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I have a 2001 h6 llbean that hesitates when it is hot outside.It is more noticible when the ac is on.It has new sparkplugs.What. would be causing this.
 

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Had new plugs two months ago and all fluids changed.Valve cover gaskets were changed when plugs were changed.
 

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I have a 2001 h6 llbean that hesitates when it is hot outside.It is more noticible when the ac is on.It has new sparkplugs.What. would be causing this.

The AC pulls a load on the engine. Any issue with the proper "tune" of the engine will be magnified with the AC on. The issue is the hesitation.

A few things to look into.

The temperature sensor - It does more than work the gauge on the dash. It feeds info to the ECM to control idle, fuel trim, timing and shift points of the transmission, to name a few. If it is feeding improper info, the ECM only knows to react to what it receives. If it is feeding the correct temperature, then make sure the engine is running at the correct operating temperature averaging 195-205F.

Carbon on the throttle body. Even the slightest bit of build up will effect the measurements of the TPS. Try to clean it every 15k miles or so, or at least check to make sure its clean.

Fuel Filter. With heat, fuel expands. The added pressure of the fuel on the system will effect flow. Also, are you getting any "knocking/pinging" from ignition on acceleration? This would be an indication of a fuel starving.

Are you using a "cheap" gas? Quality gasoline, highest octane. Always.

What condition is your battery in? Amperage output, not voltage. The computers need a total of 450 amps from the battery for proper operation. Low amperage means the computers and sensors work slow, or not at all. Kind of like a laptop or phone with a low battery or poor connection to its signal.

Forget the knock sensor. If there was a problem with the knock sensor, the ECM would pick up on it immediately after start up or when the error occurs. The moment it loses the base signal from the sensor, MIL illuminates. As I have stated in prior messages, the knock sensor has one function and one function only, to alert the ECM of a detonation by means of altering the resistance back to the ECM via plate seperation. It is a single wire sensor that does nothing but vibrate and when the vibration is altered by detonation, the ECM alters timing to correct for it. The knock sensor fails when the connector is damaged or corroded, if the wire breaks, or the sensor cracks which is the #1 cause of the sensor failure. It will not cause hesitation.
 

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Knock sensor if starting to fail does not always show up on the ECM.

There are posts that reflect this.

How old are the PCV valve (replace).

It wont be the alt, don't know where the 450 amps comes from, thats cranking amps of the battery, especially considering the car does not even put out that much current via the alt. A subaru Alt is 90 amps stock. If it was an alt issue he would have the problem with everything on, not the light electrical load of just the AC.

The knock sensor is a pizioelectric sensor, sort of like a microphone. If the plastic body starts to fail it will not "listen" properly and send an incorrect signal to the ECU. the ECU is not smart enough to know if this is a vlaid signal or a malfunction. it can only tell if there is or is not a signal.

The fuel pressure regulator adjusts the fuel pressure. If it was a fuel issue there would always be hesitation under acceleration, not just in the warm weather, and not just with the AC on.

It would not be the engine temp sensor (seperate from the gauge) as the car runs fine again in cooler weather. a 40 degree temp change would not affect this, nor as much as a 60 degree change.

Modern engines run lean, hence hotter then normal and are prone to ping, you can not always here the ping but the knock sensor can. Ping (pre ignition) is just what it sounds like. A hot spot develops in the cylinder and an uncontrolled explosion occurs. When the explosion happens at the wrong time it can damage internal parts. Some ping (inaudible) is acceptable and the ECU adjusts the timing. The knock sensor can get loose in its housing from thermal expansion over many many years. Eventually it begins to hear the wrong signals, or misses them all together. This can cause the knock sensor to over adjust the timing.

This is a classic symptom of a tired knock sensor on modern engines. My first response would have been plugs, but the OP said those have been replaced.
 

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With all due respect NIPPER> I have been building and repairing all types, styles, sizes and brands of vehicles for 25 years. I do not have comebacks. I do not replace anything unnecessarily. I have a very large customer base.

You would be surprised at how many vehicles I get in my shop that have been elsewhere and were misdiagnosed by a mechanic. A parts exchanger that didn't pay attention to math and science.

Electrical theory. Study. When is the last time you changed a fuse that was rated V? Its amps. I didn't say anything about an alternator.

The hesitation is there and is amplified by the operation of the AC. Therefore, the AC is out of the equation. It is narrowed to Air, Fuel, Fire and the controls of each one in relation to the timing of what the operator wants and what the engine can provide.

The fuel map of any modern fuel injected engine is mathematically calculated using information gathered from the temperature sensor, AF sensor or short band O2, camshaft position sensor, MAF or MAP depending on the vehicle setup for calculating air input or intake pressure, the TPS, gear position and speed sensor. It calculates the engine load using the same inputs.

Fuel pressure and flow are two different things.

The temperature sensor can effect operation at high ambient temperature but not low in a case where the output is fixed all the time. Case in point, a 99 outback with 2.5 dohc won't start unless the throttle is held slightly open. Had a dealership tech call me after he replaced the expensive idle control motor and he had the same problem. No start. I told him to look at the temp output. He called me back and said it started right up after replacing the ECT. The old one was feeding 365F to the computer so the computer was keeping the valve closed to prevent it from starting and damaging the engine.

If you don't know how a computer system and all the relative inputs function for each system, guessing is your worst option.

That's why I said check.

I have posted a good technician reference to fuel trim on this forum. You may want to read it. Modern engines do NOT run lean unless there is a problem. All ECU/ECMs are programmed for the engine to run at the prime stoichiometric equation for the fuel it runs. If it can't, there is a problem.

Cheap gas, high ethanol content, water, trash, dirt, clogged filter, poor pump effect this equation. As well as the sensors I've already mentioned.
 

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Discussion Starter #9
I just replaced the pcv valve a month ago. The battery was new in 2007 but checks out fine.I wish the engine light would come on so I could figgure this one out
 

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If the complaint is transmission engagement related, and given the recent work done, I'd check the transmission fluid level.

otherwise - what they said ^^^
 

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I just replaced the pcv valve a month ago. The battery was new in 2007 but checks out fine.I wish the engine light would come on so I could figgure this one out
Five year old Battery????? Check the battery amperage output with the engine off. Advance stores have a meter that test amperage, the same they use to check loads, and will check it for free. If its low, get a new battery and start fresh. Oreilly's sells a meter just for checking conductive amperage flow, but its around 250. Same with tool vendors, but it comes in handy on many occasions. At least I use mine many times a day.
 
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