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Oil Pressure



Just replaced the throttle body on my '86 L98 67,000 mi. Once the engine reaches 180 degrees, and I'm idling at a stop light, the oil pressure drops to 0 psi and a red light comes on. The idle fluctuates between 500 and 600 RPM. Once I increase the revs to 700 rpm, the pressure increases to about 10psi and the red light goes off. Oil pressure is healthy between 40 and 54 psi during normal driving speeds.

I've replaced the oil and filter, using 5-30W Synthetic Castrol but have had no improvement in oil pressure at warm idle. Does this sound like an oil pressure switch problem?

I'm considering increasing the idle speed to 700 rpm but am I masking another problem by doing so?

Need some advice.

When did this happen??

Hi there,
Well, did this start after you installed the throttle body?????
A few tips here. Please remember that using synthetic motor oil will lower your oil pressure, because it does NOT contain any waxes or parrafins that conventional oil has. So my question would be, what was the oil pressure before your oil change to synthetic????
As for the rest, it could be a sending unit, however, the only way to check for sure is using a mechanical gauge directly on the oil pressure tap, located at the back, where the oil sending unit is screwed into the block. This should be your next course of action.
Please keep us posted, c4c5
Thanks for your response.

In answer to your question, 2 years ago, I had a leaky intake manifold gasket replaced, but also had a large mouth intake manifold installed with a 52mm throttle body. larger runners and exhaust headers. The oil pressure had dropped (same pressure drops at idle) and the mechanic who had done the work, increased the idle at that time to compensate, about 100 revs. Recently, a month ago, I had to replace the throttle body since it was malfunctioning and put the 48mm throttle body back on. So, now I'm back to the low idle pressure.

So, if I replace the oil pressure sensor, and put regular oil back in, am I barking up the wrong tree? Does going back to a 52mm throttle body help?
Hi there,
Yes, you can start with the conventional oil, and a new sending unit.
The throttle body will NOT cause this issue, in any way, unless you replace it and reset your minimum air rate.
I would concentrate on the oil pressure, and then proceed.
Please keep us posted, c4c5
I was just clearing my inbox and forgot to respond to your answer. For all the help you offerred, I felt you should know the outcome.

I replaced the oil pressure switch and voila, the pressure returns to 12 psi at idle. Just a worn out switch.

Thanks for all your help.
What should the proper oil temperature be in the neighborhood of? Also, would using synthetic oil lower the temperature as opposed to regualr oil?

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