Nothing sucks worse than heading out with a group of friends, packed with camping essentials for a weekend getaway, only to discover that your engine caught an illness along the way and is in need of diagnosis. Aside from being the guy who halts the group from the trailhead, you now have to unpack the tools and get your hands dirty. No vehicle is immune to this scenario, and despite ample preparedness, even the most accomplished mechanic can find himself searching for answers. For this story, we consulted the professionals at Overkill Jeep Fabrication in Campbell, California, to extract years of knowledge from shop owner Jeff Arabia. Follow along as we highlight some of the most common issues associated with Jeep vehicles and suggest ways to identify and resolve them.
Problem: Running Hot
Most cooling system issues are usually the result of poor maintenance and/or a lack of general upkeep.
Aside from spiking the temperature gauge, a hot-running engine will usually let you know it’s unhappy by venting off pressure in the form of steam. However, other signs can include a loss of power, a knocking or pinging sound, and even a loss of fuel economy.
First, ensure that airflow to the radiator is not restricted by foreign materials such as mud, leaves, or other debris. Next, make sure the fan is working properly. Inspect the condition of the coolant. Coolant discoloration is a sure sign of contamination. If left unchecked, contamination results in efficiency-robbing buildup inside the system. Another cooling issue is when excessive air gets trapped inside a high point of the system. This is known as cavitation. Keep in mind that the cause of overheating may be a combination of issues, but with a few simple steps you can figure out the best plan of attack.
Obvious issues such as contaminated coolant or a faulty fan clutch are easy to identify and fix. However, when it comes to part failures such as blown head gaskets, a bad thermostat, or an ineffective water pump, the diagnosis can be tricky. Start by checking coolant temperatures on each side of the radiator. A stuck thermostat will prevent the transfer of liquid between the radiator and engine block. If a large difference in temperature is observed, chances are that the jammed thermostat or worn water pump is not allowing the fluid to move. Otherwise, you may discover that the head gasket has failed and is letting engine coolant escape from the system. In this case, you need to determine if the gasket is leaking. There is more than one way to do this. You can do a compression check using a compression gauge that installs in place of the spark plugs to measures the specific pressure of each cylinder. Another proven method is a combustion leak test. To do so, a simple hydrocarbon test kit can be purchased for right around $50. This device will expose hydrocarbons that may have escaped from the combustion chamber through the head gasket. If engine overheating is caused by cavitation or air bubbles in the system, you may simply need to fill the radiator on a steep incline to allow trapped air to burp out of the system. By making the radiator cap the highest point in the system, air bubbles typically can find their way out. If you do this and the problem continues, chances are you have a coolant leak somewhere.
Problem: Bad Sensor
All Jeep engines require fuel, air, and a spark to run. Such essentials are typically easy to check in the field if you have the appropriate tools. Electronic sensors, such as an idle air control valve (iac), throttle position sensor (tps), or an O2 sensor, can falter and negatively impact engine performance. With a common multimeter tool, a wiring diagram, and a few specific pieces of information about the acceptable voltage or ohm range for a given application, anyone can diagnose a bad sensor.
Failing sensors will typically result in poor or erratic idle, hesitation during acceleration, or diminished fuel economy. These symptoms can usually be attributed to one of the following sensors: idle air control valve (iac), or throttle position sensor (tps). Malfunctioning oxygen sensors (O2) can also wreak havoc on fuel economy, and we’ll address them as well.
Instead of 12 volts like everything else in the modern automotive world, sensors operate between 0 and 5 volts. Most tps sensors on Jeep vehicles have three wires: positive, negative, and a computer signal or output wire. Prior to testing, make sure that the vehicle is not running, but that the key is the “On” position. Connect the negative lead of the multimeter to the negative battery post. Next, probe each wire with the positive lead of the multimeter to establish which wire is the output to the ECU. The wire you want usually reads between 0-5 volts. Keep the positive lead on the output wire. For a tps, you can manually cycle the butterfly valve on the throttle body and watch the multimeter sweep the range from 0-5 volts. If you are using a digital meter, do it somewhat slowly. Smacking it wide open and dropping it closed won’t normally register on a digital meter. If you notice any significant numerical jump within the range, you have found a “dead spot” that will cause hesitation during acceleration. The iac valve is a stepper motor that moves a pintle in or out to adjust how much air can bypass the butterfly valve. Over time, the interior of the throttle body gets gummed up; this interferes with how much air flows past the pintle of the iac valve. Treating the throttle body to a quick solvent bath usually fixes this. However, in some cases you will need to remove the iac from the throttle body and clean the inner air passages along with the pintle and seat. There are two ways to test O2 sensors—with the engine running at normal operating temperature or with the key and engine off. While running at normal operating temperature, the output voltage should fluctuate between 0-1 volts consistently. If the sensors voltage remains constant with variation to engine rpm, the sensor is probably bad. With the engine off, you have to check the sensor for resistance. First, you need to know the specific ohm range associated with your sensor. You can get this information from any decent technical manual.
