I've noticed a lot of threads over the last several months about fuel pump problems. One thing that stands out in almost all of them is a misunderstanding of the function of the oil pressure switch and how it runs the pump. A lot of folks have the idea that the oil pressure switch will shut off the pump in the event of low oil pressure. THIS IS NOT TRUE!
If the system that runs the pump is operating as designed, it works like this: When the ignition is first turned on, the ECM will energize the fuel pump relay for a few seconds and then shut off. When the key is turned to "start" and the ECM sees reference pulses from the ignition system (indicating the engine is turning over) it will re-energize the fuel pump relay. After the engine starts and the key is released to the "on" position, the ECM will keep the fuel pump relay energized as long as it continues to see the reference pulses from the ignition. When the key is turned "off", the ECM will de-energize the fuel pump relay. The point to remember is: The fuel pump relay is engergized the entire time the engine is running.
Now, what about the oil pressure switch? It has a set of contacts that will close when it sees oil pressure. This completes a circuit that will engergize the pump IN PARALLEL with the relay. According to the GM factory shop manual, the purpose of the oil pressure contacts is to act as a BACKUP in case of a problem with the relay. This wlll allow the pump to run with a bad relay, but the engine will take longer to start since the pump won't be energized until oil pressure builds.
I have personally unplugged the oil pressure sending unit on an 84 SE and an 88 GT and proceeded to start and run the engine with no problem. (The 84 didn't even have an oil pressure switch for nearly a year while I had a mechanical gauge hooked up.)
I think that only the "Lotus" suspension on the 88's is more misunderstood.

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Rod Schneider, Woodstock, Ga.
White 88 GT :)

EXACTLY!!-A redundant circuit I call it. Oil press switches are prone to failure, as indicated by all the calls in any fuel related thread to check/replace them. IF it is a safety device to kill the fuel pump any time oil press drops, there would be far more liability placed on the manufactorer if the op sw 'died' while you are haulin as*, keeping up with traffic flow. Your car suddenly loses power, and the cars following you plow into your soft underbelly(rear end). You don't hear of this happening, it is always, 'starts, then dies" or "won't start". Not, 'I was driving along, and it suddenly died right in the middle of I-95 at 70MPH traffic" I've been asking for a year for someone to show the diagram & explain how it is not a parallel (redundant) circuit.

Ok, so what happens if the engine dies and the oil pressure goes to zero? Pump stops right?

Then why does the pump run for a few seconds with the key on and engine off and then stop?

Why does the pump keep running then the engine is running?

So put those together.

Back in my AMC Hornet 360 days I switch over to an electric fuel pump. The literature with the fuel pump indicated that to be legal an oil pressure safety switch had to be use in order to be legal for street usage. No oil pressure, no fuel.

Separating misinformation from facts is a good thing to do! Thanks for the correction. Suppose I need a GM shop manual.

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I always get a kick out of hearing the Fiero had plastic lug nuts; because of the plastic covers.

terryK,
To answer your questions:
"Ok, so what happens if the engine dies and the oil pressure goes to zero? Pump stops right?"
When the engine dies, the ECM no longer sees reference pulses from the ignition system and de-energizes the fuel pump relay. There will be a short time until the oil pressure bleeds down during which time the pump will run. (Usually just a couple of seconds unless the engine is cold and the oil is thicker)
"Then why does the pump run for a few seconds with the key on and engine off and then stop?"
The ECM will engergize the pump for about 2 seconds in order to build up pressure for easy starting. If it doesn't see a reference pulse after that time period, it will shut off the relay.
"Why does the pump keep running then the engine is running?"
Again, as long as the ECM sees reference pulses from the ignition system, it will energize the fuel pump relay.

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Rod Schneider, Woodstock, Ga.
White 88 GT :)

Actually the oil pressure switch is a back up to the relay. Check the schematic the fuel pump can get power even if the relay should fail. Looking at the schematic after the car starts you should be able to unplug the relay and the car will still run.

LS1swap,
You're exactly right. You can unplug either the relay OR the oil pressure switch and the engine should still run.

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Rod Schneider, Woodstock, Ga.
White 88 GT :)

quote Originally posted by terryk: Ok, so what happens if the engine dies and the oil pressure goes to zero? Pump stops right? Then why does the pump run for a few seconds with the key on and engine off and then stop? Why does the pump keep running then the engine is running? So put those together.

(mis-read the 1st statment so I edit)
Re your first question/statement-Yes, if the engine dies, the pump stops, because the ecm is no longer getting ign pulse from the dist and the oil press switch no longer senses press over 4psi.

2nd question: ECM cuts the fuel pump off after 2 seconds, if it does NOT recieve ign pulse from the distributor.

