Was just wondering what the normal operating pressure should read on a fully stock 89 350 TPI Formula Firebird? I recently did a oil change. Now when first turning the car on it reads in the middle at 30 give or take. After driving for awhile it drops below that. When at a stop (like a traffic light or stop sign) it will read just a few notches above the red bar. When driving (depending how hard/fast it will read between the red and 30 marks. Prior to the oil change it was the opposite. still in the middle area before car was fully warm, but when driving it was constly on the higher side of the 30 and responded more to me getting on the gas. It would shoot way up near the top when on a highway. Was just wondering what it should be at and should I be worried about anything. My buddy said that I might be losing pressure some where. Maybe a line or something. I dont know where to start with that issue. What line would cause this? Where is it located or where do I look if thats a potential issue? My dad said that it might have to do with the oil change. I used different oil this time. Also while Im her, what type of oil should I be running on my car? I have never used synthetic. I know the cap says 5w30, but I was told by my uncle to use 10w40 and thats what I was using the past couple of years until this last oil change where they were doing a deal and I put in 10w30. any and all insight/help would be greatly appreciated.

 

I had a similar pressure swing and solved the problem by replacing the faulty oil pressure sending unit. It gave me false readings.

Try that for starters. It was more prevalent in my '96 3.8L however. I try to find oil pressure sensors made in the USA vs some foreign land.

 

Where is it located? Is this a job one could do themselves? By that I mean the level of difficulty. I do alot of my own work, but there are some things I just dont tackle because of a mixture of knowledge/time/tool restraints. Also is there a way for me to determine if this is the problem without having to buy a new one and installing it?

 

10W30 is fine. As an engine gets older, parts start to wear. Oil pressure is maintained by the main bearing clearance. Using 10W30 instead of 10W40 and having lower pressure probably means your main bearings are seeing their age and clearances are greater than what they used to be. .001" difference can mean a lot. Being in a climate that can see cold winters, I wouldn't recommend using anything thicker than 10W40. If you were in a much warmer climate, using something like 15W40 all year round wouldn't hurt an older engine. If you want to use synthetic, 5W50 may be a good choice.

I'd confirm the actual pressure with a mechanical gauge before replacing anything. The sending unit is normally mounted at the back of the block beside the distributor. There may also be a pressure port down near the oil filter.

A rule of thumb is that an engine only needs 10 psi per 1000 rpm. Since your street engine rarely sees over 4000 rpm, 30psi while driving is fine. At an idle, 10 psi is fine. More pressure through the rpm range won't hurt but it's not really required.

 

Ok so forgive me if I sound redundant or dumb. Im relatively unfamiliar with all of this. I never dealt with any oil pressure situations in the past. Im trying to break down what you are saying into "idiot" terms for myself to better understand you (Everything I know was taught to me by backyard mechanics who call everything the "thing", this is why I get so confused).

Changing the oil thickness might be directly related to pressure difference Im currently seeing? Im saying "might" be because one cant know for sure until one checks it for himself.

 

You always need to confirm something is faulty before replacing it. A gauge or sending unit can be diagnosed if one or the other has failed but even a working gauge or sending unit can give a false reading. Confirming actual pressure with a mechanical gauge can confirm how accurate the dash gauge can be.

If the engine is running and the gauge on the dash says 30 psi then you connect a mechanical pressure gauge and recheck. If it also says 30 psi then you know the dash gauge is accurate. If the mechanical gauge says 60 psi then you know the dash gauge isn't correct. The dash gauge will usually work or not work. Very rarely will they be inaccurate. That means the sending unit would probably be sending the wrong reading. There's also a possibility of defective wiring creating too much resistance and giving a faulty reading.

The oil pressure gauge works off a variable resistance to ground. Simple test to check if the gauge is working is to turn the key on and disconnect the sending unit. Check where the gauge reads (probably zero). With the key still turned on, connect the sending unit wire to a good ground and check the gauge (probably pinned right over to the high side). That basic test can tell you if the gauge is working. When you start getting into multi wire sending units (transducers) troubleshooting becomes more difficult.

It's all basic automotive troubleshooting. If in doubt, have a mechanic inspect it.

 

Sounds like my car, only it reads 10 psi when I first start it up and nothing once it's warm. Obviously this is wrong as the whole engine would be making an awful noise if it had no oil pressure.

As others have said, the proper procedure is to check it with a mechanical gauge. Personally, if a sending unit is cheap, I'd just replace it.

 

Sounds like the oil pressure sending unit. Mine when bad and the gauge started reading low and twitching. Just replace the sending unit. There probably about $20 or less. If it doesn't go back up you know you've got a different problem. Either that or by a mechanical gauge to check it with. The sending unit is located on the back driver side of the engine between the engine and firewall. It's shaped like a bell and has one wire connected to it.

 

sorry to kind of hijack this, but I have a very similar problem on this topic and it's described pretty much like this.
at a stop in idle in gear, it reads 0 I guess (all the way back in the red), and then in park it twitches. I think it's also reading low while driving.