QuestionMy 318 fi '88 Dodge Ram 250 van starts and runs well in warmer temps, but backfires while running.
As soon as temps drop,engine will turn over,but won't start.
A mechanic checked the engine with his diagnostic
computor, but no codes were found. He installed a new coil,dist. cap, and rotor. The engine started,
but by then the temp had gone up +10 degrees into the forties and the next morning the same no
start problem was still there.Money out-problem not corrected. I have installed new Bosch plugs and a new air intake filter recently. Thats about it. If you are unable to answer this question, could you relay my info to the Dodge person. Thanks!
AnswerHi John,
I don't have any experience with the 318 V-8 FI but I am familiar with the smaller V-6 FI engines. The main sensor that sets the fuel mixture for temperature conditions is the coolant temp sensor. It is usually located very near the thermostat housing (it is screwed into the head) and has 2 wires (black/light blue, tan/white). So try and find that and open its disconnect plug. Then with an ohmmeter check what the resistance across the terminals of the sensor is: It should read 7,000 to 13,000 ohms at 70F and probably in the range of 20,000 ohms near freezing (though that is an extrapolation). Once the coolant is warm (200F) it should read 700 to 1,000 ohms. There may also be a temp sensor on the intake manifold if it is a multiport injection system (black/light blue, black/red) or the throttle body if it is a throttle body injector only system. Those sensors would read very similar ohms as the coolant temp sensor I listed above. You could have a sensor that is off value but not so much as to set a fault code. The other possible reason is a faulty MAP sensor which similarly could be off value but not enough to ser a fault code. A mechanic with a Chrysler diagnositic readout box (DRB II) can actually read the MAP and tell if it is measuring the local atmospheric pressure correctly or not as a test of the calibration. I had a MAP sensor fail in that way on my 2.5L FI engine; it wouldn't start, didn't set a code, and the Chrysler tune-up mechanic diagnosed it and put in a new MAP with only 15' of labor charge.
And then there is the oxygen sensor, which given the backfiring I would wonder about as well. It too should set a code, but I don't believe it has any role to play until the engine is warmed up so it wouldn't keep you from starting.
So that is my best suggestion. I can't get to Pete Lacey (dodge guy) any easier than you can, unfortunately.
But good luck on checking out those resistance values. The other thing you might want to check is the pressure in the fuel injection system. If it is multiport it should be around 55 psi.
Roland
