Hey all.

84 TA, 355 sbc. 700R4

My oil pressure has suddenly dropped from a normal operating pressure of 40psi at idle and 80 psi while driving to almost zero psi at idle and 40 psi while driving.

How safe is this? What causes it? Possible the pickup fell off? I don't know.

I do have full oil so it's not low on oil.

Pictures below.

 

Pickup falling off has happened with my 2nd gen with a Big block.
Even with the full pan, pressure was near destructively low!! I was lucky
to have seen it when I did.
Have heard but not seen silicone clogging screens.

If you don't mind spending a few $, put in a couple quarts oil & see if it
makes a difference. If it holds normal at idle, very very slowly take RPMS up
just a lil bit, maybe to 1500, 2000. If it holds I would say tube fell.
Either way, pan is coming off, so maybe just drop it anyway.

No mention of gassy smell in the oil.

( edit )
I have also heard but not experienced
10lbs is minimum at idle.
I know more experienced will give more direct, Good Luck

 

Thumb rule is like 10 psi for every 1000rpm. Worn bearings reduces restriction to oil flow. Worn oil pump. Faulty oil press gauge and or sensor. Verify oil press with known good mechanical gauge.

 

Well, it's still at the trans shop, had some other issues that cropped up when I test drove it.

I wonder if they knocked something loose or damaged the sender in anyway while pulling the trans.

If that all checks out, then I don't know where to go with it. I'm not as worried as I would be if there was just zero oil pressure.

 

I had the oil pump work three of the four bolts holding it together loose, which caused very, very low oil pressure. Of course the gauge wasn't working quite right at the time and I'm lucky that it didn't cause any major damage (at least none that's shown up yet). I'd definitely verify pressure with a mechanical gauge, running with low pressure is not something I'd take chances about.

 

If the sender is in the normal place near the distributor , and the low readings coincide with the transmission remove/replace , yes it's quite possible that the sender got damaged . Especially if they pulled the distributor to gain some room for the engine to tilt rearward , this could have led to the sender getting squished in there as the engine was tilted to a favorable position for transmission removal .

I'll second someone972 and say to put a known good mechanical gauge on it well before considering pulling the pan to look for a fallen pickup , you may just save yourself a buttload of unneeded work ! ....

 

If it was holding good pressure when it went into tranny shop did they test drive the car after the tranny was fixed or serviced? Not trying to point the finger but unless you really know them then we'll you know... But as previously stated I would pull the pan and inspect. when you do go ahead and replace pump and screen while pan is off. Did the motor hold the same pressure after first cranking and after car was driven for a good period of time? SBC doesn't need much oil pressure to keep things running but the more pressure you have the better the bearings and all moving parts will be and last a lot longer. Hope you find good news.

 

any links for a good gauge to buy?

It'll pull 40psi on the gauge while driving at 70...think it's ok to drive it 80 miles home from the shop?

 

Unless you think you'll need one again go to auto zone and rent a oil pressure gauge. You pay something like $25 you use it, bring it back, $25 back to you.

If the engine honestly has that low of oil pressure, 80 miles will turn the inside of your engine into a war zone.

 

Just swap the sender out.

WAY less trouble and expense than dinking with a mech gauge.

The old racer's "rule of thumb" doesn't apply to a situation like this. (or really, much of any situation at all, IMO; but that's a discussion for some other time) What it ACTUALLY MEANS is, if you're anticipating some particular max RPM, then the OP needs to be ... that ... AT THAT RPM. Of course it assumes that the bearing clearances are somewhat correct and the engine is otherwise in good working condition. Has nothing to do with a stock motor driving down the road. Even more, has ABSOLUTELY NOTHING WHATSOEVER to do with a situation where potentially a bearing has failed and the clearance is now much larger. You could put a pump in there that will give 100 psi per 1000 rpm, and if a bearing has spun or stacked or just plain wiped out, it's STILL gonna eat the crank.

40 psi at cruise is typical SBC pressure. That's what an out-of-the-box unmolested M55 will produce. Virtually ALL SBCs ran with 35-40 psi regulator springs for decades.

0 at idle, OTOH... tends to indicate a problem.

If changing the OPSU doesn't change the reading, there's a motor pull in your immediate future. I would NOT recommend even starting it up any more after determining that, LET ALONE driving it on the highway. No sense in tearing up more stuff that might not be bad yet, or filling the whole engine with metal shavings.

But, no sense in getting all alarmist yet... just swap the OPSU and see what happens. Go from there.

 

Yea, i dont see any problems driving with 40psi oil press at what <3,000rpm. Just no spirited driving. I wouldnt tow it until it was <20psi but thats me - use your own discretion and resources.

Funny u ask as ive been shopping for gauges lately (well trans temp). I couldnt tell u what size or color u want but the full sweep gauge is the way to go. Myself i prefer electrical as ive had and seen to many mechanical oil press gauges leak inside the people compartment - though nothing wrong with a mech gauge. Now the expensive electrical gauges are over $200 but use an accurate and fast response stepper motor to drive the needle where the cheaper air core gauges <$75 maybe accurate enough but not as fast or stable as the stepper motor gauges. Its what u think u need. The stepper motor gauges are great for tachometers, maybe EGT, A/F monitors and maybe boost. But usually anything beats the stock OEM gauges installed as u can easily calibrate a aftermarket gauge any time.

U cant beat Summits catalog filters to help choose what u want but i found the best pricing on Amazon (better than FleaBay too).

Just thought to pass that along as the stock OEM gauges are always questionable. Many large technical companies do have their own tool shop with calibration equipment and u maybe able to calibrate your own gauge itself or find someone at work that can do it.