What would cause an engine to have low oil pressure?

 

Define low!!!!

 

15 pounds

 

Define RPM's and temperature at which this pressure occurs.

 

Do not know the RPM's, no tach, car is not driven above 60 MPH. At all speeds and all temperatures guage only shows 15 LBS.

 

Worn bearings
Worn oil pump
Thin oil (~5w20)
Weak relief valve spring
Bad oil pressure gauge

Is it EVER higher, such as when first starting cold, or when revving high (especially when cold)?

Does it drop at idle when hot?

- Eric

 

Car was registering around 20 lbs, drove about 20-25 miles to repair shop to have some work done. Picked up car and now gauge registers 14_16 Lbs. Engine only has about 300 miles on it since rebuild, records from company that rebuilt engine shows new bearings installed. Gauge always shows 14-16 Lbs. Does not drop at idle. The same at all temperatures. Idiot light is still operating and it only comes on at startup.

 

Then it comes down to determining the answers to unknown questions.

Did some kid working in the shop put radically wrong oil into it when they built it (unlikely, but should be excluded)?

Did they put in a new oil pump? Did it need one? Was there good pressure before?
Was it installed correctly? Any chance they didn't tighten it properly or forgot the gasket?

Did they screw up the relief valve when they put it together?

How was the pressure before they got to it? Was it a running motor when you gave it to them?

Has this gauge read normally at other times with other motors?

Finally, did this motor ALWAYS read no higher than 20psi after the rebuild?
Because if it did, there was something wrong from the start.

- Eric

 

Not sure if gauge is correct. Answers to questions are unknown as I bought the car as is. Machine shop is in Michigan where I bought car. Talked with owner of shop and he said there was nothing wrong with motor either before or after rebuild. Can you help with some direction here, can another gauge be attached to check actual pressure. Should I change oil or have repair shop check oil pump and change oil also? Can a heavy duty or high performance oil pump correct the problem?

 

Getting past analysis and into action, I am hesitant to give concrete advice, due to the fact that you might follow it.

However, seeing as how nobody else has chimed in here, I would say:

• Since this engine is a total unknown, with allegedly 300 miles since an alleged rebuild, and
• since it has apparently always had too little oil pressure, and
• since tearing down the engine to look inside and see what, if anything was done to it, is a big pain in the butt, and
• since 20psi may be acceptable as an oil pressure in an old, worn out motor, but not in a newly "rebuilt" motor,

I would first do a confirmatory test to be sure you've got reason to dig deeper, ie: I would go to AutoZone or Advance, or wherever and buy an oil pressure gauge for about twenty bucks and hook it up in place of the one there (just connect it in the engine compartment, don't install it...) and see if it read the same.

Then I'd probably change the oil on principle - oil changes are good after recent rebuilds - but be sure to use a high-zinc oil (see Popular Mechanics 10/2010 "The Missing Zinc"), mostly just to see what came out, and whether it looked too thin.

Then, having eliminated the "easy" possibilities, I'd go to the guy who built it, screaming and cursing a blue streak and see if he could be convinced to open it up and check his work.

Something is rotten in the state of Denmark.

- Eric

 

I agree with md!! It might just be as simple as the wrong weight.

 

Ha, yours is better than the junk 307 motor I just picked up.
It has 22psi cold, 9 psi hot, and 0 psi in gear.

 

Do you recommend changing the oil pump? What are the odds it could be the oil pum?

 

Unfortunately, it could be a number of things. Bearing clearances, worn or mis-adjusted pump, oil galleries clogged or missing. Everything will have to be torn down and inspected.
May be more trouble than it's worth depending on what motor it is.

 

No.

Just changing things when you've got a problem is NOT the way to fix things.

You've got to figure out what's wrong first, and THEN correct it.
As J said, it could be a clogged gallery, and as I said much earlier, it could be an improperly installed oil pump.

First: Confirm that you do have a problem.

Second: Eliminate cheap and easy causes (like thin oil).

Third: Start digging.
Since you were told that this engine was "rebuilt" 300 miles ago, and since low oil pressure may be a sign of improper rebuilding, going to the guy who "rebuilt" it (baseball bat in hand) would be a logical step.

- Eric

 

I can't go to the rebuilder as he is in Michigan and I am in Ohio!

