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Vexing Oiling Issue - 90 TBI

 
HCT
I love my Chevy Chevette!
Posts: 11
Joined: 04/14
Posted: 04/01/14 10:14 AM

Hello to the forum.  This is a great site; I've lurked on and off for a long time and had many problems explained or solved.  I have a new one for which none of my regular sources have no answer.  My 1987 GMC Suburban is a 5.7 TBI, automatic 4x4.  It sat garaged for the last nine years before I purchased it last year.  Overall it needed very little other than fluid changes, a fuel pump and coolant temp sensor.  It has about 119,000 miles on a virgin engine.  No leaks, no stumbles or any other issues except this one:  whenever it sits after driving it to full operating temperature, it loses oil prime after a few hours (8-10) and has to crank an inordinate amount of time to prime the engine and reach minimum pressure for the fuel injection.  What is so vexing is if I just start it and shut it down cold, it will sit for days without losing prime and will start ASAP.

So far I've changed the oil sending unit and pulled the oil filter adapter housing to check the bypass valves for the filter and oil cooler.  They are fine.  Indicated oil pressure is at 30 psi on the road and only drops about 5 psi at idle, and the original owner says the gauge read that way always.

Ideas and comments welcome before I change out the oil pump.  I'd hate to go through that only to find out it didn't make a difference.  Thanks in advance, Harry.  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/01/14 05:09 PM

fuel injection systems in general motors cars and trucks use a 2 or 3 wire oil pressure switch in parallel with a fuel pump relay...



you can see that the Orange wire is constant power and has a fusible link...

the tan and white or pink and black wire supplies the fuel pump fuse.. and then on to the fuel pump...


Gmfuelpumprelaywiring Zps4c5f7051

here is a slightly different version for cars with oval relays...

how it works is the same...

Gmfuelpumprelayoval Zpsa6266fe9

i bet your fuel pump relay.. is either bad.. or the socket terminals are LOOSE..  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/01/14 05:41 PM

look for an oval relay with an ORANGE wire.. a PINK and black stripe wire.. a Green and white stripe wire and a black and white wire..

you can in some cases.. remove the cover by gently prying the relay out of the bottom of the case..

please be careful to NOT let the relay touch ground.. as you will blow the fusible link in most cases..


and.. YES... if you run a positive wire to PIN G on the left lower end of the ALDL connector with the ignition off.. you can test or prime your fuel pump...

i normally test the oil pressure switch contacts with the engine running.. by pulling the relay connector off... if the engine continues to run.. the oil pressure switch is good...  

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HCT
I love my Chevy Chevette!
Posts: 11
Joined: 04/14
Posted: 04/02/14 12:21 PM

Thanks for the schematic; that's going to useful.  I replaced the fuel pump relay last summer when I replaced the fuel pump but I have never tried pulling the relay while the engine is running.  I'll try that this weekend.  Also, my 87 still has the big can-style oil pressure sending unit with a single wire connector.

The oil pressure gauge stays on zero while I'm cranking for a good 10 to 15 seconds.  Long enough that I know I'm stressing my starter.  Eventually the needle comes up off of zero and at about 5-10 psi indicated the engine starts. As I said, it will start right away after that as long as it doesn't set too long.  I'm afraid it's a tolerance issue between the pump gears and end plate.  But hopefully something simpler than pulling the pan and hopefully not a crack somewhere in the block that opens up when it gets hot.  

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HCT
I love my Chevy Chevette!
Posts: 11
Joined: 04/14
Posted: 04/02/14 12:21 PM

Thanks for the schematic; that's going to be useful.  I replaced the fuel pump relay last summer when I replaced the fuel pump but I have never tried pulling the relay while the engine is running.  I'll try that this weekend.  Also, my 87 still has the big can-style oil pressure sending unit with a single wire connector.

The oil pressure gauge stays on zero while I'm cranking for a good 10 to 15 seconds.  Long enough that I know I'm stressing my starter.  Eventually the needle comes up off of zero and at about 5-10 psi indicated the engine starts. As I said, it will start right away after that as long as it doesn't set too long.  I'm afraid it's a tolerance issue between the pump gears and end plate.  But hopefully something simpler than pulling the pan and hopefully not a crack somewhere in the block that opens up when it gets hot.  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/02/14 01:45 PM

if the oil pressure sender is a large SINGLE wire version..

then look carefully.. there is a second oil pressure switch with 2 terminals parallel.. orange wire and a tan wire probably..  same coloring as the fuel pump relay..

do you have the old fuel pump relay.. can you extract the guts and check to see if the contacts close for 2 seconds when you turn your key... and that they close while cranking.. and that when you depress the moving part .. the pump turns on..

the proper part number for the fuel pump pressure switch is..  ACDelco - Engine Oil Pressure Switch Part # D8035 Alternate/OEM Part Number(s): 89057488

your oil pressure switch is probably working or the truck would not start after cranking for a while.. its the relay or the wiring to the relay thats the issue..

because the relay is mounted probably on the left inner fender.. it might have gotten soaked by rain coming thru the gap between the hood and fender.. or some other issue.. rodent munching on the wiring..    loss of the ground connection for the relay..  black and white stripe wire..  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/02/14 02:03 PM

let me post a little theory..

