Z06_BluByU

Driving home last night My check gauges light came on (blinking) as I watched the oil pressure drop to 0!!! :eek: I immediatly hit the key and coasted over to the side of the road..

I towed it home and removed the intake manifold.. tons of oil in the lifter valley.. Then I used a coreless screwdriver to spin the oil pummp drive shaft and I had a good amount of oil flowing out of the holes (which leads me to belive it has plenty of pressure).. does this indicate its the oil pressure sending unit? This is what i'm thinking..

Don't LT1 ECM's cut power to the fuel pump if there is low oil pressure? Could someone please confirm this.. I already ordered the new sending unit and intake manifold gaskets.. I hate to think i pulled the intake for no reason at all.. although it will make getting to the sending unit a snap!'

thanks for your responces and information! :cheers:

dlmeyers

This area can get confusing. Have seen many people confused about repairs to various oil pressure sensing devices.

The oil pump pressure switch/sensor (keep in mind there are two units back by the distributor) is wired in parallel with the fuel pump relay. GM did this thereby providing an additional fuel pump power circuit.

Lots of people believe loss of oil pressure will cause complete loss of power to the fuel pump. Not true. Remember, the switch is wired in parallel with the fuel pump relay.

When looking back by the distributor, you should see an "oil manifold" with two electrical connectors mounted. Here is where the fun begins.

The "Oil Pressure Switch/Sensor Assembly" (around page 8A-20-6 Model Year 90 L98 electrical supplement) sends a signal to the analog oil pressure gauge. This is the unit's sensor portion. The schematics show the "switch" part heading for the fuel pump. This switch closes around 4 psi and provides the back up fuel pump power.

Continuing on, the other little widget mounted back there is the "Oil Pressure Switch". For this critter, head for page Central Control Module 8A-50-4 (MY90). You will see the circuit provides a ground for the "Check Gauges" indicator control and is CCM/panel light oriented. Keep in mind, the schematics differ on pre 90 units as they had no CCM but do have oil pressure warning lights. Not sure how they are wired. Don't have the manuals. Pretty sure the 90/LT1/LT4 are similar. They both have CCM's and mixed dash.

There are several good threads on this subject. Archive search "fuel pump" or "fuel pump relay", etc, should produce some good reading. Hope I haven't beat this to death but some people get really off track and schematics tell the story.

You noted both the Check Gauges light and decreasing analog gauge readings. That suggests loss of oil pressure. Look at the disributor shaft. Is the drive pin/gear intact? When you hook up the drill and run the oil pump, understand you have pressrue? It is best to pull the fuel pump fuse when doing this. With open fuel lines and the battery hooked up, fuel pump will fire up at around 4 psi. Now without checking the manual, and assuming the distributor drive unit is intact, maybe this would work. Pull the fuel pump fuse. Hook up the battery, turn the key on, have someone watch the analog gauge and warning light. Run the drill and see what happens. I'm curious now. And be careful with open fuel lines. I know some real smart people who get fooled by this system.

Good luck with your repair and hope everything works out.

dlmeyers 90 coupe zf6 3 speed shocks

scorp508

I've seen 0psi and my fuel pump kept a runnin'

Z06_BluByU

dlmeyers - So your saying if both the check gauges and analog gauge said the same thing then chances are it isnt a bad sensor.. now thats confusing

Now with the LT1 we dont have a distributer but we still ahve a little dsitributer gear thingy under the manifold that ties the cam drive sprocket to the oil pump shaft.. I pulled that thing out (after i removed the intake manifold) and it appears in order.. the pin is in and the gear look ok..

Next I took a cordless screwdriver and spun the oil pump shaft and visually saw the flow of oil that pumps out onto the cam shaft flowing.. wouldnt this indicate the presance of oil pump pressure?

I've been experiancing a intermitten short in the wiring at the back of the engine.. could a short cause this condition?

Dave C. '04 Z06

If your fuel pump shut off at 0psi oil pressure, then it would be really hard to start your car. :)

-Dave C.

dlmeyers

OOOPs, sorry about using the distributor as a reference.

Intermittant short at the back of the engine, the plot thickens.

OK, what I meant by suggesting loss of oil pressure. If two inputs both report loss of pressure, then one has to consider pressure was lost. If one is satisfied the mechanical pump system is intact, then checking the electrical circuit(s) is definitely in order. I doubt if both components (switch/sensor and switch) failed exactly at the same time, likely a circuit problem. If that little manifold got plugged up, that would explain it but very unlikely.

If you have experienced any electrical problems in that area, one could certainly be suspicious of the electrical circuits, given the warning system/gauges are electrical. Put them on the list for rounding up the usual suspects Get the schematics, check out the circuits, bad grounds, open circuits, etc.

Check the trouble shooting schematics in book 8A.

