LT1 oil psi drop under heavy load - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 06-01-2007, 02:19 PM
Registered User
 
Join Date: May 2007
Location: north carolina
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
LT1 oil psi drop under heavy load

Originally Posted by ztwntyn8
I have a 94 Z28 6 spd with 80K miles on the clock. One day I was passing on the highway in 2nd gear. Out of the corner of my eye I saw the oil psi guage gain psi when I let off. I thought huh. After that I started to observe the guage during acceleration and it realy was dropping. It seems to happen after about 3K rpm and ONLY during a very heavy load. For example: it can happen on flat ground at wot or up a hill at about half throttle. I am using a ported MAF (which I believe is a mistake) throttle body bypass and other than a flowmaster it's stock. When I changed the oil it was like water but didn't smell like gas. The enging progressively looses oil psi as the oil life decreases. In the winter it's at it's worst. I feel that the porting of the MAF has made it super rich (no black smoke) at an idle and cold weather makes the problem worse. So, I change the oil and get some oil psi back but never what I used to have. I've never put a mechanical guage to it but if you take it upstairs at lite throttle it dosn't drop psi. I switched from 5W-30 to 10W-40
and have about normal psi now (maybe a little high). I believe there is a clearance issue now in the bottem end. A fellow once told me to expect the engine to sieze soon but it hasn't with another 15K miles or so. He told me with a stick that the input shaft attempts to move the crank against the rear main bearing via pilot bearing and ends up moving the crank away from an oil passage which bleeds off psi. I have done alot of WOT shifting and I still beat on it but I'm scared. lol This is the only forum where I have seen my exact problem. Oh, and the engine has never been apart. So, let me know what you think guys. Thanks alot!

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 06-01-2007, 06:05 PM
Registered User
 
Join Date: May 2007
Location: KANSAS
Posts: 9
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
We usually do use a mech gage to verify degree of problem. I would be curious if using the BG Chemical product MOA would boost the life of your oil. ( if that is the problem) Have you ever tried using a good quality synthetic (Amsoil or Mobile 1) ? I would stick w/ 5/30 or 10/30. That very well may help if the oil breakdown is the problem. If that helps, Amzoil has quite a choice.
ROOKY
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 06-01-2007, 06:45 PM
65 crusier man
 
Join Date: Jun 2007
Location: california
Age: 50
Posts: 39
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Just a thought, I had a chrysler 300 that did the same thing. If I remember correctly our issue was the oil pump gears had too much clearance and uder heavy throttle pressure would drop. I am no expert and that was 20+ yrs ago but with 80k on the engine that may be another item to inspect. let me know if i am way off base here.
Joe
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 06-01-2007, 08:01 PM
Bryan59EC's Avatar
Car? Truck? Who Cares
 

Last journal entry: First week of December
Last photo:
Join Date: Jun 2006
Location: San Antonio, Texas
Age: 54
Posts: 2,375
Wiki Edits: 0

Thanks: 2
Thanked 34 Times in 26 Posts
My 67 Chrys 300 only had oil pressure issues at idle.
4-spd trans and a heavy Borg & Beck clutch finally wore the thrust bearing
face thru to the block. But never enough to lose pressure at cruise or under a load.
Push the clutch in on that car and you could hear the crank counterweights hit the bock webbing.
Sure miss that car---stolen in 88---got the shell back a year later.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 06-03-2007, 10:18 AM
Registered User
 
Join Date: May 2007
Location: north carolina
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thank you all for your insight. In response to Rooky.... I have used Mobil 1 for quite a while but don't any more. Even on a fresh oil change where pressure is better it still drops when I get on it hard. In response to Joe Johnson... I think this could be more along the lines of what's happening to me. One time when the radiator was out of the car I had to move it at our bodyshop.. the phone rang and the temp guage was about pegged when I came back. I have heard that there is a spring in the oil pump that can fatigue due to overheating. I wonder, is that an old wives tale or could that be what happened to me? Thanks guys!
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 06-03-2007, 11:38 PM
Member
 
Last photo:
Join Date: May 2003
Location: Shreveport LA
Age: 65
Posts: 5,103
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I have a 350 that has done that same oil pressure trick since the pickup screen fell out of the oil pump and began rattling around in the bottom of the oil pan. Next time you have the oil all drained out of it, hammer your fist against the bottom of the pan. I can hear mine bounce when I hit it.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 06-04-2007, 12:55 AM
Registered User
 
Join Date: May 2007
Location: north carolina
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
right on, I'll definately try that next oil change. I wonder how that would cause an oil psi drop though. Not trying to say that you are wrong, just understand. Thanks!
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 06-04-2007, 02:58 AM
Member
 
