Oil Pressure issues--4.0 SOHC rebuild

[IronHorseDriver]

(Sorry for the lengthy post--I wanted to provide as much information as possible in hopes that someone on this forum can help)

OK, here is the background: I purchased a 2003 Ranger Edge project last winter. The engine is the 4.0 liter SOHC. The vehicle has 157K, and I bought it from the previous owner because the timing chain guides went, causing the timing to jump, and that was it for that engine. I decided to rebuild it myself (first rebuild FWIW), farming out the machine work to a highly-rated machine shop in this area. When I disassembled it, there was almost no wear on the bearings, and I trusted the machine shop to get the right bearings. According to them, everything was in standard limits, and the only work done was a polishing of the crankshaft. They did the heads, which included replacement of the bent valves.

I have an official shop manual on DVD, so I did everything “by the book” when I disassembled then reassembled the engine. I purchased a new DNJ brand oil pump. I used engine assembly lube when reassembling it. Finally, we got the engine back in the vehicle, and lo and behold, it started right up and sounded good.

However, I have an oil pressure issue. I am sure most, if not all of you, know that the oil pressure gauge is nothing more than an “idiot light”, and the pressure sender is a go/no-go pressure switch. When I started the engine cold, the oil pressure gauge showed normal, but after the engine reaches operating temperature, the pressure gauge went to “zero” (same as an oil light coming on) at idle. I did not want to run it with a “low” reading, but there were no noises that indicated the engine was starving for oil.

I finally got my mitts on a professional-grade oil pressure tester so I can get the precise oil pressure readings at different RPMs both cold and hot. Below are the readings.

(BTW, the shop manual states that there should be a minimum of 20 PSI at 2000 RPM, but no other pressure readings).

COLD READINGS:

1000 RPM: 8 PSI
2000 RPM: 18 PSI
3000 RPM: 19-20 PSI (highest reading)

HOT READINGS:

Idle (800 RPM): 4 PSI
1000 RPM: 5 PSI
2000 RPM: 11-12 PSI
3000 RPM: 19-20 PSI

So, as you can see, the oil pressure is below normal both cold and hot (and worse when hot).

I am looking for some advice. I obviously do not want to pull the engine out again. When I assembled the engine, I did not do any plastigage readings because I trusted that the machine shop did all of those tests (and one of the people who recommended that machine shop said that he had them do quite a few of his engines, and he stopped doing plastigage readings because he trusts them so much). Nevertheless, I am beating myself up for not doing that as a matter of insurance that the bearings were in proper tolerances.

In desperation, I changed the oil and filter. I decided to use a straight 30-weight non-detergent oil for initial break-in, but when I changed the oil I used 10W30 detergent oil.

But I have been thinking a lot about the oil pump. As previously mentioned, I purchased a new DNJ pump (standard, not high volume), and despite the american flag and eagle head on the box, the pump is made in China. I did an initial prime of the pump prior to install by pouring a bit of oil into the inlet then spinning it. Initially, it was very hard to spin. After the engine was assembled and installed, I made a makeshift oil pump primer (it uses a star shaft rather than a hex shaft so I had to make my own). I used a cordless drill to spin it until oil first came out the pressure sender port, then I installed the pressure sender then primed it until I got a pressure reading on the gauge.

I am open to suggestions. I would really love to hear from someone who may have built a 4.0 SOHC. What do others think about the pump? I know the basics of oil pumping and how pressure is developed, but could there be a possibility of a bad pump? Has anyone ever experienced issues with DNJ pumps? I am thinking of purchasing a Melling high-volume oil pump; will that possibly cure the issue? As for replacing the oil pump, I may be able to swap it out with the engine still in the vehicle (getting the cradle out will be a bear, but according to the shop manual I would need to lift the engine a inch or so and remove one or more cross members).

After all of this work, I am extremely depressed over this situation. I would greatly appreciate any advice that can be given by others.

 

[adsm08]

Well, that is a tough one. I have never dealt with DNJ parts, I tend to use Melling or Sealed Power if I don't use OEM parts.

A new pump may fix it, but then it might not. Don't beat yourself up about plasti-gauge though. The 4.0 SOHC is a modular engine, meaning that the block doesn't assume it's final shape until everything, including the oil pan, has been bolted down. This makes plasti-gauge inaccurate.


If your easiest and cheapest option is a new pump, try it. But I would be prepared to pull the engine again for bearings. But I like to expect the worst, that way I am happy and surprised when it doesn't happen.