3.9 v8 seems to be loosing prime

rlynch356

Well-known member
Friends 3.9... he drive it down 3.5 hours with 5-30 (I think) in it and right before it got raleigh the oil light started flickering at idle.
We changed the oil to 20-50 vr1, and he lost prime, got that sorted out, then he drove home 3.5hrs with no issues. Next day the oil light is on.
Level is fine,
Engine just rebuilt (maybe 1500 miles on it)
New oil pump gears were installed

My guess is one of 2 things

1) clogged pick up in the sump
2) the pressure relief valve is stuck open

I did remove the spring on the pressure relief valve to load it up with vasoline when we were priming it, the piston did not come out (does it with out removing the entire assembly? It did not seem to nor did I force it).
Prime was done via the disti removal and drill method poring 90wt into the switch sender hole (resealed before spinning it)

The spring was returned and the bolt screwed in after I loaded it up with vasoline.

I think #2 is most probable and the oil filter / both pump assemble will need come out and it be cleaned or replaced.

Other ideas?
 

Uncle Douglas

Well-known member
Callsign: delete
Marlon ?
I'd start with the switch/sensor. Have seen lots of trucks with oil light on where replacing the < $20 switch/sensor was the fix.
That sensor just switches ground to the dash light. If it grounds out, light is on.
 

Uncle Douglas

Well-known member
Callsign: delete
Tell tale, if oil pressure is actually low the valve train / lifters start making noise @ idle.
If it sounds good, believe your ears and suspect the sensor.
 

rlynch356

Well-known member
I swapped out with one of my spare sensors... no joy...
when it left it was fine, (and had sat over night) seemed to loose prime over night which is weird.

I’ll get him to check and make sure the sensor wire is on but the switch was good sat at least
 

MountainD

Technical Excellence Contributor
V-belt or Serp? If it is a V-belt, it can very well have low oil pressure--there are two main causes in my experience and I have been through the ringer with the V-belt set up (I eventually changed to the vastly superior serp belt front cover).

the two main problems I battled were a stock pressure relief valve and worn gear housing--not gears.

The pressure release valve should be free to move in it's bore. If not, and it won't come out and is stuck, then what i ended up doing was taking a spare valve, putting a bolt in the hollow recesses, and using some polishing paste to smooth out the bore and polished up the valve body/cylinder. I have it still together so I will let you know the bolt size as I also use that to retrieve the stuck valve.

Replacing worn gears is only 1/2 the battle--the bigger issue is that the gears also wear down the housing. Sure, I trued up the cover plate, but you still have the internal wear and you need to true up the flange on the housing/front cover in order to get the proper clearance for the gears. So you can replace gears and still have low pressure. Particularly at idle.

Make no mistake about it. I firmly hate the v-belt set up. As many do.

Or it could be the gauge... but all my issues were verified running a VDO gauge. First mod I made on my 110 was to get all new VDO gauges for everything...
 

rlynch356

Well-known member
Ok, when I pulled the spring on the pressure relief valve, the piston did not come out, it moved some in there but not a lot, and it did not come out (didn’t force it either)
So I think it’s stuck. And is not retracting against the spring.

vep time a VDO - I have one for the 110 on the bench and just waiting till I can put in the center dash raptor console.
 

rlynch356

Well-known member
Marlon is going to replace the spring and switch today. Does the piston come out when you remove the spring (should it is probably a better question)?

If so he can polish it and reinstall, otherwise he may need to disassemble the oil pump/filter to get to it and possible replace that with a used one (seems NLA)

Thanks
 

MountainD

Technical Excellence Contributor
Yes it comes out. Forcing it out can scour the aluminum bore so yo need to be careful. I used a 7/16” bolt in the end of mine. This is my spare I used to help smooth the bore— I also used wet/dry Emory cloth on my pinky iirc— can’t recall which, but ultra fine as you don’t want to remove material. Once that bore is scoured, it can become problematic and every now and then it’d still stick. I changed my cover eventually. There might be better ways— I didn’t find them. I strongly recommend going to SERP cover for better oil pressure=engine longevity. I’d rule out other issues first. But it needs to be able to move in its bore
 

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