Low oil pressure at idle with 4.0l

[Gary Fraser]

I have read a few threads about this on here before i joined but no one ever says if they figured it out or not. My engine is a 2008 mustang 4.0l v6. Fresh build less than 300 miles break in and main bearings went because of low oil pressure. The motor went together text book with great care and all clearences were good. 50psi at startup cold and drops to almost nothing hot. Anybody got any ideas? My engine only had 20,000 miles on it. I have had this motor apart a few times because I race the car and I put forged pistons and steel rods on camshafts and ported and Polished heads better valve springs Etc. But this oiling it problem is something new. Because of the low mileage I have been using the stock oil pump with no issues until now something is causing the oil pressure to drop. I will be replacing it with a melling 328 high-volume oil pump.

 

[massacre]

Hope you get her sorted
I replace the oil pump any time I rebuild an engine

 

[Gary Fraser]

Only had 20k on it oil pump is good but soething else is going on. I have read posts here about it but no one ever follows up with the anwsers.

 

[donalds]

My opinion is if the engine is torn down that far why not replace the pump
When the bearings ground down all that metal went through the oil pump

My shoc gets 60 psi at startup idle
And 30 when hot at idle

 

[Gary Fraser]

My opinion is if the engine is torn down that far why not replace the pump
When the bearings ground down all that metal went through the oil pump

My shoc gets 60 psi at startup idle
And 30 when hot at idle

You have the cart before the horse my friend. I am not using the same pump after the bearings went bad. I reused the pump because there was nothing wrong with it because it was a different issue the last time with the head not the bottom end. I do intend to put a high volume pump in it this time and will not reuse the stock pump because it may have damaged from the bearings going bad but the filter did its job I found no bearing material anywhere else in the engine.

 

[donalds]

You have the cart before the horse my friend. I am not using the same pump after the bearings went bad. I reused the pump because there was nothing wrong with it because it was a different issue the last time with the head not the bottom end. I do intend to put a high volume pump in it this time and will not reuse the stock pump because it may have damaged from the bearings going bad but the filter did its job I found no bearing material anywhere else in the engine.

At least you didn't find to much metal

 

[massacre]

Could there be a leak between the oil pump and pickup tube?

Seen that happen before

 

[Gary Fraser]

Could there be a leak between the oil pump and pickup tube?

Seen that happen before

Well there are no gaskets from the factory where the pump bolts to the block or where the pickup bolts to the pump. Maybe there should be. I see in the melling adds they show gaskets.

 

[massacre]

There’s no o-ring?
If there is no o ring or gasket seal to the pickup tube, it’s possible that the pickup is not picking up the oil, or not enough of it

 

[Gary Fraser]

There’s no o-ring?
If there is no o ring or gasket seal to the pickup tube, it’s possible that the pickup is not picking up the oil, or not enough of it

I am not talking about where the plastc pickup tube seperates in the middle but where the actual pump bolts to the block with two torx screws. Only the two center main bearings are trashed the rest are not terrible and the rod bearings are ok.

 

[ptf18]

Have to say I'm not up on the internals of the 4.0l so take this with a grain of salt.
Do you only get the 50psi on the first ,cold start? OR do you get the 50psi with every start (engine warm/hot) and then the pressure drops?

 

[Gary Fraser]

Have to say I'm not up on the internals of the 4.0l so take this with a grain of salt.
Do you only get the 50psi on the first ,cold start? OR do you get the 50psi with every start (engine warm/hot) and then the pressure drops?

Every start . When the engine was in the car that is. Now the engine is on a stand.

 

[boominXplorer]

Trashed main bearings would cause low oil pressure but didn't you say it had low oil pressure after the build?

If damage is isolated to middle main bearings there's either a clog or crack inside the oil passage or in the crank. Other possibilities could be mismatched crank bearings (had that happen to me once) or overturned crank in those areas.

I did a crankshaft on a inline 6 jeep once and only had 8psi hot idle pressure. Took it back apart and found 2 standard bearings and the rest were .20 oversized. It was pretty apparent as the standard bearings didn't even have any wear on them. I bought them in a sealed package set so nothing I could of switched up installing them. I only used a plasigauge in 2 places instead of on all bearings and learned my lesson as I would of found the problem then instead of 4 days later.....

 

[ptf18]

Every start . When the engine was in the car that is. Now the engine is on a stand.

How long does the oil pressure stay at 50 psi before it drops? And you get 50 psi after a start (both cold start or hot start) everytime you start it, correct?

So.... you start it (either hot or cold) , get 50 psi, then the oil pressure drops to what pressure? I know you said next to nothing but not ZERO, correct? And you are you a accurate gage, correct?

If you restart it, do you get 50 psi again and then it drops, again? Or when you start it again you ONLY get next to nothing?

 

[donalds]

Isn't the standard rule of thumb 10psi per 1000rpm

50psi hot
not unless he's running like 60wt oil LOL

 

[Gary Fraser]

When I start it it has 50 psi when it gets hot it drops down to 0 or near 0 I use a mechanical gauge made by autometer. Every time I would start the car whether it was hot or cold it would go to 50 psi and then drop like I have said. It doesn't really matter at this point that's getting a complete rebuild again with a high volume pump from melling. All the clearances were correct in this motor but for whatever the reason the centre bearings went for a crap. This motor was cleaned several times before it was put together. It was first soda blasted inside and out and then blown out with compressed air and then it was washed in hot soapy water and blown out with compressed air then I went through every gallery with wire brushes and Brake clean and then washed again in hot soapy water twice after blowing it out with compressed air and all the cylinders were wiped down with clean oil. After letting it sit in the Sun for an hour to dry off the outside of the block completely I painted it and ready to get for assembly.

 

[ptf18]

Um... Whats ODD is that you loose oil pressure AFTER the engine gets hot/warmed up. I would think that if the loss of oil pressure is due to the pump you would not get oil pressure at all ( assume its a gear pump not having taken one apart) ie...if the pump is turning its going to produce pressure unless it is internally so worn out...rather doubtful.

Have you looked at the pressure relief valve now that you have the engine torn down? I assume it has one. If so do all the parts meet specs and is its bore in spec?

From your first post I assume this engine is in a Mustang, correct? Does it have a external cooler? If so is there a temperature controlled valve within the part that contains that valve (oil filter mount housing, I think)

As I mentioned I have never had one of these engines apart so I'm winging this. In the "old" days prior to running a rebuilt V8 we would "prime" the oil system by removing the Distributor and "driving" the oil pump with a drill motor and long connecting drive shaft.

Is it possible for you "prime" your oil system prior to install to ensure that your getting a constant oil pressure with this engine?

 

[Gary Fraser]

Yes i primed it before it was fired up. It also was fully lubed during assembly . Pump only has 20,000klms on it so not worn out. I am leaning towards an internal crack and still have to get it checked and pressure tested when time allows. To be honest this is not my first build by far. I have experience building my motors for several racecars and tow vehicles etc.

 

[ptf18]

Sounds that you have the experience. Keep us posted on what you find.

 

[Gary Fraser]

Will do but its always nice to have more opinions incase a person misses something. My 23t i built from scratch. 440alky injected 800hp.