Bad G.C. Oxygen SensingQ:I replaced an exhaust manifold in an acquaintance's 4.0-liter 1997 Jeep Grand Cherokee. Since then, the Jeep's been running rough and the Check Engine light is on. I scanned the vehicle and found a fault for the MAP sensor and an oxygen sensor. He's since taken his Jeep to the dealer for diagnosis and was told that because the MAP sensor had the fuel trim too rich it ruined the catalytic converter and seized the O2 sensors. Can the MAP sensor cause the cat and the O2 sensors to fail?
| expert Advice oxygen Sensors
A:Nothing's absolute, but it's almost a given that when a vehicle enters a repair shop running fine and leaves running poorly, the technician involved accidentally caused the problem. He may have cracked a spark plug, knocked off a vacuum hose, etc. MAP stands for manifold absolute pressure; the MAP sensor reads the engine's level of intake-manifold vacuum. Manifold vacuum is typically high at idle and decreases as engine speed increases. The oxygen sensor is mounted within the exhaust system and measures oxygen content in the exhaust gases. This information helps the Powertrain Control Module (engine's master computer) determine how rich or lean the air/fuel mixture is, so it can make the appropriate adjustments to fuel delivery. With that said, a vacuum leak or a misfire will have an effect on both engine vacuum and exhaust oxygen content, while causing the engine to run poorly. These conditions can often set trouble codes referring to particular sensors, even though the sensors themselves haven't failed. A MAP sensor causing a rich condition, which could damage the catalytic converter and an oxygen sensor, would certainly be considered atypical. There may be a difference in oxygen-sensor readings before and after the catalytic converter indicating a problem with the converter, but you want to be absolutely sure. Catalytic converters aren't cheap. Double-check everything you touched during the exhaust-manifold replacement. It may even be worth a second opinion from another Jeep technician.
RAM Van Hardware ProblemQ:While driving my 2001 Dodge Ram Van 1500 in the rain, I went through a puddle and the Check Engine light came on. At an emissions-test facility, code P0601, Internal Control Module Memory Check Sum Error, came up. The van runs fine, starts fine, and gets the same gas mileage as before the light came on. I then took it to a repair shop, where a technician cleared the computer and the Check Engine light came back on. He didn't reflash the computer--he just cleared the light and said the van needs a new PCM. I must get it fixed to pass the emissions test or the state will cancel my driver's license and revoke my license plate (isn't Illinois great?).
| expert Advice ram Van
A:While I'm not up to date on Illinois emission inspection regulations, I know every vehicle in the U.S. needs a valid inspection certificate. The technician's recommendation to replace the PCM was correct. Engine code P0601 indicates an internal PCM failure--that's hardware, not software. A reflash, or reinstallation of software, isn't going to fix the problem--it's like reinstalling Windows on a PC to repair symptoms from a cracked motherboard. This is one of those "no ifs, ands, or buts" trouble codes. The PCM on the 1500 series van is mounted inside the engine compartment, against the firewall, near the wiper motor. Why there? Good question. A lot of other vehicles' PCMs are located under the dash in the passenger compartment, where they're better protected from heat, dirt, and moisture. Control units located under the hood utilize a more insulated and weather-resistant housing, but that wasn't enough in this case. It seems crashing that puddle sent a good-size wave right where you didn't want it to go and contaminated the PCM. Retail price on the part can range from $200 to $600 depending on the specific application and where you buy it. The labor cost shouldn't be too bad for installation and reprogramming.
Trailblazer Blower MotorQ:I have a 4WD 2004 TrailBlazer EXT LS with the inline-six. All the vents work except the ones on the dashboard pointed at the passengers. I read somewhere that the blower-motor resistor might be the problem, so I replaced it. It still worked the same--the air is cold, but doesn't come out of those vents. I'll check the fuses, but have no idea which one does what.
