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Rejuvenating vehicles

You're in charge of giving power to cars that take it without giving back.
Tuesday, March 1, 2011 - 01:00

You're in charge of giving power to cars that take it without giving back.

Motor Age Garage 2001 Pontiac Aztec engine power vehicle power fixing vehicle repair shop training technician training automotive aftermarket

While some customers might accept the high estimate as necessary and open their pocketbooks, others will pay the $100 diagnostic fee, trash their copy of the 29-point inspection sheet that urged $900 in needed service and drive away to have their Service Engine Soon light taken care of somewhere else. Then they'll tell all their friends not to bring their cars to the shop that finds huge amounts of work to do on every set of wheels that rolls through the service aisle, especially if that disturbing estimate was handed to them the first time they used that shop. Customers will, however, accept a pricey upsell estimate from a shop they trust. But they generally won't trust a shop that tries to upsell too much too early in the relationship.

All that being said, anybody who wrenches for a living knows there are multiplied thousands of vehicles in drastic need of some fairly extensive (and expensive) service by default, because warning lights and minor symptoms often are ignored until the owner perceives that driving the car has become dangerous. Sometimes the catalyst for change is triggered after a test drive by somebody who is alarmed at how the car drives because they haven't been lulled to lassitude by the gradual deterioration of the vehicle.

One day at the Ford dealer, I drew a ticket on an early 1980s Thunderbird with no warning light (Ford had no Check Engine Light until 1988, but it did give a stalling-at-stops symptom with about 140,000 miles on the clock. This one had Throttle Body Injection with that square black EGR valve position sensor (remember those?) that was prone to causing stalling concerns. In the course of my diagnosis, it tossed a TP sensor code and the Idle Tracking Switch in the ISC motor was floating 40 to 60 ohms of resistance when it should have shown a dead short across its contacts at off-idle. The O2 sensor was a flatliner even after three minutes at 2,000 rpm.

Three of those four parts conceivably could cause a stalling concern, and from the way things looked under the hood (dusty and cooked but otherwise pristine) the car hadn't even been checked for codes or concerns since EEC IV was born. The estimate (this was in the very early 1990s) was just shy of $500. I have no doubt that I could have combed that car from bumper to bumper and produced a $1,500 upsell ticket, but our shop policy (as well as my own) was to handle the stalling concern first.

The customer's favorite car salesman came storming into my bay complaining that I was trying to sell them too many parts. During our discussion, he demanded, "Surely just one of those parts will fix what's wrong!" He had sold that family five other cars and figured he needed to intercede on their behalf. I suggested that because he had made so much money selling them cars that he might want to pay for some of the needed repairs.

He wasn't amused, but it turned out that the customer had no problem with the estimate. The car ran like new when all the parts were replaced.

No Joy to Drive

This month's 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 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 speedometer 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 speedometer topped out at 55 mph and the tach seemed to hit a 3,500 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 the engine actually ran worse when it was hotter. 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 recovered fairly well on a no-load engine rev, we were reading 3 to 6 psi of exhaust backpressure. To a subjective eye, that 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 3,000 for about 10 seconds. GM's rev limiter engages at 4,000 on this vintage, so that has to be factored in during diagnosis.

Drawing on past experience, I remembered a Monte Carlo we repaired for a very similar concern a few years back (one with basically the same engine, same year model). It passed the temperature gun test with flying colors (320 degrees F at the cat inlet but 433 at the outlet), but it 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 inlet should be hotter than its outlet if it's restricting exhaust flow. That makes sense, but it only proves true if the catalyst is totally clogged.

As mentioned earlier, the Monte Carlo's cat passed the temperature test but had some 15 psi of backpressure. When we removed that converter (the brick was easily visible), we found particles of sand-like vermiculite clogging the honeycomb.

Removing the converter 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 catalytic converter 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 to 59 is spec), along with 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 percent 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 might 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), 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. 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 typically are the ones in the most hostile environment. Well, in this case the hub bearing and sensor were OK, 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 tribe for which it was named.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail Richard at

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