Hi, new member here. Looking for any members who may have had a similar issue, or answer to my problem. This may be long & boring, but having all the info is important. Here goes:
I am the original owner of a 1998 Sierra SLE K2500, 5.7L, ext cab, long box. The truck currently has 278,000 miles (always driven in my rural area, 90% highway miles). It is no longer a daily driver, only used on occasion for hunting, fishing, etc, since 265,000. Always changed oil and did other maintenance, never "let it go". Maximum mileage in it's lifetime without oil/filter change has been 5K miles. In the 18 years I've owned it, it's spent "maybe" 5 nights not garage parked, so it's in otherwise excellent shape for it's age & mileage. Lots of parts been replaced between 200K and now. Bearing hub assemblies, alternator, starter, steering componants, ball joints, radiator, rear brake lines/wheel cylinders/drums... pretty much a partial "rebuild".
About a month ago, I had a common problem that the 5.7 & others have... failure of lower intake manifold that "milkshaked" my oil. Same thing occurred at 78,000, but was an external leak only that time. It was a sudden failure (had just checked coolant level & oil a day or so before) that I believe I discovered after only about 5-7 miles of driving. Oil was always clean, wasn't losing any coolant prior.
I had the truck towed in when I discovered the problem, wouldn't even drive it the 2 miles to the repair shop I've used for 25 years. I had the gasket replaced with the Felpro "problem solver" set. Also had a new distributor put back in, and had a new water pump installed. Engine was flushed, oil & filter changed twice. Compression tested good.
It runs just great, no noises, vibrations, loss of power etc. But, after about 15 minutes of engine running, or highway driving, oil pressure drops about 10psi on the gauge, and at 600 rpm idle & in gear, "check gauges" light comes on. I brought it back to the shop, and had pressure mechanically checked with engine at normal operating temp, driven only 5 miles or so. It was reading 15psi at 600 rpms, 35psi at 1500rpms, which is very good for an engine with that mileage. They replaced sending unit (original).
Oil pressure gauge on dash reads just under 40psi on a warm engine at 1800rpms highway speed, drops to just a bit under 20psi on dash gauge at idle, in park. Until it's driven about 15 miles, then it drops down to about 30psi at 1800, and just at the point where the "check gauge" light will flicker (5psi?) at 600rpm idle. When that happens, I shut it off. After sitting for half hour or so (it's cold here) it's back to normal, then it repeats after driving awhile.
Possible causes we can come up with, from best case to worst case:
Stepper motor on gauge itself is bad. Fuel gauge has looked like a mini-vibrator for a few months, but reads accurately. Plan of action is to drive it until readings drop again and then have pressure checked mechanically again, immediately, while it's still showing low on dash. The shop has a gauge that attaches in line with oil filter, so both gauges (dashboard & mechanical) can be read simultaneously... just to eliminate the dash gauge as the problem. When gauge reads normal pressure, pointer "sweeps" steadily. When reading lower pressures, pointer is "jerky", like the second hand on a clock - instead of sweeping, it "jumps" a few lbs of psi in it's movement up or down. That's why I thought sending unit was clogged or bad. But... problem wasn't there prior to gasket failure, so more likely something internal caused by coolant/oil sludge in engine.
Clogged sump screen on pump, loose sump tube - starts sucking air, bad pump or pressure relief valve spring fatigued enough to open at 40psi? Oil cooler clogged?, which could lead to overheating of oil and lowering viscosity & pressure. Has 5W30, been changed 3 times since milkshake and engine flush.
Worst case, bearings are bad and creating friction that's overheating oil after 15 or so miles of driving. Engine has ZERO odd noises... not even lifter noise when pressure drops low enough to turn warning light on, so my thought is, overheating bearings wouldn't keep 40psi on a warm engine for 15 miles at 55mph, then pressure suddenly drops off. If it weren't for the milkshake, I'd never even consider bearings.
I'm fully prepared to have a crate engine installed if it comes down to that, but if it's not necessary, I'd prefer to save the thousands of dollars that'll cost.
Any ideas, or just bite bullet and replace engine? Already have a new distributor, water pump, starter, and oil sending unit that can go on the new crate motor.