 

Can you tell me where the oil pressure sending unit is on a 1966 "400" in a 442

 

1st step.....Confirm the readings with another gauge. The sending unit is right next to the thermostat housing. 15 lbs at a hot idle is fine.

 

Use a mechanical gauge that connects with a tube to the hole where the sending unit is not installed. May just be a bad sending unit

 

Does the oil pressure hold steady at idle or does it jump around a lot? Generally, when I have seen problems with seals, or valves the needle tends to jump around when the car is idling.

 

Gauge does not jump around, holds pretty steady. Drove car last night and when engine reached operating temperature the pressure actually went up a couple of pounds!

 

Man,don't be doing any driving until you get this figured out,or it might show you for itself what is wrong! ---bil

 

What weight oil should I use when I change it? Should I use 10W30 or 10W40or something else?

 

Follow the advice and put a different gauge on it. Use the new fittings as well. Don't try to get around doing this. Replace ALL gauge parts. I had a line get clogged recently. My oil press is normally 70ish when cold 45-50 when hot. One night the gauge read 20. Then it dropped to 0. I was in my drive at the time. I pulled the line off the gauge and tried to blow through it towards the motor. No go. I replaced the line and all is well. Replace everything when you check it. Can't stress that enough.

 

What he said. Just do it.

General instructions here.

- Eric

 

Still would like to know what weight oil to use? I'm sure its going to have to be changed anyway.

 

10w40 is pretty standard for a low-mile engine, I generally use 20w50 in a worn one.

That being said, during the break-in period, be sure to use a high-zinc oil (not a regular oil for Toyotas and Fusions), as discussed in the Popular Mechanics article mentioned earlier (you may still be able to find a copy on newsstands).

- Eric

 

pick up an inexpensive oil pressure gauge kit to confirm the pressure. just hook it up, no need to permanently install it. the sending unit or dash gauge may be faulty

eliminate the easy stuff first

 

I suggest Pennzoil Racing Oil 25W-50, good weight and all additives included.

 

I've been hearing problems with a specific brand-name oil filter changing pressures on Olds motors - I believe it was Fram.
Change that when changing oil, and I'd cut it open to see if there's anything out of the ordinary.
Someone else mentioned the oil relief valve - if the spring was 'thrown into the pan' that got hot-tanked w/ your other parts, it could've been weakened, causing your problem!
Replace it!

 

Started car this morning and it must have taken a good 30 or more seconds to produce any oil pressure. I thought the pressure was gone. Oil pressure finally went up and now its 15 lbs instead of 10. Talked with previous owner last night and he said he can't remember if he used 10W-30 or straight 30 weight oil. He used off the shelf Pennzoil and yes he used a Fram oil filter. Said he blocked a bypass valve that when pressure is too high it bypasses the filter so that the oil always goes through the filter. Claims to have installed a new oil pump but can't remember any particulars about it. Took car to a shop to have a mechanic look it over. I am praying that its nothing serious.

 

Let us know what you find!!!!!

 

What ACDelco or Wix oil filter number would fit a 1966 "400" in a 442?

 

I still don't see where you say you pulled out the send unit and screwed in a known good gauge. Your entire problem could be a simple as a bad send unit. In fact, I'd call this the most likely suspect.

The blocked bypass valve is just stupid. Couple that with a weak oil filter and you might ruin the filter (or even burst it) on a cold start when the oil is thick. That bypass valve is there specifically to prevent the filter from ever seeing pressures too high. And what's the point? To make sure that all engine oil goes through the filter even if it's clogged? If your filter is clogged, you've already got big problems.

 

I recently had a similar problem and fixed it with a 1/4 inch of teflon tape on my oil pressure gauge tube connection.

 

To Rickman48: It was the oil filter, and yes it is a Fram. Mechanic checked for the bypass valve and it has been removed. To all who questioned the accuracy of the oil pressure gauge, you are also correct, the gauge in the car is about 8 pounds lower than a correct gauge. To all: Thanks, but I still need to know what ACDelco or Wix filter to use.

 

wix 51045

 

Your friendly neighborhood auto parts store should be able to give you ALL the part numbers for various oil filter manufacturers!

 

I would like to add that I have always used Fram filters because I buy the oil and filters at the discount stores. I will never use a Fram again after going through this!

 

Very glad that everything worked out for you.