A:The blower-motor resistor assembly turns the blower motor on or off, and changes blower motor speeds. It has nothing to do with directing air to the appropriate vents. There is a series of electronic actuators (motors) and doors which handle air delivery. A fuse that supplies voltage to these actuators is fuse #39, marked HVAC 1, located in the rear fuse block beneath the second-row seat. However, if turning the defrost vents on and off and controlling air temperature work, don't bother going under the seat--fuse #39 must okay because it also powers the defrost and temperature-control actuators and doors. What we're zeroing in on is the mode actuator. When the mode actuator is turned on it positions the mode door to direct air out the front panel vents. When it's turned off, airflow is directed to the lower vents for heat. In your situation the mode door may be stuck in the heat position, the actuator itself may be no good, or it's just not receiving the appropriate electronic control signals, if any. This is where it gets technical and requires special tools and experience to efficiently diagnose the problem. The HVAC (heat, ventilation, and air-conditioning system) is controlled by the HVAC control module. Like other control modules we know and love, this one stores trouble codes in its memory when a failure is detected, and supplies other valuable diagnostic information to technicians using the appropriate computer scan tool. Bottom line: The problem involves the mode door, and in all likelihood it's going to take more than a fuse to fix it.
Missing NISSAN Twice a YearQ:My 1998 4WD Nissan truck with the 2.4-liter four-cylinder engine has 118,000 miles on it. On a hot day, over 90 degrees F, after driving for at least two hours, the truck will start to misfire badly. The computer doesn't show any codes and after it cools down it runs great. This has only happened twice in the last year.
| expert Advce nissan Misfiring
A:Twice a year isn't bad. With high mileage comes a longer list of potential causes of an intermittent misfire. That it occurs at high temperatures might be a clue, but a lot of failed components can be affected by expansion and contraction from changes in powertrain and ambient temperatures. Begin with the basics--starting with the ignition system. With six digits on the clock you shouldn't hesitate to replace the distributor cap, rotor, and ignition wires, if you haven't already. Also carefully inspect the ignition coil for burn marks where voltage may be leaking through. The ignition coil is an integral part of the distributor, and some of the distributors installed on Nissans like yours have had issues with moisture drainage. a pool of water collects on the floor at the front passenger seat. I assume a line is clogged somewhere. Is there an easy fix or should I have a professional look at this?
A:Give it a shot first. Condensation is formed on--and drips off--the air-conditioning system's evaporator (cooling component within the dash) and collects in the bottom of the evaporator/heater core case. The water is then routed through the firewall, into the engine compartment, and down to the street through a rubber drain tube. When this drain gets plugged up with debris, the water level will rise and eventually overflow onto the passenger-side floor. Carefully jack up and support the right front of the vehicle, crawl underneath and look for the evaporator drain tube pointing downward at the street. It's usually made of rubber, removable, and pulls off easily. But don't forget to duck. If the tube itself is obstructed and holding back the water, it's going to be a gusher. If you're lucky, and that's all there was to it, you can clear the tube of debris, reinstall it, and be on your way. But there may be more debris inside the evaporator case still plugging up the drain path, and that's going to require compressed air to blow it out. Worst-case scenario is that the drain path is clear, and condensation is entering the passenger compartment through a cracked evaporator case, or a bad seal against the firewall. Second-to-worst, there's so much debris collected in the case that it continually plugs up the drain over short periods of time. This requires removal of the evaporator case for a thorough cleaning. Either way, it's always a good idea to carefully inspect the plastic screening at the cowl, right at the bottom of the windshield beneath the wipers. This is where outside air enters the A/C system, and any missing, damaged, or misaligned protective screens will allow leaves and other debris to enter the evaporator case, consequently stopping up the evaporator drain.
Rough-running JeepQ:My 4.0-liter 1997 Jeep Cherokee hesitates and idles rough for a brief period, not long after starting it, then runs okay. It does this every time I start it. I had the computer, MAP sensor, cam sensor, and crank sensor replaced. No lights come on, and when it was put on the machine no codes came up. What could be the problem?
