Bad fuel pump? - Ford Mustang Forum
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post #1 of 25 (permalink) Old 4 Weeks Ago Thread Starter
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Cool Bad fuel pump?

In addition to my bad ECU, I think the fuel pump is going out. I bought the car as a 100k vehicle but in reality it might be 200k. I use to turn the key and it would fire right up and fast idle at 1100 rpm then drop after 30 seconds to 800-900.
Now, I turn the key, it fires up then dies. I have to feather the throttle in the 1800-2000 range to keep it running. After a couple of minutes it settles in and runs and idles fine. Also the FP makes an inordinate amount of noise!
I replaced the FP relay about a year ago. I've never changed the fuel filter (figure I'll do that when I change the FP).

I have a Walbro that was in Thunderstang with about 4k miles on it however, it's been sitting on a shelf for the last 5 years. I was contemplating installing that. Are there any seals/O-rings inside the pump body that may have dried out/shrank/etc? It's not too hard to drop the tank. I figure if I can get away with running that one I will have saved $80, if not, I'll drop the tank again and buy/install a new one. Let's hear your thoughts!


'93 5.0 LX ragtop - Donor car for '54 Ford Customline
'89 LX ragtop with turbo'd TC motor - Thunderstang!
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post #2 of 25 (permalink) Old 4 Weeks Ago
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Maybe check the fuel pressure? I guess you could check key on engine off and then again when it finally idles.
I had a fuel pump go out in a cougar that I had. The motor shut off and wouldn’t start. It didn’t “get weak” and then die. It was instant death. A louder than once was pump would make one think ones on its way out though. If it is the pump surely you’ll have a no start condition soon.
You don’t think the stalling etc is related to the bad ecu?

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Yeah, it could very well be the ECU. With the loud FP, I figure it's probably on it's way out so I'll just replace it and repair the ECU at the same time.

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I'd put a pressure gauge on the fuel rail and turn the key forward see if pressure ramps up and if it holds then see if it holds while cranking... If not , figure out where (maybe at fuel pump relay output ) if the relay loses power , I'd blame the ecu or that combination-module that also controls fan speed..... If you have one... Not sure if we're talking fox or sn95.

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Originally Posted by guntek18 View Post
I'd put a pressure gauge on the fuel rail and turn the key forward see if pressure ramps up and if it holds then see if it holds while cranking... If not , figure out where (maybe at fuel pump relay output ) if the relay loses power , I'd blame the ecu or that combination-module that also controls fan speed..... If you have one... Not sure if we're talking fox or sn95.
I don't think it'd be a relay really since it starts. If a relay was bad and it wasn't getting power I'm pretty sure it'd just crank and not start. I think his is a fox so no CCRM, but the CCRM is just a box with a bunch of relays in it. So if anyone with a '94/'95 has an intermittent problem the CCRM is probably not to blame.
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Sometimes when the pump is getting ready to go out,the pump check valve will cause fuel pressure to bleeddown as soon as you turn the key off.So if you turn the key on and take a reading,then turn the key off,wait 60 seconds then take another reading,a loss of more than 5psi in that 60 second period means the check valve is bad or the fpr is bad.If pressure bleeddown,as mentioned, does occur,you can verify its the pump that's at fault by pinching a soft section of the fuel return line closed,then do the same pressure test and if pressure still continues to bleeddown,that verifies the pump is bad. Keep in mind there are other things that can go out in the pump also besides the check valve.So even if the bleeddown test passes, the pump could still be bad.Make sure the iac valve is working properly too as its one of the primary causes of immediate stalling upon startup.The iac is commanded fully open by the ecu during cold starts to bypass air around the throttle blade so the engine will idle.Internal engine parts create more friction when cold.

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Make sure the iac valve is working properly too as its one of the primary causes of immediate stalling upon startup.The iac is commanded fully open by the ecu during cold starts to bypass air around the throttle blade so the engine will idle.Internal engine parts create more friction when cold.
That's a good idea and makes perfect sense! Is there a testing procedure for the IAC?

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That's a good idea and makes perfect sense! Is there a testing procedure for the IAC?
Unplug it and see if it runs better, worse, or the same. The red wire should be getting 12v w the key on
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That's a good idea and makes perfect sense! Is there a testing procedure for the IAC?

