Contour Enthusiasts Group Archives Forums General Troubleshooting 2.5L Duratec bogs at 4500 RPM!

I have a `98 Mystique LS w/2.5L Duratec and about 145K on the odometer. I purchused the car about 20K ago knowing it had a few problems, all of which I have solved, but this one.


Ok on to the problem!


With the car in park, you can bring the engine RPMs to exactly 4500 and it begins to bog! By bog I mean the engine actually sounds as though it shuts off then surges back to life (Funny how hard it is to explain a problem with a car on the net!!). Also, while traveling on the highway, the car is great! Using the overdrive, to get the extra boost, when needed to pass. When it comes to excellerating from say, 45 to 80 (merging into traffic on the interstate) the engine surges enough to kill any chance to gain any power. As for the bog at 4500 mentioned earlier, once in gear, the bog is gone at that RPM and replaced by the simple surge while getting on the gas. Also, while under the hood, if you snap the throttle open you can hear a rush of air at the rear of the engine bay (by the firewall).

About three weeks ago, my CEL came on so I went by the local Autozone and it was a P0420. I soon found that figureing out which O2 sensor to change was enough for me not to change it. You really need a scanner to figure that out! Most aftermarket parts outlets only carry the upstream and not the downstream, the dealer wanted $130 for either of them and told me that the part was nonreturnable for any reason. I will add that the car acted no differently while or after the CEL came on. The light came on 2 times and has not come back on since. I did pull both downstream just to visually check them, but found nothing out of the ordinary.


So what have I done so far to address this...?

I have new plug wires (Bosh) and new Autolite Platinums. New Frame air filter. Have run several bottles or fuel injector cleaner through the system. Checked all wiring harnesses and vaccum lines for breakage/wear. I have also checked the IMRC for proper operation. I have removed each of the downstream O2 sensors, seperately, and rev'ed the engine and it still bogs at 4500 - so I don't fill that it is soley the cat.

Any input would be of much help.

Thanks,
Phillip

P.S. Just moved to NE MS about a year ago and love the twisty roads up here! I find myself sounding like a navigator (Like the ones in WRC) on familiar roads alot and this Mystique loves every turn.

The Bog at 4000-4500 in neutral is normal. The computer prevents overevving in neutral/park by cutting fuel and spark at that RPM. When in gear, the limiter doesn't kick in till redline. Also, you are speed limited to 112 MPH (The speedo may lie and say your going faster, but the computer knows and you can't exceed the 112 MPH.

When you hit 4000 RPM (3600 RPM or so at Wide open throttle), the secondary intake butterflies open. That is the power surge you feel. Ain't it great?!! I love that surge. The secondaries are controlled by the computer too.

We have dual tuned intake runners, If you look at the intake, you see 12 tubes. The long ones are used at low (<4000) RPM and the short ones are added above that. That is when you get the surge. The runner is tuned for best performance for the engine at high RPM. The system gives us better low end torque and better high end power.

From the sound of it, you don't have any problems.

Your code is for the Bank 1 Catalyst Monitor, meaning you need to replace one/both of:

• Downstream O2 sensor - the one in the Y pipe, very easy to get at
  
• Rear bank exhaust manifold - a very hard thing for a home mechanic to do, not cheap to pay for (labor and parts)
  

So, go to www.oxygensensors.com and get what you need for $50; replace the O2 sensor and see if the code comes back within a week or two. If it never comes back (reading reading same code again) then you are good. If it comes back, you need a new manifold and you are screwed unless you are under the 8/80 factory emissions warranty (which you're not) and it's free (which it won't be). Get used to the Check Engine Light if it comes to that.

Originally posted by unisys12:

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With the car in park, you can bring the engine RPMs to exactly 4500 and it begins to bog! By bog I mean the engine actually sounds as though it shuts off then surges back to life (Funny how hard it is to explain a problem with a car on the net!!).

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As was mentioned above, there is a limiter (aka governer) on the engine while in Park.

The following is from the Ford TSP:

Engine RPM/Vehicle Speed Limiter

The Powertrain Control Module (PCM) will disable some or all of the fuel injectors whenever an engine rpm or vehicle overspeed condition is detected. The purpose of the engine rpm or vehicle speed limiter is to prevent damage to the powertrain. The vehicle will exhibit a rough running engine condition and the PCM will store a Continuous Memory DTC P1270. Once the driver reduces the excessive speed, the vehicle will return to the normal operating mode. No repair is required. However, the technician should clear the PCM and inform the customer of the reason for the DTC.

Excessive wheel slippage may be caused by sand, gravel, rain, mud, snow, ice, etc. or over revving of the engine while in neutral or while driving.

Thanks guys, you have really placed my mind at ease.

PA, thanks for the link. I will check it out.

After the responses above, I am really thinking of rotting out the precats, removing the resisonater and replacing the downstream bank 2 O2 sensor and see what happens. That will make for one heck of a fun weekend! It does look like the bank 1 exhaust manifold would be a real bugger to get to, but I will just have to go slow, be careful and carry a bucket of WD40 in my pocket.

Any thoughts or "be sure to look out for"'s on these subjects.

Phillip

If you are considering guting the pre-cat, you'll need to do both the front and rear bank. If you gut the pre-cats, you'll need to get mil eliminators and plug them in where the downstream sensors plug in. (No more downsteam sensors needed ) Mil eliminators are like $20 and there are ads for them on some of the mustang and cougar sites. To gut the front pre-cat, you can drill a hole in the bottom of the elbow to get a tool inside to break up the brick. I think Brad Noon has already cleaned and smoothed out manifolds he sells on an exchange basis.