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post #16 of 25 (permalink) Old 09-10-2019 Thread Starter
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OK, new FP is in and works good however, the car still does the same thing at cold start. Fires up, then dies and barely runs until it gets up to temperature. Once warmed up, it runs fine. Took it on a 90 mile cruise yesterday!
So.....I tested the IAC. First thing I did, once it was warmed up, is disconnect the IAC. The motor didn't drop any RPMs at all!
Reconnected the harness and probed the red wire with key on. I got 12.5v.
Motor running. Red meter probe to red wire, black meter probe to wht/lt blue wire. I got 4.35v - 4.65v.
Removed the IAC entirely and ohmed it's terminals. I got .012 ohms both ways.
While the IAC was off I soaked it with Brakeclean and even scubbed a little inside the ports. Reinstalled it and it does the same thing!
Please tell me it's just a bad IAC!


'93 5.0 LX ragtop - Donor car for '54 Ford Customline
'89 LX ragtop with turbo'd TC motor - Thunderstang!
'86 Turbo Coupe (daily driver)
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Quote:
Originally Posted by evintho View Post
OK, new FP is in and works good however, the car still does the same thing at cold start. Fires up, then dies and barely runs until it gets up to temperature. Once warmed up, it runs fine. Took it on a 90 mile cruise yesterday!
So.....I tested the IAC. First thing I did, once it was warmed up, is disconnect the IAC. The motor didn't drop any RPMs at all!
Reconnected the harness and probed the red wire with key on. I got 12.5v.
Motor running. Red meter probe to red wire, black meter probe to wht/lt blue wire. I got 4.35v - 4.65v.
Removed the IAC entirely and ohmed it's terminals. I got .012 ohms both ways.
While the IAC was off I soaked it with Brakeclean and even scubbed a little inside the ports. Reinstalled it and it does the same thing!
Please tell me it's just a bad IAC!
Stack a penny & a nickel and place them between the stop screw and the "arm" on the TB so it holds the TB open a little. If it idles better I think you're on the right track thinking iac.
*edit*
I think doing so kinda takes the IAC out of the equation. Will yours start with the IAC unplugged? Mine would not, but I believe it’s supposed to be able to.

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Last edited by 90lxwhite; 09-10-2019 at 07:09 PM.
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Quote:
Originally Posted by evintho View Post
OK, new FP is in and works good however, the car still does the same thing at cold start. Fires up, then dies and barely runs until it gets up to temperature. Once warmed up, it runs fine. Took it on a 90 mile cruise yesterday!
So.....I tested the IAC. First thing I did, once it was warmed up, is disconnect the IAC. The motor didn't drop any RPMs at all!
Reconnected the harness and probed the red wire with key on. I got 12.5v.
Motor running. Red meter probe to red wire, black meter probe to wht/lt blue wire. I got 4.35v - 4.65v.
Removed the IAC entirely and ohmed it's terminals. I got .012 ohms both ways.
While the IAC was off I soaked it with Brakeclean and even scubbed a little inside the ports. Reinstalled it and it does the same thing!
Please tell me it's just a bad IAC!

I assume this is the 89 lx ragtop you're working on,thats listed in your signature?? If yes,& if your meter was set on the correct ohms range,your (.012) reading indicates a defective iac valve.On 89-93 models,the oem iac or remanufactured iac should ohm out between 7-14 ohms,with the meter leads connected one way,& OL or a very high ohms reading,with the meter leads connected the other way (i.e.- reversed) since the iac has a diode inside of it on these (89-93) year models.If your meter was set on the wrong scale and your reading from earlier was really 12 ohms,instead of .012 ohms,your iac is reading in the correct range.On 86-88 models,the meter should read 7-14 ohms both ways,since the diode was placed on the iac harness instead of the iac itself on these (86-88) year models.You should test the iac for shorts too.To do that,touch one meter lead to the iac housing,touch the other lead to one iac terminal,take a reading,then move the lead to the other iac terminal and take another reading.The meter should read OL or a very high ohms reading for both tests. So to recap:

** Terminal To Terminal Test **
(86-88)= the meter should read 7-14 ohms both ways
(89-95)= the meter should read 7-14 ohms one way and OL or a very high ohms reading the other way
** Terminal To Housing Test **
(86-95)= the meter should read OL or a very high reading for both terminal to housing tests

1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's
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post #19 of 25 (permalink) Old 09-10-2019 Thread Starter
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OK, my bad! Firstly, the '89 went to that big Mustang graveyard in the sky a couple years back when some idiot rearended me at 40 mph and totalled the car. The current project is a '93 LX 5.0 ragtop...................



