Johnnyoutlaw

Hello everyone. New guy here! Just got me a super clean 99 suburban.
paid 1600 for it....
I have had it a few weeks now and absolutely love it.
Got a issue I have never dealt with before....
FIrst off this suburban is super clean - there is no rust anywhere to be found - iv triple checked...... There is not a leaking or seeping gasket to be found - no oil leaks from oil pan / valve cover gasket - transmission pan.
It looks like a vehicle that has maybe 20k on it...... It drives amazing - no shakes - no rattles - no pulling..... No smoking - engine doesn't tick or sound noisy.
Again it acts like it has 20k miles........
WHen I first went to look at it - I was suspicious some one had been doctoring the oil..... It showed - 60 plus psi on start up and once warmed up - fell to around 40 plus..... After driving it for a bit - I decided everything was good and maybe the guage was just goof.....

​​​​215k miles on the dash..... That kinda oil pressure just seemed crazy for a engine with so many miles.......
but again the thing drove amazing......

I made the purchase. Took it immediately on a 300 plus mile trip..... Like literally put gas in it and took off....... Had zero issue. Have had zero issue since.........
Since the purchase - I have towed the race trail ( 12 foot x 6 foot ) 3 times - once to the track - the hard way through the hills and normal highway on the way home...... Not one single issue..... Even after towing the oil pressure would be 35/40 at idle in gear...... Impressed.....
However I kept it in the back of my mind someone may have been using a higher oil weight or such.....
So Tuesday when I got off work I changed the oil.....
I got Valvoline high mileage - 10/40...... Did the oil chang - everything appears normal - no ticking or anything crazy but now the oil pressure gauge is almost pegged all the time.......... On start up in the morning - it shows about 70 ( never makes it to 80 even when reving ).......
I let it run for about 5 mins and take off - by the time I get to work ( 10 mins drive on the highway ) when I get parked - it idles at 60 psi......
So my assignment is - 10/30 is what the previous owner was actually using......

My question is should I change the oil back to factory - I thought about switching to royal purple 10/30....?
These pressures seem high. But it is mind boggling there this high with this many miles......
I only kicked the oil weight up - because it's oklahoma and we see high temps - plus the age and miles..... Also towing - I wanted that extra protection.

Also another thing - the engine appears to never fully warm up - even after towing - the coolent temp reads 150 - 160....
This thing gets better gas mileage the my 5.3 avalanche - I think I was only getting 12 or so out of it - this thing appears to be getting 18 to 20...... Round trip from Okc to Muskogee ok with a trailer - I burn about 50$ in fuel with fuel left over for 2 days of work......
No check engine lights - iv checked for codes with snap on code reader......


This is new to me - iv dealt with plenty of engine with low oil pressure but never any high.....

Options and thoughts please?

Never gets warmer then 160.
Oil pressure - 70ish cold start - 60 ish warmed up after 10 mins drive....
about 60 to 70 when driving down the road.
Thanks guys!
​​​​​

Johnnyoutlaw

Johnnyoutlaw

oldchevy

I would try a mechanical oil pressure gauge to see if the factory gauge is giving you a correct reading.

hanky

Anytime we have any question regarding oil pressure readings , we always confirm them with a mechanical gauge. Good advice.

repairman54

Drove my '99 5.7 Yukon to 200k miles on 5-30w SuperTech synth. oil and sold it running like a champ no issues.
No need for any other weight oils or overpriced brand names as the Walmart oil is API certified.
Engine not getting up to 195 deg. will keep oil pressure higher also. Verify and change the T stat. Motor is designed for 195 which usually puts temp gauge around the 210 mark.
Your old 5.3 truck needed new O2 sensors, my '11 5.3 mileage dropped to 16ish, new front O2's got it back to 18-19 highway.

Johnnyoutlaw

Thanks bud - was pretty much thinking the exact same thing......
I changed the oil last night back to - castrol 10w30 full synthetic. Oil pressure is normal again.....
I think the gauge is a lil goofy / sensitive - it'll idle at 40-50 psi and a light touch of the throttle kicks it back up to 65ish - not even 500rpms more.....
I know rule of thumb is 10 psi for ever 1000 rpm...... It's weird.


I'll change stay this weekend.

Thanks bud.