Hi all,

My GMC has run well, until recently after the last oil change. It has the 5.0, with 128,000 miles on it. I did a oil change with 5W-20, and used a WIX filter. I usually use WIX, but I always have used 5W-30 in the past.

Ever since I bought the truck a few years back, the OP has been anywhere between 40-75PSI while driving. Well, I noticed today, that when it is cold, the O/P is normal, but when it's warmed up, the O/P gauge stays around 40, and sometimes drops as low as 20 or so while idling (and even while driving.) The O/P gauge has always been somewhat erratic, so I don't trust it entirely, but would switching from 5W-30 to 5W-20 for one oil change cause that big of a drop in O/P? It runs fine, but 20PSI at idle and such scares me a bit. The engine doesn't "sound" any different, but I don't want to cause any damage either. I do believe I added a bit more than was needed (thought it took 6 quarts, for some reason) so draining a quart out may help, but I don't think an extra quart would partially inhibit the oil pump from working properly. Could easily be wrong, however.

Any insight here?

 

I'd say that the drop in oil viscosity alone is enough to change the oil pressure. I can't remember what I put in mine last, but I know that what the PO used to put in was thicker than what I used.

If your're in doubt of your gauge, clean the connection to the oil sender and double check.

Worst case, swap the oil out again and use 5w30 this time. I wouldn't bother with the filter, as it's still a fresh oil change ... My 5.0 takes 5 quarts.

 

Where is the O/P gauge located? On my Town Car it's right above the fuel filter, not sure where it would be on the Sierra. I need to get some more De-Oxit to clean it with. I suppose it's possible forthe gauge to be accurate, but it's bounced around from 75 to 45 to 80 to 60, etc. all the time. If the O/P is doing that, the truck needs to be sidelined ASAP. It's been that way ever since I got it. Sometimes it's fine, usually it's all over.

 

Well it'll fluctuate with RPM, but usually at idle is should be pretty steady. The oil pressure will also drop when the oil gets warm, because it gets thinner.

Also, yes, it's above the oil filter on the driver side of the engine right by the exhaust manifold. It's one wire connected to the top of the actual sender.

 

I'm aware of that, but, for example, say I'm doing 60MPH, maintaining a (nearly) steady 2,000 RPM's. It'll bounce between 50-75 as I watch the gauge. sometime it stays the same, but usually it just does it's own thing.

 

Mine does the same thing I guess. Never thought too much of it really. I figured it's just because my gauge is funky ... jumping back and forth when I'm driving.

I know mine's pegged whenever I cold start it. Then will go to about 40PSI after it's good and warm. I've seen it as low as 30 PSI, but again, never thought twice about it.

I say if it gets low enough to where you're uncomfortable with it, just change your oil to a thicker variety. If the gauge still shows low, then you could have an issue.

 

Could be lots of things. Wouldn't chance it and have an actual oil pressure test done.