I have a 97 gmc 6.5, when i start it when its cold the oil pressure is good but as i drive it the engine temp goes up and the oil pressure goes down ,at idle to almost nothing.when you accelerate the pressure goes up again until you let off,???

Unfortunately to me it sounds like you have bearing problems. Main rod or cam bearings may be worn pretty badly.

first your best bet is to install a mechanical oil pressure gauge and compare the readings from that gauge to the stock gauge. if the mechanical gauge shows good oil pressure then its stock gauge issue(OPS, gauge itself, etc)

when the oil pressure drops to almost nothing do you hear a sound change in the engine? the sound would be very noticeable as you'd be getting metal on metal contact with that low of an oil pressure. if the engine doesn't sound different(in a bad way) then i'd say try replacing your OPS.

Those OPS is not the most reliable. That OPS also controls the LP. So if it is bad, you are starving your IP. Actually, this is a better outcome than really having a low oil pressure.

How's your oil level?

Define ALMOST zero?

Taylor,
I see this is your first post so I am guessing that this truck is kind of new to you.
I have no idea what kind of vehicles you have had in the past, but I have to ask, is this your first vehicle with an oil pressure gauge as opposed to just a light?
The readings you describe are actually fairly normal. That is not to say there is not a problem, but without actual readings ( 8 psi at idle 40psi at 1800 rpms) it is hard to say.
When you start your engine in the morning the oil is going to be colder and it is going to take more pressure to pump it through the engine. As it warms up the viscosity (resistance to flow) drops. At low pump speeds, like at idle you will see low pressure. When the RPMs go up the pump is trying to force more oil through the engine and the pressure climbs. The oil pump has a pressure relief valve in it to keep the pressure from going so high it blows the seals out.
If the oil pressure is too low, a couple of possible causes would be....
1) Oil pump wear
2) Oil pump pressure relief valve stuck open
3) Worn main bearings

My truck with 235,000 miles doesnt fluctuate that much. It is higher when cold, which as stated is normal, but it barely drops below 40psi even when hot. It too rises with RPM but it is not a HUGE rise. Probably no more than 5 or 10psi if that...

It will vary with the oil you are using and how many miles you have.
My current engine I have less than 50K miles on. Using Mobil 1 right now I will get 40psi at idle at start up. Driving it will go up to about 50-60 psi and when warmed up at idle I am showing ( I think ) maybe 10 psi. If I was running straight 30 weight I would expect all of those to be higher.

It will vary with the oil you are using and how many miles you have.
My current engine I have less than 50K miles on. Using Mobil 1 right now I will get 40psi at idle at start up. Driving it will go up to about 50-60 psi and when warmed up at idle I am showing ( I think ) maybe 10 psi. If I was running straight 30 weight I would expect all of those to be higher.


those numbers are quite low for an engine with less than 50K miles. my engine has 165,000 on it and cold idle my factory gauge sits at 60 psi. the lowest pressure i've ever seen is 39 psi hot idle(~650 rpm but it goes right back up to ~43-45 psi once the engine hits 900 rpm) and thats after running down the highway so the oil is quite hot. what weight oil are you using? i'm running shell rotella 15w-40. that low of an oil pressure at hot idle on that new of an engine doesn't sound good. when the motor is hot your oil pressure is a rough indication of how good(or bad) your main bearings are.

The readings you describe are actually fairly normal. That is not to say there is not a problem, but without actual readings ( 8 psi at idle 40psi at 1800 rpms) it is hard to say.


those readings don't sound normal at all. that low of an oil pressure at idle, regardless of what almost zero is, is way too low. it could be something as simple as a bad OPS(I've heard the OPS you can get at napa auto parts makes the oil pressure gauge go screwy for lack of a better word so a bad one could show low oil pressure at idle) or it could be something like a worn oil out pump that only creates good oil pressure at a higher RPM or the main bearing could be shot.

Iamdave,
That (8psi at idle 40...) etc was not meant to be actual numbers, just an example of what he should tell us so we can better assess his situation.
As far as my numbers, those were just off the top of my head. I will try to make note of my actual readings later today. But for reference my readings will probably be lower than your as I am running ( I think) 5w-30

Iamdave,
That (8psi at idle 40...) etc was not meant to be actual numbers, just an example of what he should tell us so we can better assess his situation.
As far as my numbers, those were just off the top of my head. I will try to make note of my actual readings later today. But for reference my readings will probably be lower than your as I am running ( I think) 5w-30


