98 Chevy k1500 350, low oil pressure and misfire.

[Cgbranch1]

I am needing assistance with the next step to take with my truck, 98 k1500 350 with 159k miles. It cranks hard, and has a random misfire while idling. Stutters while driving just enough to be noticeable and has extremely low oil pressure when warm, I have replaced plugs, wires, distributor cap and rotor button, IAT and OPSU. Oil pressure still reads 5 lb and below at idle and never over 25 lb even at 2500rpm. No noticable rattling or pecking as if it doesn’t have oil pressure. Is the misfire issue and the low oil pressure connected? Any help would be appreciated, thanks.

 

[WICruiser-97]

It is not likely that the two problems are related but never say never.

Get a second gauge reading on the oil pressure. If it is reading correctly you have something worn enough that oil is being "leaked" reducing the pressure. Most likely rod bearings but there are a number of other potentials. Try running a higher viscosity oil (if you are using 5W20, try 15W40).

I would work on the oil pressure before trying to do anything about the random misfire and shutter.

 

[name]

Make sure the truck is putting out enough volts, low voltage may make the gages read low & and give you a weak spark.

 

[Schurkey]

98 k1500 350

has a random misfire while idling. Stutters while driving just enough to be noticeable

1. Verify fuel pressure during prime, at idle, and under load. How old is the fuel filter?

2. Connect a scan tool, look at the misfire COUNTS for each cylinder. Find out which cylinder(s) are acting up.

3. The OEM poppet-valve fuel injector spider and regulator assembly is infamous for rough idle, poor power, poor starting, etc. Similarly, the intake manifold gaskets are also infamous for misfire (rough running especially at idle) and coolant leakage

and has extremely low oil pressure when warm

Verify oil pressure with a known-good "shop gauge" connected to the pressure port.

How old is the oil? Did you use a quality oil filter last time--the filter media didn't collapse?

Consider an oil change with a known-quality filter, and proper oil viscosity.

Oil pressure still reads 5 lb and below at idle and never over 25 lb even at 2500rpm.

If the engine idle speed is low due to misfire, oil pressure can be reduced also. But that does not account for low oil pressure at higher RPM.

 

[Cgbranch1]

It reads just over 14 volts while running, I changed the oil this afternoon with 10w30 (because that was the thickest gas engine oil they had) and a new Mobil 1 filter and this made no change to the reading. I am going to get a manual gauge and check the oil pressure as soon as possible.

 

[Cgbranch1]

Took me a few weeks but I got a manual gauge on it, this is oil pressure with 100 degree water temp, at idle.

 

[Cgbranch1]

This is oil pressure at idle. It is at operating temp, and has been running 15 minutes, just drove down my street and back.

 

[Road Trip]

This is oil pressure at idle. It is at operating temp, and has been running 15 minutes, just drove down my street and back.

Good on you for getting a 'real' reading from a mechanical oil pressure gauge.
This removes a lot of variables, especially for those of us who don't get to
be next to the running engine.

IIRC, the classic oil pressure rule of thumb for the SBC was 10 lbs per 1000 rpm.
I would be interested in what your readings are using this gauge while
holding the engine at 2000 & 3000 rpm.

If you come back with 20 lbs & 30 lbs respectively, then with the
oil you are currently running your engine is still 'serviceable'.

By the way, 15W-40 oil in the colder weather and 20W-50 in the
summer may give you a few lbs of cushion and thereby give you less
stressed motoring.

And yes, a properly assembled new engine will give you 25-30+ lbs
at a hot idle, and hit the bypass of 55-60psi by 2500 rpm. But I've
seen in several places that 10 psi of oil pressure per 1000 rpm is
what you need in order to operate properly. (And you did mention
that the engine is still quiet, both valvetrain and lack of rod knock?)

****

I know personally that even if I know the mechanical gauge gave me
'Go' readings, driving around with only the electric dash gauge showing
0 at a hot idle would still be a bit unnerving. If you are going to
continue to drive the truck as-is, I for one would either put your
test gauge inside the cab, or purchase a decent mechanical gauge
and install that in your field of view while driving. (See attached.)

For what it's worth --

 

[Cgbranch1]

It makes no noises, does not burn or even color the oil. Will probably go up to at least a 15w40 oil soon. The engine stays right within the 10 PSI per 1000rpm. This is around 2800 to 3000 rpm

 

[jjester6000]

This is oil pressure at idle. It is at operating temp, and has been running 15 minutes, just drove down my street and back.

Throw a quart of Lucas in there and see what happens.

I personally run 10w40/Lucas on most of my engines.