Inconsistent oil pressure gauge

[Washington_XJ]

A few days back I was driving along watching my oil pres. gauge and wondering if it was acting normal. Off the gas the oil pres. needle would drop down to about a quarter to one-third of the gauge but on the gas it would rise all the way up to almost three-quarters. What was really weird is if I let off the gas after flooring it, the needle would delay for a second and then spike to full oil pres. (all the way to the right), then drop back down like normal.

Ok so here's the strange part, I changed my oil today and now the oil pressure gauge just stays at 50 the whole time - even off the gas. Does anyone have a clue as to whats causing this?

p.s. I changed oil weights from 10-30 to 10-40

[Allen]

How long was the old oil in the jeep? It sounds like it might have been breaking down and loosing it's viscosity. Thin oil would act kinda like that.

Oil pressure in an engine isnt like pressure in a hydraulic pump. usually the hydraulic system is closed with oil pressure waiting at some valve to do some redirection for you, causing motion in some part somewhere. Oil pressure in an engine is backpressure. It flows freely unobstructed thru the block and pressure is only obtained because of the small porting of the oil journals. Thinner oil would pass easyer thru the ports in the oil passageways causing low pressure when the pump is running at idle. Also there could be a spike when it changes from high load to idle quickly, as the journals have not yet depressurized.

There could be other factors that cause oil pressure to vary, but they would be consistant regardless of oil. Things like cams or pumps with issues.

[TRobertsRN]

What Jeep and engine do you have?

What you describe is very typical of an AMC 360 or 401.

If you have one of these I am very confused as to it going away with a switch to 10-40.

If you don't have one of these engines is it possible this started shortly after your oil change previous to this and you didn't notice untill recently? Maybe someone put in the wrong oil, but I can't imagine a lube shop would have anything less than a 10-30.

[RalphA]

I can only add that our TJ oil pressure gauge went through the same motions for a month. I was going to change the pressure sending unit on the engine but a friend took an electronic cleaning spray and cleaned inside where the harness wires hooked to the unit. The next week all was back to normal. That was four months ago.

[GRUNT]

I too had this problem, I just got a new pressure sending unit from NAPA and put it on. Getting my big hands down there to change it was the only had part.

I didn't think about cleaning the connections...but mine was 14 years old. :( No matter though as it is a cheap part.

[XJINAZ]

On my 4.0, I can move the wire around at the connector and effect the gage reading. I know my readings are questionable due to a bad connection. I would add that if you break the circuit the gage will read high.

[Washington_XJ]

Thank you all for the replies and sorry for the delayed response as I have been at disneyland for the last few days.

To answer some questions I have a 1994 Cherokee 4.0L - it is a very clean engine with less than 60k miles on it. And I do all my own oils changes so there is no question as to the oil that was put in last. I have used nothing but 10W-30 Castrol Syntech (full synthetic) oil since I bought the jeep about 2 years ago. The inconsistent oil pressure IS a new problem as far as oil changes go, however as someone mentioned it is possible it started shortly after the last change and I just didnt notice it until before my switch to 10W-40.

My question - is the oil sending unit actually pressurized by oil through all the way up to the gauge or does it just send an electric signal?

[TRobertsRN]

Nearly all modern vehicles have a electrical oil pressure gage that gets it's signal from a sending unit near where the oil filter goes.

Mechanical gage is more accurate and dependable.

So likely you have an electrical gage.

Thank you all for the replies and sorry for the delayed response as I have been at disneyland for the last few days.

To answer some questions I have a 1994 Cherokee 4.0L - it is a very clean engine with less than 60k miles on it. And I do all my own oils changes so there is no question as to the oil that was put in last. I have used nothing but 10W-30 Castrol Syntech (full synthetic) oil since I bought the jeep about 2 years ago. The inconsistent oil pressure IS a new problem as far as oil changes go, however as someone mentioned it is possible it started shortly after the last change and I just didnt notice it until before my switch to 10W-40.

My question - is the oil sending unit actually pressurized by oil through all the way up to the gauge or does it just send an electric signal?