'89 F150 2WD 5 Speed 5.0L / 60K Miles
Changed oil , filter and oil pressure sending unit , cap and rotor good , wires good , plugs good , new TPS , cleaned IAC , have also checked wiring to Oil Pressure Sending Unit.

The problem I am experiencing is Oil Pressure Gauge will fluctuate and fall all the way down past low , I have checked wiring and replaced sending unit , when this happens there is no change in motor operation , idle remains the same and no valve train or other bearing noise what-so-ever.
Also intermittently will spit / pop back in intake at low rpm under load , sometimes runs normal and sometimes acts like extremely lean / weak with backfire through intake.
I am curious whether Oil Pressure Gauge is controlled by ECM , I have been told that these trucks suffered from gauge cluster problems as well as ECM issues , my thought is if sending unit is fed into ECM and ECM controls signal to gauge then I may very well have a ECM problem .
When cold all seems to be fine , oil pressure gauge issues seem to increase the longer truck is running indicating a possible breakdown regarding temperature , yesterday the gauge was pretty much constantly doing this fluctuating and dropping all the way , I had a friend watch and report gauge readings while I stood by motor with hood up , again with pressure dropping below "L" on gauge , there was no idle rpm change , continued running smooth , and no valve or bearing noise ....................... I find it hard to believe it would be the gauge cluster seeing that all other gauges and cluster lighting are working just fine , I am thinking that gauge is controlled by ECM via Sending Unit signal and that I have a bad ECM which may explain the other intermittent problems as well .
If anyone could give me some input on this I would greatly appreciate it .

 

No the ECM does not control the oil pressure gauge and has nothing to do with it.
Just stick some new rod bearing and mains in it and run it for another 150,000 miles.

 

Test the oil pressure with a manual gauge and see what you get, its worth a shot.

 

So you believe the motor needs new main and rod bearings @ 60K ?
I will put a mechanical gauge on it first , I find it hard to believe it is the motor when it has no knocking on a cold start and no noise or rpm difference when gauge drops to 0 , I believe there may be another issue , I know the dash has been out , maybe I need to inspect the wiring better through the firewall , it does this intermittently as well , if it were a bearing issue I would imagine it would be a consistent problem , not a intermittent one , don't you agree ?

 

you did not say it was an intermittent problem. You said it has good oil pressure cold an low oil pressure when warmed up.
That sounds like a loose engine to me.

 

My mistake , the only time I will see this oil pressure problem is when it is normal road operating temps but it will come and go , sometimes worse than others but it comes and goes , it can be often or it can be once a day , or every 30 minutes , but if the motor was really worn out in my experience when the pressure dropped below the "L" on the gauge it would be making noise ( valvetrain and or bottom end , when the pressure drops it keys the engine light , it has to be tied to the ECM for that to happen and that would also indicate the gauge is working per the the ECM but it does not rule out the ECM ................. , I am pulling codes tomorrow after clearing codes , going to see what it has loaded after a fresh run yesterday , also there is no idle speed change which would normally happen as well if oil pressure were to drop to what would appear 0 level , I will get a galv. nipple and tee and run the analog gauge with the sending unit , I hate to do it but in the absence of all the above , I feel it is a gauge issue , whether by ECM or a possible short or break somewhere I have not seen , you never know , I will also trace the oil pressure sending unit signal wire yet further , maybe I can find some online wiring schematics, after I put the analog oil gauge on it , I will be able to see both at the same time . I will let you know , but if any other ideas please let me know , yesterday I had a friend reporting the gauge readings in the cab to me and when it was below the "L" I was over the motor and no noise whatsoever and no idle change either , just does not sound like the gauge is telling me the truth , but we will see , I am hoping maybe someone out there has had the same issue and can send me to the problem as well , or maybe when I pull the codes it will give me a bad computer code / ram failure, I just have a hard time believing the motor is worn out at 60K , I got the truck from a one owner - 89 yr old country gentleman , got complete maintenance records , I just have a hard time accepting it is the motor I guess , will know more tomorrow

 

Low oil pressure will not trigger a Check Engine Light. The PCM has no feedback from any sensor to trigger the light. The factory gauges are well known, especially the oil pressure sensor, to be considered "for reference only".

