Why!?!? :o

So we're working on a 99 GMC Sierra 1500
(the Chevy Silverado's twin)
We changed the oil pump, cuz it started out just taking it's time to bring up that oil pressure in the mornings.
One morning it just built no pressure when started.

We already had the oil pump so we got workin

(For anybody that hasn't done this job, it sux. badly)

So after tearing away the water pump, getting the Harmonic Balancer out of the way... and all that fun stuff, we replaced the pump, and put the sucker back 2gether.
Now... We're STILL not getting any oil pressure...

WHY!!!!!!?????

Any ideas? suggestions? etc?
Will be greatly appreciated. ^_^

Thanx a bunch everybody!

Is the engine making any knocking sounds or rattling?
How are checking oil pressure?
With a manual test gauge or just by the dash gauge?
If dash gauge.
Put a manual test gauge on it.
And post back pressure readings.

Motor is rattling very loudly.

It's all top end noise, although i know the factory guage isn't the most accurate creature in the world, it was showing me pressure b4, then when it didn't come up i got the top end rattling, I know it's not a knock ^_^

Could something be stuck in the pickup tube?

Also, we did prime the pump b4 installing it. Was there any other steps i may have missed? And is there any other check valve etc i should have checked/replaced?

Still need pressure readings to see if pump is pumping?
Have you checked the oil filter?

well, I was tempted for a while to have some fun with this, but my conscience told me otherwise.
sorry, you'll have to remove oil pump cover, and pack housing up the eyeballs with petrolium gelly. AKA, vaseline. when you replaced pump, you let air into the pump housing. that's why there's no suction. you have to kick start it with a "filler', which is vaseline.
I learned this the hard way on 84 buick. fortunately, that one had pump housing outside, so all I had to do was to pull the cover off, replace just ruined gasket, pack it with vaseline, and be happy.

with that being said, whilst I re-read my post, I have a stupid question. Oil pump is just a set of 2 interlocking gears. How can that break? On my Buick, gasket was pouring oil down, that's why i had to replace it. You sure you fixed the cause?

Other things to check is to pull valve cover and see if lifters are bleed down.
Pull oil filter and put a dry one on and see if it is pumping oil to oil filter.
Was the old pump bad or had a stuck relief valve?
Pull dist and spin oil pump rod to see if new pump is pumping.

stupid question...what engine does this have ?

j cAT,
This truck came with the 4.8 liter V8

MT-2500,
we pulled the valve cover and actually pulled the oil filter off and started the engine, NO PRESSURE. not a drop of oil came out.

ukrkoz,
Thank you 4 not messing wit my head! I really appreciate the hint...
You're absolutely right, the gears don't "wear out" and look exactly like the new ones lol I believe that's why they integrated that valve...
(of course... there's also a lightbulb that has been lit 4 over a hundred years! But that wasn't smart 4 the company! That's why we have lightbulbs that last 600 hours and oil pumps with a valve that gets stuck and cost you $900 just in frikkin labor, at any shop, other than the dealer of course...)
ANYWHO, here's the problem, we took it all apart and primed it with vasoline, worked, however now at idle i'm getting no pressure and the top end clattering. Accelerate a little bit, like right over 1K and then it quiets down and all i hear is the sweet and innocent rumble of that v8!
wtf!?!?
So that means that i can cruise around all day fine but soon as i hit a stop light or park, the needle just dwindles from it's healthy 40+ to nothing! and then the clattering begins...

WHY!?!?

Check for a leak in the oil sump pickup tube assembly...bad seal or not seated fully. I'd also suspect a partial blockage to the hydraulic lifter feed. Is the clattering coming from one or both banks? Maybe some Seafoam or other oil cleaning additive will help break up the deposits, but if not then you'll have to perform more exploratory surgery...better suggestions are welcome. Good luck!

It sounds like the oil pressure valve is stuck open.
Get your repair book out and check where it is and check it out?

