Oil Pressure Question

Okay guys, I think I need some pointers here. 'BlueTwo' has started misbehaving on me. I was driving it to Church this morning and just a mile or two before I got there (12 miles one way) at a stop light and the oil pressure gauge drops to the red marks and the light comes on. I throw the car in neutral quickly to let the RPMs get up. Pressure rises but not by much. Make it the rest of the way to Church and park the car.

It sits for a couple hours. I check the oil, level is fine and still mostly clean. Didn't start it, went back inside. Came back out about another hour later and start the car. Oil pressure is over the 30 mark but not by much. Putting the car in drive has little effect on pressure.

So I head home keeping a close eye on the gauge. As the engine reaches operating temps I can see that the pressure is dropping some but still midway between the 15 and 30 marks. Two hashes below 15 is red and where the light comes on. As I get home, I notice that the pressure is dropping to 15 or just barely below. Park car and let it sit while I work on another.

Hour or two later, get the car up on stands so I can check underneath. No oil leaks. Everything looks fine under hood and under car. While adding air to the tires I let the engine run to get back up to operating temps. Once hot, the engine will idle (in Park) with the oil pressure just over the 15 mark. Put it in gear and idle, the pressure drops and will 'bounce' around light comes on once or twice. A tap on the throttle and the pressure rises with the RPM jump.

Now, this poor car was abused by it's previous owner. It was bought by me as a repossessed car. When I did my first oil change, it took forever to drain as it was practically sludge. I've changed it a couple times since with no noticeable issues. Oil pressure has been good until now and consistent with my other Cruiser ('92 with 150K miles). This one has just over 103K miles and an oil change less than 1K ago. No knocking or ticking in the engine that I can hear, runs fine. Minor fluctuation in engine speed at idle.

So, now that I've rambled on. I'm thinking my oil pump is going bad. But before I get into replacing it I would appreciate some other opinions or ideas as to possible problems. What do you guys think?

My guess would be one of 2 things, the pickup screen is clogged up, form the sounds of the oil, I wouldnt doubt this at all. Or the Pump is on its way out the door. Either way, I would give it one of the old Redneck cleanups. Drain about 1 qt out and add 2 qts of fresh Kerosene and let it run till warmup.Drain and add fresh oil and filter and see what happens. If this doesnt seem to help, gre prepaired to drop the pan and change the pump and clean the pickup. I would go for a high volume pump with the replace.

Mike:

Before leaping to conclusions, make sure it's not the sender unit. Mine is all over the map. I had an old mechanical guage that I "tee'd" into the system and discovered it was the sender.

Note here. Go to the dealer for a new sender. There is some real problems with the aftermarket part numbers. Guess they all assumed that our cars only have idiot lights.

Bill

Thanks for the tips guys! Bill, I hadn't thought of a bad sender. A very good point though and certainly easier to replace at least for troubleshooting purposes. Am I correct that the gauge and the 'idiot' light get their signals from the same sensor? If there are two sensors that might rule out a bad one though as both are indicating the same thing, i.e. low pressure.

Keep 'em coming!

EDIT: Looking at the FSM, looks like there are two sensors. One by the oil filter and another, probably on the head.

my 92 9c1 does the same thing as long as i owned it 4yrs ,now the car has 367k on it , it starts out with good press then drops, ive run 20-50w didnt really help,i keep it close around town and have aaa plus with 100 mile towing lol , if i get around to it i may jack the motor up and drop the pan and change the pump, i ordered a new senser from rock auto, they had a few types in ac/delco i got it but it was a short version , mines long, but b4 i do all that ,ill hook up a mech. gage, i got one somewhere ......

DBeaSSt wrote:

Thanks for the tips guys! Bill, I hadn't thought of a bad sender. A very good point though and certainly easier to replace at least for troubleshooting purposes. Am I correct that the gauge and the 'idiot' light get their signals from the same sensor? If there are two sensors that might rule out a bad one though as both are indicating the same thing, i.e. low pressure.

Keep 'em coming!

EDIT: Looking at the FSM, looks like there are two sensors. One by the oil filter and another, probably on the head.

