Hello I'm new here, great forum! I've recently acquired a 1992 3.1L 4x4 isuzu rodeo automatic (GM'* LG6 engine as found in olds silhouette, lumina van, pontiac transport van, etc.). I'm wondering if anyone has had this issue and what they've done to address it. When I am underway and driving the oil pressure is very steady at about 40 psi. However, when I stop at a red light the pressure becomes erratic ranging anywhere from 20 to 110 psi (the maximum value of the gauge). Once the light turns green, steady value returns. I have done some research and everything seems to point to the oil pump. I doubt the gauge is faulty because of the needles erratic nature (sometimes it will stay at the high value for a few seconds then drop down, etc.). I wish I had a simpler explanation as I don't want to disassemble so many parts to get to the pump but this may be inevitable. Any words of wisdom? Thanks in advance for any suggestions you all may have!

Hi Mack Donald! Welcome aboard!

Wow, I didn't know they put that engine in the Rodeo!

Most likely cause: Have you changed the oil pressure sending unit?

How many miles on this vehicle?

Thanks for the warm welcome cathedral cub! The rodeo has 180k miles on it. I have not changed the oil sending unit. Is it located near the oil filter? Would a faulty oil sending unit be causing the erratic oil pressure when i'm at a stop light but not underway? Like i said, it runs great underway. I should also mention that when i am at a stoplight and noticing the erratic readings i can feel/sense that the engine is slightly/intermitently harsh. Hard to describe...almost like really minor backfires, stuttering or something. Then when i hit the gas and move, those symtoms disapear and all is well at around 40psi. Could this be attributed to the sending unit? I may be lucky if this is the root of my problem. Thanks again and have a good one!

My first thought was the sending unit as well. When warm, it'* typical for oil pressure to drop to 20psi, and if there is an issue with the sender that is only apparent at 20psi, that'* how it would look.

If the stepper motor in the gauge is on its way out, it'* also possible, but much less likely, and would typically be apparent despite the pressure.

Definitely replace the sending unit.
On My Pk Ave coming home one night (600mile trip) the warning ding happened and the oil pressure reading dropped to zero, in the middle of no-where!
Ya that got my immediate attention!
The oil was at full line, & all I could say was well it is either the pressure sending unit or I'm about to have a bad engine day!
Continued driving home no problems, replaced the sensor & all was well again!

The sending unit is a common issue and is the problem

Thank you all for the advice. I am planning on replacing the OSU this weekend, will let you all know how she turns out. Have a great day!

Don't rule out low voltages while sitting at a red light.