With the engine at operating temp,when you disconnect the iac harness,the engine should stall or the idle should drop slightly, depending on how far inward the throttle body idle adjustment screw is set.If the iac is just dirty,& not defective,idle will normally bounce around surging up and down. However a defective iac can also cause this,if cleaning it doesn't help.
Mechanical idle is the rpm value you set with the tb screw,with the iac harness disconnected.Computer controlled idle is the idle rpm value the ecu sets,with the iac harness reconnected.Mechanical idle should be set between 550-650rpm then the ecu will control idle above that value.672rpm is the target value the ecu looks for.If mechanical idle is set too high,the ecu might not be able to lower idle to the ecu target.

There are several tests for the iac/wiring. With the key on,the red iac wire should read 10.5-12v.Use the engine block as the ground for the multimeter.With the engine running, the white/lt blue iac wire should toggle between 12-.5v,with the meter leads between the red and white/lt blue wire terminals.If there is a short to ground in the white/lt blue wire or the iac body to internal wiring,the meter will usually not read above 1v during the w/lt blue wire test.With the harness disconnected,meter set to ohms, touch each iac terminal with a multimeter lead.The meter should read between 7-13 ohms one way then when the meter leads are reversed on the terminals,the meter should read OL or open loop,since the iac has a diode in it.This is for 89-93 & all remanufactured iac's.86-88 models had a diode inline with the iac harness instead, while 89-93 models (& all remanufactured iac's now sold) had a diode in the iac itself.If its a 86-88 iac you're testing,both test directions listed above with the meter, should read 7-13ohms.Now touch one meter lead to the iac body and touch the other lead to one iac terminal then move it to the other iac terminal.The meter should read OL or a very high ohms reading for both terminal tests.

91 LX 342ci (10.5:1)
TFS 190cc (stage ll)
GT40 (tubular) intake
Lunati 61011(.549"/.565")
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Weld ProStar XP's

Last edited by wbrockstar; 3 Weeks Ago at 08:09 AM.
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post #11 of 25 (permalink) Old 3 Weeks Ago Thread Starter
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Cool

Thanks, you guys! I installed my used Walbro FP last week and after installation I still had the starting and dying issue until it warmed up a bit. Took the car for a drive and got about 20 miles before the FP started singing like a canary and would barely run! Called AAA and towed it home. My new Walbro just came in the mail today so I'll be installing it this weekend. After that I'll give the IAC a test and clean. If that's not it I'll have to go for the remanned ECU.

A little update on the ECU. I Pulled it and took the cover off. All 3 capacitors looked good and weren't leaking. I then took it into the local electronics repair guy and he looked it over real good and said he couldn't see anything obviously wrong with it. I called Autozone and it turns out their $158 remanned ECU is now unavailable however, I can send in my ECU and their supplier will rebuild it for $235 with a 5 day turnaround time. May have to go that route.

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Quote:
Originally Posted by 90lxwhite View Post
Quote:
Originally Posted by wbrockstar View Post
Quote:
Originally Posted by evintho View Post
That's a good idea and makes perfect sense! Is there a testing procedure for the IAC?

With the engine at operating temp,when you disconnect the iac harness,the engine should stall or the idle should drop slightly, depending on how far inward the throttle body idle adjustment screw is set.If the iac is just dirty,& not defective,idle will normally bounce around surging up and down. However a defective iac can also cause this,if cleaning it doesn't help.
Mechanical idle is the rpm value you set with the tb screw,with the iac harness disconnected.Computer controlled idle is the idle rpm value the ecu sets,with the iac harness reconnected.Mechanical idle should be set between 550-650rpm then the ecu will control idle above that value.672rpm is the target value the ecu looks for.If mechanical idle is set too high,the ecu might not be able to lower idle to the ecu target.

