4.3 Oil Pressure Problem

Hi All,
We put a used 4.3 in my son's '89 S-10 Blazer and the engine runs fine, does not smoke and has good oil pressure when cold (60psi). As the engine warms up the pressure drops. I instantly thought "Sending Unit"! But as it warmed up and the pressure dropped the lifters started clattering. I was going to put a mechanical gauge on it but since it started clattering, that told the story. My question is, do you folks think it is the pump? Maybe the pressure relief valve in the pump? We are running 10W-30 in it. The oil and filter are fresh.


Thanks,
Brian

It could be that the gears and/or end plates of the pump are worn. I would not suspect a pump pickup problem, since there is good cold oil pressure, and they seem to suck more air when the oil is thicker.

A new Melling pump and pickup tube set is a relatively inexpensive item. If the oil pan is reasonably easy to remove, it might be worth investigating. I'm not sure what's involved on the '89 engines, but I'm fairly certain they still had the stamped steel oil pan.

If you don't have any idea what the engine mileage is, however, there could be deeper problems. Then again, once the oil pan is off, it's easy enough to crack off a main and rod cap and check bearing condition.

Thanks BB!

I agree with you on the pump. It does have a stamped steel pan. I was going to check bearing condition but I've been thinking that if it were a bearing clearance problem, it would seem like the pressure should be low all the time. It's hard to beleve that the clearances would open that much when heated up to make the pressure drop to zero. Do you think it's possible?

Thanks,
Brian

Thanks BB!

I agree with you on the pump. It does have a stamped steel pan. I was going to check bearing condition but I've been thinking that if it were a bearing clearance problem, it would seem like the pressure should be low all the time. It's hard to beleve that the clearances would open that much when heated up to make the pressure drop to zero. Do you think it's possible?



Thanks,
Brian

Wow, You didn't mention it went to Zero. Did you reuse the old oil pump when you did the engine swap? The exact same thing happen to me 30+ years ago when I re-used the old oil pump on a rebuild. My oil pressure would drop to zero upon braking for a stop, then jump right back up. I learned my lesson and put a brand new oil pump in whenever I swap or re-build an engine. I won't even let them sell me a rebuilt oil pump. The cost difference is worth the money of a new pump. You'll thank yourself later.

Thanks BB!

I agree with you on the pump. It does have a stamped steel pan. I was going to check bearing condition but I've been thinking that if it were a bearing clearance problem, it would seem like the pressure should be low all the time. It's hard to beleve that the clearances would open that much when heated up to make the pressure drop to zero. Do you think it's possible?

Thanks,
Brian

Hey Brian,

Might want to look at this thread:

http://www.automotiveforums.com/vbulletin/showthread.php?t=714888

Knock at warm is from the oil thinning out and not providing enough of a cushion between bearing and journal. When you get the pan off would be worth pulling a rod cap and a main cap a plastigauging them. Will give you a ball park figure about where your clearances are at. I would bet your are on the loose side, especially connecting rod farthest from the oil pump (#1). Low oil pressure is a dead give away that your clearances have opened up to far. A new pump will delay the inevetible, but not forever.

I think you may have gotten a bum engine from the yard.

Good Luck,
Sparky

Hey Brian,

Sparky makes a good point here. Please post back with the actual oil pressure when hot and idling, and also when hot at say 2000 RPM. Come to think of it, you mentioned 60 PSI when cold, which seems high to me. I seem to remember my 89 was about 45 lbs. cold, and about 20 lbs. hot and idling. You should not drop to zero. If you are dropping to zero, when does this happen? How long does it stay there?

I would agree, I'd say this is a loose engine with a worn out oil pump. You may be able to keep it going for awhile longer by putting a high volume oil pump, but it sounds like you got a dud.

As a reference, my '89 would be at 55psi cold and hover around 35-40psi warm idle.

Also what kind of oil filter are you running on it?

I agree with Rick, even if its an used engine, I myself always change the oil pump, clean up the pan /new gasket. Its so easy to it when it out of the vehicle.

Well, I fired it back up. The oil pressure came up to about 50psi. It ran for about 10 minutes until the pressure started to drop. It dropped to around 40psi. and stayed there for a while and about after 15 minutes it dropped to zero. When it hit zero it started knocking and the "Check Gauges" light came on. The temp still had not reached 195. It was barely over 100 deg. The weird thing is that when it would drop the gauge bounced down to the next pressure. It bounced up a couple of times when it was at zero and the light would blink off and on... I did change the pan when I did the swap because the engine was out of an Astro Van. I did check the clearance between the pump gears and the pump body when I had the the pan off. The clearance was less than .004" between the gear teeth and the body and around .002" between the end of the gear and the cover. Now, the only thing that I didn't do was remove the pressure relief valve in the pump cover and clean it. :banghead:

Looks like I'm going to put a set of bearings and a new high volume pump in it.
That sure would have been easier on the stand! :-(


Brian

My son bought the oil and filter, I think that they are both Advance Auto Brand.