Last photo:
Join Date: May 2003
Location: Shreveport LA
Age: 65
Posts: 5,103
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
No problem. I make no guarantees, just something you might look out for. I figure mine might be sucking some air if the pump inlet hole gets uncovered when the oil puddle moves toward the rear during acceleration. It does it less if I run an extra quart of oil. It also tends to drop 10-15psi if I hold a steady speed over ~4000rpm, but not as much as it drops under acceleration.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 06-04-2007, 10:41 AM
Registered User
 
Join Date: May 2007
Location: north carolina
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Awesome! I thought it must be an air problem of sorts I just couldn't figure out how. I can't wait to change the oil and see if I can get it to rattle in there. If that's the problem would the fix be a new pickup or whole pump? If that's not the problem, what do you think about a fatigued spring in the pump due to over heating. I don't know what spring or if it's possible, jsut something I've heard from oldschool hotrodders over the years. Thanks man.
Nate~
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 06-04-2007, 02:10 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,088
Wiki Edits: 0

Thanks: 4
Thanked 540 Times in 456 Posts
Quote:
Originally Posted by ztwntyn8
Originally Posted by ztwntyn8
I have a 94 Z28 6 spd with 80K miles on the clock. One day I was passing on the highway in 2nd gear. Out of the corner of my eye I saw the oil psi guage gain psi when I let off. I thought huh. After that I started to observe the guage during acceleration and it realy was dropping. It seems to happen after about 3K rpm and ONLY during a very heavy load. For example: it can happen on flat ground at wot or up a hill at about half throttle. I am using a ported MAF (which I believe is a mistake) throttle body bypass and other than a flowmaster it's stock. When I changed the oil it was like water but didn't smell like gas. The enging progressively looses oil psi as the oil life decreases. In the winter it's at it's worst. I feel that the porting of the MAF has made it super rich (no black smoke) at an idle and cold weather makes the problem worse. So, I change the oil and get some oil psi back but never what I used to have. I've never put a mechanical guage to it but if you take it upstairs at lite throttle it dosn't drop psi. I switched from 5W-30 to 10W-40
and have about normal psi now (maybe a little high). I believe there is a
clearance issue now in the bottem end. A fellow once told me to expect the engine to sieze soon but it hasn't with another 15K miles or so. He told me with a stick that the input shaft attempts to move the crank against the rear main bearing via pilot bearing and ends up moving the crank away from an oil passage which bleeds off psi. I have done alot of WOT shifting and I still beat on it but I'm scared. lol This is the only forum where I have seen my exact problem. Oh, and the engine has never been apart. So, let me know what you think guys. Thanks alot!
The whole reason the factory went to 0W and 5W-XX oils is to improve their EPA mileage averages. This watered down swill they sell for lubrication is dangerous stuff to use if you spend much time pushing into WOT territory. This concept that oil needs to be as thin as water is a Madison Avenue advertiser's wet dream, the stuff has no redemming value in a hard working engine, save it for mom's run to Safeway for groceries.

Unfortunately for you the damage is done. Reading everything you've posted and that of others, you guys are into two principle problems. The first of course is that factory recommended oil weights are too low. You can get away with less if you're using a good synthetic but not a mineral base oil. I'd say nothing less that a 10W-XX for synthetic and 15W-XX for mineral, and a 20W-XX would ease my breathing on a mineral oil.

The second issue is clearly one of oil escaping the pump pick-up upon acceleration and sharp maneuvering. Stock pans just aren't designed to protect the oil intake sufficiently well. Hard acceleration combined with high RPMs conspires to put a lot of oil in circulation, drawing the sump level down to a quart or less. Add some hard acceleration and or cornering forces and what little oil is in the pan is climbing the walls not laying where the pick-up is. There are several solutions to this and all of them require getting inside the engine. Certainly a windage tray is high on the list to separate the high speed flailing crankshaft from the sump, giving the oil time to settle and de-aerate before being recirculated. Along with that is a scraper to catch crank fly-off and direct it into the pan instead of allowing the crank to build a couple quarts up on the case walls. Secondly, the pan needs a larger capacity sump with baffling to restrain the oil from escaping the pick-up. Consider in this design that hot thin 0W-5W swill is harder to keep around the pick up than a heavier bodied oil. A deeper sump with a top baffle to keep a couple three extra quarts is a real good idea, but due to body height limitations doesn't work well for missing the pavement. An upside down "T" pan with vertical baffles to either side of the pick up and a top baffle to prevent the oil from climbing out of the sump is most helpful.

While you're in there replace the plastic collar aligned oil pump drive shaft from GM with a good steel aligned piece from the aftermarket.

As far as pumps go, I don't recommend a high volume pump unless you're running racing clearances on the main and rod bearings. Other wise the pressure just goes up and serves no useful purpose. Plus it drives you into pump modifications to insure the by-pass oil goes back to the intake side while not disturbing inlet flow. This is tricky to do.

