350 SBC Oil pressure question/problem... '79 vette
 

I've got a 79 vette that definitely is having a lack of oil pressure problem. I'm trying to figure out what is causing it, and what I should do about it.

When I first got the car, the oil pressure needle sat at 40 any time the engine was on and never moved. I suspected a bad gauge or a bad sender immediately. It took me a while, but I finally replaced the sender.

The motor does burn some oil (about half a quart per tank, so probably about a quart every 500 miles). It doesn't have any visible oil leaks, so I have to assume it is going out the tailpipe even thought I can't smell it in the exhaust.

The result? Hot, 15psi pressure at idle and 20 psi above 2000rpm. That is IT. It never went above 20. That was running 10w30 oil.

I was a little concerned, so I said "Hey, it's an old engine, let's just try a heavier weight oil." I drained it, replaced the filter and added 10w40 oil. The result was 20psi at idle and 35-38 above about 1500 rpm. I was reasonably happy with that.

Fast forward about three days, and now I'm getting only 18psi at idle and 22-24 above 2000rpm. This happened midway through one tank of gas (~100 miles). The oil on the dipstick is still CLEAN! It hasn't burnt any either, I figure probably because the heavier weight is having more trouble getting past the rings.

What gives? Bearing clearences do NOT open up that fast, at least without odd noises and metal in the oil. I am thinking it might just be the oil pump dying. Anyone else have any ideas?

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"Beware he who would deny you access to information, for in his heart, he dreams himself your master"

'84 Black Camaro ZZ4 M5 (V-6 in a former life) -- street beast
'91 Medium Blue Metallic Z28 A4 (UB-HSTRY's old ride) -- currently broken
'99 Navy Blue Metallic Firehawk #120 M6 (99.44% stock) -- daily driver

first off, do a compression test, and make sure all the rings are holding up ok.

second, consider that a 79 vette is a smog 350. Probably like 8.XX:1 compression, and low oil pressure in general. I could believe 20psi on a 122k motor with that low compression.

the difference between 3 days ago and now could be attributed to engine temp or outside temp.

You should be running 20W50 on an engine that old. Other than that there really isn't much you can do, it's just age.

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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

If isn't knocking, run 20w50 and start saving for a rebuild. It will run along time with that oil pressure.

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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

I disagree. That motor, if it's still the original one, should have the factory equivalent of a M55A in it, and should have the same pressure as any other small block with that pump... 20-25 idle, 50-55 cruise. With that few miles on it, unless it's been thrashed, it should have good pressure with 10W-30. And oil pumps don't just "die" slowly; about the only oil pump failure I've ever seen, is the stupid plastic sleeve that holds the drive rod tot the pump during assembly, failing and causing the pump not to be driven. They either work, or they don't. No slow fade.

If the oil isn't getting blackened and the engine otherwise doesn't seem to be in a low oil pressure distress condition, I'd suspect the gauge. You might want to hook its sender up to another car's gauge, or check it with a mechanical one if you have one. If it's really that low, it's time for an oil pan removal. Main bearings can do what you describe without making much noise; that's what I'd suspect, the front one specifically.

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"So many Mustangs, so little time..."
ICON Motorsports

Here's a little more info.

No noise, knocking, or other signs of imminent motor death. It actually has MORE power than it did a couple of weeks ago, but I changed the choke and moved to the 10w40 oil at roughly the same time.

Car has always had detonation problems at high rpm/high load especially at high ambient temperature. Decarboning helped some. I figure it's carbon deposits and oil burning that is causing the problem.

The car JUST started hesitating/bogging again on a full-throttle downshift. It feels just like it did the last time the primary choke pull-off was bad. This is probably a tuning issue since this happened right after swapping in the electric choke.

I'm not suspecting the sender since it is brand new, and the oil pressure went up almost 20 psi after changing to 10w40, but it didn't stay there for very long. Whether the numbers are accurate or not I don't know, but the fact is that it now has quite a bit less oil pressure than it did.

I've been "saving up" for a rebuild for a while now because of the detonation/oil burning problem, but anything cheap that I can do to keep it going would be a huge help. I can drop the pan pretty damn easily (no crossmember) and replace the pump if anywone thought it would help. If the pressure is just that low because of the bearings, I'll just run a nice thick oil and live with it. I'm mostly concerned with the pump just crapping out on the interstate.

You might just need a new pickup if your pickup screen is cloged. the pickup screen can get cloged and cause low fuel preasue. If that is the case like the man above stated "start saving for a rebuild" is correct.

