Ok guys I've got a 2005 Envoy with 192,000 miles the other morning my wife came home and told me the oil pressure gauge dropped and the engine started knocking, I went out and restarted the car with no knocking but no oil pressure either. Did some research and replaced the oil pressure sensor and changed the oil (it was due anyway) now when it gets warmed up the oil pressure drops and it develops a rattle, if you shut the engine off and restart it the oil pressure goes back up and no engine noise but after running for a bit oil pressure drops and rattle comes back. Any help and any need to worry about damage? She drove it about 3 miles with a slight knock I'll add.
Oil pressure drops after engine warms up
- Thread starter Bigt62097
- Start date
Mooseman
Moderator
First of all, welcome to the Nation.
The oil pressure gauge on the 4,2L is fake. The sensor is just an on/off switch, like the old days of idiot lights, and the gauge readings are fabricated by the PCM. If it reads anything but 0 (zero), you have oil pressure. If it's 0, you don't have any and you get the oil light as well as a message to shut off the engine on the DIC (if you have one).
Was the oil level OK when you first checked it? If it was low and running on low oil pressure all the way home (like most wives will do!), the engine may have been damaged.
The oil pressure gauge on the 4,2L is fake. The sensor is just an on/off switch, like the old days of idiot lights, and the gauge readings are fabricated by the PCM. If it reads anything but 0 (zero), you have oil pressure. If it's 0, you don't have any and you get the oil light as well as a message to shut off the engine on the DIC (if you have one).
Was the oil level OK when you first checked it? If it was low and running on low oil pressure all the way home (like most wives will do!), the engine may have been damaged.
It was low and took 2 quarts to get full again just concerned of if I need to look for a different vehicle because we don't have the money to replace an engine and with 200k on it other things are going to eventually go wrong
I'm thinking the oil pump is bad but I'm not real used to these newer vehicles all my experience is with pure 1980 lol
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Mooseman
Moderator
2 quarts is not bad since it holds 7. How did your wife know the oil pressure was low? If it's by the gauge, it's fake, remember. If she got the light and message, then possibly the pressure was low.
Oil pump or oil pressure failures are rare on these engines. 99.99% of the time, it's a failure of either the sensor or the stepper motor of the gauge, which is fake.
These engines do tend to be "rattly", especially when cold.
Oil pump or oil pressure failures are rare on these engines. 99.99% of the time, it's a failure of either the sensor or the stepper motor of the gauge, which is fake.
These engines do tend to be "rattly", especially when cold.
therealsethallen
Member
A good chunk of higher mile 4.2s are extremely noisy when it's cold, for whatever reason it's just the nature of the engine. Now that doesn't mean running the engine low on oil didn't cause some kind of damage. First suggestion, fresh oil change with a new sending unit, I'd avoid the aftermarket senders and go right for the ACDelco.
The gauge and the stop engine light was on, I changed the pressure sensor and oil/filter today and took it for a test drive got it up to operating temp brought it back home and added the 7th quart just as I add the 6th on my truck (both show full right right after oil changes before the final quart for some reason) so I always put the last one in after my test drive, today as I was adding the 7th quart the engine got a rattle in it after running quiet I checked the dash and the low oil pressure light was on so I immediately shut it down a and restarted it and the oil pressure came back up and engine was quiet again outside of the cam take its had since we bought it.
What has me puzzled is the engine is pretty quiet unless first start up
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Mooseman
Moderator
It would need to be tested with an actual test gauge to see what is happening with the pressure. If it is the pump, it is a major job to work on, probably beyond what it's worth.
littleblazer
Member
- Jul 6, 2014
- 9,267
I would guess some damage was done, like others have said an oil pressure tester is necessary but it sounds like it's dropping it. Maybe a bypass is sticking I don't really know the oil system on these trucks well.
Mooseman
Moderator
The oil pump is built into the timing chain cover so it's a big job. It could be several things like the pickup tube o-ring, pickup blockage, bypass stuck, leak in the oil galleries or the pump itself.
doesn't sound good. As suggested get an oil pressure sensor installed and monitor... to confirm what your ears and eyes (light / gage) are seeing. Of course, if it is confirmed you are kind of stuck a bit anyways. you might want to consider something like doing a "seafoam attack" in the oil to see if a cleaning will change anything depending on what the issue is. It might not resolve anything but at a cost of another change (plus foam), it might help especially if something is stuff somewhere. Just a suggestion / thought. Good luck either way... hope its just your ears ... 
Njg425
Member
Did the oil light come on in the dash? if not I would put money in it being a bad stepper motor in the gauge. You could get one of the cheap obd readers to check the actual oil pressure easily. It's tough to say without actually hearing the knocking what is going on there, could be anything.
Mooseman
Moderator
Nope. There is no sensor so no pressure reading AT ALL. It's just an on/off pressure switch.
6stringbass
Member
I've had a similar problem. My 2006 TB has close to 298000 miles. During a serious cold snap in October I had the same issue. Put oil in the engine and drove home. After a two day wait with warming temps I put in a AC Delco pressure sensor. Everything seemed fine until 3 weeks ago. Same conditions and same situation. Engine was about a quart low. Put one in and...same thing...guage bobbling up and down like a roller coaster. Got the car to the dealership (local shop where a friend worked didn't have an adapter to do an oil pressure test) and after diagnoses found I had extensive main bearing wear and that was causing the low pressure. The service manager (who I have known for years and trust) told me it is not uncommon for TBs near 300000 miles to have this problem. So it is a rebuilt engine Christmas for me. I'm waiting for the work to be completed. (Why did I go that route? Simple....I just put new tires on it!) I know you have lower miles but it might be the same thing. Have your favorite shop do a mechanical oil pressure test and cross your fingers. P
I know it's just a pressure switch but I find it interesting that the oil pressure gauges in both of our TBs (we're a two Trailblazer marriage) go up as the rpm rises. Not a lot but it happens
Moderator edit: Please use the edit button instead of multi-posting. Posts merged.
I know it's just a pressure switch but I find it interesting that the oil pressure gauges in both of our TBs (we're a two Trailblazer marriage) go up as the rpm rises. Not a lot but it happens
Moderator edit: Please use the edit button instead of multi-posting. Posts merged.
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Mooseman
Moderator
That's because the PCM is faking readings by changing the reading with the RPM. It's all trickery.