I found this post. Not sure if it will help you but the workshop I take my vehicle for services has been very helpful. They are fighting with my insurance to have the throttle body unit as well as MAF sensor replaced under warranty.
The ETM issue is well-known among those lucky Volvo owners who have the 5-cylinder engine in models produced in 1999, 2000 and 2001. Like me . Volvo has extended the warranty on original throttle bodies, for up to 10 years or 200,000 miles, and that discussion has, I’m sure, been had many, many times and is beyond this post. In my experience, throttle bodies last around 70,000 to 90,000 miles, though I did get 118,000 out of my original one.
The air intake path, at least on a naturally aspirated (NA) engine, is as follows:
Air Filter --> Mass Airflow Sensor (also called the Air Mass Meter, or MAF) --> Throttle Body --> Intake Manifold
The throttle body contains a metal “throttle plate” that pivots along its diameter, allowing more or less air through. The car’s computer determines the plate’s position based upon, among other factors, engine speed and load, engine temperature, and accelerator pedal position. Within the throttle body is a “throttle position sensor” which, I understand, is nothing more than a strip of resistance film, over which the throttle place moves. Depending upon the location of the throttle plate’s edge, the electrical resistance through the sensor strip tells the computer the exact throttle plate position. Minor adjustments to this position are made by the MAF, which senses temperature and humidity. For example, hotter, more humid air has fewer oxygen molecules available for combustion (hot air less dense, oxygen displaced by water vapor), so the throttle plate opens slightly more than “normal” in this situation. If it did not, then you’d have less oxygen in the cylinders, which would either cause the engine to run “rich,” wasting fuel and possibly fouling spark plugs or catalytic converters, or the car’s computer would reduce the fuel charge into the cylinder, with a corresponding loss of power. More precisely, your car would perform noticeably different depending on temperature, humidity, altitude, etc.
Eventually, after the throttle plate rubs over the throttle position sensor millions of times, the resistance film begins to deteriorate. As time goes on, the computer will be unable to determine the throttle plate’s position. My thought is that the MAF tries to compensate for some of the early throttle position sensor failure, and, in turn, it’s quite common for the MAF to fail shortly after the throttle. By “shortly after,” I’m talking, perhaps, 5,000 miles – much too long for most people to connect the failure of these two components – but, since I often drive that much in less than a month, and have had these components fail several times, the connection has been more than obvious to me. As for the throttle position sensor itself – my experience has been that the throttle body will last longer if you drive a variety of speeds, and it lasts the shortest time if you, for example, do the vast majority of your driving at, say, highway speed (minor fluctuations, rubbing the throttle position sensor, all happen at the same place on the resistance film).
Symptoms of ETM and MAF failure:
By the time the ETS light comes on your dashboard, and the car goes into “limp home mode,” the throttle has totally failed, though turning off the engine and restarting it will often clear the ETS light and enable you to drive normally for a short time.
But, here are the earliest symptoms of ETM failure, most of which would probably go unnoticed:
1. The earliest symptom happens in this circumstance – after driving steadily for a while at highway speeds, you come off the highway and have a series of stops – at traffic lights, or whatever – each time, the engine speed will drop noticeably below the 900 rpm idle speed, then come back up to 900, with an ever-so-slight surge. The feel is almost as if your transmission is downshifting from second to first gear too late. A normal throttle will not do this – if you watch your tachometer as you slow down, your engine speed will almost never go significantly below idle speed. Also note that the original idle speed for these engines was 850 rpm – this was increased by 50 rpm during one of the earlier “throttle software upgrades” – which has the effect of masking this particular symptom, at least for a while. My experience is that, once a throttle starts doing this consistently, though it can be intermittent, the throttle has around 25,000 miles left on it.
Later in this stage of deterioration, you’ll also notice that your engine speed drops too low when coasting at moderate or highway speed. For example, normally my tach will read around 1500 rpm when I’m coasting at 40 mph. When the throttle starts acting up, my tach will read as low as 1000 or 1100 rpm while coasting at 40 mph, though the engine is in no danger of stalling. This symptom is also intermittent.
2. The next symptom is “hunting at idle.” This means that, when you experience symptom #1 above, but are now stopped at, say, a traffic light, you’ll sometimes see the idle speed fluctuate slightly. Again, a normal throttle, with a warm engine, will not do this. Here, the feel is like, say, the a/c compressor is cutting in and out, which often momentarily raises or lowers the engine’s speed. But – the throttle symptom will happen even when the a/c is shut off. You will not be able to create the hunting situation at this phase of failure, though you will be able to induce it later.
3. Eventually, you’ll feel momentary hesitation when driving at highway speeds, almost as if there’s a gust of wind pushing the car back. When this happens, I look at shrubbery to see whether there is a significant wind – and, if there isn’t, then it’s likely to be the throttle, especially if it has lots of miles on it. At this stage, the car is in no danger of stalling, but it’s one of the last signs that you’re getting close to a true failure – figure 5 to 10 thousand miles remaining on the throttle.
4. When the throttle actually starts “failing,” you’ll feel a significant jerking motion as the engine tries to stall, often at highway speeds. Stepping on the accelerator pedal will, momentarily, do nothing. This can be quite unnerving to the uninitiated driver, or for passengers. However – you CAN get out of a potentially bad situation, merely by manually shifting the transmission down one gear – this increases the engine speed, which means that the throttle plate will now open to a place where the throttle position sensor is not as badly worn, and the hesitation episode will be over. You can upshift and chances are that the throttle will not fail again at that point – though you’re now on borrowed time, and a complete failure can happen at any time. Once the throttle has reached this point, hunting at idle will be very common, even with the engine cold, and the car may also be hard to start. It’s quite likely the there would be no codes stored in the car’s computer, although a failure to start may leave something behind.
I have experienced the above but only after starting and trying to accelerate from standstill – again intermittent.
You will also be able to induce hunting at idle, especially if the car is warm – with the car stopped, one foot on the brake, and the transmission in drive (I’ve also had this work with the transmission in park, but I’ve found it’s more likely to happen with the transmission engaged), use your other foot to step on the accelerator to raise the engine speed to, say, 2000 rpm. A normal throttle will return the engine speed to 900 rpm in one smooth movement, with little or no overshoot. A failing throttle will often allow the engine speed to significant undershoot 900 rpm, then have the speed go well over 1000, drop well under 900, and continue to fluctuate with no further driver intervention – until the car stalls, or it settles in, sometimes with another tap on the accelerator. I have also had this happen spontaneously – once my car did a command performance on a test drive with a Volvo shop foreman as my passenger – needless to say, that scenario produced an automatic throttle replacement, no questions asked J .
5. Failure of the MAF appears much like those of paragraphs 3 and 4, minus the hunting at idle – except that downshifting does absolutely nothing because the problem is not that you need to run the engine at a different speed to use a different portion of the throttle plate’s path, but that the throttle is getting faulty information from the MAF sensor. Also, MAF failure often presents itself as a staccato of multiple hesitations or attempts to stall, a fraction of a second apart, while ETM failure is often one such hesitation per episode. And MAF failure is more likely to show itself on extreme weather days, since the sensor would normally be providing the most correction to the ETM. Again, early failure often does not leave any codes in the car’s computer.