If you have a GOOD fuel pump relay & associated circuit, the eng will keep running , irregardless of oil press. I suspect there are a lot of people who have a bad fuel pump relay or wiring, and their eng will die if they lose oil press because of this unknowingly bad fuel pump relay. They never realize the relay is bad, until the oil press switch goes bad, then the car will not start, because BOTH parallel circuits are now dead. Most of us keep the ign system, & inj system in good enough shape that we really wouldn't notice the slightly longer start times caused by a bad fp relay.I'm not going to drain the oil out of my car while the engine is running to test this, but I have un-plugged the three wire connector off the oil press switch and the eng will continue to run fine. Look at the diagram. A safety circuit just isn't shown.
A back-up circuit is. This is my view of it, just haven't seen a schematic or diagram yet to show me any different.
RodrV6, you started this can of worms and then ran off?? (oops, sorry. I see you haven't)
Don

[This message has been edited by maryjane (edited 03-03-2002).]

[This message has been edited by maryjane (edited 03-03-2002).]

I'm right here, maryjane. (Although I've got to go shortly)
Actually, I addressed terryK's questions a couple of posts up the page.

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Rod Schneider, Woodstock, Ga.
White 88 GT :)

Sorry, I'm a slow reader & typist.

The Key point to remember here is: if you turn the key to the one position (not to start) you should here the fuel pump kick in...if not then I would be checking the fuel system (relay, wiring, pump, etc)...if it does, then there is no problem with the fuel pump.

Tim

Give it a whirl Rod... I've tried to explain it about 10 times in the last year but new people keep coming up with the same oil sender saftey device theory. I think because of the confusion that later models (non-Fiero) have an inertia device to trip a breaker like electrical device so the pump stops until the trip is reset.

[This message has been edited by 87GTZ34 (edited 03-03-2002).]

holy smokes mind scape likes amc sc hornets AND fiero's????whts this world coming too???but seriously i love both as well. i was raised on ramblers and loved fieros..i currently own a 72 jav amx w/ 475 hp 360 and a whole bunch more good stuff.(now how to get my 360 andf 727 trans in one of these things???)

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fun is rowin throo my 5-speed

Hrm, so if we just disconnect the fuel pump relay on our cars, does this give us a 'pre-oiler?'

On an BMW 850, the engine turns over for about one second before spark is even fired, to pre-lube the engine before explosive stress is placed on it.

Best!
Ben.

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Ben Cannon
88 Formula, T-top, Metalic Red. "Every Man Dies, not every man really Lives"
88 Formula, Northstar, Silver, In-Progreess. -Mel Gibson, "Braveheart"

I am getting the ole "starts then dies" with my 88 coupe Iron Duke. But only when it's hot, (engine and weather).

Now we haven't had much hot weather here lately, so my fuel pump relay has been working just fine. When I turn the key to run I hear the fuel pump run for two seconds.

But when we had a couple of warm days a while back, (I'm in Houston) it would start then die immediately. After reading these threads I suspect I have a thermal issue with my fuel pump relay. It is physically located in the engine compartment, is it not?

Yes the fuel pump relay is on the firewall, near the left deck lid hinge support. should be 2 identical relays, side by side. one is the f/p relay, the other is the a/c relay(if you have a/c)

Interesting, I always thought it was relay to start and sender to run.
So if you unplug the oil sending unit before cranking it will not continue to run, but if you unplug it after the engine is running it will? Thats if the relay is good of course.

Consider this. I had a problem with an 85 GT that would crank but not run. The green/white wire was cut at the ECM. Was the switch at start activating the relay for the initial pump run?

Cool idea, artherd!
Only thing is you might get rear-ended. When your engine suddenly dies. On the highway. At 90 MPH. Because the oil pressure switch fails
Seriously, pre-oilers are a pretty common thing on hi-performance engines and really reduce wear.
Does artherd have something here?

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Bob Cardone
Suncoast Fieros

'85 2M6 SE Targa

Wolfhound,
You can also unplug the oil switch before starting the engine and it should start and run just fine--I took the oil pressure switch completely off of my 84 to hook up a mechanical oil pressure gauge and drove for about a year like that.

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Rod Schneider, Woodstock, Ga.
White 88 GT :)

Pre-oiler - well one can always put in a delay for the relay that turns it on after the engine is running and therefore avoid the problem with the car stalling out when the sending unit dies

...just a thought

Tim

Rod is right, for about a year straight my fuel pump ran non stop, you wanna guess why???? My oil sender was stuck, but it was only stuck a little (it only has to be 7 PSI to start the fuel pump) I had to unplug the wires on the sender so my battery would stop going dead if I didn't drive the car everday. My car is an 87 GT. I replaced the oil sender and problem solved.

Pre-oiler, Try holding your gas pedal to the floor, your engine will not start until you release it (clear flood mode)

What if the relays are good and the op switch is good but the pump won't come on for the inital 2 secs so the motor has to crank and build op for the motor to start?
Bad ECM?
I've gotten around it by putting a switch on the fuel pump and controlling it manually.

