My 85 GT - 120K miles, auto trans, one owner-me, regularly services, runs great, BUT.. after warming up if the rpm's get below 1,000, the oil pressure gauge drops into the red. Blipping the gas will pop it back up, but only as long as rpm's are above 1,200. Have had the gauge and sending unit checked by two different garages and it appears that both are working. The only advice I have received from both mechanics is to run it till it blows and then to have the engine replaced. (seems like a valve job is a thing of the past!)
Would hate to spend the money for a new (re-manufactured) engine, but will if that is the only answer. Any suggestions seriously considered and appreciated.
Thanks. - Yep! same GT having door lock problems. : )
So, you took this by TWO different garages and neither one of them ran a compression test?!?!?!?!? That's the first thing I would have done/would recommend. That'll tell you if your engine really is "ready to blow".
I know when I had my stock V5 w/ stock auto trans, it would dip lower in the summer as the temps heated up. One factor that can come into play is the weight/viscosity of your oil. If you really are getting ready to blow, it may not fix the root cause, but it could buy you some time.
The other thing I would do if you got some decent miles on the engine oil is get a used engine oil analysis from blackstone labs.
If low still can be several things: Iffy bearing(s) for cam, crank or both. Iffy oil pump. Maybe Iffy distributor. And combination or all of the above at same time.
Engine could run w/ this for days, weeks, or longer.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Folks, Thanks for all your replies. Neither of the shops recommended a compression check as I recall - its been in storage for a little over a year - but, for some reason I think one may have been done, without alarming results. I will revisit that. I do know that both places did use a mechanical gauge to verify that the on-board gauge was working, and it was. Even in the Red, both said that it was still getting "enough" oil to keep it running. ?? If I can get the door issue resolved, I will get a sample of the current oil tested, and get the oil changed. I can also say that I have never seen any evidence of a leak or smoking, and have not had any abnormal oil usage in the 102K miles its been driven. Thanks again. I'll let you know if anything turns up. Doc
If their mech gauge said same then engine have problem I posted above.
Driving engine with OP problems that you have will damage it over time. Could be days to even years depending how you drive, how low the pressure, etc.
The 87 GT I picked up a month or so ago has the oil light on after I drive it for 10-15 minutes. It runs fine, drives fine, etc. Previous owner said the light has been on like that for 3+ years. Never did anything about it. I'm going to guess the gauge is started to be junk after 29 years.
Substitute a quart of Lucas for one quart of oil at the next oil change. As long as you are at 5 psi at idle and the engine carries 10 psi per 1000 rpm you will be good to go for a long time.
My 86 gt had the same issue, when the shop used Dexos (5w30 water) for the first oil change, changed again to 10w40 quaker state, was better but would still run low at idle. I am now running 10w40 royal purple and it has not dropped since the last change which was approx. 500 miles ago.
Had issues with the old style sending units being inaccurate or becoming inaccurate over time, installed a fluid filled gauge and verified that my sending unit was garbage. Upgraded to an 88 sending unit with the correct weatherpacked pigtail from gm. Perfect. I always run 10w30 and have 240k, no issues.
Originally posted by Patrick: I'm just curious how a broken oil ring could affect oil pressure.
It can't.
Sorry but claiming broken ring doesn't wash. Rods of most cars do not carry pressurize oil to the pistons. (Some rods have spray holes on crank end. Others nothing.)
Fix bad rings will require to remove pistons, that will require you to remove rod caps etc. Likely remove other parts too. Even if you don't replace crack/rod bearings, Removing and reinstall parts can "fix" problems. If you have Oil Pump etc leaking a bit, remove/reinstalling can fix the leak.
Is like shutdown and power on a PC "fixes" many issues. In Fact, many Windows Servers are programmed to reboot at least once a week to stop many memory bleeding/leakage problems.
Drop the pan, Replace the mains and rod bearings, then that will probably do the trick. Too much bearing clearance. If no blow by and good compression, that's probably the only issue.....old age.
