Hello,

The past year and a half I have been trying to track down a problem with my '98 Explorer V6 OHC motor.

I have replaced all ignition components, Most fuel components, and all emmisions sensors with the exception of the Downstream O2 sensors. The engine is clear of all Vacum leaks and all parts are cleaned regularly I.G throttle body, MAF etc.

The symptoms:

• Weak Idle around or under 500rpm, park and drive
• Sluggish, bogging acceleration up to but no at WOT
• Occasional hard Shudder in Idle
• Poor Gas milage, can almost see the gauge drop when hard acceleration is attempted and motor bogs.
• No Check engine light.
• No obvious problems mechanics can find.
• Vehicle runs much better in very cold weather.
• Runs awful in hot weather.
• Problem occurs mostly in the mid range power band, specifically around the 3000rpm range.

It seems these are all the symptoms of a bad O2 sensor. A Ford Dealer half arsed OBD scanned them and they were switching, but who know how fast.

I think this coulc have happened.

A year or so ago I was cleaning my K&N filter and due to ignorance, heavily oiled both sides of the filter and stuck it back in the housing. It seems some of the oil stuck on the MAF sensors and became baked on ruining it. I did not have the money to replace it, so I had to drive the vehicle til I could earn enough.

As you can guess without the correct MAF readings vehicle ran very rich for five months of hard driving until I had enough money to buy a new MAF. I wonder in this time period if the unburned fuel could have ruined the O2 sensors?

I had the tow upstream replaced but it did not solve the problem. Looking for advice, maybe I need to replace the two downstream ones as well?

 

The rich running could have plugged the cat. converter. Should set the cel for cat efficiency but may not - these systems are not perfect. Also possibly intake manifold gaskets (o'rings).
Just my 02 (pun)

Scucci

 

Thanks for the suggestions. Had the intake smoked over the weekend, no leaks. Also had the O rings replaced just for the hell of it.

How would I know if the cat is bad? No CEL codes.

 

try this... remove both upstream oxygen sensors (in front of the converters).. unplug them and everything and set them aside.

Go drive the car... yes it'll be VERY LOUD and you'll set O2 sensor codes in the process, but feel for a performance improvement. If you have restricted converters you should notice a considerable power gain by allowing the exhaust to flow better (by having two large holes in front of the converters).

I wouldn't recommend driving it very long though.. I get worried about hot exhaust gas blowing up onto wire harnesses, etc... and also you might get a ticket haha.

Good luck.

 

Testing A Catalytic Converter
Unlike the old-fashioned muffler, the catalytic converter cannot be considered in working order simply because there are no rust holes.
Catalytic converters are fitted on cars using unleaded petrol to clean exhaust gas. Apart from shaking it to see if it rattles, the Motor Vehicle Repair Industry Authority says there are some tests which must be done by a licensed repairer with instruments.
Connect a vacuum gauge to a manifold vacuum source on the motor and rev the engine to 2500 rpm. The vacuum gauge should drop momentarily and then return to normal. If it drops and does not return, then the converter may be damaged and pieces of catalyst can block the exhaust system.
Or, use a pyrometer which is a high temperature thermometer by placing the probe on the inlet of the converter, about 50 mm in front of the unit and read the temperature. Then place the probe on the outlet side of the converter, about 50 mm to the rear of the converter, and again read the temperature. The outlet should be at least 40 degrees Celsius hotter than the inlet. If there is no difference, the converter is faulty and should be replaced.
Next use an emissions analyser. Check that the carbon monoxide and hydrocarbon readings are close to normal. Enrich the mixture with the addition of propane to the inlet air. The converter should hold the CO and the HC readings down to normal. If not, then the catalytic converter may be faulty.

 

Thanks for the great replies. I am going to give it a shot.

 

Also, chunks of the convertor honeycomb material may have broken off and plugged up the exhaust/muffler. This happened on a ranger i had a few years back. Best bet is to take it to a muffler shop and have them take the exhaust pipe off the back of the cat and check it that way, they can also check for obstructions in the pipe at the same time.