And, my electrical skills are abysmal at best so please bear with me. I set my meter on the lowest setting, I think, which was 2k when I ohmed out the IAC and got the .012 readings.


'93 5.0 LX ragtop - Donor car for '54 Ford Customline
'89 LX ragtop with turbo'd TC motor - Thunderstang!
'86 Turbo Coupe (daily driver)
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OK, my bad! Firstly, the '89 went to that big Mustang graveyard in the sky a couple years back when some idiot rearended me at 40 mph and totalled the car. The current project is a '93 LX 5.0 ragtop...................



And, my electrical skills are abysmal at best so please bear with me. I set my meter on the lowest setting, I think, which was 2k when I ohmed out the IAC and got the .012 readings.


Oh ok.Youre still fine as far as the testing goes that I listed in my previous post.If you would have had a 86-88 model instead,your test results would have been different than the ones you got for a 89-95 model.IOW-
86-88 should have the same ohms value regardless of which way you connect the leads.89-95 should have totally different values when you test the terminals one way then reverse the leads and test the terminals the other way.

1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's
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post #21 of 25 (permalink) Old 09-11-2019 Thread Starter
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So, I have a defective IAC, correct?

'93 5.0 LX ragtop - Donor car for '54 Ford Customline
'89 LX ragtop with turbo'd TC motor - Thunderstang!
'86 Turbo Coupe (daily driver)
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So, I have a defective IAC, correct?
It sure sounds like it.If its a oem iac from a 89-93 model or a remanufactured iac & its reading the same ohms value with the test leads connected in both directions,its defective.Only the oem iac's from the 86-88 models should read the same (7-13) ohms value in both test directions.Reasoning???
86-88 models have a diode wired into the iac "HARNESS" while 89-93 models have a diode built-in to the iac "VALVE" instead.(Reman iac's have the built-in diode too) If you're not the original owner & the previous owner installed a 86-88 oem iac,that could also make it read the same value both ways too.However if the correct iac is currently being used and your tests showed .012 both ways,the iac is definitely bad.....

**NOTE**
Is the 2k setting the lowest ohms value your meter has or does it also have a 200 or 20 ohms setting too?? If it has a lower setting than 2k,turn the knob to that setting and retest your iac in both directions.You want the meter to be set on the ohms value that is slightly higher than the value you expect to see on the component under test.
Example: if the component is supposed to have a value of 100 ohms when tested,you'd want to set your meter on the 200 ohms setting (if your meter has that setting).Since the value youre expecting to see on your iac is between 7-13 ohms,you'd want to set the knob on the 20 ohms setting (if your meter has that setting) since thats slightly above the value of 7-14 ohms you're looking for.My meter starts at 200.

Another test for the iac is to let the engine idle at operating temp,turn the ac on max & listen to how the idle reacts.When the ac is turned on,the ecu commands more duty cycle to the iac to compensate for the additional load of the compressor,so if the idle drops & it doesn't immediately recover, the iac is indeed suspect,as long as the iac wiring is ok.

1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's

Last edited by wbrockstar; 09-11-2019 at 04:53 PM.
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post #23 of 25 (permalink) Old 09-11-2019 Thread Starter
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As you can see from the pic, I have 200k, 20k and 2k followed by 200 with the 'beep' symbol. I tested it on the 2k graduation and got the .012. I tried the 200 also and got a constant 'beep' and I think the reading was 7. And yes, I tested both ways, red lead on one terminal and black lead on the other then, I swapped the leads and got the same readings.