As far as I know, there is only one sender. It does three things. Shows oil pressure, turns on idiot light and acts as the fuel pump shut-off/turn-on. Could be on your car (mine are 91s) they use a seperate one for fuel pump control.

Bill

Well the stock gauge can lie, trust me. My engine spun a bearing at 176k-ish with a 3 year old oil pump that had around 40k miles on it. Now I did have a knock for quite some time prior to the failure so it was not a surprise that it happened. What was a surprise was the fact my gauge never dropped below 30 psi, even when the motor was hot, the entire time that I have owned the car. When it was cold it would read close to 60 psi, so I figured I was good when it came to me bearing health. On the night of the failure the gauge never moved and the light did not come on, the engine just died. There are several variables as to why my engine spun a bearing and I am not going to blame any one at this time as I simply do not know what the root cause was. I think I am going to look into adding a mechanical gauge just to be on the safe side from now on.

If it does turn out to be an oil pump problem stick with a standard volume pump. High volume pumps can spell trouble for stock engines with higher mileage as in some cases the oil can not return to the pan as fast as the pump is moving it out of the pan. If memory serves the other sending unit should be on the back of the block, near the distributor, at least that is where every other small block chevy I have ever worked on has had the sending unit or feed line for the gauge.

waynes91: Yes, my Cruisers have always read a little low but I've never had them go this low. Nor have I had the light come on thus I'm a bit nervous.

Unlike Uber's experience, my gauges do move so there is some kind of signal getting there. Though I agree, there is the possibility that it's just wrong. As I go through the manual further, I'm thinking it's less likely to be electrical though (sender or gauge). However, that is much easier to replace so I'll probably try that first.

I re-checked my FSM and the exploded parts view shows two oil pressure senders/switches. However, it may be a generic small block parts view. In looking at the electrical schematic for the instrument cluster, it only references one sender which is for the fuel pump switch/oil pressure sender. I had forgotten about the fuel pump connection here.

So, if I understand the fuel pump connection. If the switch senses no oil pressure, it will cut off the fuel pump and thus engine dies. Simple test here then would be to unplug the sensor and see if the engine runs I guess. When my light flickers I'm not noticing any stumbling of the engine.

Hmmm, well I guess I know what my job is this evening after work!

yeah mike my wagon when i first got it did the lowwwwww reading it was the sensor, and i second on the dealer sensor. i have worked for a few aftermarket parts places and well they leave alot to guessing. second, the stock chevy has a great oiling setup, i fully agree on the stock pressure pump if that is it. i put the high flow high pressure ones on hipo car engines i build and any that are drag cars. the stock one works excellent and with like said it being a stock engine with some wear, it can do more damage then help. keep an update because mine is getting up in miles and i will follow what happens next. hopefully is just sensor. best of luck to you.

matt

Mike:

This is the wiring for my 91 Roadmaster. It should be similar. I have seen it where the pressure sensor circuit has died but the fuel pump circuit remains good. In other words, low pressure indication including oil light, but the fuel pump remains on.

[You must be registered and logged in to see this image.]

The only other oil related sensor I can find is the oil level, located 1/2 way down the oil pan. The pressure sender/fuel pump switch is located at the back of the engine on the drivers side if memory serves. Different on the diesel.

Bill

Well, looks like you guys saved me some money and hassle! I pulled the pressure sender (above the oil filter) from the parts car (92 Cruiser with L03) and swapped it with the one in 'BlueTwo' The parts car had always shown higher oil pressure and never once dropped much below 30 on the entire drive from NY here.

Well, the issue followed the sender! I let both get good and hot with their 'new' senders. 'BlueTwo' now runs between 30 and 45 just about all the time. Where I put the parts car in drive after it got warmed up. Gauge dropped to zero, light came on and a warning bell 'bonged'. As you suspected though Bill, engine never shut off (fuel pump) and still no ticking.

So, looks like it is a bad sender. I'll stop at the dealer (ugh) and order a new one tomorrow as the one from the parts car is a bit rusty and looks like it's been there for the full 300k miles. I'll post up the part number too so whoever needs one can get it online cheaper.

