Q: Missing and popping under heavy acceleration.

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I have a 1996 Dodge ram 1500 with a 5.2 318. It is missing or popping when I floor it. I just replaced the engine and tranmission with freshly rebuilt ones from a local shop. The plugs and wires are new. The only thing i can think of is the fuel sync from the distributor is slightly off from taking it out. Any ideas? I can not drive over 80 or even pass someone without it popping and missing. I know the intake plenum gaskets go bad on this year model but it was replaced when I did the engine swap. I'm just lost. No obdII codes are illuminated.

My car has 2000 miles.
My car has an automatic transmission.

Popping back through the intake, along with misfires, is a sign that the engine is running lean (not enough fuel). Here are a few things to check:

  • Check the fuel pressure: There is usually a port on the fuel rail. The pressure should be 45 to 54 psi.

  • If the fuel pressure is low: Replace the fuel pump and filter.

  • If the fuel pressure is high, replace the fuel pressure regulator on top of the fuel tank.

  • If you have access to a scan tool with data, you can verify if it is running lean on a test drive.

  • Look at the readings from the upstream O2 sensors, B1 S1, B2 S1.

  • Look at the readings when at full throttle and the engine is popping.

  • Lean is 0.100 and rich is 0.900. These are rough numbers, it could be anywhere in between. This step is optional, but I like to see verification before I move on.

I wouldn’t worry about the distributor right now. It does not control ignition timing like on older vehicles. It is used as a camshaft position sensor. It would set a code if you were too far off.

There are two sensors that control most of the fuel delivery: TPS (throttle position sensor) and MAP (manifold absolute pressure) sensor. You can test with a multi-meter if you don’t have a scan tool. A lab scope, however, is the best tool for measuring the full range of the sensor, but in this case, WOT (wide open throttle) is the only reading we are interested in.

Both sensors have 3 wires. 5 volts, sensor reading, and ground. The sensor reading wire is in the middle on both. With the key on and engine off, measure the voltage on the middle wire with the sensor connected. This is call back-probing. TPS at idle should be around 0.5 volts, at WOT it should be at least 3.5 volts. Do the same with the MAP. With the engine off, the middle wire should be at least 3.5 volts.

If you still haven’t found the problem, there are some more possibilities. It could be clogged fuel injectors, a wiring issue, or a defective Engine Control Module (ECM). A qualified mechanic from YourMechanic would be happy to assist you with this; a diagnostic inspection of your misfire condition would be a good place to start.

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