Alright, if I am not mistaken the pick up screen/tube on the blazer for a 4x4 is different from the astro and will not work with anything else. So I guess my next question is, is it a 4x4?

Yeah, we changed that too. :)

Well, I fired it back up. The oil pressure came up to about 50psi. It ran for about 10 minutes until the pressure started to drop. It dropped to around 40psi. and stayed there for a while and about after 15 minutes it dropped to zero. When it hit zero it started knocking and the "Check Gauges" light came on. The temp still had not reached 195. It was barely over 100 deg. The weird thing is that when it would drop the gauge bounced down to the next pressure. It bounced up a couple of times when it was at zero and the light would blink off and on... I did change the pan when I did the swap because the engine was out of an Astro Van. I did check the clearance between the pump gears and the pump body when I had the the pan off. The clearance was less than .004" between the gear teeth and the body and around .002" between the end of the gear and the cover. Now, the only thing that I didn't do was remove the pressure relief valve in the pump cover and clean it. :banghead:


Looks like I'm going to put a set of bearings and a new high volume pump in it.
That sure would have been easier on the stand! :-(


Brian

Brian, something is really wacky about this situation, I would expect if the bearings were loose that you would start it and at some point the pressure would begin to drop as the oil warms up, not all of the sudden the pressure drops from 40 to 0. The knock is definitely from no oil pressure but I am not sure if the no oil pressure is from the bearings.

I wonder if there is a crack or a leak in the pickup and when enough oil gets trapped upstairs the pump starts to suck air and the pressure drops. Maybe if you jack the front of the truck up and shift the oil toward the pump you can rule this idea out.

Like I said this is a strange one. Another idea is maybe the pressure relief is sticking.

I admire you for checking the pump clearances before you put the motor in, those values sound pretty good without having a manual to verify.

I'm interested to hear from some of the others about thier ideas.

Sparky

I think I'd be back talking to the outfit that sold you the engine. It sure is sounding more and more like excessive bearing clearance. I wouldn't put this off. At Zero pressure, you could destroy that crank along with just about everything else in the engine.

Sparky, LOL, I am an old drag racer. I used to build my own engines and checking the oil pumps was one of the things that I was really fussy about. I've returned new Mellings pumps because they would not meet the specs.
I'm thinking the pressure relief valve too. The pressure acts goofy!:screwy:

Rick,
We put the engine in last winter and parked it, the yard had no history on the engine. I only payed $75 for the engine. We were going to sell the Blazer but my son decided he needed it for a second vehicle. I think I'm going to slide some new inserts into it and get a new pump. It should be less work than changing the engine again... 3rd!!! :banghead:


Brian

Thanks guys for the ideas and support, I'll keep you posted in my findings if you want...

Brian

I've heard elsewhere that tjhe "new design" Mellings are not what they used to be. I guess an M55 isn't an M55 any more. The old standard was an M55 (standard volume) with a 45# spring. In your case, an HV pump might be warranted.

If you're lucky, you can pull the pan with the engine in (4x4s seem to be easier that way) and crack loose a couple caps to check condition. I'm thinking that for 75 bucks, you really aren't going to get much of a warranty.

And remember if you do use Plastigage, to be sure to remove all traces of oil from both the journal and the bearing being checked beforehand. Oil will mess with Plastigage. Don't forget to relube the journal and bearing before putting it back together.

Silicon...,
You are right about keeping things clean but a little trick I used to do is, clean the journal and bearing, then I'd smear a very thin coat of grease on the bearing journal. Then I'd lay the plastigage in the bearing cap and bolt the cap on and torque it to the specified torque value. The grease acts like a release agent on the bearing journal so all the plastigage would be on the bearing in the cap.

Blue Bowtie...,
Yeah I still like the Mellings pumps but they are just like anything mass produced. Once in a while a bad one slips through. Also, most of the pumps will work well for the general public. But if you are blueprinting a race engine or any engine for that matter, you want (and need) the best of everything. You are right, I think I'm going to use the HV pump. I have used the Big Block pumps but I doubt that this V-6 would have enough HP to turn the pump! LOL!!! (probably cut gas mileage by 75%)