5w-30? please excuse me but are you crazy?? these engines call for a 40 weight oil (15w-40, 5w-40, 0w-40 are all fine). that thin of an oil cannot be good for these engines as the oil gets well over 200 degrees when running, hence the need for the external oil cooler. 5w-30 is not even rated for a diesel engine because it cannot handle the soot a diesel produces when running. that oil is thinner cold than 15w-40 is luke warm. not a good idea at all. if you want to run an oil other than 15w-40 i'd say switch to amsoil syntehic 15w-40 or 5w-40. running 5w-30 in these engines cannot be good for them. SAE 30 can be used in certain temps however.

other diesel rated oils i've seen

castrol texicion extra 15w-40
mobil delvac 15w-40
royal purple full synthetic 15w-40
amsoil full synthetic 15w-40 and 5w-40
shell rotella T 15w-40
shell rotella T full synthetic 5w-40


the ones in bold can be found at your local walmart and most parts stores. a few parts stores carry royal purple 15w-40 and i've only seen two stores in the area that carry amsoil.

According to figure 1-a3 in the Haynes manual #10330, the thinest oil that should be used in a 5.7/6.2/6.5 is SAE 10W-30 in 60 degree or lower F temps.

I run Rotella 15-40 year round. Thats what its had since it was new, and after 235k, it still doesn't use any oil and holds good oil pressure. This is what i would recommend to anyone.

Just FYI, Amsoil have a 5W-30 version for diesel, it is called HDD. They are closer to xW-40 but it is rated as xW-30. There are a few CI4+ rated diesel oil with xW-30. Most of them are 10W-30. The challenge is, user have to make sure that they used the diesel rated oil instead of gasser oil.

GM Spec warm (operating temp 180-200F) for a '97

10 psi at idle

40-45 psi at 2000 rpm

The oil pump relief is something like 80 ish psi and would only open when cold. When warm its just too easy to flow oil through engine. Therefore unless cold you are not hitting near 55-60 I'd say relief is ok. Assuming gauge is ok.

The dash gauge has been reported by many to vary greatly compared to a mechanical gauge for the idle pressure. And then be closer at 2000 rpm. I think some of it is GM gauge errors to show a bit more pressure at low rpm so people don't freak and say its a problem.

If oil pressure really drops low maybe below the minimum oil pressure of 10 psi above I think the check gauges light comes on? Not sure about that though. On oil changes my check guages comes on until it registers a little oil pressure.

By my dash gauge I get something like 60 psi cold at idle and maybe 20 or 30 warm it goes to 40-45 psi at 2000rpm. I probably have lower oil pressure at idle than my dash guage reads I guess but again according to GM the spec is 10 psi.

This has been discussed several times. Issues have been gauge, cam bearings, main bearings, and a crack near a lifter that "opens up" when warm.

Can't find the bottle, but from their web site I am using the Mobil 1 Diesel Truck 5w-40.
Going by the dash gauge at idle on cold start up I am two marks to the right of 40. At cruise it is just to the right of that second mark, after running three minutes, stopped and in gear it reads two ticks to the left of the 40.
Now mind you, I am in the Houston suburbs, so my "cold start" this afternoon has ambiant under hood temperature of over one hundred degrees.

Now thats it's warmer would it be ok to run 20w-50 in an older motor? I'm running 20lbs at idle and 40lbs cruising too. It would be more reassuring to have more pressure especially when towing in hot weather.

Popps, pressure for the sake of pressure is not so important. As you want oil to flow. And pressure is a measure of the resistance to flow. Oil cools the lower half of the engine. Mainly the pistons but other stuff too.

You want as much oil to circulate through cooler and onto engine to do its job. Its the circulation thats important (once minimum pressures are satisfied) 20W 50 is probably too thick. Your pressures look good so a good quality 15W-40 is plenty. If pressure drops while towing its probably temp related and you might be better off with bigger oil cooler or switch to synthetic (its more viscously stable vs temp).

I have read reports of the oil temp being too high on the 6.5.

popps, if you want a better heat resistant, use 15W-40 Synthetic. The main thing is to use a diesel rated oil, usually Cx. The latest one is CI-4+ or CJ-4. Rotella Synthetic is a good oil which is called Group III oil, a very refined dino oil. A step above the Rotella Synthetic are the Group IV oil, true synthetic called PAO. Among this category are Mobil1 Turbo Diesel or Delvac 5W-40 (not the Delvac 1300S), Amsoil AME 15W-40 and Schaeffer 9000 5W-40. These groups of oil have a higher flash point and more resilient to high temperature, in other words, the viscosity will hold longer to higher temperature and the oil does not wear out as fast.

Schiker is correct that a higher pressure may not necessarily provide better protection because the oil may not flow as good. Hence higher viscosity may not be desirable. One thing you need to make sure is NOT to use a gas rated oil even at 15W-50. So far, I have not seen 15W-50 oil with diesel rating, although I can be wrong. The trend has been to have 5W-40 for Diesel synthetic.