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    First I’d like to say that I’m new here and not sure if I’m in the correct place to be asking this or not, but I am in need of serious help with this engine...


The truck is a 1999 LS silverado 1500 with a 5.3L vortec v8 lm7 in it. 265K miles.

When I first got the truck this wasn’t an issue it just progressively got worse and worse until it just did it fully 24/7.

I know this is a very common issue with these engines, but the symptoms I’ve been having are not the same as what Ive been seeing other people having.


The truck started having a low oil pressure problem, where it would be fine when cold full around 25-30 psi, then as it warmed up / as I drove it, the pressure would slowly drop until 0psi, sometimes this wouldn’t happen till after driving 50+ miles, sometimes it would happen in 10 miles. Now after replacing the pump it does it in literally 1 mile. After it would drop to 0psi the lifter chatter/tap/knock (whatever you want to call it) would finally start up. I would stop the truck kill the engine then restart it. Most of the time after id restart the engine, the pressure would rebuild back up (lifter tap would go away) and I’d go for a mile or two, and then the whole process would just repeat itself. 


Make a long story a bit shorter... 


I ended up replacing the oil pump, with a mellings 295 pump, cleaned out as much gunk as I could, got the oil pan nice and squeaky clean... The truck did fine for a couple hundred miles, had really good oil pressure. it did lose all oil pressure entirely on my first test drive but it never had a lifter tap or made any noise when that happened I just pulled over killed it, started it back up and bam it had normal oil pressure.... After that happened, the truck did fine for a couple hundred miles, then out of nowhere it would just randomly start ticking even when it had OK oil pressure 15-25 psi.


Prior to replacing the pump when the engine was cold it wouldn’t even build up oil pressure right after starting, usually took like 10 seconds + of it running for it too build pressure. And it would only build up to about 25psi and it stayed there even when warm. Now after the oil pump replacement, when the engine is cold the oil pressure will build immediately and be at around 60 psi, then as it warms up it slowly drops to 40, I’ll get out onto the road and as I barely make my way down the road, it slowly drops until it gets down towards 5-10psi and then the lifter tap will start up, doesn’t usually drop fully down to 0 like it did prior to the oil pump replacement. It usually atleast has a couple psi, but that ain’t enough obviously lol cause it’s ticking.


When it does this, I have no choice but to pull over, kill the engine, start it back up, and it builds back a little bit of pressure. But it still has the lifter tap it doesn’t go away like it would before “sometimes”.... And it’s bad, I can definitely feel a power loss, also can notice a change in the way the engine sounds other than ticking, like a ****** in the timing or something? I know it’s NOT good and the ticking means there’s no oil getting to the head, so please save the lecture, please...


 I’m just at a loss as to why this would still be happening after replacing the pump? The pump doesn’t do what it did before I replaced it, which was, it would fail to build pressure after starting the truck when cold, that’s what made me think the pump was failing so I replaced it, and the new pump DID fix that issue... lol but that’s it... so I know it can’t be the pump right? I mean it’s literally brand new... I also tried to make sure the o ring wasn’t pinched when re-installing the pump... 


I really just need some good insight on this issue from someone that knows these engines well, this is killing me! This is my only vehicle I need to get this figured out ASAP. I did all the work myself, 23 year old guy, grew up working on bikes then transferred over into cars/trucks, not an expert by any means but I’m no dummy that’s forsure hahahah. Any help is appreciated!


& The truck is a great truck, it’s an all around clean truck, the last owner definitely took care of it, so it’s not like it’s a big ass pos. Well kinda I guess lol.. 

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Well, I'm no expert, but I have torn down a 5.3 a time or two.


If yours is a 99, you dont have AFM, which is a BIG plus.


 As far as a loss of oil pressure is concerned, I would say that you need to consult someone that's more knowledgeable that I am, but I will add what I can.


Start with the bottom and work your way up.


Do you have enough oil in the pan?

Do you have the correct length oil dip stick?

Is your oil sump screen plugged?

Is your oil pickup tube cracked?

Is your oil pickup tube oil seal bad?

Bad oil pressure sender?

Blocked oil return passages?

Incorrect oil weight?

Plugged oil filter?

Have you tried a motor flush? (I like motor medic oil flush)

What else.... Hmmm.


Is there any chance you have ......... Uh..


How about trying different oil weights, or different brand oil filters?


What about removing the oil sender, and installing a mechanical gauge, to confirm that its a supply problem rather than a return problem.


Try over filling the motor by 1 quart and parking it nose down on a STEEP hill. Then run it for a while. Then try parking nose down on a steep hill, and running it for a while - any difference on oil pressure?



Sorry you are having trouble with your rig. the fellas on this site are cool, and they will try to help you. Hopefully they can provide more insight that I can.



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2 hours ago, Supreme Pizza said:


Blocked oil return passages?





3 hours ago, 99vortec said:

I ended up replacing the oil pump, with a mellings 295 pump, cleaned out as much gunk as I could, got the oil pan nice and squeaky clean...


265K on it?

Might want to give the lifter valley and rocker returns a hard look? 

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I am not an expert, but as I have to do this myself soon and have been researching what I need to do, I have come across some symptoms of low oil pressure.


1st and most common, the oil pick up tube O'ring.  Did you repace this, and if so when you put the new one on did you lube it, and make sure it was seated correctly?   One of our fellow subscribers to this sight, took his truck to a dealer, who did this, and then told them he thought it was the oil pump, changed that too, put it back together, no oil pressure.  It turns out they didnt get it seated correctly and had to redo the procedure. 


2) gunky lifters.  Some of the vids I have been watching that were causing problems with oil pressure were lifters that were pretty gunky/sludgy.  The procedure for this fix was simply removing the valve covers, and you would be able to get to the lifters from here, keeping in mind they need to be kept in order and put back exactly how they came out. Carb cleaner seemed to clean the lifters, also whilst you have the cover off at this time spraying carb cleaner to remove gunk in this area was recommended, especially down the holes for the lifters.  Do an oil change after you do this as it will dry out all of your working parts.  lightly lubing this area with marvel mystery oil might help too before reassembly.


3) priming your mellings oil pump and O'ring application 


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The oring problem on the pick up tube has been discussed at length recently here on the forum, it sounds to me that the pick up screen is getting clogged and when you shut it off enuff crud falls out of it that it has op again

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Please tell me you didn't keep reusing the original O ring....I went through this last summer, I have told the story here on the forum many times. I did everything you did and I replaced the O ring AND the pick up tube. I put the Mellings Hi volume oil pump in, with the O ring Mellings recommended, (yes I called them to MAKE SURE I used the right O ring as they provide 2 O rings in the kit), cleaned and filed down the aluminum slag in the high and low side of the oil pan to speed drainage. I also used a yoke shaped bracket (like the one on your new pick up screen)on the the other side of the pump, pick up tube connection joint of the pick (just like the one on the Mellings pick up tube) up tube and pump, I got it on Amazon, $5.00. It looks just like the one already on the tube. I followed Mellings instructions to prime the pump. I have had no problems with oil pressure. Now the engine holds 40 +- 3 or 4 lbs. at idle when fully warmed up. I used a thread locking compound EVERYWHERE (just like GM does) and a torque wrench.  Now the remaining leak is back at the rear main seal, no point in going through all that unless you replace the rod bearings on the engine crank as the crank will be below it's spec'd home without new rod bearings.

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  • 3 weeks later...
10 hours ago, tiffany31 said:

thanks for sharing with us, this will help more people.

The smell of spam is strong with this one. ^^^^  :spam:




New Delhi, National Capital Territory of Delhi, 110001, India


Edited by txab
  • Haha 3
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