Last of course is a dry sump, but I'm considering this as rather expensive for the street.

Bogie
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 06-04-2007, 02:51 PM
Registered User
 
Join Date: May 2007
Location: KANSAS
Posts: 9
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Listen to oldbogie! Good ideas.
Any chance oil is not getting back down from topside? I guess I got hung up on the problem worse as oil life increases. I have had problems w valve stem seal pieces plugging up oil return. (years ago)
ROOKY

Last edited by ROOKY; 06-04-2007 at 03:00 PM.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 06-04-2007, 03:51 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,088
Wiki Edits: 0

Thanks: 4
Thanked 540 Times in 456 Posts
Your thoughts of oil not getting back down from the topside are in the neighborhood. While this isn't usually as big a problem as the magazines make it into, it's still a problem. Crankcase windage does a lot to interfere with return oil flow at high RPM. Most engines vent off the valve covers which means surging air flows in the engine are trying to get up past the returning oil. This tends to hold the oil up in the rocker covers more so than is desirable. Plus the major return path for oil is at the back of the valley. The engine sits several degrees low at the back side, so natural oil drainage is down the back of the engine. Work spent encouraging flow into the timing case is not effective as the forces of acceleration and the drooping back end conspire against this as a return path of any consequence. The holes along the valley floor are mostly moving air between the valley and crankcase, so while some oil is draining here, it's not a major amount. That's not to say that reworking these holes and those connecting the timing case isn't important, they are, but more as a passage that allows communication of crankcase pressures than as an oil return. But increasing the size and smoothness of these holes reduces gas turbulence and increases flow area which reduces velocity which in turn permits what oil is passing that way a freer return path. The place to pay good attention is the rear returns. This starts with the head, increasing drain back hole size by an eighth inch is most helpful, as is deburring the pushrod passages of the head to smooth and slightly increase the breathing area. Reworking the rear drain back holes is most useful to increase the drain back area as this is where most of the oil is. There's always the problem of this impinging upon the crank as it falls back, but there's already plenty of oil around the crank because of throw off from the bearings. It's not uncommon to find dry sump competition engines that block these return passages to oil flow allowing the oil to puddle in the valley. Then with a suction pipe in the back of the valley pull the collecting oil into a return pump and to the external sump. This reduces the flood onto the crankshaft while insuring that top end oil get back into circulation without first flooding the top end.

Typically at moderate speeds there's a quart in the galleys, another in drain back and a couple in the pan. As RPMs get upwards of 4000, at any moment there's still a quart in the galleys but drainback is getting to be a problem with 2 or more quarts somewhere in drainback, with a quart in the pan. Push that up to 6000 and there's still just a quart in the galley's , but there's 3 quarts or more flooding the engine and next to nothing in the pan. At this point the engine is right on the edge.

Depending on what your doing to the car, between 4 to 6 grand if you're seeing surges in oil pressure or just a steady slow of pressure, you can expect that the pan is running out of oil.

A 5 quart supply, with one in the filter, just isn't enough for an engine that gets beat on.

Bogie
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 06-04-2007, 04:07 PM
Registered User
 
Join Date: May 2007
Location: north carolina
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks Oldbogie, this is becoming alot more clear to me. However, I would have to ask now, why did it take 80K miles to become a problem. I suspect that tolerances are wearing and enlarging somewhere and like you said the damage is already done. So, what would you suspect has been damaged? Do you think I can solve the problem by instaling new main and rod bearings or possibly a damaged crank too? I appreciate any advice. Thanks!
Nate
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 06-04-2007, 04:20 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,088
Wiki Edits: 0

Thanks: 4
Thanked 540 Times in 456 Posts
It's wearing out, the "damage done" is excess wear on the rod and perhaps main bearings. So internal oil leakage is going up putting more demand on the pump to hold pressure which leads back to a greater sensitivity to the amount of oil surrounding the inlet. You're seeing starvation that's always been there under some conditions but now the margin of the lubrication systems performance is closer to the edge.

A heavier wt oil will hide this somewhat, but time isn't on your side. I think you should start to consider at least a bottom end rebuild with attention paid to windage and oil control before a rod is delivered out the side of the block.

I hate to sound so strident, but I've been down this road a few times and know where it comes out.

Bogie
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
oil change- step by step instructions for women and men Fireball Hotrodders' Lounge 38 08-30-2008 08:39 AM
What is the correct oil pressure psi for my motor? batman09 Engine 1 09-15-2004 08:04 PM
Oil pressure drop under load knizet Engine 9 01-13-2004 03:26 PM
'72 Buick 455 Oil psi Drop buick455gearhead Engine 3 04-15-2003 06:19 PM
Should I change my oil? 48Man Engine 2 01-29-2003 08:28 AM


All times are GMT -6. The time now is 09:08 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.