Oil PSI drops/engine stumbles, on warm, 1988 2.0

[borlax]

When the truck is warmed up completely the oil PSI is usually right at 40 and it idles nicely. After driving it for a while sometimes the oil PSI will drop down to about 20 and the engine struggles to idle, it does this for a while and sometimes will go away. Usually after shutting it off when I start it again it sits at 40 for a while then struggles again. It goes down to about 10 PSI when I get off the interstate going no faster than 65. and its died a couple times. There's no computer controls really besides the duraspark box, it has a weber 32/36. I'd like to fix this before I install a 2.3 roller cam, MSD box, racer walsh intake, etc. Thanks for any help

 

[RonD]

Oil pressure is back pressure from bearing gaps when there is too much oil.

The oil pump should provide more oil than can be used by the bearings, this extra oil causes a back pressure in the system which is what we read as oil pressure.

Oil pump flow changes with RPM.
General rule of thumb is that an oil pump should create enough flow for 10psi per 1,000RPMs
But it is a curve more than a linear rule.
A warmed up engine idling at 650RPM should be 6-10psi, and at 6,000RPM 40-50psi

If oil pressure gets too high it can cause dry spots at the bearings which will lead to failure.
Below 6psi and gravity can prevent oil from reaching the top of the engine, which you can hear in most engines as "rocker tapping" noise, lower bearings are still being cooled and lubed but it is a sign of a problem.

Your oil pressure seems too high at idle assuming 650-800rpm idle
But your oil pressure dropping is normal if RPM is dropping, and not the cause of the RPM dropping, just a product of the oil pump flow dropping with RPM.

The carb is probably responsible for the drop in RPM, might be spark but my money would be on the carb.

 

[borlax]

Since you're betting its the carburetor, what do you think would be the problem with it, causing PSI to drop & the engine to stumble? Especially after constant high RPM for about 45 minutes like on the interstate at 65MPH. I had also considered the oil pump or the head gasket being the culprit. I happened to notice some white foam in the radiator a couple of months ago but none as of recently. Thanks again for the input.

 

[RonD]

Oil pressure drops because RPMs do, oil pressure doesn't cause the RPMs to drop.

So you need to look at what controls the RPMs of an engine.
And that is pretty much the throttle plate and air/fuel mix, so the carb in your case.
Could be a loose throttle plate or idle cam on the linkage allowing throttle plate to close too much after longer drive, this could cause idle to drop down to 400-500rpm.
I can't speak about your specific carb, don't have one, but they all work the same and all fail.
When you have a carb engine you usually need to adjust it at least 2 times a year, but at this age every 2 months would be the minimum to keep MPG up and other issues down.
EFI(electronic fuel injection) is sweet in that respect, "set it and forget it".

EDIT:
I did see that the weber 32/36 doesn't like mechanical fuel pumps, pressure is too high at speed, so you would need to have a pressure regulator if using a mechanical pump.
There are quite a few articles on "weber 32/36 tuning"

Spark is more of an off or on thing, however that being said the spark advance changing rapidly could cause a sudden drop in RPMs.
Not sure what the '88 2.0l would have, Duraspark and vacuum advance?
If so check the vacuum hose, make sure it is air tight.
And pop the cap and make sure rotor is tight and not sloppy, loose.

 

[tomw]

Could have a worn distribulator bushing, sticking centrifugal advance, weak advance springs, hanging throttle plate, worn throttle plate bushings, and a myriad of other things.
If you get off the gas quickly, the fuel inside the passages wants to keep going and going, possibly over-feeding the intake and causing misfire due to over-rich condition. Older carbs had a 'dash pot' that eased throttle plates closed for the last bit, preventing some of the above.
It is possible to overcome just about any 'transition' circuit that tries to handle the change of fuel & air flowing through the carb. Most of the transition stuff is a patch and an adjustment of one sort or another that affects the carb through the rest of its operation, partially anyway, so the carb is mal-adjusted for idle, possibly for cruise, and definitely for WOT. Just because it works. And was cheap to make back when.
This is not a knock on the carb as it performed well and was diddled with a lot by the designers to overcome the problems that operating at multiple rpm ranges along with changes in power demand. Some designs were better than others. The HW most 2.3 engines got were actually a pretty decent design. Weber did their work, and Holley built a large pile of carbs for Ford and, I think, GM as the Vega also had a HW. From memory, anyway.
tom