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Old 03-29-2007, 11:46 PM
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low oil pressure question _chevy 4.3

My chevy 4.3L has low oil pressure. I have not had the pressure tested with a professional gage. I am only going off the in dash gage.the engine has @100K miles. My question is a bit general, but here goes. Where is the loss of pressure most likely coming from? I am pretty sure the entire engine is worn, and worn consistently, but it sounds good, and runs good. Also, there seems to be no play in the distributer shaft. It seems that there is no excessive wear at that point.
Here's what is happening: I have @ 40 psi when the engine is cold at idle.
As soon as it worms up the pressure is 0 at idle. The pressure stays in specs for most of the rpm range. But at idle and higher rpms the pressure falls too low. Should I look to the main bearings and oil pump first? I want to get the pressure back up and get as many miles as possible out of this worn engine.
Not looking to do a rebuild at the moment. It really doesn't sound like an oil starved engine. oh yea,..the compression is perfect and consistent. 150 psi 1-6, give or take a few pounds.

I am praying that the oil pressure sensor is wrong!

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Old 03-30-2007, 12:06 AM
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briansansone,

As an engine wears out over the years, all of the bearings in the engine wear out. So you loose pressure from the mains but also from the cam bearings and connecting rod bearings. The distributor is not a wear indcator so just because it is good is good news but that is about all.
The reason you have good oil pressure when the engine is cold is because the oil in the crankcase is "thick" and doesn't leak past the worn bearings so you get a good reading. As the engine gets warmer the oil thins out and yeah you guessed it the oil pressure drops. I am not so sure that it is really 0 PSI at idle because you would hear the valve lifters rattling and ticking and since you say it sounds good then I think you have at least 4 pounds of oil pressure.
Now what you want to do is bring the oil pressure up all the time without rebuilding the engine. Well here is what we did WAY BACK IN THE DAY.
Put in some thicker oil like 40 weight. Not 10-40 but straight 40 weight. I once put in 90 weight gear lube to get one of my cars home from college. I made it home but I knew I was going to rebuild the engine as soon as I got home so I was just worried about getting home.

You could put in new main bearings but you would want to replace the connecting rod bearing too. The only problem with this is if there is any ridge in the cylinders the top ring will more than likely strike that ridge and break the ring and then the engine will start smoking. Oil pump would help too but this is a lot of work for an engine that is on it's last legs, RIGHT?

Good Luck
Scholman
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Old 03-30-2007, 01:35 AM
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thats the advice i was looking for. i am not "ASE", but i know FAR more than than the average grease monkey, and more than most "certified" mechanics. I hear no "clackity click click click" from the top end, but i do know the engine is very worn-nearing the end.I was hoping to get away with new crank bearings, rod bearings, oil pump, and maybee some top end work - anything i can get away with without removing the engine. but if its going to cost too much ill probably have a rebuilt 4.3 installed. if the oil pressure does check out to be too low, how long do i have? and is it worth my time to try and save the engine w/ new bearings?
I once tried to install new crank bearings with the engine and crank in place. it did not work. I cuoldn't get the top halves to rotate into place. I have heard roomers that it is possible.

thanks so much for the input. i have fixed problems that "certified" mechanics could not solve. but i am confused on what i should do next w/ this engine. FIX OR REPLACE???

thanks

bri
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Old 03-30-2007, 02:09 AM
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heavy oil rebuttle

oh yea. I poured a BUNCH of the Lucas oil stabilizer in there, and it made no difference at all. i am running thick oil/w stableizer-no help. I'm going to pull a bearing soon to see what it looks like. my guess is that it will show a lot of wear.....but i have never seen a running engine with oil pressure that low.

you seem like you know what your talking about. I would like to pull a bearing, look , and report back to you. i am pretty swift with the mechanics.
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Old 03-30-2007, 04:32 AM
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The stock oil pressure gauge is proably not telling the truth. The sending unit that's at the back of the block there by the dist. is the cause.
Before I'd pull anything apart... go get a cheap oil press. guauge (mechanical) at Autozone or Advance (similar) and take out the stock sending unit and hook the mechanical one up to see where it "realy" is.
I've seen 4.3L's go well over 200,000mi.
They have soft cyl. bores and the TBI's coke up on the PCV system along with a few other annoying things but overall run decent for a long time.

If you've tried thicker oil and Lucas and did not see a difference in the gauge AND have no noise I'd do a manual gauge to confirm pressure.