There are several tests for the iac/wiring. With the key on,the red iac wire should read 10.5-12v.Use the engine block as the ground for the multimeter.With the engine running, the white/lt blue iac wire should toggle between 12-.5v,with the meter leads between the red and white/lt blue wire terminals.If there is a short to ground in the white/lt blue wire or the iac body to internal wiring,the meter will usually not read above 1v during the w/lt blue wire test.With the harness disconnected,meter set to ohms, touch each iac terminal with a multimeter lead.The meter should read between 7-13 ohms one way then when the meter leads are reversed on the terminals,the meter should read OL or open loop,since the iac has a diode in it.This is for 89-93 & all remanufactured iac's.86-88 models had a diode inline with the iac harness instead, while 89-93 models (& all remanufactured iac's now sold) had a diode in the iac itself.If its a 86-88 iac you're testing,both test directions listed above with the meter, should read 7-13ohms.Now touch one meter lead to the iac body and touch the other lead to one iac terminal then move it to the other iac terminal.The meter should read OL or a very high ohms reading for both terminal tests.
Evin, sorry about the thread jacking.
I tested my IAC today with a noid light like the guy did in the video. It lit up when I turned the key on, but it did not stay illuminated like the one in the video. It just came on and went back off. I probed the red wire at the plug and it read 12v (12.5 I think it was). It read a constant 12, it wasn’t “on and off” like the noid light was. I reconnected the IAC and I had someone turn the key on while I listened to the IAC body with a mech’s stethoscope and I heard a click click. Would y’all diagnose this as a working IAC or is that still TBD?
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Evin, sorry about the thread jacking.
I tested my IAC today with a noid light like the guy did in the video. It lit up when I turned the key on, but it did not stay illuminated like the one in the video. It just came on and went back off. I probed the red wire at the plug and it read 12v (12.5 I think it was). It read a constant 12, it wasn’t “on and off” like the noid light was. I reconnected the IAC and I had someone turn the key on while I listened to the IAC body with a mech’s stethoscope and I heard a click click. Would y’all diagnose this as a working IAC or is that still TBD?

Use the neg battery post or the engine block as the ground for your meter when you test the red wire voltage.Its supposed to remain at a constant/fixed voltage with the key on,since it gets its power supply from the eec relay.After that test,use the iac white/
lt blue wire as the ground for your meter.It should read a fixed voltage between .5-12 volts or a fluctuating voltage between .5-12 volts,since this wire is a ground thats cycled on/off by the ecu.When this wire is grounded,the iac valve opens.When its ungrounded,the iac valve closes.The fact that your noid light turned on for a nano second,then turned off just as fast,indicates the ecu isnt holding this ground on at startup,like its supposed to do when the engine is cold.The ecu could have an issue affecting pin 21 or the white/lt blue iac wire could be suspect.If you disconnect the iac harness and ecu harness then use a meter to test the white/lt blue wire for continuity between those two points,and you get a reading less than 5.0 ohms,that verifies the wire is ok.If its above 5.0 ohms,the wire is open.If the wire is shorted to ground,voltage will not go above 1 volt with the ecu harness reconnected.

91 LX 342ci (10.5:1)
TFS 190cc (stage ll)
GT40 (tubular) intake
Lunati 61011(.549"/.565")
QA1 Coilovers
Weld ProStar XP's
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Evin, sorry about the thread jacking.
I tested my IAC today with a noid light like the guy did in the video. It lit up when I turned the key on, but it did not stay illuminated like the one in the video. It just came on and went back off. I probed the red wire at the plug and it read 12v (12.5 I think it was). It read a constant 12, it wasn’t “on and off” like the noid light was. I reconnected the IAC and I had someone turn the key on while I listened to the IAC body with a mech’s stethoscope and I heard a click click. Would y’all diagnose this as a working IAC or is that still TBD?

Use the neg battery post or the engine block as the ground for your meter when you test the red wire voltage.Its supposed to remain at a constant/fixed voltage with the key on,since it gets its power supply from the eec relay.After that test,use the iac white/
lt blue wire as the ground for your meter.It should read a fixed voltage between .5-12 volts or a fluctuating voltage between .5-12 volts,since this wire is a ground thats cycled on/off by the ecu.When this wire is grounded,the iac valve opens.When its ungrounded,the iac valve closes.The fact that your noid light turned on for a nano second,then turned off just as fast,indicates the ecu isnt holding this ground on at startup,like its supposed to do when the engine is cold.The ecu could have an issue affecting pin 21 or the white/lt blue iac wire could be suspect.If you disconnect the iac harness and ecu harness then use a meter to test the white/lt blue wire for continuity between those two points,and you get a reading less than 5.0 ohms,that verifies the wire is ok.If its above 5.0 ohms,the wire is open.If the wire is shorted to ground,voltage will not go above 1 volt with the ecu harness reconnected.
Would the red get a constant 12v if it wasn’t grounded?
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Good info!! The new FP is going in today. I'll test the IAC after that.
90lxwhite:
No worries on the thread hijack! This forum is all about information sharing. I just wish the mods would figure out the Imgur issue.


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