Hey guys, just have a little issue with my truck and wondering if anyone has some input. Sometimes, not all the time, when i come to a stoplight and my truck idles the oil flow gauge drops to below zero and my check engine light comes on. My truck seems to run a little rough when idleing so I am wondering if there is a bigger issue I should be looking for. I recently changed out my oilpan gasket so my oilpump was removed and put back it. Everything went back into the truck in the right place [as far as i could tell]. I obviously did not do something right as I did not have this issue before I did the oilpan job. The strange thing is that this does not happen all the time. Anyone have any ideas? Thanks. Truck is a 1987 4.9 I6 Standard

 

Oil pressure gauge in dash is just a glorified idiot light. It will register anywhere between "L" and "H" when the sender registers at least 7psi of pressure. I would check oil psi with a mechanical gauge to verify.

Is there any valvetrain noise or unusual engine noise now when its running? I'm thinking your electrical output on the truck is dipping or otherwise showing its a** and causing high resistance, which would affect the physical location of the needle. Higher resistance, the gauge will read high, and low, to the other end.

Yours being a 4.9, I'm betting on poor grounds or higher resistance caused by another problem. For example.. when I saw my oil gauge read really high, it turned out to be the Fender mounted relay solenoid. I have no idea, but swapping it out for a new one worked.

Case in point, I chased a High coolant temp reading on the dash for years, even installing an aftermarket Autometer analog swep gauge, but it still would run into the warning of "H"more accessories added to the problem. 6 years went by... After the PCM/ECM failed (4.9l trucks are plagued by the leaking/blown capacitors) earlier this year, I did lots of routine maintenance. I removed the "T" that threaded into the OEM temperature sender location and removed both the Autometer and OEM senders from the "T". IT was then I realized why my gauges weren't reading correctly....the threaded portion of the block was rusted in about 7-8 threads which is where most of the contact for the "T" and the OEM temperature sender was located. I grabbed my battery cable cleaning tool and cleaned the inside of the threaded portion well. Now, I never have a high resistance problem or an overheat condition shown by the dash gauge or the Autometer gauge.

 

Thanks for the reply! There seems to be a fair bit of vibration when idling but this is all the time, not just when the oil flow gauge drops down. Im thinkin its normal vibration as it is an old truck and another contributing factor to the vibration is one of my engine mounts is almost toast.

It very well could be corrosion on a sensor but it just seems odd how the gauge work fine 90% of the time and the other ten percent the gauge drops only when I am idling. I guess my next course of action will be to refer to my Haynes manual and check where the oil flow sensor is and see if there is corrosion along the lines somewhere.

 

'87 still used a variable resistance sender for the gauge. As timbersteel mentioned, a mechanical gauge is the best way to know for sure what the oil pressure is doing.

 

Okay thanks guys. Im going to look into maybe getting a mechanical gauge this weekend. Ill let you guys know how it works out.

 

What's your tach say? sounds like your oil pressure drops because your engine drops to very near stall speed at idle, not a oil pressure or gauge problem but a problem in the way your truck is running.

Intermittent or not running very weak at idle foot on brake, rpm's drop so low about to stall but doesn't. Slow rpms causing near complete loss of oil pressure also causing check engine light to light up, find what causes that correct oil pressure issues same time. fair bit of vibration when idling is not normal I don't care how old it is!

 

My RPMs seem normal when I am idling. They sit around 1000 RPM. As far as the vibration goes I definitely have more than I should so mabye it is tied into the oil pressure issue.