So I guess I thought I was too cool...

[bp71k5]

The fuel pump should turn on for just a few seconds when you turn the key on. The oil pressure sensor is only used when the relay goes bad. Sounds like your relay (or its wiring) and the CTS need to be fixed before anything else.

-Brian

 

[4X4HIGH]

I am almost convinced to drop the tank tonight. But I had a brainstorm walking out the gate at work... I am not hearing the pump kick on when the key is in the "on" position. I know some body has mentioned that the computer needs to sense oil pressure in order for it to activate the pump. I'm probably over generalizing this, but I hope I have the jest of it.

Well last night when I had the fuel pressure gauge hooked up, with the key ON, engine off... I got zero fuel pressure. Engine had to be running to achieve a reading

I know there is a way to test the fuel pump relay by running 12v to the red wire hanging by it. So I will do that first.

To really monkey the situation.... I had an issue a week or so ago with the coolant temp sensor. Problem was the PO had pulled the sensor out to "install a temperature gauge" the sensor was left hanging by it's wires in the front of the engine.

Well guess what other gauge is "installed" in the truck... Oil pressure gauge. I can only imagine the hack job on my poor oil pressure sensor back there

Has this issue been resolved yet? If not that is the VERY FIRST thing that needs to get corrected.

 

[VitaminC]

Sorry guys, I have been under the hood nonstop...

It has not been resolved. The cts was replaced when I found the sensor hanging, I'm not sure it's fixed but it does have a new part. lol from autozone. I have now discovered another connector disabled, well two actually. Did these trucks come with both the single spade and the two wire weather resistant plug for the oil pressure switch. The reason for these being disconnected is someone needed an aftermarket oil pressure gauge and said f the computer.

Picture of said single wire spade connector that is disabled

Picture of said two wire plug that is disabled

Both of those connectors are at the back of the engine, very close to each other, and both can reach where the ops should be on the driver side under the distributor.

Okay... Troubleshooting I've done.
My main issue is when I turn the key to "on" position the fuel pump kicks on for one second, then I hear the relay click off, I never do hear the actual pump. I had someone watch the fuel pressure gauge while I jumped 12 volts to it. It jumps to 11 psi and holds. As soon as I remove jumper, the pressure bleeds off... In my mind it should hold there, but the connections to the gauge are not air tight. So that is a variable. Shouldn't I hear the pump though, all my chevy pumps make a lot of noise, and none of them make that click of the relay popping.

I'm also concerned about the disabled connections for the oil pressure switch. I grabbed a circuit tester attacked the single spade connector. When grounded... Nothing. When voltage applied, a dim light on my tool is achieved, and then "oil sign" dummy light lights up. Okay, that's the oil pressure switch right? Then what the hell is the plug for?

I also found a splice in the hot side of the battery that runs to the starter.

I am buried in wires guys. Please help.

 

[bp71k5]

I think you should stop and get the aldl cable. You can use it to figure out which sensors are plugged in or not working.

-Brian

 

[crawlink5]

EDIT: Just thought about it - Did you by chance wash the engine? If so change the TPS. Do not EVER get the TPS wet or damp.

EDIT2: I am assuming you have a TBI in the 1987 Blazer. I don't think that was mentioned here but may have been. With more info maybe could get closer to a solution.

==============================================================

I did not read all the posts above so bear with me.

The biggest question here is do you have the factory stock cam that the ecm is expecting or an aftermarket? If there is a lobe separation greater than about 110 deg forget a perfect idle unless you get a chip burnt. Or get a factory cam. Do a search for cam specs and ask others how much this can be as I don't remember all numbers. Don't take ANYTHING for granted from an idiot PO.

I would not depend too much on codes thrown on a vehicle where an idiot bypassed the temp sensor (without custom burnt chip) and oil press sensor. You don't know what else he did. Try to get everything hooked back up - or bypassed completely with a burnt chip.

Has this wiring been butchered up? If it has then you may want to find a replacement.

The oil pressure sending unit does play a part in the fuel pump wiring. That said, if your vehicle is running then it does not play a part at that particular run time. It will only turn the pump on or off or ignition on or off. If it is running then worry about it as concerns the relay and startup sequence. But FORGET about it as concerns the idling. Unless the unlikely event that it is turning the ignition on and off real quick.

I would get the temp sensor working first as it WILL play a part in idling since the ecm uses this to determine if the engine is hot or cold. EGR valve needs to work correctly.

After that make sure the TPS is set correctly and that the idle screw is set correctly. If you do have an aftermarket cam, opening the butterflys a little more seems to help but is not a Fix-all. Reset TPS after every move of the butterflys.

O2 sensor. Usually this is a bad enough problem you can tell it.

