Mustang 1995 GT - RPM fluctuating while driving and surging - Ford Mustang Forum
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post #1 of 12 (permalink) Old 4 Weeks Ago Thread Starter
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Unhappy Mustang 1995 GT - RPM fluctuating while driving and surging

Hello guys, so I stumbled across this problem and it's been a riddle, since I just took the car to my local mechanic and we were unable to identify the problem.

Here's what's going on: The CHECK ENGINE light is ON and after a couple of minutes driving, the RPM needle goes crazy and the car starts to shake a lot (surging). The exhaust even pops sometimes (backfire?) and all of this happens only while driving, never during idle. Speaking of idle, after a short drive the RPM won't fluctuate at idle, but will hang at a bit over 1000 RPM's and the engine sounds a bit weird. When the car is cold it idles fine (around 650 RPM's). Also, the car even died while I was driving, even though it was a single time and at a low speed. Anyway, it shakes A LOT (as you can see in the video). Spark plug/spark plug wires are good, fuel injectors are also good. We put a new ICM but the problem persists.

I was thinking maybe IAC? MAF? something on the PCM? Thanks in advance, here's the link to the video:


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post #2 of 12 (permalink) Old 4 Weeks Ago
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1st check for codes then follow the diagnostic procedures...……..


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1st check for codes then follow the diagnostic procedures...……..
Already did it, no codes shown. I honestly don't know what to do anymore.
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post #4 of 12 (permalink) Old 4 Weeks Ago
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Which connector did you try to pull codes from? The one under the hood, or under the dash?

1995 Mustang GT (5.0), daily driver
1995 GMC C2500 (5.7), alternate daily driver
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Which connector did you try to pull codes from? The one under the hood, or under the dash?
Both, to no avail.
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Did you use a scanner or did you use the jumper wire method instead?? If it was a scanner,use the jumper wire method.The link at the end of this paragraph details how to run the tests & how/where to connect the jumper wire.You should at least get the "system pass" (111) code,if the test initiated & ran,but no codes were present.If you cant get the test to begin at all,a wiring or ecu issue could be present.This can cause the test to not initiate when using a scanner.This is why the jumper wire method is oftentimes better,just in case something is amiss like I mentioned above.So go ahead & try the diy method now.Connect the jumper wire between the STI terminal and the SIGNAL RETURN terminal then turn the key on & the test should begin.If the test runs,count the flashes,write the codes down then post them.If you turn the key on & nothing happens,move the jumper wire from the SIGNAL RETURN terminal to the negative battery terminal then try to initiate the test again.It should work now (This connection method is shown in the diagram below at the bottom right corner) If the test doesn't run when connected between the STI and SIGRTN terminal,but it runs when connected between the STI & neg battery terminal, the wiring and ecu need to be checked.

DIY KOEO/KOER/Cylinder Balance Self-Test Procedure



**THINGS TO DO B4 YOU RUN TESTS**

1) Engine must be at operating temp
2) Transmission in P (aod) N (manual)
3) Floor the clutch pedal during tests
4) AC is turned off
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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; 4 Weeks Ago at 07:47 PM.
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Run a cylinder balance test. Use a scanner or paper clip in the OBD port under the hood. It’ll let you know which one if any cylinders aren’t firing.
https://www.stangnet.com/mustang-for...e-test.695039/

*edit*
I just read where you have a check engine light on. Are you sure you stuck it in the right hole? The port under the dash isn’t hooked up to anything. There’s another under the hood.

Last edited by 90lxwhite; 4 Weeks Ago at 09:53 PM.
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Originally Posted by 90lxwhite View Post
Run a cylinder balance test. Use a scanner or paper clip in the OBD port under the hood. It’ll let you know which one if any cylinders aren’t firing.
https://www.stangnet.com/mustang-for...e-test.695039/

*edit*
I just read where you have a check engine light on. Are you sure you stuck it in the right hole? The port under the dash isn’t hooked up to anything. There’s another under the hood.
Yes, but I think I found out the problem, my computer is kinda "messy". Is it possible that the computer can cause this? Cause everything else seems fine. Think I gonna get a new one.
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Quote:
Originally Posted by Paulo Fli View Post
Yes, but I think I found out the problem, my computer is kinda "messy". Is it possible that the computer can cause this? Cause everything else seems fine. Think I gonna get a new one.
What do you mean by messy?? Covered with corrosion at harness connection, external case corrosion or internal board corrosion?? Is there a moisture leak of somekind??

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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Cool

Quote:
Originally Posted by wbrockstar View Post
Did you use a scanner or did you use the jumper wire method instead?? If it was a scanner,use the jumper wire method.The link at the end of this paragraph details how to run the tests & how/where to connect the jumper wire.You should at least get the "system pass" (111) code,if the test initiated & ran,but no codes were present.If you cant get the test to begin at all,a wiring or ecu issue could be present.This can cause the test to not initiate when using a scanner.This is why the jumper wire method is oftentimes better,just in case something is amiss like I mentioned above.So go ahead & try the diy method now.Connect the jumper wire between the STI terminal and the SIGNAL RETURN terminal then turn the key on & the test should begin.If the test runs,count the flashes,write the codes down then post them.If you turn the key on & nothing happens,move the jumper wire from the SIGNAL RETURN terminal to the negative battery terminal then try to initiate the test again.It should work now (This connection method is shown in the diagram below at the bottom right corner) If the test doesn't run when connected between the STI and SIGRTN terminal,but it runs when connected between the STI & neg battery terminal, the wiring and ecu need to be checked.

DIY KOEO/KOER/Cylinder Balance Self-Test Procedure



**THINGS TO DO B4 YOU RUN TESTS**

1) Engine must be at operating temp
2) Transmission in P (aod) N (manual)
3) Floor the clutch pedal during tests
4) AC is turned off
Is that procedure the same for a '93? If so, I'm gonna give it a shot. I have pretty much the same problem and cannot nail it down.

'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
Quote:
Originally Posted by wbrockstar View Post
Did you use a scanner or did you use the jumper wire method instead?? If it was a scanner,use the jumper wire method.The link at the end of this paragraph details how to run the tests & how/where to connect the jumper wire.You should at least get the "system pass" (111) code,if the test initiated & ran,but no codes were present.If you cant get the test to begin at all,a wiring or ecu issue could be present.This can cause the test to not initiate when using a scanner.This is why the jumper wire method is oftentimes better,just in case something is amiss like I mentioned above.So go ahead & try the diy method now.Connect the jumper wire between the STI terminal and the SIGNAL RETURN terminal then turn the key on & the test should begin.If the test runs,count the flashes,write the codes down then post them.If you turn the key on & nothing happens,move the jumper wire from the SIGNAL RETURN terminal to the negative battery terminal then try to initiate the test again.It should work now (This connection method is shown in the diagram below at the bottom right corner) If the test doesn't run when connected between the STI and SIGRTN terminal,but it runs when connected between the STI & neg battery terminal, the wiring and ecu need to be checked.

DIY KOEO/KOER/Cylinder Balance Self-Test Procedure



**THINGS TO DO B4 YOU RUN TESTS**

1) Engine must be at operating temp
2) Transmission in P (aod) N (manual)
3) Floor the clutch pedal during tests
4) AC is turned off
Is that procedure the same for a '93? If so, I'm gonna give it a shot. I have pretty much the same problem and cannot nail it down.
Yes.
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post #12 of 12 (permalink) Old 4 Weeks Ago
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Quote:
Originally Posted by evintho View Post
Is that procedure the same for a '93? If so, I'm gonna give it a shot. I have pretty much the same problem and cannot nail it down.
Yeah its the same procedure.Only difference is the location of diagnostic port and the # of digits output for each code depending on which year/model

86-93 Mustangs/Pre 91 Fords= 2 digit
94-95 Mustangs/Post 91 Fords= 3 digit

(Diagnostic Port Location)
86-93= driver strut tower
94-95= passenger strut tower

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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