Finally someone with a nice thorough understandable description of the problem! :)
First of all, if you still have the old sending unit, have a Dodge technician tell you if the correct one was replaced. I can't remember if your engine uses one or two units. I think it's one dual unit. If yours has two connector pins, one is for the gauge and the other one is for the "Check Gauges" warning light. The part for the gauge is a rheostat, and in those dual units, Chrysler had a real lot of trouble with them doing exactly what you are describing. The clue was that the warning light never came on because that section was working fine. That half is just an on / off switch that turns on, as I recall, at around 7-15 psi.
Some engines had two separate sending units, one for the light and one for the gauge. Each had only one terminal. Is it possible your guy changed the one for the light by mistake? It is pretty small compared to a sending unit for a gauge and is very inexpensive.
A worn oil pump will cause low pressure all the time once the oil is warmed up and thinned out. If you ever see high or normal oil pressure, you know the pump is not worn out. Some things that could happen though include debris in the pressure relief valve or a loose pickup screen.
Something stuck in the relief valve could cause it to dump oil back into the oil pan, or it could block the passage causing higher than normal oil pressure.
A loose or mispositioned pickup tube and screen could allow it to draw in air instead of oil, but I doubt this would happen at idle or low speeds.
The worst thing I can think of is a spun crankshaft or connecting rod bearing. You would normally hear a loud knocking noise, but as the bearing spins, it will at times block the oil feed hole thereby preventing loss of oil pressure momentarily. I don't think this is the problem either because it wouldn't have lasted very long.
I would suggest finding someone who would install a mechanical oil pressure gauge in place of the warning light switch, and tie it to the front or rear of the hood so you can watch it while you drive. By comparing the readings to your dash gauge, you can figure out if it's a gauge or oil system problem.
Low pressure at high engine speeds has also been known to happen when the bottom of the oil pan is dented blocking the pickup screen. One other thing to look at is if the technician used teflon tape on the new sending unit. Teflon tape is used to help seal the threads against leaking, but it also insulates the electrical connection. Both parts of the sending unit are grounded through the threads. Teflon tape usually won't make an intermittent connection. You also can't tell if the connection is good by observing the Check Gauges light with the engine off and ignition switch on because the stopped charging system will also turn that light on.
Friday, March 20th, 2009 AT 3:54 PM