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Discussion Starter #1
'05 TB. 4.2L. 90k. Have not had long but seems to be getting about 10.5 MPG currently.

I just replaced the fan clutch because I was getting MIL from the fan speed sensor.

Replaced with brand new Behr part number 376734021. MIL went away.

However, the clutch does not disengage on a cold motor. Now getting P0128. (Engine taking too long to heat up) It's stiff and will not continue to spin when kicked over but you can move it with your hand.

Is this the proper behavior of this new clutch or do I need to have the ebayer replace it?

TIA
 

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2003 chevy trailblazer_lt_ext
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Many people believe that only the GM AC Delco replacement fan clutch will work correctly with our vehicles. Some folks have even needed to go to dealer to have the computer re-flashed to make new fan clutches work correctly.
 

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Discussion Starter #3
Thanks. The Behr clutch I installed is the updated OEM part. I'm asking if the clutch being stiff on a cold engine is normal behavior. I would assume not but I figured someone else with a working fan clutch could verify theirs is stiff or not on a cold motor.

Many people believe that only the GM AC Delco replacement fan clutch will work correctly with our vehicles. Some folks have even needed to go to dealer to have the computer re-flashed to make new fan clutches work correctly.
 

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At startup my fan clutch will start engaged (roar) but will quickly release (a few seconds) and the noise settles down to a quiet idle.

Hope that answers your question.

hopefully your part settles in and starts to work as expected
 

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Ok, kind of. I just put a couple hundred miles on it and I'm certain its still not right. It roars when you start it. It does quiet down just a bit. But even at highway speeds its still roaring. You can feel engine lag and the vibration of the fan in the steering and pedals. Pretty ridiculous that new manufacturing is such garbage.
 

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2004 chevy trailblazer_lt
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OK . A P0128 would kill your fuel mileage and set that DTC with what your saying if the motor never fully warms up to about 205-210 degrees F on a scanner. Never read a gauge in the cluster as it can be inaccurate.

What happens >>> If the thermostat fails to close, the engine may never
achieve normal operating temperature. This will prevent it from going into closed loop feedback control of the fuel mixture
(which makes the engine run rich and waste fuel).

See it on the scanner to check feedback control open or closed.
A Bad thermostat is common issue.
Verify.

What code # was set to determine that the fan clutch was defective?
 

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Discussion Starter #7
Yes, P0128 if I remember correctly was the engine taking too long to heat up. It does get to 200+ indicated by Car Scanner, it just takes 15-20 minutes of run time to get there. I figured this could be and probably is due to the fan running all out as it never threw P0128 prior to replacing the clutch. The old clutch wasn't locked up, intermittently, it would fail to engage when stopped.

I do have a replacement stat/housing here anyway. As well as both O2 sensors, and a set of plugs. OEM components. I'll get to them next weekend. 13MPG avg wont cut it.

P0526, I believe was what the clutch was throwing. It went away after installation of the new clutch.

I notice DOWNSTREAM O2 doesnt display a fuel trim but does display voltage within scanner. Is this normal?
 

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2006 chevy trailblazer_lt_ext
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Voltage is raw data and is used by the PCM to calculate STFT and LTFT. The B1S2 HO2S should show relatively constant voltage at constant throttle. B1S1 should swing back and forth. But now you're wandering from the original diagnosis and just throwing parts.

Start with a new thermostat and fresh coolant and leave the O2 sensors on your bench. If the engine does not reach proper operating temperature, the emissions control system defaults to open loop mode. It will increase the pulse width of the injectors. Fuel trim and the HO2S sensors do not come online until the vehicle switches to closed loop.
 

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:iagree: Thermostat needs a swap. Think how many cycles they do.

Now on the O2 Keep in mind its possible may not throw a code but yet be defective.
The O2 sensor reads unburned oxygen in the exhaust, and generates a voltage signal that is proportional to the amount of oxygen in the exhaust. The signal can vary from a low of about 0.1 volts up to a high of about 0.9 volts.
A low voltage signal indicates a lean fuel mixture. A high voltage signal indicates a rich fuel mixture. The engine computer uses the O2 sensor's
input to balance the fuel mixture during closed loop operation. A bad sensor may prevent the system from going into closed loop, and usually causes the fuel mixture to run rich causing an increase in fuel consumption and emissions.


You asked about fuel trim on the downstream but its only a tattler for the converter. * Note* A downstream oxygen sensor built in or behind the catalytic converter works exactly the same as an upstream O2 sensor in the exhaust manifold. The sensor produces a voltage that changes when the amount of unburned oxygen in the exhaust changes. However it only monitors the efficiency of the Cat itself.
The PCM monitors converter efficiency by comparing the upstream and downstream oxygen sensor signals. If the converter is doing its job and is reducing the pollutants in the exhaust, the downstream oxygen sensor should show little activity (few lean-to-rich transitions)).
The Downstream sensor's voltage reading should also be fairly steady (not changing up or down), and average 0.45 volts or higher.
 

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Another thing i would look at.
Bad fuel mileage could be a faulty coolant temp sensor that always reads cold causing the fuel control system to run rich and waste fuel ?
Look up the specs and measure the resistance of it @ cold and @ operating temp.
If the resistance of a coolant sensor is within specs and changes as engine temperature changes, but the engine is not going into closed loop, the fault is in the wiring or PCM.
You can also check into the Voltage specs too.
A cold coolant sensor will read a few volts then as the engine warms up and reaches operating temperature it should gradually decrease its voltage.
 

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2006 chevy trailblazer_lt_ext
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Yes agree with Grinder.

The #1 input to fuel economy in fuel injected engines is the engine temperature. Whether that is actual temperature, or reported temperature is to be determined. I've attached the temp-resistance table below. You will have to measure engine temperature with a separate thermometer, or remove the ECT to known ambient temperature and then measure resistance across the sensor.

But first I'd replace the t-stat and re-check. It and fresh coolant are probably needed anyway unless this has been done within the last three years.
 

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Discussion Starter #12
Voltage is raw data and is used by the PCM to calculate STFT and LTFT. The B1S2 HO2S should show relatively constant voltage at constant throttle. B1S1 should swing back and forth. But now you're wandering from the original diagnosis and just throwing parts.

Start with a new thermostat and fresh coolant and leave the O2 sensors on your bench. If the engine does not reach proper operating temperature, the emissions control system defaults to open loop mode. It will increase the pulse width of the injectors. Fuel trim and the HO2S sensors do not come online until the vehicle switches to closed loop.

Nah, not throwing parts at it. I was just curious how the sensors should read. Thank y'all for the explanations and chart. The O2s appear to be functioning ok. But I'm still replacing them, and the plugs. Haha.

It's nearing 100k and pretty certain they've never been done. Had these parts on list for maintenance after acquiring the truck. Not in lieu of P0526 or P0128.

Coolant temp on warm engine in car scanner reads ~197F while dash gauge is at 210. I'm aware they don't mesh. Just an observation. I'll check the coolant temp sensor for giggles and replace if necessary. There should still be a lot of miles left in this truck with good care.

I'll do the stat and fresh coolant first. Though coolant has probably been changed as I know a water pump was put on in the last couple years. Probably due to the broken fan I replaced when I did the clutch.

But about that fan clutch... another on the way.
 

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In MANY situations where P0128 is thrown, the ultimate problem has been the thermostat failing to regulate coolant flow correctly - ie. stuck open or opening too early. You can easily check the operation by pulling the thermostat and heating it in a pan of water, monitoring the temperature, and seeing at what point it opens - or if, in your case, it even closes when cold.
Since you already have one in hand, i wouldn't even bother testing it and just swap it, since you have to pull the old one to test anyway. It's an easy swap.
 

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Discussion Starter #14
In MANY situations where P0128 is thrown, the ultimate problem has been the thermostat failing to regulate coolant flow correctly - ie. stuck open or opening too early. You can easily check the operation by pulling the thermostat and heating it in a pan of water, monitoring the temperature, and seeing at what point it opens - or if, in your case, it even closes when cold.
Since you already have one in hand, i wouldn't even bother testing it and just swap it, since you have to pull the old one to test anyway. It's an easy swap.
Thanks. Looks to me like the stat might be a pain. Is it not necessary to remove the alternator? And it looks like the alternator would be easier to remove with the fan out. Since the coolant temp sensor may require alternator removal as well, I went ahead and ordered one up and will swap both. Cheap enough.

The fan clutch seller shipped out a replacement pretty quickly. Wasn't expecting it today. Home from work at 5 and just replaced it. The new clutch is much BETTER. But still doesn't seem QUITE right. The P0128 disappeared almost immediately. Need to run it from a cold engine again to be sure.
 

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Fan Clutch

I replaced the fan clutch and thought that would do the trick. I later found that a mouse had eaten some wires to the fan clutch under the engine compartment fuse box. You really need to make sure that the signal is getting from the computer to the fan clutch. If the signal is not there, the fan clutch will remain engaged constantly. Some of the aftermarket fan clutch replacements do need reprogramming or they will come on full speed at first and then eventually settle down.
 

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When I did my stat, I did remove the alternator because it's easy to do - just three bolts (after removing belt) and it's off. I believe some do it without removing the alternator by going through the wheel well and reaching under, but IIRC it does make accessing at least one of the bolts a little awkward. Either way you do it, it's an easy job, even though some make it sound like a PITA.

The temp sensor is a much bigger PITA if you do it because of the location (under the exhaust manifold), but mine is a 2007. I believe the earlier ones are more accessible near the stat.
 

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'07 TB, I had a fan speed sensor error, let it go for many months cause I was having no issues, the fan engaged when hot and stopped in traffic, then released when I accelerated. Then suddenly I had a small knock noise in the bottom of the motor, 2 mechanics decided it was better to just replace instead of ripping apart and discover the problem and fix it for $5000 plus, instead of $1650 plus labor to replace with a used motor and 15k fewer miles. At the time of the motor swap, I had them replace the fan clutch to get rid of the code which it did. I now had the same problem you have. It was a Hayden clutch for $225, after discussing and letting them test drive they agreed it should disengage, options discussed and I decided on the AC Delco direct OEM for 400 and it works perfect, exactly like before but without the error code. My advice, get the AC Delco brand.
 
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