I noticed this morning when I started it up, the problem seems to have gone away! It fires and runs about 1100 rpm then drops after a few seconds to about 800. I guess the cleaning helped it. I do notice however, that once it gets to 800, there's a surging idle. Probably 100 rpm in each direction. I'll bet that has something to do with that old, dirty IAC! Cheapest Motorcraft I've found seems to be RockAuto @ $61 shipped. I think I'll just bite the bullet and go for a new one. wbrockstar, you are a wealth of knowledge! Thank you very much! Thanks to others who chimed in, also!

'93 5.0 LX ragtop - Donor car for '54 Ford Customline
'89 LX ragtop with turbo'd TC motor - Thunderstang!
'86 Turbo Coupe (daily driver)
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Quote:
Originally Posted by evintho View Post
As you can see from the pic, I have 200k, 20k and 2k followed by 200 with the 'beep' symbol. I tested it on the 2k graduation and got the .012. I tried the 200 also and got a constant 'beep' and I think the reading was 7. And yes, I tested both ways, red lead on one terminal and black lead on the other then, I swapped the leads and got the same readings.

I noticed this morning when I started it up, the problem seems to have gone away! It fires and runs about 1100 rpm then drops after a few seconds to about 800. I guess the cleaning helped it. I do notice however, that once it gets to 800, there's a surging idle. Probably 100 rpm in each direction. I'll bet that has something to do with that old, dirty IAC! Cheapest Motorcraft I've found seems to be RockAuto @ $61 shipped. I think I'll just bite the bullet and go for a new one. wbrockstar, you are a wealth of knowledge! Thank you very much! Thanks to others who chimed in, also!

You can try doing a base idle reset now or wait until you get/install the new iac. Anytime the base idle is set above 650rpm,w/ iac harness disconnected, surging issues can begin.This may/may not be your issue.The following copied/pasted instructions should help.


Summarized/Corrected Base Idle Reset version
1) Clean TB with carb cleaner and nylon brush (if needed) Engine off, fully open TB blade and spray/brush TB until all gunk and oil residue is cleaned.


2) Allow to dry, or close TB blade and start engine until it clears all carb cleaner fumes/liquid.

3) Remove the spout plug,just to make sure the ecu doesn't have any input on timing during the reset procedure.Let engine idle...if it doesn't, increase idle speed via TB stop screw until engine stays idling on its own.

4) Disconnect IAC valve, if engine dies repeat 3 with IAC disconnected.

5) Set idle speed to the lowest setting possible between 550-700 rpms with IAC disconnected. It is key to use the lowest possible to prevent idle surge, rolling, etc once the IAC is connected as follows.

6) Turn engine off, and reconnect IAC

7) With Ign On Eng Off, check TPS voltage output....if it's between .7vdc - 1.0vdc, it's OK. This voltage check should be done between the Green and Black wires at the TPS side connector as shown in the pic below....(-) terminal of the DVOM on the Black wire and (+) terminal on the Green wire.

8) Disconnect battery (-) for 3-5 minutes.

9) Reconnect battery, start engine, allow it to idle for 2-4 minutes to confirm setup,

10) If idle speed falls too low or stalls, increase idle speed via the TB set screw a little at a time.

11) Turn engine off for 20 seconds, re-start engine and repeat 9 - 11 if required.

12) Reconfirm TPS output is within .70 - 1.0vdc range.......you're done!...

..no need to reset ECM KAM (reset computer) b/c the TPS minimum value used for idle control, is automatically reset by the EEC-IV every time the ign is cycled on-off for 20 seconds-on, and as explained in the next post

13) Allow a 10-20 minute "relearning" period under normal driving conditions (drive cycle).....now you're done.

1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's

Last edited by wbrockstar; 09-13-2019 at 11:30 AM.
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post #25 of 25 (permalink) Old 09-12-2019 Thread Starter
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Great info! I'll try that.

'93 5.0 LX ragtop - Donor car for '54 Ford Customline
'89 LX ragtop with turbo'd TC motor - Thunderstang!
'86 Turbo Coupe (daily driver)
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