Is this fan setup worthless? Also, code 91 question.

It looks like your fan covers less of the radiator than mine. I have a Volvo fan and about a 1 1/4" up top isn't covered. Mine runs high in traffic also. I try not to look at the gauge too much. It looks like your running underdrive pulleys to. That's less power to drive the fan.

Keep in mind the stock guage sucks. Get an autometer. If you're running around 200 F, you're fine. Looks like a good fan setup. Hide the wiring though (tacky). The stock gauge won't give you a true reading. The car will get warmer on a stop/go because air isn't pushing into the car to provide additional cooling.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Cman88 said:

The fan is electric, not driven by the engine.

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Given his hypothesis, I think he was trying to infer that alternator output might drop (let alone water pump issues from spinning slower with U/D's) at idle.

hook a good guage to it and see what it reads... make sure the fan pulls through the rad aswell....

you dont want the fan pushing air forward....

Given what you're seeing, I would follow the advice to install a real gauge (even if you dont want to keep it, just set it on the console for a few days till you can see what gives). A mech gauge is 10-15 bucks at the parts store.

FWIW, my stock gauge read ~35* too cool at 180*F. It certainly left something to be desired.

Another thing to try once you know exact engine temps: if it will run too hot for you in the garage, put a big box fan in front of the car and see if it lowers the temps. If so, your fan isnt cutting it. If it makes no difference, you gotta figure your fan is doing its best.

Good luck.

Cman88 said:

Maybe so... but I had the exact same issues before I put those pullies on also. I don't think the small decrease in speed from the pullies is the root of the problem. Wouldn't the upper section of the gauge be closer to 270 degrees... or should I just completely disregard anything I see from these gauges?

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I wouldn't disregard what the guage says, it's just not accurate like an autometer. You hit 270 and I don't think you're hoses would be attached. You want to try and stay under 200. Anything above is pushing it. I stop and go on my setup and it may hit 200 on a hot day. I would however, ditch the under drive pullies. They will cause your pump to turn slower, resulting in an overheat. How many CFM's is your fan pushing? Mine was 2200, which wasn't enough for me. I had to add a big Giffin radiator and an additional fan, making two total to run cool.

Good luck!