I am the original owner of a 1998 Sierra SLE K2500, 5.7L, ext cab, long box. The truck currently has 278,000 miles (always driven in my rural area, 90% highway miles). It is no longer a daily driver, only used on occasion for hunting, fishing, etc, since 265,000. Always changed oil and did other maintenance, never "let it go". Maximum mileage in it's lifetime without oil/filter change has been 5K miles. In the 18 years I've owned it, it's spent "maybe" 5 nights not garage parked, so it's in otherwise excellent shape for it's age & mileage. Lots of parts been replaced between 200K and now. Bearing hub assemblies, alternator, starter, steering componants, ball joints, radiator, rear brake lines/wheel cylinders/drums... pretty much a partial "rebuild".
About a month ago, I had a common problem that the 5.7 & others have... failure of lower intake manifold that "milkshaked" my oil. Same thing occurred at 78,000, but was an external leak only that time. It was a sudden failure (had just checked coolant level & oil a day or so before) that I believe I discovered after only about 5-7 miles of driving. Oil was always clean, wasn't losing any coolant prior.
I had the truck towed in when I discovered the problem, wouldn't even drive it the 2 miles to the repair shop I've used for 25 years. I had the gasket replaced with the Felpro "problem solver" set. Also had a new distributor put back in, and had a new water pump installed. Engine was flushed, oil & filter changed twice. Compression tested good.
It runs just great, no noises, vibrations, loss of power etc. But, after about 15 minutes of engine running, or highway driving, oil pressure drops about 10psi on the gauge, and at 600 rpm idle & in gear, "check gauges" light comes on. I brought it back to the shop, and had pressure mechanically checked with engine at normal operating temp, driven only 5 miles or so. It was reading 15psi at 600 rpms, 35psi at 1500rpms, which is very good for an engine with that mileage. They replaced sending unit (original).
Oil pressure gauge on dash reads just under 40psi on a warm engine at 1800rpms highway speed, drops to just a bit under 20psi on dash gauge at idle, in park. Until it's driven about 15 miles, then it drops down to about 30psi at 1800, and just at the point where the "check gauge" light will flicker (5psi?) at 600rpm idle. When that happens, I shut it off. After sitting for half hour or so (it's cold here) it's back to normal, then it repeats after driving awhile.
Possible causes we can come up with, from best case to worst case:
Stepper motor on gauge itself is bad. Fuel gauge has looked like a mini-vibrator for a few months, but reads accurately. Plan of action is to drive it until readings drop again and then have pressure checked mechanically again, immediately, while it's still showing low on dash. The shop has a gauge that attaches in line with oil filter, so both gauges (dashboard & mechanical) can be read simultaneously... just to eliminate the dash gauge as the problem. When gauge reads normal pressure, pointer "sweeps" steadily. When reading lower pressures, pointer is "jerky", like the second hand on a clock - instead of sweeping, it "jumps" a few lbs of psi in it's movement up or down. That's why I thought sending unit was clogged or bad. But... problem wasn't there prior to gasket failure, so more likely something internal caused by coolant/oil sludge in engine.
Clogged sump screen on pump, loose sump tube - starts sucking air, bad pump or pressure relief valve spring fatigued enough to open at 40psi? Oil cooler clogged?, which could lead to overheating of oil and lowering viscosity & pressure. Has 5W30, been changed 3 times since milkshake and engine flush.
Worst case, bearings are bad and creating friction that's overheating oil after 15 or so miles of driving. Engine has ZERO odd noises... not even lifter noise when pressure drops low enough to turn warning light on, so my thought is, overheating bearings wouldn't keep 40psi on a warm engine for 15 miles at 55mph, then pressure suddenly drops off. If it weren't for the milkshake, I'd never even consider bearings.
I'm fully prepared to have a crate engine installed if it comes down to that, but if it's not necessary, I'd prefer to save the thousands of dollars that'll cost.
Any ideas, or just bite bullet and replace engine? Already have a new distributor, water pump, starter, and oil sending unit that can go on the new crate motor.
Last edited: