First off, yes I'm aware the stock oil pressure gauge is considered as innacurate as the stock boost gauge. I don't believe this. It may be slow to respond and each tick can mean the difference of as much as 20psi (when compared to a mechanical one), but like the stock temp gauge, it is accurate enough to tell you when things are normal, and when you have a problem.
Second off, I have no want/wish to pull the dash on every customers car who comes in asking me to "replace the stock gauge cause it reads low" with an aftermarket one.
Precurser: It's always bothered me why it seems most of the cars that come into the with motors that were just rebuilt would have "lower than normal" oil pressure. Both my grey car and Vinnie's car got brand new shortblocks and still our oil pressure would read second or third tick above the bottom large one while cruising. Throw on a mechanical gauge and everything reads great!
More so, I've wondered why the NA's cars would have 160K+ on the origonal/unopened engine and the oil pressure would read literally 1/2 way between the 2 large ticks on the gauge while cruising. The main reason an NA motor lasts longer than a TT is because it's not exposed to as much HEAT. This got me thinking about it how other components wear out faster, like the alternator, plugs, & wires...
Guys have tried replacing the stock oil pressure sending unit to find it made minimal difference. Some people have found a more noticable increase by simply cleaning the harness that plugs into it (common for bearing grease from an open axle boot and oil to get on it). However, on my grey & green cars, simply cleaning harness and replacing the oil pressure sender made almost no difference.
I think I found the solution.
Note: If the bearings in your motor have worn or you oil level is too low, this will cause low oil pressure. If you have bad lifter tick and the oil light flickers at idel, chances are motor is on its way out. This fix will only ensure that the gauge is reading properly. It will have nothing to do with actually improving the oil pressure inside the engine. Sorry.One last side track before I post the fix: Over time, wiring degrades as it's exposed to underhood temps. The reason you need to replace your plug wires is because as they age (exposed to heat over time), they don't conduct as well, and you get higher resistance.
Same thing happens to the wiring harness, specifically for the stock oil pressure gauge and low oil (dummy) light on the dash.The wire harness for the stock oil gauge starts by the throttle body on the intake plenum. There are 3 plugs. One is for the knock sensor (black plug), one is for the rear feul injectors(grey plug), and the last one (blue and black plug) is the harness for the stock oil gauge, dummy light, and the PS idel bump up wire attached to the PS pump. This harness can be replaced for about $35-45 from the dealer.
The test cars were a 95 Stealth TT and 95 VR4. The stealth has 90K on the origonal motor, the VR4 has barely 4000 miles on the new shortblock.
I wanted to replace the harness on my car because the connector kept comming off the sending unit and was partcially cut. For the heck of it, I measured the resistance of both the old and new harnesses. I don't remember the exact numbers as it's been some time, but I remember the resistance being close to 1/3 as much on the new harness as the old. (I could remessure for the technical guys who love to see numbers)
Simply replacing the harness and oil pressure sender, my oil gauge went from reading the bottom large tick, to almost 1/2 way between the large ticks. This is on a completely stock 90K mile TT motor. For the hell of it, I asked the owner of the 95 VR4 to experiment and also purchase a new harness. The results were the same. Gauge now reads closer to the top tick (about 2-3 ticks below the top one to be exact) and barely drops below the bottom large tick at idel on his car.
Conclusion: If you have no want or wish to buy an aftermarket oil pressure gauge, and the stock one is reading a little lower than you're comfortable with, consider replacing the whole harness when you replace the oil sender. (note: it's much easier to remove the intake plenum to replace it than to leave it on) It's probably cheeper to fix the stock one than it is to replace it with an aftermarket one too. If anything, it's easier to pull the intake than it is to pull the dash.
If you do end up doing this, PLEASE POST YOUR RESULTS! Also include the year, model, and mileage of your car.
