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Jerry
Jerry, Master Mechanic
Category: Dodge
Satisfied Customers: 1740
Experience:  ASE Master tech, 30 years exp. troubleshooter, driveability tech
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1999 caravan 3.33 =backfires when ac on or acelerates

Customer Question

1999 caravan 3.33 =backfires when ac on or acelerates
Submitted: 1 year ago.
Category: Dodge
Expert:  Dodgerench replied 1 year ago.
Hi, my name is Ed. Welcome to JustAnswer!. I'd be looking for a secondary ignition system problem for this sort of complaint. By that, I mean... a burned out plug wire or a bad ignition coil. This is a "distributorless" ignition-type engine, one that uses a sequential firing of three coils to service six cylinders. What it does is to fire an individual coil once per engine revolution to send spark to two cylinders at the same time. Since only one of the "companion" cylinders will be on a compression cycle at the time, the other cylinder receives what we call the waste spark, a firing of the coil that completes the circuit between it and the companion cylinder, but normally does nothing on the cylinder on its exhaust stroke. That's unless the companion cylinder that's on its waste cycle hadn't fired earlier. When the cylinder misfires earlier, it's given another chance to ignite during the exhaust (or valve overlap) period, when there isn't normally a combustible mixture available. Now that it's given that opportunity, waste spark may ignite that unspent mixture at a time when both the intake and exhaust valves are open at the same time (overlap period), allowing the ignited fuel to backfire through the intake manifold. It goes POP. It's usually a bad plug wire that causes this, but a broken coil tower (or one shorted internally) may shunt spark away from being sent to the intended cylinder. I'd check each spark plug for electrical continuity by using a digital ohmmeter first. Expect to see 20,000 (20K) ohms of resistance OR LESS on each plug wire between ends. More resistance is bad, less checks OK. I just set my meter to the 20K scale and let it be the judge... actual continuity in any amount less than 20,000 ohms constitutes an actual circuit and more than 20K will mean you have a burned-out plug wire. Chances of a bad coil pack are pretty slim, compared to plug wire failure. It becomes a bit more difficult to test coil output than plug wire impedance, so let's start with the plug wires first. You may get lucky and find that the wire actually crumbles in your fingers when trying to disconnect it from either the coil or plug wire, which is where failure usually happens... at the outer ends. Crumbling plug wires (with blackened soot ends) definitely simplifies matters. Let me know if you have any problems or questions and I'll be glad to help. Good luck! Ed