Hi XXXXXXXX,
Have you checked the oil pressure in the motor during the first couple minutes when runs well versus the time after warm up when it lopes? Or versus above 30mph when it stops loping?
With 310,000 miles on the engine I would be looking at things other than sensor malfunction, even though a bad sensor could cause the problem. Without a reliable oil guage getting past step one with testing rather than experimentation is going to be tough. Have you tried replacing the oil pressure sending unit in the past to correct the guage problem?
Due to the mileage on the truck oil pressure could very easily cause what you are experiencing. When you first start the truck up the oil is cold and therefore thicker, the thicker oil creates higher pressure than after a couple minutes after it warms up and thins out. As you increase the RPM the oil pressure will climb again though. If you dont have enough oil pressure your hydraulic lifters wont "pump up" and open your valves all the way.
Before going through the process of testing sensors and going through the ignition and fuel systems I would perform an oil change on the truck and add heavier weight oil such as SAE 50 or using the oil you normally would but adding a thickening additive such as Lucas Oil. If the thicker oil either improves or corrects your loping condition you have found your problem. If that is your problem you can continue to run the heavier oil to put off repairs for a while, you could put in a higher volume oil pump from a company such as Melling or just opt for the inevitable engine rebuild that you are coming due for soon.
If that doesnt improve or correct the problem, let me know and we can start walking through the sensors, fuel and ignitionsystems.
No the oil pressure shouldnt affect anything with the computer directly. The only time the oil pressure will really trigger anything is if it is causing your lope problem as I described and was in turn triggering the knock sensor.
The sensors that can affect the system and cause issues are the oxygen sensor (located in the exhaust pipe) and the MAP sensor located on the intake. Either of these sensors could potentially cause your problem and also be affected by engine temperature.
A fuel problem or ignition problem could cause loping as well however...
A fuel problem will tend to get worse with higher engine RPM as the motor needs more fuel, not correct itself after 30mph. An ignition system problem shouldnt be affected by engine temperature and also be sensitive to RPM both.
Dont worry about getting out of my hair....this is what I am here for. Technically any of the sensors should kick a code. Not having a code, however, doesnt mean that the sensor isnt malfunctioning.
The MAP sensor does fit to an extent...my fault in the MAP is the consistency of the problems changes. Failing sensors tend to me more erratic, but that doesnt mean the MAP isnt the cause.
The timing chain could begin to act up as the engine got warmer and the chain more easily stretched but shouldnt cause a problem that would go away after 30mph...it should get progressively worse. As the engine turned faster the more stress that would be on the chain which would cause it to stretch more.
Well a worn distributor could cause the loping problem but just being worn wouldnt cause a problem that would go away above 30mph. A bad distributor advance could cause the lope at a specific RPM but shouldnt be affected by the enigne being cold or warm.
Its the combination of the heat of the engine AND the fact that its RPM specific that leads me to the oil pressure.
I would rank possibilities in this order:
1 Oil pressure
2 Map, O2 or knock sensor
3 Odd acting vacuum leak
4 Fuel presure
5 Distributor
There are a lot of other things that "could" cause the problem but the probability is extremely low.
Chevy Mechanic
20 years experience