 

So why would the engine light flash each time my oil pressure dropped ?
Guess I will have to wait to get the codes and see what they are and what relation they could possibly have with the cluster if that is the case , all other gauges are working fine , just oil pressure that is acting up and each time it has dropped the engine light has flashed , guess I am on a mission . I will let you know what I find .

 

The lamp that flashes is the "Engine Warning Indicator".
It has nothing to do with codes or the computer.
It comes on with the Coolant Temperature sender (not the ECT sensor) or the Oil Pressure Sender.

The Check Engine Light (CEL or MIL) is a different lamp on the cluster that is connected to the computer.

 

Well that is odd , because every time the oil pressure gauge dropped that light flashed , the ECT is new , was replaced by the previous owners shop less than a year ago , guess I will see if it has codes on it in the morning and see what they are if any , if not any pointing to a problem then I will put the analog oil gauge on and check what that has to say , other than that if that light is flashing at the same time as the oil pressure is dropping and I confirm pressure with analog gauge then I guess that would indicate that the cluster is bad , I will see what happens tomorrow .

 

A wiring diagram of your "Engine Warning Indicator":



/

 

Thankyou for that schematic , that shows me exactly what you have said and shows me the role of this warming light , this is my first Ford ever so I am in unfamiliar territory regarding controls and such , I am going to see if there are any codes this morning and will get a analog gauge to double check the oil pressure issue , I had a friend tell me this morning he has seen clusters fail and do this , I have seen that older ones had a voltage regulator that could be replaced but then I am only having a single gauge issue , if it were a "voltage regulator" for the cluster I am fairly sure it would affect the entire cluster .
First things first though , will check codes and then install second gauge to verify loss of oil pressure , thanks allot for that schematic .

 

Your truck does not have a cluster voltage regulator.

 

Also from what you have said and this shows me , the "check engine" is not included in the cluster , does this vehicle have one ? And if so could you tell me about where it is , and if it detected a fault would it stay on ?
This truck sat allot previously and various controls have failed since I have started driving it regularly which is normal for the most part , the main high pressure fuel died , the tps failed as well as the clutch starter switch , I have not seen a "check engine" light come on at any time as I thought the warning indicator was the "check engine" light , the bulb may be bad or possibly owner removed bulb , something else I will have to look at if it has one because I have not seen it at all , the ECT looks brand new also and I do have the reciepts showing recent replacement of that and cap , rotor , wires and coil , obviously people were sitting on hood at the fender wells because hood was pushed down on each side badly which I fixed with hood hinges , when it rained water would fall on top of coil and I am thinking that is what killed coil more than likely being in a very wet environment , the rest was probably wear and tear .
I know I am a broken record regarding the motor but I am hoping it is not the motor , I would much rather replace the cluster than a motor , I just hope I have not been duped regarding mileage on this vehicle , the motor is clean , not power washed clean but clean and like I said when the pressure drops there is no engine noise that indicates the drop which I would think would be present especially if it was high mileage and worn out .
I will find out more today , I am going to get the stuff here very shortly and get some more info to go on , at least verify the loss of oil pressure or rule it out .
Really appreciate your help , I was a member of another Forum and I did not get any real help there , I found you guys and registered and I am glad I did , Thankyou for your time and patience
I used to turn wrenches but have always been a GM man , worked at a GM dealership as a mechanic for years , now I am a Ford owner for the first time in my life , so out of my comfort zone .

 

Exactly , I had figured that much out in my searches but it really did not matter , even if it did , if it were bad it would have affected all my cluster , not just one gauge , Thanks again for your help !