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Discussion Starter #1
I've got a new "to me" 2002 PT touring, auto 2.4L. 104k easy miles. Run and drives great, gets poor fuel mileage (16-17MPG). no codes stored, has fuel trim for downstream 02 sensor pegged at 99.03%, which intermittently, flashes 0 and then goes back to 99.03% (max)

History:

[repairs by me]: new plugs/wires/timing belt and tensioner update, new thermostat,new dorman exhaust manifold, the old one was badly cracked in several places, resealed valve cover and upper and lower intake gasket o-rings.

[recent repairs] just before I bought it: new radiator (due to overheat), new factory UPSTREAM 02 sensor.

note: downstream o2 sensor looks original, I've disconnected the battery many tmes during troublehsooting, fuel trim for downstream always goes to max.

note2: my wife claims the car intermitently runs poorly/wants to stall after a hoat soak. I've never had it miss a beat, runs great even when it had wasted original plugs in it!

Might this vehicle need down stream sensor? Mopar techs, help!

Thanks!
KJ4
22 year GM tech, former HHR owner.
PT is a nice car!!!
 

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Downstream sensor doesn't affect operation. It's an indirect indicator of whether the catcon is working properly, in that the downstream sensor should measure noticeably leaner exhaust than the upstream one, if the HCs are being broken down by the catcon. A catcon failure, an exhaust leak or other even might show similar readings for upstream and downstream, which should trip a fault code.

Any chance that the overheat damaged the head gasket? A small coolant leak into the cylinders might not be noticed yet, but could poison the O2 sensor (upstream would affect performance). Is it using any coolant at all?
 

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The 2.4 in the PT Cruiser doesn't need the major burping like the 2.2/2.5 or 2.0 even, never had a problem with that in either of my '02 PT Cruisers. Given there was a busted up exhaust manifold I would look at getting the injectors cleaned right now. One cylinder/injector being bad can throw off the O2 sensor and try to overcome the problem and mess things up all around. Burned valves and definitely the cracked exhaust manifold indicate this to be the main problem, computer controlled engines maintain strict fuel/air ratios to prevent this from happening and it may not be throwing a code if one cylinder is running lean. The bad gas mileage occurs when the computer tries to compensate by running richer to cover the lean condition and things remain out of kilter, 16-17mpg is the minimum mpg these engines get, so go with this as the next step.

Being the engine was so far past its sparkplug change interval, can't use them to get an idea if it is one or more injectors with a problem. The temp gauge should be at the center mark all the time, (just a hair below the top edge of the center mark), mine have always run at that position. The only other thing, as Dave and Bob said, would be a bad or plugged converter, but go with the injectors first, you know how to check for bubbles for a carbon tracked head gasket.
 

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If the downstream O2 sensor voltage pegs high or low, it will eventually set a fault code. I would replace it with an OEM NTK/NGK replacement and then see how it looks.
Over time the downstream O2 will shift the fuel trim, but if you keep disconnecting the battery to reset the PCM, it will never accumulate adaptive memory.
A cat fault code takes a long time to set.
 

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Discussion Starter #6
Thanks to all who replied, I'm going to revisit the injector idea- not a lot of maintenance was done on this vehicle, save for oil changes. I visited our local Chrysler dealer, when I mentioned the long term trim pegging out at max to their master tech, the first thing he said was "Did anyone just do a timing belt?". After a conversation with him, I believe I may have gotten the exhaust cam timing off a tooth- I'm no stranger to timing belt and chain repairs, but had never done a Chrysler. If I recall correctly, i had the belt on about 4 times before I got it where I 'kind of liked it'. The marks are just not clear and precise on this engine, and the access doesn't make it any easier to view (excuses, excuses!). At any rate, my plan is to retime it some day soon. Also, I have already replaced the downstream O2 sensor with a factory-type NTK- same condition. My gut is that the exhaust cam is retarded one tooth. We'll see- I'll post here when I'm done.

Thanks again all! Great forum!
 

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Since it difficult to see the cam timing marks down in there, I use a small mirror and a straight-edge across the marks and cam bolt centers to make sure that everything is in line.
It is easy to be off a tooth and not know it as the cam sensor only watches one of the cams.
 

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get one of them cheap bluethooth elm 324 obd2 interfaces and
torque light( free ) foryour your smart phone and you can see and logg
both of your o2 sensors...
 

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Off Topic: I've had a very distracting & continuing upstream o2 sensor code and ya'll have gave me a idea to visit the injectors values and the suspected downpipe leakage by following this discussion. Cool.

These Forums are great for looking at a problem thru the eyes of others.

My best to KJ, anyone who does his own work needs encouragement.!!
 
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