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97 formula w/6speed,cags skipshift eleminator, hurst billet pro shifter, 30#injectors,adjustiable fuel presure regulator,Moroso cold air induction,ported mass air sensor,accell300+ ignition box, accell 9000 wires, bosch platnum4 plugs,flomaster crossflow muffler,Lingenfiter 52mm throttle body,TPIS L98 SuperProfile cam,Crane 1:6 full roller rockers,Edelbrock subframe connectors,Edelbrock strut tower brace.

Other car is a 89 GTA w/700r4w/stage 2 shift kit, corvette servo, B&M mega shifter, TPIS bigmouth intake, TPIS large tube runners, TPIS 52mm throttle body,Edelbrock ported centerbolt heads,LT1 24# injectors,GMPP LT4 hotcam,CompCams Pro magnum roller rockers,Edelbrock headers, flowmaster crossflow muffler,MSD 6A, accell coil, bosch platnum plugs, this car is currently under rebuild and restoration.

Put a mechanical gauge on it! It's nothing to get 2 or 3 bad electrical gauges in a row, especially from GM. Electrical gauges suck and can't be trusted! Until you put a real gauge on it, there is no telling what your pressure really is.

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'93 2wd truck
408 sb, vortecs, 454 TBI, Comp Cam, dual exhaust

First, no offense to this forum, but maybe you should be posting your '79 Vette question over on Corvetteforum.com. I am a member of both. They are a great bunch of guys over there and are as willing to offer help and support as the guys are here. Unfortunately, I've been away from this forum far too long (probably 10 months?)dealing with the problems associated with my Vette, and believe me there are many. I also have a '79 Vette (see signature). I don't see you Vette in your sig, so I don't know whether you have the L82 or L48. L82 has 8.9:1 CR. When fully warmed up, your engine should have about 38-40 psi. At idle, around 20 psi. What Vette's are notorious for are bad console gauges. You might try checking the gauge or circuit board first. In regards to your detonation and timing issues, I would contact a guy named Lars on Corvetteforum.com. He is a wizard with Vette tuning, distributors and carbs and will give incredibly detailed assistance. Just a suggestion.

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'84 Z28 w/ T-tops, 420HP Stroked 383, Trick Flow aluminum heads, Tremec 5 speed, 3.73 posi rear, Edelbrock 750 cfm carb, Dual Snorkel air cleaner w/ K&N filter, Edelbrock headers & 3" exhaust, Edelbrock Strut Tower Brace, Hotchkis LCA's and Panhard Rod, there is much suspension work to come.

1979 Corvette - Black/Black - 350 CI L82 w/ MM4 close ratio 4 speed manual

"Smoke 'em if you got 'em"

First, no offense to this forum, but maybe you should be posting your '79 Vette question over on Corvetteforum.com.

It's not really a Vette question, it's a small block chevy question. The motor could be a 305 in a 82 Camaro and there wouldn't be much appreciable difference.

I don't see you Vette in your sig, so I don't know whether you have the L82 or L48. L82 has 8.9:1 CR. When fully warmed up, your engine should have about 38-40 psi. At idle, around 20 psi. What Vette's are notorious for are bad console gauges.

It's the low-output smogger motor (L48 I believe). It's not even my 'vette, it's my mother's. I just do the work on it. That's why it isn't in my signature. And FWIW, the console gauges in thirdgens are pretty crappy too... I've had trouble with mine before (I can't count which # instrument cluster I'm on, not all of which were replaced due to problems).

You might try checking the gauge or circuit board first.

In regards to your detonation and timing issues, I would contact a guy named Lars on Corvetteforum.com.

I actually posted the detonation question on corvetteforum a long time ago, and basically came to the conclusion that I posted above. I don't actually remember who it was that I ended up talking to. It's really a moot point now. I just want the damn thing to hold together a while longer until I can afford a rebuild. The motor is really to weak power-wise for me to care about it otherwise.

check the pickup screen. oil pumps are cheap so you might as well replace it. i've banged my head against the wall before trying to figure out this problem on some older vehicles i've had. i even rebuilt an entire ford 351w and still had the problem until i pulled the pan off again and realized that the pickup screen was clogged with sludge! take an awl and remove the sludge. i would definitely replace the pump anyway. if your screen is clogged, you may have worn out that pump. oil pumps have very little tolerance for wear.

Yep, thanks everyone. I've already got a new pump. Damn thing is cheap enough. And it looks like a 15 min job to get the oil pan off, because I don't see an engine crossmember.