Bruce

...well I'd start checking the wiring..is the signal getting to the relay to turn on? How about the relay socket (is it making good connection)?, etc.

Tim

OK, let me back up. More clues:
With everything hooked up normal the pump would never shut off, so I'd have to pull the fuse, so I put a switch (and fuse) across the fuse terminals.
I also had to put a jumper across the terminals at the OP switch because if plugged into the sender the pump wouldn't turn on til OP came up.
Weird stuff....

Bruce

Well, start by checking if there is power on the load side (fuel pump side) of the relay with the key off (disconnect sensor and relay from pump to make sure you have no feedback) if you got power then I would be looking at the relay (stuck on, short in the plug, etc - something supplying power to that side). Also check power coming from the sensor (should be none if car is not running). Basically you need to disconnect the items in question and trace back to see where are are getting the power from.

how that helps

Tim

so you turn the key, the pump comes on...
you don't start the car....

what shuts it off? when it reaches a certain psi? a timer in the ecm?

quote Originally posted by artherd: Hrm, so if we just disconnect the fuel pump relay on our cars, does this give us a 'pre-oiler?' Best! Ben.

Yes, I believe so. It would be simple enough to wire in a kill switch to the fuel pump relay. Kill the relay, the crank her over. The engine won't fire till pressure builds. Flip the switch back to return to normal operation. Pretty good idea after oil changes, etc.

Wiring in a delay would be a bit more complicated. I wouldn't want to remove the relay completely.

quote Originally posted by jscott1: I am getting the ole "starts then dies" with my 88 coupe Iron Duke. But only when it's hot, (engine and weather). Now we haven't had much hot weather here lately, so my fuel pump relay has been working just fine. When I turn the key to run I hear the fuel pump run for two seconds. But when we had a couple of warm days a while back, (I'm in Houston) it would start then die immediately. After reading these threads I suspect I have a thermal issue with my fuel pump relay. It is physically located in the engine compartment, is it not?

Wow this thread came back...I have since replaced my fuel pump relay with a brand new one from the Fiero Store. I cleaned the contacts before putting it in. Well, it seemed to fix my "starts then dies" problem, but it has occurred once since then.

There must be something else going on to cause this. Probably the wiring to the fuel pump relay or the ECM?

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1988 Coupe
Fiero - It's like an Actual Miniature Car

[This message has been edited by jscott1 (edited 04-09-2002).]

Gee-it came back to life
JD86GT350--The ECM has a timer that will shut off the pump relay after 2 or 3 seconds if the ECM doesn't see the "reference pulses" from the ignition system which would indicate the engine is running.
jscott1--you might want to check the wire connections at the distributer and coil. If the ECM doesn't get good signals from the ignition system, it may be shutting off the relay before the oil pressure builds enough to keep the pump running.

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Rod Schneider, Woodstock, Ga.
White 88 GT :)

FWIW, I can see where the "safety switch" misconception probably came from. At least in my case.
I had a 76 Chevy Monza. Had a carburetor with an in-tank electric fuel pump. The pump *was* wired through a circuit in the oil pressure switch.
This wasn't a show stopper with carb'd engines, though, because if the oil pressure went away and shut off the fuel pump, there was still a bit of gas in the carb. Peobably enough for you to get to the side of the road after the "oil" light came on.

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Raydar

From the Department of Redundancy Department.

Those older cars had to have an oil pressure cut off switch for the electric fuel pump for safety. They were carburated engines. The fiero is computer controlled and if there are no reference pulses from the engine running, the fuel pump shuts off.

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- Robert Beaubien
- 87 GT 5-speed
-

My pressure sending unit failed, and unfortunately it failed in such a manner that it indicated that there was oil pressure. So my fuel pump continued to run even though the key wasn't even in the ignition!

Needless to say the fuel pump fuse got pulled when the car was parked until I figured it out.

I have an 84, and similar to jscott1, it tends to stall only on hot days, usually sitting still for a while (long wait at traffic lights). Once the ignition is turned off and on again, it starts and runs fine for as long as it is moving. Similarly, prolonged coasting down a hill in gear will cause the engine to stall.

From the other posts, it seems that this problem would most likely occur if say:
1) the resistence in the fuel pump relay increases in hot weather to prevent a signal from reaching the fuel pump or ...
2) the heat prevents proper "reference pulses" from the coil/distributor from reaching the ECM.
3) but the oil pressure switch is probably stuffed as it doesn't "backup" during conditions where there might be low signal from the fuel pump relay.
4) or there is a genuine low oil pressure at idle or coasting which doesn't close the oil pressure circuit.

So regardless, something is wrong with either the reference pulses getting to the ECM, or the fuel pump relay. Is this correct?

cheers
dio

And when someone has a Fiero with a bad fuel pump relay AND a bad oil pressure sending unit AND a bad ECM AND a bad pickup coil, and no mechanic can get it started, you get the car for free!

(It was a b!tch to troubleshoot)