FUEL PUMP PORBLEM AT STARTUP (Page 1/1)
Francis T OCT 31, 09:26 AM
Problem with My 86GT turbo. At startup -especially when cold- my fuel pump cuts off at 30-35 lbs; which is not enough to fire the injectors. If I spray a little gas or starting-either into the intake manifold it fires right up and maintains 40+ lbs, runs fine and will restart np.
My Question: is the problem a faulty oil pressure relay switch? I'm going to replace the fuel filter too. I am going to sell my 86GT soon and keep my 87GT and want to resolve this issue ASAP.

thanks

[This message has been edited by Francis T (edited 10-31-2019).]

Gall757 OCT 31, 11:13 AM
Try this:

Power the fuel pump at pin G on the ALDL. If it stays on, you have a bad fuel pump relay (or maybe socket or wiring). I don't think the oil pressure sender is a factor.

[This message has been edited by Gall757 (edited 10-31-2019).]

olejoedad OCT 31, 11:28 AM
Relay and OP circuit see the same voltage, fed from same source.
spad OCT 31, 11:32 AM
Oil pressure sensor's switch is there just for backup. A relay, controlled by ECM, is the main source for fuel pump power.
I can't see how clogged filter could be issue. It is located before fuel rail measuring port, and in that case pressure should drop from baseline when injectors start spraying.

ECM will feed fuel pump about 3 seconds after turning key to ignition, even if you don't crank the engine. Could it be, that ECM will not feed relay or relay is bad? After engine has started, pump will run on oil sensor backup-feed. Oil will get hot and sensor reacts faster.

ALDL connector pin G should show you voltage, that relay will supply to fuel pump. It should be around battery voltage (0...1 V less). Voltage should be there for that 3 seconds, or when ECM detects engine rotating, or when oil pressure sensor will supply backup. It may be good idea to disconnect oil pressure sensor before tests.

Fuel pump relay is located in engine compartment, driver's side firewall, right one.

http://spad.dy.fi/fiero/man...6_Service_Manual.pdf page 596, 663 and so on...
Francis T OCT 31, 12:30 PM
Thanks to all for the quick helpful replies.
I will check the voltages at the ADL. BTW: when I turn the key ON, I can hear the pump run for a few seconds, but it always cuts out at 30- 34 lbs? which is why I was thinking its the switch and not the relay.
And I doubt that's its filter related, but I have a new one so wtf may as well change it.


quote
Originally posted by spad:

Oil pressure sensor's switch is there just for backup. A relay, controlled by ECM, is the main source for fuel pump power.
I can't see how clogged filter could be issue. It is located before fuel rail measuring port, and in that case pressure should drop from baseline when injectors start spraying.

ECM will feed fuel pump about 3 seconds after turning key to ignition, even if you don't crank the engine. Could it be, that ECM will not feed relay or relay is bad? After engine has started, pump will run on oil sensor backup-feed. Oil will get hot and sensor reacts faster.

ALDL connector pin G should show you voltage, that relay will supply to fuel pump. It should be around battery voltage (0...1 V less). Voltage should be there for that 3 seconds, or when ECM detects engine rotating, or when oil pressure sensor will supply backup. It may be good idea to disconnect oil pressure sensor before tests.

Fuel pump relay is located in engine compartment, driver's side firewall, right one.

http://spad.dy.fi/fiero/man...6_Service_Manual.pdf page 596, 663 and so on...
olejoedad OCT 31, 01:43 PM
The Fiero ECU gets no feedback relative to fuel pressure.
The ECU runs the pump at key ON for a specified time period, will not turn it back on until the ECU detects the engine running.
theogre OCT 31, 02:39 PM
FP has power, low power, or none.
ECM FP Relay and OP sender is On or Off and ECM does not care about pressure.

Low pressure could be low volts, iffy pump, iffy lines or combi of all three.

Low power can be a lot of other problems. Iffy contacts in the relay, OP sender or both are possible but not the only thing or worse burnt contacts can be a symptom not the root cause.
See my Cave, Electric Motors
So reading Volts on ALDL G might help or not. Can see normal volts at the test point but FP have iffy grounds you won't see that problem testing this way.
Jump G to lighter + may help but remember FP needs 7-10a normally so make sure anything jump to 12v handles that. Lighter socket + term should be ok but many use wire is too small.

Iffy Filter often sees Normal Pressure at idle but can't Flow enough gas at higher RPM. Fiero Filter is Huge and needs a lot of crap to block them but can happen. Bigger/heavier crap sinks to filter bottom as wider space slows flow. Fine crap might stay and block the media. Very big crap should be blocked at the pump sock.

FP, rubber hose and line inside of tank can leak causing low or no pressure.
If FP replace before then likely has cheap hose in there that often has problems because Ethanol can eat the hose.

Note: If you run out of gas... FP looses its coolant and lube and that can cause FP problems very quick or just shortens lifetime often a lot. You never want to run out for this reason on most cars. If you even think you run out like gauge is lying add 2-3 gallons before trying to turn the key.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Francis T OCT 31, 03:32 PM
I think you're spot-on, whereas the car ran out of fuel a few times recently! I will change the filter and check the relay etc first but it will probably be the pump. Funny how the fuel pressure is fine when the engine is running? Fortunately, I now have a lift in my garage so dropping the fuel tank won't be as hard as it was the first time.
I'm getting over a case of pneumonia -in my 76YO case, oldmonia-, I just need to get feeling good enough to do it.
thanks

QUOTE][B]Originally posted by the ogre[B]
FP has power, low power, or none.
ECM FP Relay and OP sender is On or Off and ECM does not care about pressure.

Low pressure could be low volts, iffy pump, iffy lines or combi of all three.

Low power can be a lot of other problems. Iffy contacts in the relay, OP sender or both are possible but not the only thing or worse burnt contacts can be a symptom not the root cause.
See my Cave, Electric Motors
So reading Volts on ALDL G might help or not. Can see normal volts at the test point but FP have iffy grounds you won't see that problem testing this way.
Jump G to lighter + may help but remember FP needs 7-10a normally so make sure anything jump to 12v handles that. Lighter socket + term should be ok but many use wire is too small.

Iffy Filter often sees Normal Pressure at idle but can't Flow enough gas at higher RPM. Fiero Filter is Huge and needs a lot of crap to block them but can happen. Bigger/heavier crap sinks to filter bottom as wider space slows flow. Fine crap might stay and block the med I preety sure i replaced the fuel pump awhile back/,

FP, rubber hose and line inside of tank can leak causing low or no pressure.
If FP replace before then likely has cheap hose in there that often has problems because Ethanol can eat the hose.

Note: If you run out of gas... FP looses its coolant and lube and that can cause FP problems very quick or just shortens lifetime often a lot. You never want to run out for this reason on most cars. If you even think you run out like gauge is lying add 2-3 gallons before trying to turn the key.

[/QUOTE]

[This message has been edited by Francis T (edited 10-31-2019).]

Gall757 OCT 31, 04:16 PM

quote
Originally posted by Francis T:

I think you're spot-on, whereas the car ran out of fuel a few times recently!

Note: If you run out of gas... FP looses its coolant and lube and that can cause FP problems very quick or just shortens lifetime often a lot.



I think that's it. The pump has oldmonia too....