Aso if this is the type where oil pump gears slip over the crank shaft check the key if it has one and make sure you have the front crank damper torqued down good and tight.
If no key in pump gear the crank pulley has to be tight to turn oil pump gear good.

low oil pressure. times sticky lifters. It will not be a complete cure, but, at least, will unstuck the lifters - pour can of Seafoam into the cranckcase and drive on it. Totally safe to do, will clean anything anywhere oil goes. I drove 2000 miles on it in my Ranger.

Otherwise, listen to other posters. Something's causing undue pressure drop at the low RPMs. You can change to heavier weight to help some too. 10W40, eg.

low oil pressure. times sticky lifters. It will not be a complete cure, but, at least, will unstuck the lifters - pour can of Seafoam into the cranckcase and drive on it. Totally safe to do, will clean anything anywhere oil goes. I drove 2000 miles on it in my Ranger.

Otherwise, listen to other posters. Something's causing undue pressure drop at the low RPMs. You can change to heavier weight to help some too. 10W40, eg.


No crank case cleaner is very safe.
On a sludged up engine it cleans all sludge and drops in to the bottom of oil and and then into oil pump screen and oil pump and pressure valve and into rods and mains and lifters.

No crank case cleaner is very safe.
On a sludged up engine it cleans all sludge and drops in to the bottom of oil and and then into oil pump screen and oil pump and pressure valve and into rods and mains and lifters.

Seafoam is not cranckcase cleaner. It's Seafoam, one and only.
With all due respects, considering that I have contacted Seafoam manufacturer directly and had it all explained to me, how and why, I must repeat myself: Seafoam is safe to drive on in the cranckcase. Transtune, their other product, is not recommended to be driven on.
I can vouch for this not just from my Ranger experience but, also, I pre-clean all of my cars with Seafoam before oil changes, by driving roughly 500 miles with it in the cranckcase before oil change. I have been doing so for yrs. Give or take, 5 vehicles in the stable on average. Of course, I do not use cheap gas and cheap oils, so I am not even anticipating sludge buildups in any of those engines.
Once again, due respects. Peace.

Seafoam is not crankcase cleaner. It's Seafoam, one and only.
With all due respects, considering that I have contacted Seafoam manufacturer directly and had it all explained to me, how and why, I must repeat myself: Seafoam is safe to drive on in the crankcase. Trans Tune, their other product, is not recommended to be driven on.
I can vouch for this not just from my Ranger experience but, also, I pre-clean all of my cars with Seafoam before oil changes, by driving roughly 500 miles with it in the crankcase before oil change. I have been doing so for yrs. Give or take, 5 vehicles in the stable on average. Of course, I do not use cheap gas and cheap oils, so I am not even anticipating sludge buildups in any of those engines.
Once again, due respects. Peace.

If it cleans it is a cleaner.
Sea foam is a good gas stabilizer.
The only safe crankcase cleaner is good fresh oil and a good filter.
The manufacturer of slick 50 said the same thing about slick 50 even after the rods and mains started knocking after putting that stuff in a high mileage engine.
Any cleaner or dope added to the oil dilutes the oil base and takes away lubrication.

[quote=jdmccright]Check for a leak in the oil sump pickup tube assembly...bad seal or not seated fully. quote]


the oil pump screen needs to be installed into the pump body securely. the pipe end you apply sealant. then tap the pump screen into the pump body..the screen must be parallel to the pan bottom....

if the pickup is not installed correctly you could have these problems of low/no pressure at lower rpm.....

low oil pressure. times sticky lifters. It will not be a complete cure, but, at least, will unstuck the lifters - pour can of Seafoam into the cranckcase and drive on it. Totally safe to do, will clean anything anywhere oil goes. I drove 2000 miles on it in my Ranger.

Otherwise, listen to other posters. Something's causing undue pressure drop at the low RPMs. You can change to heavier weight to help some too. 10W40, eg.

I know you love this seafoam product ....you say here to pour a can of seafoam into the crankcase....you know that this is well above the recommended amount....I would follow the instructions IF this engine does indeed need a cleaning....maximum is 1.5oz seafoam/quart of engine oil....