"Boy I hate winter!" It's easier to drop the tank if it's not full and the nuts for the straps aren't rusty. Before you go through that work though, borrow or rent a fuel pressure gauge to be sure you're on the right track. A lot of parts stores rent tools or borrow them for just a security deposit. Pressure regulators have been a huge problem for GM but I've never heard of a bad one on a Chrysler product.
Injectors will not cause the symptoms you described unless one is partially shorted. In that case, the engine computer will shut that circuit down to protect its driver transistor and you will have a check engine light and a diagnostic fault code for a single cylinder misfire. Here again, Chrysler has almost no trouble with injectors. GM just grabs a handful out of the bin and throws them into an engine on the assembly line. They WILL flow at different rates and later cause some cylinders to be lean and some to be rich. The oxygen sensors report a proper mixture but the lean cylinders cause a misfire condition. Chrysler gets theirs from Bosch after they have been flow-matched. Because they are a matched set, they rarely cause trouble.
What you should find with the pressure gauge is pressure drops when you are coasting but remains up during acceleration. Here's a copy / paste version of my story:
Think of a fellow about to jump out of a perfectly good airplane. Two forces are acting on him. The vacuum from the air flow past the open door, and the huge guy behind him pushing him out. If vacuum increases, that guy pushing him doesn't need to be so big.
Now compare that to a molecule of fuel waiting to jump from the tip of the injector. Two forces are acting on it too; intake manifold vacuum and fuel pressure. When manifold vacuum goes up during coasting, that pulls harder on the molecule and his buddies so more of them are pulled into the engine. That would result in an excessively rich mixture during coasting. To prevent that, a vacuum hose going to the fuel pressure regulator causes the fuel pressure setting to go down. The net difference in the two forces stays the same so no rich condition occurs.
The fuel pressure regulator is a spring-loaded diaphragm with a return port to send unused fuel back to the tank. When increased vacuum helps that spring relax, it is much easier for the extra fuel to get through the port and go back to the tank. That means the pump is pushing fuel against less of a restriction so it is easier to move more fuel.
That's why the pressure varies. What you should look for when the problem occurs is the more you press the gas pedal, the higher the fuel pressure goes. If it fails to increase or starts to drop off, suspect the pickup sock in the tank.
The other clue leading to the fuel supply system is that's the only thing that isn't monitored by the engine computer. Everything else will cause a diagnostic fault code to set in the computer's memory, and most of those codes will cause the Check Engine light to turn on.
Another approach is to connect a scanner that can record live sensor data during a test drive. The "record" button is pressed when the problem occurs, then the recording can be played back later in the shop to look for problems. Because the data passes through the scanner's memory, the recording actually starts a few seconds before the button was pressed. Most scanners, such as Chrysler's DRB3, will display injector pulse width. The longer the pulse width, in milliseconds, the more time it has to spray in fuel. If no other problems are observed, but pulse width suddenly increases, that is an indication not enough fuel is entering the engine and the computer is trying to make up for that the only way it can. That's what will happen when pressure drops unexpectedly. Modifying pulse width can make up for a little drop in pressure but that's not the purpose of that capability. As pressure drops too low, fuel stops squirting from the injectors and there's nothing the computer can do to overcome that.
All of this assumes the engine is only losing power and not developing a misfiring single cylinder. Usually one misfiring cylinder is hard to feel, especially on the V-10, but the computer detects it from the slowdown of the crankshaft's rotational speed. When we feel a misfire, it's because of that slowdown. Beginning with the "On-Board Diagnostics, version 2, (OBD2) emissions systems in 1996, all vehicles can detect misfires and the stored fault code will even specify which cylinder. Only three things can cause a misfire. Loss of spark, fuel, or compression. Mechanical problems such as burned valves won't be intermittent. Same with loss of compression. That leaves spark and fuel. Sometimes the fastest way to figure out intermittent problems is to switch the injectors between a good and a bad cylinder to see if the problem moves to a different cylinder. Coils can be switched that way to on the newer engines that use coil-on-plug individual coils.
Saturday, November 27th, 2010 AT 8:04 PM