Let us know.

dlmeyers 90 coupe zf6 3 speed shocks


[Modified by dlmeyers, 11:33 AM 10/16/2003]

Z06_BluByU

not to sound like a maroon.. but what is book 8a?

SunCr

Did the engine actually stall and/or did you try to restart it? Pump relay is grounded by the PCM unless there is a no start. Pressure switch is the backup and is not connected to the PCM or the Relay. If there is a fault in the relay or the PCM driver, the Pressure switch will keep the circuit completed as long as oil pressure is =/> than 4psi. If the Pressure Switch fails, pump will continue to run. Did you check for a trouble code? Is there fuel pressure with the key on and does the pump run for 2 seconds? Quick test for a bad switch is to run it and then disconnect the relay. If it doesn't continue to run, the switch is bad. If it did quit running and it wouldn't start, I'd guess that switch was allready toast and the Relay or PCM went south. Otherwise, sounds like the circuit within the switch that drives the gauge may have failed and that's a little difficult to test. Most senders vary the voltage with 0 ohms equalling 0 pressure and 90 ohms being full or high on the display. Disconnecting it should peg the needle. If it doesn't, the wire to the display is generally the fault, but of course none of this means that you didn't experience 0 pressure.

Section 8A is a reference to the Service Manual Electrical Diagnosis Section. For an LT1, it's probably a separate book.

FullShred

8a is chapter 8a in the Corvette service manual . It's a separate book from the rest of the service manual.

ittlfly

Almost.....chapter 8A is in Helms Book #2 covering emissions towards the back. 8A is not a separate book..just a chapter in book 2. It covers wiring diagrams. Hope that is not to confusing.


[Modified by ittlfly, 11:46 AM 10/16/2003]

Z06_BluByU

there ya go with the helms manual again.. do you sleep with that thing under your pillow ;) :jester

To answer the question about did the engine continue to run or not... the engine did continue to you and did not stall.. I thought it was supposed to stall if 0 psi was reached and then that would have told me the sensor was bad.. wishful thinking i guess.. I'm going to check for a short tonight when I get home.. as well as go and buy a mechinical gauge to check the actual pressure reading..


can anyone tell me.. if I visually saw oil flow does that mean I have oil pressure? I'm reasonaing that oil can't flow without some kind of pressure.. right? (just so i can stop the nasty thought that i actually had 0 oil pressure..) I would hate to think i took the intake manifold off and spun the oil pump shaft to check for flow if that was a worthless exercise..


[Modified by BluByU, 1:04 PM 10/16/2003]

Z06_BluByU

last night i went and bought a sunpro oil pressure gauge.. I hooked it up and spun the oil pump drive shaft with the screwdriver.. it got as high as 20psi and stayed there with jsut the screwdriver.. so I guess we can now eliminate mechnical failure.. I re-taped and installed new conduit on the wiring harness in the back of the engine.. Also, I put the key to the on position with the sensors unplugged and the gauge pegged itself.. so it appears the circuit is ok.. unfortunatly i broke the old sensor when i was taking it off to check the oil mainfold thingy.. so worked stopped shortly after that.. tonight i'm gonna pick up a new sesnor on the way home and install it.. hopefully all will be ok..

if oil pressure did really go out.. would the engine still be ok if it lost pressure for 20 seconds or so?


[Modified by BluByU, 6:42 AM 10/17/2003]

dlmeyers

If a pump fails after the car has been running for some time it is unlikely severe damage would occur after 20 or so seconds.

A automotive journalist once commented on a NASCAR practice of removing the oil, (thereby reducing crankshaft resistance) doing a fast lap for time without any oil, and using that time for qualification. Supposedly current rules do not allow that.

After reading that, I was shocked. Granted, it's a race engine however it does attest to the great durability of combustion engines.

Based on the information available, you may not have had a loss of oil pressure at all, as in an electrcial problem. If you have some type of intermittant condition, (problems with a bearing, whatever) it could show up again but I suspect you will keep a sharp eye on those gauges.

If you plan on keeping the car, I would invest in the factory service manual. Working on the car with a factory manual is much easier when you have the main book and "Section 8A Electrical Diagnosis Service Manual Supplement". If I were only allowed ONE book for my car, that would be it.

Keep us informed. Curious.


[Modified by dlmeyers, 9:21 AM 10/17/2003]

Z06_BluByU

update - 65psi at cold idle! :party:

after verifing no mechnical trouble, and the senosors worked (until i broke one taking it out).. I retaped the wiring and replaced that senosor I borke.. put the intake back on.. re-assembled everything - then disassembled everything right before i turned it on for the first time.. i forgot i left the rags in the lifter valley!! AGGHHH!!! then reassembled everything again.. hit the go key.. it fired up and showed 65 psi.. thats the highest pressure at idle i've ever seen.. also looked at the oil i drained out of the car (i changed the filter and oil) and no metal particles...

must have been a short in the wiring harness!! yippie!! all betteR! :party: :hurray: :hurray: :hurray: