Low Acceleration. (running 4.56 Gear Though)

98Mustang2017

Active Member
May 3, 2017
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#1
4.56 gears in the differential... just replaced the fuel injectors and spark plugs. wires are good too. Coil is good as well. Even the MSD StreetFire coil made no difference.
MAF is clean. No vacuum leaks, i tested the manifold vacuum. it was higher than i thought it'd be and i'm okay with that... i think. At around 25" .
Fuel pressure regulator is good. fuel pump primes, maybe i should see if it holds good pressure under load.

So 1st gear does well but not enough power spin the wheels any time i want. Like i'll floor it, it'll barely go, then at about, idunno, 3500rpm it will spin the tires a tad bit. i guess it's an intermittent power loss here.

2nd gear i've heard you could move from a dead stop with this gear in 2nd. With whatever problem is going on i would have to floor it and the car would be bucking like a bull the whole time from a dead stop.
I have slightly spun tires in this gear switching from 1st in the middle of first gear but it does it when it wants to.

3rd gear? takes me too long to get there. I might as well be using 2.73 .
I've only made a pull to 69mph.

So what could be the problem ?
 
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Neuron

Mustang Master
Nov 6, 2016
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#2
While you are pondering,I would replace the fuel filter.
 

wmburns

SN Certified Technician
Aug 14, 2009
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#3
Can you confirm?:
  • the model year of the mustang in question (assuming 1998).
  • manual or automatic?
  • is the speedometer accurate?
  • The stall speed of the torque converter
  • the RPM range that the transmission shifts
Remember the 4R70W is an electronically controlled transmission. Consider that IF the speedometer is not accurate this can affect the gear selection.
 
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98Mustang2017

Active Member
May 3, 2017
113
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38
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#4
While you are pondering,I would replace the fuel filter.
I really should check it out. Who knows if the guy on craigslist even installed the new one correctly. If one was even installed at all. Can fuel filters run with a backwards filter ?

Can you confirm?:
  • the model year of the mustang in question (assuming 1998).
  • manual or automatic?
  • is the speedometer accurate?
  • The stall speed of the torque converter
  • the RPM range that the transmission shift
Remember the 4R70W is an electronically controlled transmission. Consider that IF the speedometer is not accurate this can affect the gear selection.
Oops, left out the valuable details.

Vehicle is a standard transmission '95 Mustang base Coupe 3.8l v6

Speedometer is NOT working. I don't know if it was working prior to the gear change or not. i just bought it and installed the gear. i may have stretched the speedometer wire when i let the driveshaft hang 6inches from the car.

I'm going to check out how to test o2 sensors on Google . Could be the crank sensor or throttle position sensor. i dont know. TPS does alternate voltage depending how far it is open last time i checked .
 

imp

Mustang Master
Jul 13, 2017
531
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#5
I really should check it out. Who knows if the guy on craigslist even installed the new one correctly. If one was even installed at all. Can fuel filters run with a backwards filter ?



Oops, left out the valuable details.

Vehicle is a standard transmission '95 Mustang base Coupe 3.8l v6

Speedometer is NOT working. I don't know if it was working prior to the gear change or not. i just bought it and installed the gear. i may have stretched the speedometer wire when i let the driveshaft hang 6inches from the car.

I'm going to check out how to test o2 sensors on Google . Could be the crank sensor or throttle position sensor. i dont know. TPS does alternate voltage depending how far it is open last time i checked .
Check fuel pressure, idling and under slight throttle. imp
 

98Mustang2017

Active Member
May 3, 2017
113
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#6
Check fuel pressure, idling and under slight throttle. imp
That's what i'm thinking about doing. The only thing fuel-wise left to test is the fuel pump while under load and idle.
Guess it's time to make a trip back to the loan-a-tool place.
 

98Mustang2017

Active Member
May 3, 2017
113
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#7
Just checked out the o2 sensors visually and i nearly want to guarantee the front ones are bad.
The rear ones are different with a blue wire and the front ones look OEM.
I'm going to run with them unplugged and see what happens. That would explain the popping i hear if it's running lean.
Guess we'll find out soon.
 

imp

Mustang Master
Jul 13, 2017
531
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#8
Just checked out the o2 sensors visually and i nearly want to guarantee the front ones are bad.
The rear ones are different with a blue wire and the front ones look OEM.
I'm going to run with them unplugged and see what happens. That would explain the popping i hear if it's running lean.
Guess we'll find out soon.
In the 4-sensor system with 2 cats, the front O2s are indeed different from the rear. On OEM parts, even the connectors are "keyed" differently, to prevent wrong usage. Surprise, aftermarket sensors I bought have NO keying lugs, so mistakes could be made. I figger they do this in order to service several different makes or applications with the same part. imp
 

98Mustang2017

Active Member
May 3, 2017
113
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#9
Well i removed the o2 sensor fuse under the dash( if i pulled the right fuse diagram. #10 fuse) and it made no difference in acceleration.
Sooooo.. fuel testing will ensue.
 

98Mustang2017

Active Member
May 3, 2017
113
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#10
Alright guys. Back with some results.
My fuel pressure regulator is new and still good condition. Fuel injectors are new as well.
Fuel pressure is good and is at sustained pressure while running until revved.
With the engine running and the FPR vac. line connected the pressure is around 30psi. Disconnected it reads 40psi.

I'm thinking about moving on to the clutch. I do have a slight rear main seal leak anyways. Should i go OEM clutch or stage 2 ? or 3 ? After i replace the main seal of course.
 

98Mustang2017

Active Member
May 3, 2017
113
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#11
Forgot to mention the last owner had the fuel filter on backwards ! i can actually hear the fuel pump prime now after i reversed it.
 
Sep 26, 2003
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#12
Are you throwing a code? That's the FIRST thing you should check!
Go to Advance Auto and they will check for free w/a scanner tool.
 

98Mustang2017

Active Member
May 3, 2017
113
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#13
Are you throwing a code? That's the FIRST thing you should check!
Go to Advance Auto and they will check for free w/a scanner tool.
I need to replace my dash lights. they were burnt out when i got the car so i left them out so i could get inspection in case something crazy threw a code. But i just recently found out it isnt hard at all to replace the lights so i'll run light diagnostics myself sometime. Although the OBD2 port doesn't have power going to it (v6 model)
Either way i'm dropping the transmission and replacing the seal. Fingers double crossed clutch and seal is the issue.
 

98Mustang2017

Active Member
May 3, 2017
113
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#14
Just thought i would update:
My car is actually "slower" because it spins the tires way too easily LOL.

4.56gears and stock street tires = not a good combination.
I'm still adding a turbo for the top-end though. :)
rear_TT.jpg