Make sure your rockers are adjusted correctly.

 

[Fordum]

OK, don't know it this will help too much or not.
My schematics show a single brown wire going to an oil pressure switch which turns on the oil pressure light and does nothing else.
At least they used to. For the life of me, I cannot find that now.

I do see a tan wire going to a gauge.
Either way, I would not worry too much about that wire for now.
What you are looking for, are the wires going to the switch that keeps the pump running when it sees oil pressure.
This is where it gets interesting.
The newer trucks have a grey wire and an orange wire going to that switch.
However, back when those trucks were running carbs, the wires going to an oil pressure switch were Dark Blue, and Pink/White.
With a possible light blue attached to the dark one.

With no actual proof, I suspect that they may have recycled the wires when they switched to TBI.
So the wire that used to put power to the choke, now powers the fuel pump relay.

If that weatherproof plug has either sets of colors, its probably the oil pressure switch harness.

As for the pressure holding, I'm not sure. On a Ford with MPI, it will hold for hours after the pump is off if everything is good.
Not sure on yours.

If it is still cutting off regularly, I would consider hooking a hot wire to the pump and then cranking it.
Being sure to be able to kill that wire in a hurry if something goes wrong, like a fuel line coming loose.
Then, if it runs without stalling, you can start looking as to why the pump is getting turned off.
If not, and your pressure gauge is still hooked up, see if you lose pressure again.
If not, then the stalling is something else, if so, then you will need to drop the tank or cut an access hole.

 

[4X4HIGH]

Sorry guys, I have been under the hood nonstop...

It has not been resolved. The cts was replaced when I found the sensor hanging, I'm not sure it's fixed but it does have a new part. lol from autozone. I have now discovered another connector disabled, well two actually. Did these trucks come with both the single spade and the two wire weather resistant plug for the oil pressure switch. The reason for these being disconnected is someone needed an aftermarket oil pressure gauge and said f the computer.

Picture of said single wire spade connector that is disabled

Picture of said two wire plug that is disabled

Both of those connectors are at the back of the engine, very close to each other, and both can reach where the ops should be on the driver side under the distributor.

Okay... Troubleshooting I've done.
My main issue is when I turn the key to "on" position the fuel pump kicks on for one second, then I hear the relay click off, I never do hear the actual pump. I had someone watch the fuel pressure gauge while I jumped 12 volts to it. It jumps to 11 psi and holds. As soon as I remove jumper, the pressure bleeds off... In my mind it should hold there, but the connections to the gauge are not air tight. So that is a variable. Shouldn't I hear the pump though, all my chevy pumps make a lot of noise, and none of them make that click of the relay popping.

I'm also concerned about the disabled connections for the oil pressure switch. I grabbed a circuit tester attacked the single spade connector. When grounded... Nothing. When voltage applied, a dim light on my tool is achieved, and then "oil sign" dummy light lights up. Okay, that's the oil pressure switch right? Then what the hell is the plug for?

I also found a splice in the hot side of the battery that runs to the starter.

I am buried in wires guys. Please help.

That is normal, as long as the pump is running you'll have fuel pressure but the minute the pump stops there is nothing to keep pressure and the gauge will drop back to zero.

The two wire connector is probably the oil pressure back up switch (allows the fuel pump to run as long as it sees 5 psi of oil pressure).

 

[VitaminC]

Well guys i "worked" on the truck tonight

I really just put everything back together I had torn apart, and she still ran

I know I'm gonna get super flamed for this, but I started thinking real simple.... Voltage

Voltage at battery terminal running is 12 lol... Voltage at alternator running is 13 lolx2... I know both of these are fairly low, along with my fuel pressure of 11 psi or so (all cheap analog gauges). I am going to test voltage at battery after sitting over night tomorrow.

Hey... If this ends up being the problem, yes, I do look like a complete noob moron. But, atlases I am now A LOT more educated on every system in my truck. Built, not bought right?

-Cody

 

[4xcrazy]

Mine has ran down the the near limit of 9 volts when the alternator died once, still ran decently so i doubt you're havinf a voltage issue.

 

[VitaminC]

Bringing this ttt.

Hey guys I'm back again with another sweet find on my truck.

Btw, it still runs like shiiiiiit.

But tonight while tinkering I found the ground strap from the back of the block (passenger side) disconnected and just laying against my y-pipe. Haha probably part of my issue. I'm assuming that goes to the body. I planned on running one from body to frame on the same bolt while I'm there.

Just to recap, the truck shuts down when it's really hot outside, like 105+.

And no I still have not built my cable or bought one. Had some drama in my life lately.

Who's makin bets on that ground causing a lot of this??? Any one??? No one??

He'll I don't know

Cody