This story centers on a Pontiac Aztek that had been driven almost into the ground. It had enough miles on the odometer to have circled the world a half dozen times with room to spare, and when it finally became too much trouble to drive in heavy traffic, the owner had decided to either get it fixed or trade it in. The problem with a trade in was that the way it was running, it wouldn’t have brought $500 at the auction barn. Seriously, this vehicle ran like somebody had shoved a potato up the exhaust pipe, and the customer’s initial thought was that there was something wrong with the transmission. A low power concern can feel like a transmission concern to a driver because of the deep throttle angle during attempted acceleration coupled with a vehicle speed that has flatlined too low on the speedo for the shift schedule to select the next gear. After a quick check of the fluids, I wondered if he might have something there – the transmission fluid literally looked like engine oil and smelled appropriately nasty. Driving the vehicle, I found that even at its best (on cold acceleration) the speedo topped out on 55 and the tach seemed to hit a 3500 rpm ceiling, even though the engine was kind of loud during these maneuvers. I hadn’t even connected a scan tool or a fuel pressure gauge, but my initial instinct was to go after the air and fuel filters, which (in my experience) are the most commonly neglected performance-impacting parts. I like to get the fuel filter out of there and blow backwards through it into a clear container. This one was severely restricted and gave up a lot of black stuff, so the first part I sold was the filter, but the second test drive didn’t feel any different, and when the engine was hotter it actually ran worse. It was time to gather some more data. Running the Numbers With vacuum and exhaust pressure gauges installed we found that, while the engine vacuum seemed to recover fairly decently on a no-load engine rev, we were reading 3-6 psi of exhaust backpressure, which, to a subjective eye, might bring us to examine the catalyst a bit closer. Be that as it may, some exhaust backpressure is normal if you hold the throttle up near 3000 for about 10 seconds. GM’s rev limiter engages at 4000 on this vintage, so that has to be factored in during diagnosis. Drawing on past experience, I remembered a Monte Carlo we had repaired for a very similar concern a few years back (one with basically the same engine, same year model) and that it had passed the temperature gun test with flying colors (320 degrees F at the cat inlet but 433 at the outlet) but it had needed a catalyst anyway. Suffice to say that if it had failed the test those numbers would have been conclusive, but passing that test simply doesn’t exonerate the converter. Conventional reasoning (albeit somewhat flawed) is that the cat con inlet should be hotter than its outlet if it’s restricting exhaust flow, and that makes sense, but it only proves true if the catalyst is totally clogged. As mentioned earlier, the Carlo’s cat passed the temperature test but had some 15 psi of backpressure, and when we removed that converter (the brick was easily visible) we found particles of sand-like vermiculite clogging the honeycomb (see photo). Removing the cat con from the Aztek (it has a long curved inlet pipe) we used our camera-on-a stem inspection tool to determine that the honeycomb was as clear as could be, not only on the front end of the brick but on the rear as well. The Aztek’s cat con got a clean bill of health – our problem was elsewhere, and it was a relief not to have thrown a several hundred dollar cat at this vehicle only to find the concern was still there. With an electronic fuel pressure gauge connected and lying on the console next to the scan tool, I test drove the Aztek with a bevy of learners warming the other seats. This was a heavy duty teaching exercise and I wasn’t about to let them miss out on it. We had 57.6 psi of fuel pressure showing during WOT acceleration (52-59 is spec), along with just under a MAF that mirrored TP but topped out at just under 50 grams per second of air for an injector pulse width of 10.42 milliseconds with Long Fuel Trim readings bumping 21% and a lean O2 sensor reading. The problem with those numbers is that if you don’t have a normal vehicle (or numbers stored in your head FROM a normal ride) then you may not know what’s wrong even if it’s staring you in the face. How many grams per second did we actually need for the PCM to deliver the proper amount of fuel? More than we were getting, it seemed. I stopped the Aztek out in the country and sent the student who was riding shotgun out to unplug the MAF sensor. Well, he did, and the Aztek ran almost normally, with strong acceleration all the way up to about 70 mph. And it wanted to go faster, but I didn’t. We had pinpointed our concern. With a reman MAF sensor in the tube ($88 from Advance Auto Parts), accurate airflow information was giving the PCM what it needed to deliver the right amount of fuel. We’re Not Through With the customer’s original concern handled, we sold the customer a full transmission fluid exchange along with a new trans filter and changed the crankcase oil and filter as well. There was nothing of any significance in the transmission oil pan (fuzz on the magnet, but it was normal wear), so we moved on to investigate the Service Engine Soon light that had re-illuminated after our initial repair. A P0401 code on this one means the EGR is operating mechanically but no EGR is flowing. Using the scan tool’s Active Command to open EGR valve and neither seeing nor feeling any indication that any exhaust was flowing, we yanked the throttle body to find that the port leading from the EGR into the manifold right behind the throttle plate was totally blocked. With some Sea Foam and a piece of an old park brake cable connected to a drill, I did some work on that passage and cleared it. Cleaned the throttle plate, reinstalled the throttle body with a new gasket, and the code was a goner. Now we had another warning light we had ignored until now. The ABS and “TRAC OFF” lights were brightly shining and the message center was doing its part to let us know there was a problem. Navigating the scan tool screen to the ABS/TCS area, we discovered that the right front wheel speed sensor signal was missing, and while the hub bearings and their sensors tend to fail on these vehicles, the bearing on this one was quiet. The flexible harness that carries the sensor signal bends every time the wheel is turned, and my contention is that the components that fail are typically the ones in the most hostile environment. Well, in this case the hub bearing and sensor were okay, but one of the wires leading to the sensor wasn’t. With some solder and heat shrink, the broken wire was repaired, re-taped, and re-sheathed. How long will it be until that wire or one on the other side breaks again? I have no idea, but when that vehicle left, all the warning lights were out. Stopping Short The fuel gauge was perpetually on empty too, but for the time being, the owner would use his trip meter for a gas gauge – maybe in a month or two we can replace that worn out fuel gauge sending unit. This one had deteriorated, but we had breathed new life into an Aztek that may have otherwise been ready to join the extinct South American Indian tribe for which it was named. R.W.M.
Posted on: Thu, 03 Oct 2013 08:24:34 +0000
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