A:It could be a lot of things. Running rough shortly after starting the engine can go two ways. The problem occurring after a cold engine startup may indicate a lean condition (not enough fuel) caused by a vacuum leak, exhaust-gas recirculation occurring when it shouldn't, and so on. If the "shakes" are taking place following a hot restart of the engine, it can be the opposite scenario, maybe a leaking injector or fuel regulator sending too much fuel into the engine and causing an excessively rich condition. And depending on the mileage, some Jeep 4.0-liter engines have been victim to excessive carbon buildup on the back of the exhaust valves, causing miscellaneous misfire conditions. A temporary fix for the carbon buildup is the application of a top engine cleaner treatment; Jeep also came out with updated (stiffer) exhaust-valve springs to prevent it from happening again. Either way, it sounds like you've already invested time and money and have come up empty. You may want to get it to a Jeep service department (assuming you haven't already) before you take another guess. They're the most familiar with the 4.0-liter and have all the tools needed to perform an accurate diagnosis.
Sierra Fuel DeliveryQ:I have a 2002 GMC Sierra, 5.3-liter V 8, that starts rough, but runs fine once started. The engine catches after about 2-3 seconds. The idle isn't rough. I just changed the fuel filter and have run two tanks of gas through the system. The problem is getting progressively worse. I think it might have dirty fuel injectors. There are now 102,000 miles on this truck.
A:Dirty fuel injectors is a good guess, but it's still a guess. You have to be sure all the other ingredients are there, including good engine compression and spark from the ignition. But an extended crank is a fuel-related issue the majority of the time, and obstructed fuel injectors have been a common problem with this vintage GM system. Proper operation of the entire fuel delivery system must be confirmed first. That's all part of the correct testing procedure. Enough pressure and volume from the fuel pump to the fuel injectors is just part of what's needed for a quick start. Residual pressure is also important. When the ignition is initially turned on, the pump is activated for a few seconds to prime the system with fuel pressure. The unneeded high pressure is bled back to the fuel tank by the fuel-pressure regulator, while the correct amount of pressure is held steady at the injectors. The fuel pump has a check valve which prevents the pressure from bleeding back to the tank through the pump itself. This way there's constant pressure at the injectors when you start cranking the engine over. If a fuel regulator or fuel pump is defective and allows that crucial residual pressure to bleed back to the tank, you get an extending crank while waiting for the pressure to build up and fire the engine. Have the system tested by a professional. If the problem is with the injectors, you can choose between replacement or a specific cleaning procedure utilizing special tools and a top engine cleaner approved by General Motors. Anyone can replace the injectors. I'd be wary about a cleaning procedure being done effectively outside of a GM service department. The injector cleaner in a can which you pour into the fuel tank isn't going to fix the problem.
Too Much OverheatingQ:I have a 2001 Dodge Durango with 92,000 miles on it. This summer, it started overheating during a road trip. It's usually okay when I'm driving on a straight road, but the minute I hook a trailer to it or start pulling hills, it overheats! No mechanics have been able to diagnose the reason. I've had the following work performed on it and it still overheats: new heater core, new thermostat (even tried running without one), new water pump, new antifreeze, new radiator cap, and cooling-system power flush. I can drive it about 60 miles max, then the temp gauge rises so much I have to stop and all the water boils out all over the place. There's no water in the oil and no steam is coming out of the tailpipe, so I'm told it's not a head gasket. The fan clutch seems to be fine.
A:There are a bunch of things to check and double-check. Let's start from the front. Be sure there's nothing obstructing airflow through the radiator. Grille ornamentation, a plastic bag pinned to the front of the A/C condenser, a missing air deflector beneath the bumper, or leaf and debris buildup between the radiator and the A/C condenser, can all cut down on the essential airflow through the radiator. Also, you had the coolant-system power flushed, but what did that really accomplish? Even the best job won't clear a severely obstructed radiator. So you should bring the radiator to a fully equipped radiator shop where it can test the radiator's flow rate in gallons per minute. This will determine if the radiator still has the capacity to cool your engine. If 92,000 miles was the first time you checked your antifreeze, there could even be "grunge" partially obstructing the coolant passages within the engine block. High ambient temperatures and load on the powertrain (towing and climbing hills) maxes out a coolant system's capacity. That's why manufacturers offer towing packages with higher-capacity radiators and auxiliary transmission coolers. Is your Durango equipped? No water in the oil and no notable steam out the tailpipe isn't an all-clear on a leaking head gasket. Checking for hydrocarbons inside the coolant system's expansion tank with an exhaust gas analyzer is a more accurate test.