Low pressure on the gauge at hot is very common - I had the same on my 54k mile 86 V6. What pressure did their mechanical gauge read?
I don't recall what the psi was on my mech gauge at hot idle, but it was definitely acceptable. I replaced my OP sender and presto! no more oil light at hot idle.
[QUOTE]Originally posted by Patrick: I'm just curious how a broken oil ring could affect oil pressure.
It can't.
Sorry but claiming broken ring doesn't wash. Rods of most cars do not carry pressurize oil to the pistons. (Some rods have spray holes on crank end. Others nothing.)
Fix bad rings will require to remove pistons, that will require you to remove rod caps etc. Likely remove other parts too. Even if you don't replace crack/rod bearings, Removing and reinstall parts can "fix" problems. If you have Oil Pump etc leaking a bit, remove/reinstalling can fix the leak.
Is like shutdown and power on a PC "fixes" many issues. In Fact, many Windows Servers are programmed to reboot at least once a week to stop many memory bleeding/leakage problems.[/QUOTE]
It matters not to me whether you believe it or not. The engine coughed, the OP went to zero and back to 30 above 1200 rpm. Upon teardown the only issue found was the broken ring. No other problems including the oil pump were found.
It matters not to me whether you believe it or not.
It's not a matter of whether we "believe it or not". It's simply not possible that a broken piston ring is going to directly affect crankcase oil pressure.
The oil rings could be removed from all six pistons and it wouldn't affect the oil pressure. Might smoke like a banshee, but the oil pressure would be fine... until the oil level dropped significantly of course.
Originally posted by RockinRoger: It matters not to me whether you believe it or not. The engine coughed, the OP went to zero and back to 30 above 1200 rpm. Upon teardown the only issue found was the broken ring. No other problems including the oil pump were found.
An incorrect interpretation of events is not proof of a condition that can't happen. I own and run a performance engine shop and build everything from restorations to Gt-1 and NASCAR engines and an oil ring won't/can't affect oil pressure. I raced a V8 in a 3 hour endurance race that had been overheated early (red flag, no rad fan) and suffered broken rings in five cylinders - no oil pressure problems. Oh, and won the race.
Sounds like a wiring issue: engine coughs/jerks,wire loses contact, gauge goes to zero then contact again so gauge reads normal again. The "reboot" that fixed your problem was probably putting the connector back on the sender.
Uh, it's not actually 'crankcase' pressure either......
Not so fast there, Kemosabe.
What is your definition of "crankcase"?
Absolutely nothing wrong with referring to what we were discussing as "crankcase oil pressure"... because that's exactly what it is! At no time did I refer to it as "crankcase air pressure"... or even "crankcase pressure". Crankcase oil pressure is a phrase that's been around... forever!
quote
Originally posted by Patrick:
It's simply not possible that a broken piston ring is going to directly affect crankcase oil pressure.
Most people who are replacing the piston rings will end up replacing the journal bearings at the same time. The "broken ring" wasn't the cause of low oil pressure...but likely lead to a rebuild of something else that was.
Gentlemen, The question of the cause of the low oil pressure seems to generated a serious and thoughtful discussion. Since I originally posted, I have had the oil changed to 20W-50 and have run the GT everyday. After the engine warms up ( I presume) two things happen: the Engine light comes on, and at idle the pressure gauge drops to just above Red Line - the light also comes on over gauge. I am not sure what my next move is, but I have also noted that there is no apparent oil leakage on or around the engine, or oil leaking from bottom of engine. (that I can see) Also, there does not seem to be any real oil usage between oil changes. Would hate to just run it till it blows, but none of the local "mechanics" seem to have any other answer - except for replacing the engine ($6,000 +) which each of them would be happy to do. : ) I'll let you know what I decide to do next. I appreciate all your help and advice. John
3 main things that could drop oil pressure, bad crank bearings, bad cam bearings, worn oil pump. Dropping the pan and checking bearings would be the place to start, if the crank is in good shape slap in new ones. New oil pump would be a good idea.
