Car Has Low Rpm At Highway Speeds

apothicca

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Dec 30, 2016
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so I have seen a lot of threads about cars with low RPMs not accelerating and it is usually the Catalytic converter from what I have seen.

But my car gets to 55mph fine, but after that the rpms go back down to what it has during idle, around 700-1000.

If I floor it, it takes a second or too, and then it suddenly lurches forward. What is going on here? Any ideas? Thanks
 
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wmburns

SN Certified Technician
Aug 14, 2009
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I give up. What Model year Mustang are we dealing with?

Do you think that maybe the DTC codes have something to do with the lack of performance?

IMO the DTC code that should be attacked first is the P0340. Anytime the PCM calls out a "circuit malfunction" think there's something so wrong with the circuit that the PCM is unable to perform trouble shooting. One of the first things to look for is a wiring harness fault.

But.....it happens that sometimes a bad alternator diode can "cause" a false bad cam sensor signal. For this reason I always suggest starting every trouble shooting session with a review of the battery and charging system.


>>>From Ford manual
P0340 - Camshaft Position (CMP) Sensor Circuit Malfunction

The test fails when the PCM can no longer detect the signal from the CMP sensor.

  • CMP circuit open
  • CMP circuit short to GND
  • CMP circuit short to PWR
  • SIG RTN open (VR sensor)
  • CMP GND open (Hall effect sensor)
  • CMP misinstalled (Hall effect sensor)
  • Damaged CMP sensor shielding
  • Damaged CMP sensor
  • Damaged PCM
Harness routing, harness alterations, improper shielding, or electrical interference from other improperly functioning systems may have intermittent impact on the CMP signal

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-forums/resources/howto-perform-charging-system-voltage-drop-test.56/
 
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apothicca

Member
Dec 30, 2016
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it's a 1995 v6, I tried the test and never got a negative reading, it was also 14.5-6 volts no matter if the lights and fan were on, or the radio. Revving it also made no difference.

I don't know if they do since I think it had these codes for awhile, but it really only started to get driven a week ago. And then lack of performance was only really noticeable yesterday.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
soooooooo. What have you done to rule out a wiring fault to the cam sensor? If no wiring fault found then likely it's time to consider a cam sensor and shaft replacement.
 

apothicca

Member
Dec 30, 2016
56
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soooooooo. What have you done to rule out a wiring fault to the cam sensor? If no wiring fault found then likely it's time to consider a cam sensor and shaft replacement.
Is the wiring thing the alternator check? I haven't looked at it other than that I've been busy with school and work.

I figured out it's the transmission, if I turn off overdrive it works fine. So I checked the fluid and it was a little low, added fluid and now it is running fine. I will have to save up some money since I think the transmission might be on its way out. I'd rather fix this car then get another one that has an unknown history.

I'm debating on doing a fluid change or leaving it as is since I've heard it is bad to do it on transmissions with a lot of miles.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
If your transmission level is soooooo low that it's slipping because of low level than continued operation will only harm the transmission. Short sighted not to address (IMO).

Likely you have more than one problem. The cam related DTC isn't being caused by the low transmission fluid level.

The other DTC codes are usually related to a vacuum leak.
 

apothicca

Member
Dec 30, 2016
56
3
8
21
If your transmission level is soooooo low that it's slipping because of low level than continued operation will only harm the transmission. Short sighted not to address (IMO).

Likely you have more than one problem. The cam related DTC isn't being caused by the low transmission fluid level.

The other DTC codes are usually related to a vacuum leak.
Well, it wasn't that low, but barely any was on the stick. I imagine the transmission being on it's way out now after 20 some years is not uncommon, so I am saving up to have it serviced (rebuilt or replaced, probably rebuilt cause $$$)
 
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