that's running. You wouldn't be taking those measurements if it was running.
Now that that's all cleared up, I still suspect it's bad gas. There's no convenient test port for fuel pressure on the fuel rail, so an easy way to pump some old gas out is to disconnect the larger of the two rubber hoses where they go from the right strut tower to the engine, and run a hose into a can. The fuel pump will only run for one second after you turn on the ignition switch and during cranking. Bypass the fuel pump relay to keep the pump running. For '88 models, the relays will be bolted to the left inner fender. If it comes to that I'll have to try to find the service manual to figure out which relay is for the pump. You might be able to identify it by placing your fingers on them while a helper turns the ignition switch on. You are looking for the one that clicks on, then clicks off about one second later. There might be two separate relays that do that, the fuel pump and the ASD relays. Use a jumper wire or clip lead on the wire side of the connector to connect the two fatter wires. You should hear the pump running when you're near the tank.
Once you add new gas you can run the pump like that too to circulate it to the fuel rail after the hose is reconnected. Even just by cranking the engine the pump will be running so the new fuel will get to the engine within a few seconds. It circulates through the pressure regulator and back to the tank, but that still leaves a little old gas inside the injector. You still might have to crank a little to get that old gas out.
If you want to experiment a little, throw a little old gas on the ground and throw a lit match on it to see if the fuel burns. We had two cars come in with no-start conditions two days apart. After scratching his head for a whole day on the first one, the mechanic did that and the fuel put the match out! Turns out they both had just bought gas from the same place. One of those cars happened to be a New Yorker too. The suspicion was the station had just had an additive put in the tank prior to having it filled with gasoline, and those customers got a tankful of additive, not gas. I don't know if that's accurate but it sounded good at the time!
Once you do get the engine started, be sure to change the oil right away. There will have been a pile of raw fuel washing down the cylinders into the oil. Even though it might not be gasoline any longer, it still will dilute the oil and destroy its lubricating properties. I won't bother describing what happened to my mother's minivan but once I got it running, she made it only 50 miles before the engine came apart. Later I found the oil was two quarts overfull with fuel.
By the way, if you guys need an oscilloscope, I have ten for sale!
Saturday, June 4th, 2011 AT 11:08 PM