The noise isn't a problem but that low oil pressure is. Here are three likely causes.

Dear Car Talk

Dear Car Talk | Mar 01, 2000

Dear Tom and Ray:

I have a '94 Pontiac Transport with a 3.8 liter V-6 engine and only 29,000 miles. Ever since I got it, I've noticed that once the car has been driven awhile and the engine
is at operating temperature, the oil-pressure gauge will read about 40-45 psi while I'm driving. When I come to a stop, the pressure drops to about 20 psi. When I told
my dealer about this, he said it's fine because the oil thins out as it heats up. Lately, when the car is parked for a week or more, it makes a horrible clattering noise for
about 20 seconds after I start it, then runs smoothly. Do I have a problem? -- Herman

TOM: You would think that a horrible clattering noise would be a sign of impending doom, wouldn't you? Well, that's the least of your problems, Herman. That's just
the dried-out hydraulic lifters making noise until they fill back up with oil. And as long as it only happens after the car sits for a week, and as long as the noise goes away
quickly, that's not a big deal.

RAY: The low oil pressure, on the other hand, IS a sign of impending doom.

TOM: According to the Pontiac specifications, your oil pressure should be 60 psi at 1,850 rpm. If your pressure is only 40 psi, then yes, you DO have a problem,

RAY: And there are three possible reasons for your low oil-pressure reading. One is that it's just a gauge problem. If your gauge is faulty or inaccurate, it could be
telling you that your pressure is low even when it isn't. So you want a mechanic to check your oil pressure himself, with his own gauge.

TOM: This is by far the best-case scenario, because if the mechanic's gauge tells you that the pressure really IS up to spec (60 psi), then you can solve the problem with a
Magic Marker. Just cross out the number "40" on your gauge and write "60" next to it.

RAY: But if the mechanic's gauge confirms that the oil's pressure really is 40 psi at 1,850 rpm, then you've got either a bad oil pump or an internal engine problem.

TOM: And unless you want to take the engine apart and measure the clearances between the bearings and the crankshaft, the best step to take next is to replace the oil
pump. And if the pressure tests OK with the new oil pump, then you thank your lucky stars and drive away happy.

RAY: But if the pressure is still low with a new oil pump, then, in the not-too-distant future, you'll have two choices. The $2,000 solution (an engine rebuild or
replacement) or the $20 solution (an ad that says "'94 Pontiac for sale. Good tires. Low oil pressure."). Good luck, Herman.

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