1998 Mercury Mystique Bucks while driving

Tiny
BRIAN_T
  • MEMBER
  • 1998 MERCURY MYSTIQUE
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 128,000 MILES
We will drive the car for around a 10 mile drive and when we are coming back on the highway it will start to buck like crazy. It will keep constantly doing it over and over again. Its almost like a hard slamming hesitation. We have no check engine light on although we did a while back which eventually turned off. Also noticed that the rpms will fluctuate up and down from 1500 and 2000 and will eventually settle at 1000. I tried cleaning the maf sensor with cleaner and it seems to help prevent it most of the time. Now it does it less often than before, but the car still has fluctuating rpms at start up. Lot of people are saying it could be a catalyctic converter, but wouldn't the check engine like come on? Also heard it could be a fuel pump issue.

Also we took it to the place we bought the used car and they were saying they could try a transmission flush, which I was skeptical about.
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Wednesday, September 29th, 2010 AT 1:24 PM

11 Replies

Tiny
BMRFIXIT
  • EXPERT
Failed or partial clogged catalytic converter may not always turn on check engine light on
have to do an exhaust back pressure test

suspect fuel pump
check fuel pressure and do a fuel flow test
Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and increase engine speed to 2500 RPM. If engine will not start, cycle ignition switch several times. Measure and record fuel pressure.
Pressure should be 45-60 PSI

check for codes
if tranny issues codes should tell
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Wednesday, September 29th, 2010 AT 6:55 PM
Tiny
BRIAN_T
  • MEMBER
Check engine light just came on today and the codes stated that the catalytic converter back pressure is low. Does that mean it would be the catalytic converter for sure or could the problem that were having still be caused by something else?

I'll go ahead and try to do a fuel pressure check. I would need to buy fuel pressure gauge then right?

Also forgot to say when its about to buck the rpms will go up and it will slam
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Wednesday, September 29th, 2010 AT 10:12 PM
Tiny
BMRFIXIT
  • EXPERT
Check with your local part store as they some time let you rent it instead of buying new fuel pressure gauge

what was the code # that you have let me have it so I can look it up for you
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Thursday, September 30th, 2010 AT 6:13 AM
Tiny
BRIAN_T
  • MEMBER
We just got the code to come on again and it is P0430.
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Thursday, October 14th, 2010 AT 3:05 PM
Tiny
BMRFIXIT
  • EXPERT
P0430: Check Possible Cause Of Misfire

DTC P0430 indicates bank 2 catalyst system efficiency is below minimum requirement. Possible causes are:
Damaged exhaust system component.
Faulty HO2S.
Oil contamination.
Cylinder misfire.
Fuel pressure too high.
HO2S sensor improperly connected.

Faulty ECT sensor.
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Thursday, October 14th, 2010 AT 8:05 PM
Tiny
BRIAN_T
  • MEMBER
I was actually going to try and change the spark plugs on it to see if the bucking is caused by a misfire. I've changed spark plugs on 4 cylinder engines but not v6's. I will try to see if I can change it myself, to save on money.

Seems like the simplest thing to fix and cheapest.
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Thursday, October 14th, 2010 AT 9:07 PM
Tiny
BMRFIXIT
  • EXPERT
Rear plugs is a bit more work than the front
let me know how it goes
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Friday, October 15th, 2010 AT 1:30 PM
Tiny
BRIAN_T
  • MEMBER
Changed all the spark plugs successfully. Made sure I kept the right firing order, but still seems to run rich though. One spark plugs was wet with engine oil on it.

Should I get the codes cleared and see if it comes on again?

Do you think the bucking/hesitating/slamming we get is related to the check engine light?
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Friday, October 15th, 2010 AT 5:53 PM
Tiny
BMRFIXIT
  • EXPERT
Check for pending codes before clearing the codes if any write it down
clear and give it a road test
use a vacuum gauge and take reading at idle and at 1500
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Friday, October 15th, 2010 AT 6:22 PM
Tiny
BRIAN_T
  • MEMBER
Just had another issue where the rpm's and speedometer would just rocket up there. Went to advanced auto and they said it was a code for a bad speed sensor. Forgot to get the code number but will get it if you need it.

Should I go ahead and replace the speed sensor? Been trying to find out how to find the speed sensor in the car, but unsure on where it would be. Is it a pretty simple thing to repace the speed sensor? Just wondering on the location of it?
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Sunday, October 24th, 2010 AT 5:35 PM
Tiny
BMRFIXIT
  • EXPERT
Codes should help for sure
this is what i have on speed sensor

Vehicle Speed Sensor (VSS)

Material MERCON Multi-purpose automatic transmission fluid MERCON XT-2-QDX

Removal

Raise and support the vehicle. .
Remove the vehicle speed sensor (VSS) cable.
Disconnect the VSS electrical connector.

Remove the VSS bolt and the VSS.

Remove driven gear.
Remove driven gear retainer.

Inspect the speedometer driven gear retainer O-ring seal for nicks and cuts, install a new as necessary.

Installation

Install the driven gear.
Coat the driven gear bore with Motorcraft MERCON Multi-purpose automatic transmission fluid or equivalent meeting MERCON specification.

To install, reverse the removal procedure.


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Sunday, October 24th, 2010 AT 6:34 PM

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