engine off.. turn key to the ON position. this wakes the computer.. it gives the fuel pump relay 2 seconds of positive voltage on the green and white wire.  it will again command positive signal voltage to the fuel pump relay.. when it seens cranking speed.


the fuel pump pressure switch is in parallel.. and the contacts close when the oil pressure comes up..  the part of them are redundant..  it gets you home. one or the other..

its supposed to work with both..

so.. find the fuel pump relay..

orange wire should have power all the time.

pink and black stripe wire or tan wire goes to the fuel pump fuse and then the pump..  should have power for 2 seconds after key on.. and while cranking..  

i don't recommend probing the green and white wire.. as it blows out the electronics that control it at 1.0 AMPs..   its easy to blow it out..


and the black and white stripe wire should be ground.. you might want to ohm it.. or search out the other end of that wire.. i don't recall where they grounded it to?  

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HCT
I love my Chevy Chevette!
Posts: 11
Joined: 04/14
Posted: 04/03/14 05:51 AM

The fuel pump relay I replaced is bulkhead-mounted on the firewall on the upper right side near the A/C stuff.  I did keep the old one for a spare so I can do that drill. It could be that the new one is defective. I can also pop the fuel cap and listen for the pump.  Done that before.  As for a second OP sender, I know the look of the two wire sender because the auto shop sold me the wrong one first. Oddly enough it was $8 and the old style can I needed was $35.  You'd think it was the other way around.

I'll look around but I've been all over the engine for various reasons after I bought it, and I don't recall anything like that sender on the block.  This was the first year of TBI and it may have transitioned quickly to the systems you're describing that may not be on mine.  But I'll look again and I'll let you know what I find.  I appreciate the help.  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/03/14 07:58 PM

i have all kind of books and decades of working with these systems..

there are variations from year to year.. and you can have either a relay issue.. or a wiring to the relay issue..

even a ground issue to the relay..

small chance that there is a ECM problem. but i never rule that out.. i just do the ECMs after i prove EVERYTHING ELSE WORKS..

you don't replace the circle saw. because the extension cord fell out of the wall. or the wires are frayed..

find the fuel pump relay..  probe the wire.. you should have constant power on the ORANGE wire..

you should get power for 2 seconds when you turn the key on to the pink and black or tan wire..   if you dont pop the other relay out of its shell.. takes about a minute..   plug that one in.   squeeze the contacts and see if the pump comes on.. even without the motor or key on..  that tests the power circuit...

if you have a friend turn the key on. while the BARE relay is in your hand  and plugged in.. it should click closed for 2 seconds.. then close again this is prime the pump...

just Plug in a NEW fuel pump relay..   see if the pump primes when you turn on the key... thats the easy thing and under 20 bucks.. chances are the fuel pump relay died.. or the contacts got covered with carbon from arcing..  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/03/14 08:00 PM

oh... and if it did not have an oil pressure switch to control the fuel pump power circuit.. it would never turn on the pump after you cranked it for a while..  and got the oil pressure up..  

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HCT
I love my Chevy Chevette!
Posts: 11
Joined: 04/14
Posted: 04/04/14 05:34 AM

Yeah, I was wondering about; I thought maybe the  op signal was sent through the ECM and them to the gauge.  Your schematic cleared that up.  We'll see what we see this weekend and I'll update.  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/04/14 08:50 AM

Gmoilpressurefuelpumpswitch3

you can see the contacts in the sending unit cap..  thats for the fuel pump circuit.

the bent terminal is totally separate and for the oil pressure gauge...

Gmoilpressurefuelpumpswitch2

Gmoilpressureswitchfuelpumpswitch1

you can see the flat plate in this image.. when the oil pressure comes up.. that plate is pushed against the contacts in the cap...

i pulled one of the cube relays apart..

Fuelpumprelayinsides  

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HCT
I love my Chevy Chevette!
Posts: 11
Joined: 04/14
Posted: 04/07/14 09:52 AM

Well, it rained all weekend so didn't get under the Suburban but I did pull the connector on the relay and the engine continued running. Too wet to play with anything more than that. My relay is the oval relay.  My oil pressure sensor isn't pictured above; it's the great big can unit, almost the size of a can of tomato paste with a single terminal. It may be incorrect but that was the unit type that was on the vehicle when I bought it.  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 04/07/14 06:09 PM

just change the relay...  see if the engine starts quicker..



i posted the pictures of the 3 wire switch to allow you to see whats going on inside the

if you unplugged the relay with the proper wiring colors.. and the engine continued to run. then the oil pressure switch is OK..  were you able to view the diagrams above.. open them in a new window..  

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HCT
I love my Chevy Chevette!
Posts: 11
Joined: 04/14
Posted: 04/10/14 08:47 AM

The more I look at it the more it looks electrical.  I Re-installed the original relay and I'll see how that shakes out.  I'll also clean up the ground connections, especially the tan-white ground connection at the front of the intake manifold.  The schematics helped.  Thanks for taking the time to help me with this issue.  

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