You can replace crank bearing with out pulling the crank but it's worthless and a total pain in the butt. You'd want the crank to be machiened to get any real "Lasting" results. Plus working under a dripping oil panless engine sucks.
Do a real gauge check first...
Just built the 4.3L in my wife's GMC Typhoon a few months ago.. The factory gauge reads a little different every day that's with a new sending unit.
I'll bet the engine's ok.
Just my guess after seeing and working on many abused 4.3's.
~Scott
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Old 03-30-2007, 05:01 AM
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Don't trust a factory gauge... EVER. Like posted above get a cheap mechanical oil pressure gauge and plumb it in and see what you REALLY have for pressure. I'd hook it in to a T fitting and keep the stock sending unit though in case there is a safety feature that shuts down the fuel pump if the oil pressure drops to zero. Plus that way you can compare readings between the two gauges.
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Old 03-30-2007, 05:15 AM
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Factory idiot light = useful, factory gauge = worthless for meaningful results.

Agree with the others - put in a mechanical gauge and see what you really have.
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Old 03-30-2007, 08:45 AM
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low oil pressure

check with a meckanical gauge first, like earlyer posts factory gauges are not reliable. also replacing bearings will not move the pistons up higher in the bore damageing rings. if the bearings are worn replaceing them will have a positive efect, loose bearings allow the crank tomove farther up and down same with rods. if bearings worn to the point of low oil pressure you ar in trouble. i sugest if pressure realy low throw in a new oil pump most likely solve the problem enough to get aditional life cliff
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Old 03-30-2007, 08:54 AM
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Quote:
Originally Posted by cliff tate
check with a meckanical gauge first, like earlyer posts factory gauges are not reliable. also replacing bearings will not move the pistons up higher in the bore damageing rings. if the bearings are worn replaceing them will have a positive efect, loose bearings allow the crank tomove farther up and down same with rods. if bearings worn to the point of low oil pressure you ar in trouble. i sugest if pressure realy low throw in a new oil pump most likely solve the problem enough to get aditional life cliff
Throwing in a new oil pump will hardly ever solve oil pressure problems. The oil pump would be one of the last things I would ever check.
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Old 03-30-2007, 08:58 AM
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Quote:
Originally Posted by Classix_Lover
Throwing in a new oil pump will hardly ever solve oil pressure problems. The oil pump would be one of the last things I would ever check.
I "kind of" disagree. Replacing a stock volume for a stock volume isn't going to fix anything - I agree with that. Where you could "bandaid" a loose engine is by going to a high volume pump.
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Old 03-30-2007, 09:01 AM
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Quote:
Originally Posted by Koolaid
I "kind of" disagree. Replacing a stock volume for a stock volume isn't going to fix anything - I agree with that. Where you could "bandaid" a loose engine is by going to a high volume pump.
I know the whole "band aid" trick and it isn't worth the trouble of spending money and time where it isn't needed just to have the right pressure for only a short period of time.
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Old 03-30-2007, 09:37 AM
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Use a good mechanical gage to check pressure before getting too excited. My Greek grandpa once told me, "Don't start kicking your feet until you are on the donkey"

If your pressure is really low, then......

A high volume oil pump should kick up hot idle pressure by 5 to 10 psi.

What weight oil are you using now?

I see a bearing change in your future. You should be able to buy a crank kit and just pull the engine and flip it over and throw in a new crank and bearings. Should not take more than a weekend to do that plus crank kits are pretty cheap.
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Old 03-30-2007, 05:36 PM
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oil pressur

oil pump is a high wear item, un filtered oil goes threw it cotinuously, have fixed many such problems with new pump and pangasket,bearings are not the most comon wear items to cause oil pr loss. worn push rods worn cambrgs loose lifterbores, and mainbrgs rodbrgs. plus we stil do not know if we have low pressure.if the oilpump does not help then a comp overhaul is the answer, but run the thing to destruction which can be lots mor miles, you dont need high oil pressur to keep a old engin alive. 10ps at idel hot 25 at high idelif driven reasonably will live.
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Old 03-30-2007, 05:44 PM
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OK....cam bearings have the biggest effect of all the bearings on oil pressure. Have you ever re-placed your oil pressure sending unit? With 100K I would think it would be a good time to replace it. Sending units are cheap.....put a new one on and see how your guage reads then...if it does not come up then check it with a manual guage...some fittings a rubber hose and a manual guage will do it...Just install it in the hole that you take your oil sending unit out of.
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Old 03-31-2007, 08:33 AM
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I dunno if this will help any, but I wanted to add in any oil pressure gauge is not accurate below 20 psi. The 4.3 in my chevy pickup had 270,000 miles on it before I pulled it for a rebuild, why I did so then was since it was just starting to loose oil pressure and power but it never went down to zero. I would pull the sending unit and replace it, Clean the oil pressure bung it screws into as well. Many 4.3`s are gunked up with sludge, mine sure was when I got it, after time this stuff breaks loose, falls in the oil pan and is picked up by the oil pump screen which reduces oil pressure. I would check all these areas. Since you plan to pull the pan and if everything looks okay I would replace the timing chain, it`ll make a big difference power wise. If there is a considerable amount of sludge, give the engine a couple of baths of Sea Foam. All I did was pour in a can, run it about a week, then change the oil and filter.
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