Thanks guys! Oh, and Bill, the Cruiser FSM shows pretty much the same diagram as your Roady.

Its always a good thing when a fix doesn't involve an engine re and re.

Bill

Well that is a good deal then. I am still going to be adding a manual oil pressure gauge to my car as I am now a little gun shy of the factory one.

Just following up on this. Picked up the ACDelco sensor yesterday from the stealership. I'll install tomorrow.

ACDelco / GM part number - 10222130

List price $103.56, my 'discount' price $55.76 plus taxes. I'm sure you can find it cheaper on RMS Auto or something similar. Just posting up for future reference.

Thanks again!

DBeaSSt wrote:

Just following up on this. Picked up the ACDelco sensor yesterday from the stealership. I'll install tomorrow.

ACDelco / GM part number - 10222130

List price $103.56, my 'discount' price $55.76 plus taxes. I'm sure you can find it cheaper on RMS Auto or something similar. Just posting up for future reference.

Thanks again!

Thats about right. Rock Auto has it for $54.00. May have found out the problem with the aftermarket part numbers. Rock Auto has an oil pressure switch listed for these cars for 18 bucks. The Delco part is listed as a fuel pressure switch/oil pressure gauge sender. Maybe just the terminology that has caused the confusion.

Bill

convert2diesel wrote:

Thats about right. Rock Auto has it for $54.00. May have found out the problem with the aftermarket part numbers. Rock Auto has an oil pressure switch listed for these cars for 18 bucks. The Delco part is listed as a fuel pressure switch/oil pressure gauge sender. Maybe just the terminology that has caused the confusion.

Bill

I order from Rock Auto quite a bit. I'm thinking, next time I place an order, I might get one of those $20 parts just to see if it works. I saw that too and was wondering. It does state 'three pin' on it and looks the same. But as you mentioned, might not do the fuel pump thing too.

DBeaSSt wrote:

I order from Rock Auto quite a bit. I'm thinking, next time I place an order, I might get one of those $20 parts just to see if it works. I saw that too and was wondering. It does state 'three pin' on it and looks the same. But as you mentioned, might not do the fuel pump thing too.

Been there, done that. Fuel pump works but the gauge goes full deflection. Obviously for idiot light. Have no idea why this part is listed for the gauge equipped cars but could explain why eveyone's (NAPA, Autozone etc.) books show an aftermarket replacement. To my knowledge, Delco is the only one who make the correct sender.

Bill

Ah, I see. Thanks for the tip... again.

convert2diesel wrote:

Been there, done that. Fuel pump works but the gauge goes full deflection. Obviously for idiot light. Have no idea why this part is listed for the gauge equipped cars but could explain why eveyone's (NAPA, Autozone etc.) books show an aftermarket replacement. To my knowledge, Delco is the only one who make the correct sender.

Bill

Not to rub it in but i got my unit for 39.54 .. but i did the search and found that i had the same problem as Dbeasst.. so i took home a oil pressure gauge and did some testing.. pressure was fine on the engine.. so i orederd me up one of those delco senders and we'll see if its misson complete.. with out this search it wouldnt have been possible.. so i would like to thank my family/friends/all my fans!.. hahah j/p guys but seriously if it wasnt for this page i probably would have been goin crazy!..

DBeaSSt wrote:

So, if I understand the fuel pump connection. If the switch senses no oil pressure, it will cut off the fuel pump and thus engine dies. Simple test here then would be to unplug the sensor and see if the engine runs I guess. When my light flickers I'm not noticing any stumbling of the engine.

No.

This has been discussed before, loss of oil pressure will not necessarily cause the fuel pump to stop.

JaySS wrote:

...Most, if not all, GM's of this era are wired so that an oil pressure switch backs up the fuel pump relay. If the fuel pump relay should fail, once oil presure builds, the relay is bypassed by the oil pressure switch to permit the car to start anyway. This is probably what you've had going on for awhile, an indication is a longer than expected crank time, waiting as the oil pressure builds to energize the fuel pump...

- J