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Okay, since this is such a strange issue I will try to explain EVERYTHING with this engine.
385 ci LT4 with all the internal goodies and a Speed Pro HV oil pump, built by SDPC about 12 years ago. I have not started it until about 5 months ago, oil pressure was at about 60ish psi. I just recently started driving it around the neighborhood to start the tuning process.
385ci LT4 (10.4 to 1 CR) dynoed at 485 FWHP w/o procharger
12 rib F1A Procharger with approx 10-12 psi boost at 6300 rpm
60 lb/hr motrons, racetronix 255 Intake, boost-a-pump
24x conversion
7 qt moroso pan
ss headers and exhaust
10w30 conventional oil while I am tuning the engine, then I plan to go to synthentic
When I first started putting around it didn't seem to move much from 60ish psi (but I will fully admit I wasn't watching it that closely, once I started it and saw 60s psi I stopped really watching). Now I have been driving for longer periods of time (10-15 minutes) and as the engine heats up the oil pressure gets higher and higher to about 85 psi at idle, it goes lower when I get on it to 60s-70s. While driving it goes back down to 70s but when I stop at an intersection it goes high again to mid 80s. At an idle when it goes high if I burp the throttle it lowers the psi to the 70s.
I changed the oil pressure sensor and it is still reading the same.
I am thinking the only thing it could be is the pressure relief valve in the oil pump, but I am not too sure. I really don't want to have to change it, but if that is the next logical thing to check/change then I guess I will have to.
Or, is this really a problem? It doesn't make any sense how this could be happening (higher at idle), but I am just not familiar with how an oil pump functions.
Has anyone seen anything like this, or have any recommendations?
Does it still have the oil bypass in the oil filter adapter?
No there is no oil filter adapter, the oil filter is directly on the block.
I have started the car a couple more times since replacing the oil pressure sensor and it seems to be holding study at 75-76psi. Previously, with the old sensor it would drop all the way down to low 60's after the engine was cool.
Could it have been the sensor? And my engine just runs mid 70's psi? I saw some other posts that said the sensors can go bad and really act up when they get hot and that is what mine was doing. Is mid 70's still too high for the parts on the engine? I have no idea the original build specs on the engine, but I would assume SDPC knew what they were doing this was an engine for their own shop truck.
you wouldn't be able to tell if the filter bypass has been deleted by looking at it, the actual part the filter screws onto is removed and replaced with a piece that looks identical (except that there's a passage missing).
You'd have to unscrew the filter to see if it's been done, but the adapter would look like this if it has
Otherwise there will be a passage in the area that's beveled in the picture (side towards the block) if you've still got your bypass intact
Throw a mechanical gauge on it and confirm the dash isn't wrong. 60 to 70 on cold is up there but it def shouldn't go higher when warm just putting around.
An oil filter "adapter" is under the filter against he block. If you're stock adapter still has the stock bypass this could be your problem. You can either replace it with one like shown above or drill and tap with NPT on the stocker. Either works
No need for a HV pump on a sbc, a melling select will do anything you need and you can change the springs to get the desired pressure. I found with FI setups or dry sump motors that the oil pump will have to work a bit harder to keep the correct pressure, but a vacuum pump setup will do this too.
Mine had an issue like this except the oil psi would go anywhere from 70-75psi cold start up to like 5-7 at hot idle. After replacing the oil sending unit it's about 65ish cold start up and 20-25 hot idle. I've see it hit 18psi on a very hot day during traffic.
Investigating this oil pressure issue has now sent me down the rabbit hole on my instrument cluster setup. I started this project about 10 years ago, and I am finally close to done but about 8 years ago I thought it would be cool to make a custom Gauge cluster with a 5" Autometer 200mph Speedo and 5" Autometer Tach then use LED digital gauges for all the others, I bought Nordskog gauges (who has since gone out of business). I was checking the oil pressure with the new sensor installed and noticed that my water temp gauge was saying 51 degrees, when it was about 80 degrees out. I checked the documentation I still had and they "strongly" recommended using their sending units. I for some reason ignored that recommendation back in the day and thought my GM oem units would be fine. So I just ordered what I think are equivalent sending units (since Nordskog is out of business). I will have to find a mounting location for the new water temp sending unit and run a dedicated line to the gauge. I also bought the oil pressure sending unit, and a mechanical gauge to confirm which sending unit is accurate. Man I hate redoing stuff because I was stupid!
Does anyone have any recommendations of a location to mount another water temp sensor, since I cannot replace the oem unit that the ECU reads.
Also, is it true that 98-02 Fbody ECU's do not read the oil pressure? I looked through all my wiring diagrams and cannot find a pinout for oil pressure. If the oem sensor does not work with the gauge I can just replace with the aftermarket unit I just bought.
Throw a mechanical gauge on it and confirm the dash isn't wrong. 60 to 70 on cold is up there but it def shouldn't go higher when warm just putting around.
Btw, single 255 with all that potential power?
I will be doing the mechanical gauge check soon.
I am using the Walbro 255 racetronix kit with a Boost-a-pump, I think that is good to about 650-700 RWHP. That is all I am shooting for with my car. The parts I have would be capable of MUCH more, but I don't want to have to upgrade the fuel system, transmission (again), rearend (stronger axles and posi unit). I had to set a limit...
I am using the Walbro 255 racetronix kit with a Boost-a-pump, I think that is good to about 650-700 RWHP. That is all I am shooting for with my car. The parts I have would be capable of MUCH more, but I don't want to have to upgrade the fuel system, transmission (again), rearend (stronger axles and posi unit). I had to set a limit...
I understand it's important to set a limit but a single 255 at 650-700rwhp is far from ideal. I would have at least a pair of those using stock lines, rails etc in there.
Investigating this oil pressure issue has now sent me down the rabbit hole on my instrument cluster setup. I started this project about 10 years ago, and I am finally close to done but about 8 years ago I thought it would be cool to make a custom Gauge cluster with a 5" Autometer 200mph Speedo and 5" Autometer Tach then use LED digital gauges for all the others, I bought Nordskog gauges (who has since gone out of business). I was checking the oil pressure with the new sensor installed and noticed that my water temp gauge was saying 51 degrees, when it was about 80 degrees out. I checked the documentation I still had and they "strongly" recommended using their sending units. I for some reason ignored that recommendation back in the day and thought my GM oem units would be fine. So I just ordered what I think are equivalent sending units (since Nordskog is out of business). I will have to find a mounting location for the new water temp sending unit and run a dedicated line to the gauge. I also bought the oil pressure sending unit, and a mechanical gauge to confirm which sending unit is accurate. Man I hate redoing stuff because I was stupid!
Does anyone have any recommendations of a location to mount another water temp sensor, since I cannot replace the oem unit that the ECU reads.
Also, is it true that 98-02 Fbody ECU's do not read the oil pressure? I looked through all my wiring diagrams and cannot find a pinout for oil pressure. If the oem sensor does not work with the gauge I can just replace with the aftermarket unit I just bought.
There is two coolant sensors, one in the head and one in the water pump, one is listed as a switch and the other a sensor. Should have one for the computer and the other for the gauge.
Oil pressure is not something the computer cares about, but it is wired to the gauge, so run any sender your gauge supports.
08-11-2015, 12:33 AM
Strange 385ci LT4 Oil Pressure Issue 