Troubleshooting a Dodge Sending UnitQ:My 1999 Dodge Durango's gas gauge always reads full. How can I fix it? The auto parts store says the problem is a bad fuse, but I checked all of them. Someone else suggested I'd have to take the gas tank out and look at the float.
A:Fuse? Not likely. There would be additional components out of commission if that were the case. Without going too far into Ohm's law (I = V/R), the basic operation of your fuel gauge is just as it was 40 years ago--a variable resistor attached to a float in the fuel tank. As the float rises with fuel level, sending-unit resistance decreases and the fuel-gauge reading goes up, vice versa as fuel level decreases. But from there it gets more complex. Instead of that variable ground circuit from the fuel-gauge sender going directly to the fuel gauge in the dash, it goes to the PCM, where the data is directed to the electronic instrument cluster and translated to a fuel-gauge reading. Here's something that might help narrow things down. If you can't reach the four-wire electrical connector on top of the fuel tank without dropping the tank, try to locate a larger connector at the frame rail under the left front fender. It should have a black/pink wire going in and a black/tan wire coming out of the cavity marked #1 and a dark blue wire at cavity #4. If you disconnect either one of these and the fuel-gauge reading drops to empty (ignition on)--assuming the tank isn't full of gas--then the problem is at the sending unit in the tank, and the tank has to come out. If the reading doesn't change at all, the problem is elsewhere and requires a technician to track it down. Failures in the tank could include a bad sending-unit resistor, a poor internal electrical connection or the float hanging up, possibly due to a loose baffle within the fuel tank.
Circling the (Battery) DrainQ:I bought a 2007 Silverado extended-cab LTZ 4x4 with the Z71 option. Two dealerships and General Motors told me there are so many electronics operating all the time the battery will run down if sitting for an extended period. With mine, it only takes about three weeks. Why is no one investigating and reporting on the battery rundown problem with Chevy Silverado trucks?
A:Does your truck use more current while shut down than others? It's possible--a new four-wheel-drive Silverado has more than 25 computers on board. Nowadays, there's a module for almost everything. And yes, there was a battery-drain issue which was corrected by reprogramming the Body Control Module. It was a campaign affecting certain dealer inventory and in-service vehicles. And I have it from a reliable source that the reprogramming did correct the problem as intended. But there are a few other things to look for in your situation. One is the use of any and all aftermarket (or GM-sanctioned) accessories that have been wired into the courtesy-lights circuit. This may prevent that same BCM from powering down, therefore draining the battery dead. The correct testing for a parasitic draw (current used from the battery while everything is off) is a must to be sure it's not excessive. It's also important that the battery itself be tested accurately, or you may wind up with a borderline unit that drives you crazy. GM has gone as far as stressing how important it is to have the battery cables properly clamped to the battery. Last, but not least, you may very well be letting it sit too long, just like everybody's been telling you. General Motors notes that letting a vehicle sit for 30 days or driving only short distances once a week is abnormal use and can kill a battery. An engine has to run for long periods of time, above 1000 rpm, for the alternator to fully charge a battery. So going out in the garage once in a while and running it for a few minutes won't really help. Sounds like your truck is somewhere in the middle of all that. If that's the case, your choices are to alter your driving schedule, disconnect the battery when parked, or use a slow-charger specifically designed for maintaining a vehicle's battery charge while in storage.
How To Reach AlexIf you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at trucktrend@sourceinterlink.com. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.
Can't wait for help with a problem you're having with your Truck or SUV? Ask the expert we trust here at Truck Trend Garage--visit Alex Steele at www.RealWorldAutomotive.com.
