I have a 2008 Suburban with the 5.3 with AFM with about 147k miles on the clock. I bought it 2 years ago at 124k miles. I have done research and I am aware of the "quirks" of these engines. I am adding about a quart of oil at around 750-1000 miles due to the burn-off associated with these first gen AFM engines. My real issue is what is happening to my oil pressure after about 1200-1600 miles from the time of the oil change. After a fresh oil change my oil pressure stays between 40-60 (40 idling or driving in town...averages 45-50 on the highway...as high as 60 when i'm accelerating to highway speed). At about 750-1000 miles I replace about a quart of oil that was lost over that time. At this point, the engine runs fine. However, after a couple hundred more miles, I start to notice that the pressure gauge drops below 40 at times when it was normally 40. The pressure gradually seems to drop to 30 while i'm idling or running in town and doesn't get much above 40 even on the highway. It drops gradually and ever so slightly over the next couple of hundred miles until finally one day it decides to drop down to 20, at which point the CE light comes on and DIC tells me to shut the engine off. At that point, I shut the engine off and wait about 15 minutes and I can usually keep the pressure high enough to get the vehicle home or to the oil change garage. Immediately after I have the oil changed the pressure goes right back up to normal. I noticed that this is happening right around when the DIC indicates 55% oil life remaining. I use Mobile1 high mileage synthetic 5W-30 which is the recommended oil. I'm really simply confused as to why an oil change fixes this issue. It's been happening like this since I bought it. If there's anything I can check or have done so I don't have to keep changing the oil every 1k or so, I'm all ears. Thanks in advance!
Does your oil smell like gas? Sounds like it’s getting diluted somehow. Does it look milky brown like coffee after 1000 miles?
I never really noticed a smell of gas, but I would say that it does look a bit milky brown as you say. So it possibly may be getting diluted. I was searching around a bit more and was advised to check the air filter to make sure it is clean, which makes me feel kinda dumb because I realized that I hadn't done that yet which you would think I would have when I first bought it being that it was that high mileage. Other than that, What are you thinking? im going to buy a filter tomorrow and change it one way or another.
If its milky brown like coffee you have water in your oil. Small head gasket leak. Hows radiator coolant look? Any auto place will have or rent a head gasket leak tester find out for sure.
Thank you for that info. What do you think would be the worst case scenario to repair in terms of what they have to do and cost? Also, when you asked me if the oil smelled like gas and if it does, what would cause that?
On a Suburban having gas in your oil would be an extreme rarity. It does happen on turbocharged smaller engines where fuel and high compression causes unburnt gas to push by the piston rings. Your obviously having oil dilution, gas or water are only possibilities. In your case it’s water. I would have it checked make sure it’s head gasket and hopefully not a cracked head or block. Question would be which head is it? Hopefully not both. On some GM engines it could be intake manifold gasket to. A decent independent shop will charge about 500-750 for intake manifold, 800-1000 for a head gasket. Dealer will charge over 2,000 or more.
Also ask yourself, how much engine life have I lost because of this? If it’s definitely water you have significantly worn engine jnternals driving it that way. Something to think about before fixing it. You could just change the oil and dump it cheap.
Thank you for all of your time and insight. I've basically been changing the oil about every 1200-1500 miles as a "fix" to the problem. It's a little too early to tell, but maybe coincidentally, I changed out a pretty dirty engine air filter on it yesterday and the oil pressure seemed to be a bit higher than normal. I guess it's off to my mechanic to get a firm diagnosis. Might be time to trade it lol...I want a truck anyway...Tundra would be nice.
Ok so I have a 1k mile update. First I want to say that I do NOT have milky looking oil. I wasn't exactly sure what I was looking for when the question was asked, but I looked up a video of that situation later on and I can confirm that isn't my issue. I checked the air filter about 150 miles after the last oil change. IT. WAS. FILTHY.!! I'm not sure filthy is a strong enough word, but the engine was definitely NOT happy with its air intake. After I replaced the filter, the oil pressure indicator shot up to about 60 upon start and leveled out between 40-50 once I was driving around town. Now it pumps up to almost 60 upon acceleration and I'm not seeing it drop below 40 unless I'm stopped. It doesn't get anywhere near 30...at this moment. I'm keeping an eye on it, but so far it isn't doing what it was doing around this many miles into the oil life. I've even towed my 4k lb camper on a few 60 mile trips since then. I also tried smelling the oil to see if I can smell gas, but I honestly can't tell. I don't know if my sense of smell can pick out the difference between gas and oil. Also, I don't seem to have burned much oil as the dipstick is still showing that I am within normal levels, which is another change along with the fact that I also notice that I'm not experiencing rough idling at stop lights much if at all. I normally would have had to add at least a quart by now. If this was the fix I am going to feel really dumb for not checking the filter in first place, but also happy that's all it is. Fingers crossed...but if anyone has any insight and other things to check or look for I am all ears. Thanks again!