After performing the tests mentioned above you should have an idea of which sensor is causing the vehicle to run poorly. Replace sensors as needed.
Problem: Clogged Catalytic Converter
A clogged catalytic converter typically means a trip to the exhaust shop, unless of course you can weld a new one on at home.
When your catalytic converter becomes clogged, a rotten egg smell is usually present. That’s because hydrogen sulfide, which is produced by trace amounts of sulfur in gasoline, is no longer being converted to sulfur dioxide by your catalytic converter. In addition to the smell, failing catalytic converters will sometimes make rattle noises, and a noticeable reduction in power will occur as the backpressure increases significantly.
The easiest way to test a bad catalytic converter is by simply unbolting the collector flange up-stream of the unit. Typically, a 1⁄2-inch gap is all it takes to allow the exhaust to bypass the catalytic converter. If the symptoms go away, you know you have found the problem.
Unfortunately catalytic converters are not cheap, and because they are an emissions device, the feds will lock you up if you try to skate around replacing one (i.e., hollowing them out or removing them altogether). The best practice is to pay the piper and replace it.
Problem: Misfires, Backfires and Vacuum Leaks
When an engine runs rough, misfires, or simply demonstrates inconsistent throttle response, start with the ignition system.
A misfire is a stumble or dip in power and can originate from the ignition system. On older Jeeps, a moving part is usually the culprit. Items such as worn points inside the distributor or oxidation on the cap or rotor terminals can cause it. When a misfire occurs on a late-model, distributorless rig, chances are that it is a spark plug wire, spark plug, coil, coil pack, or a funky fuel injector. A failing fuel injector can create a big problem for your engine. They are electro-magnetic valves that dispense a metered amount of fuel. The injector has several salt-shaker-sized holes in it that can become clogged over time. Injectors can also wear out with age. In either case, you get the same result—the engine lacks power and runs poorly. Vacuum leaks can cause the vehicle to idle unusually high, roughly, or cause hesitation during acceleration.
To check the ignition system on older vehicles, pull the cap off and look for corrosion on cap terminals. As the spark jumps from the rotor to these terminals, oxidation occurs and crud starts to build up. Over time, this buildup can impede the path of electricity to the spark plugs, resulting in a misfire. The same principle applies to the points inside a distributor. On fuel-injected vehicles that don’t have distributors, spark plug wires and/or coils are the usually the culprit. Wires are inexpensive and easy to test, so we won’t get into that. Coils can be tested with the ohm function of a multimeter. However, each type of coil has a different procedure and range of specifications. Basically, you are looking for a specific ohm range between the primary and secondary coil circuits. For example, the spec on a ’97 TJ with a 4.0L should be between 0.95 and 1.2 ohms on the primary circuit and between 11,300 and 13,300 ohms on the secondary circuit. Consult a good technical manual for specifics on your particular application. If the ignition system checks out okay, turn your attention to the fuel injection. To diagnose a bad fuel injector, parts suppliers sell a device called a noid light. For under $20 you can purchase a noid light kit containing several of these simple plug-in testers that light up when the signal from the ECU reaches the injector plug. This tool helps you diagnose whether or not the problem is within the injector itself, or upstream in the signal coming from the ECU. If the noid light proves that the signal is reaching the injector properly, the next thing is to test resistance across each injector. Fuel injectors should have a minimum resistance of 12 ohms. To check using a multimeter, set the scale to 100 ohms. Next, connect the ohm meter to the two pins of the injector. Take note of each injector’s resistance reading. If you discover any one injector with a noticeable difference in resistance, it is probably bad and needs to be replaced. To find a vacuum leak, you don’t need a fancy tool. A simple can of starter fluid and a good ear are all it takes. Vacuum leaks introduce air into the system, which tends to lean out the combustion process. Spraying starter fluid around suspected vacuum leaks will draw combustible gasses into the system and return air/fuel ratios to a better state.
Once you have established the cause of the misfire, simply replace the parts necessary to correct the problem.
Finding Top Dead Center
If you need to find top dead center (tdc) on an engine but don’t have a reference mark to work with, don’t fret. It’s easier than you think. Scenarios such as replacing a distributor or checking someone else’s work when an engine refuses to run properly require you to physically check for tdc. The first step is knowing when the number one piston is on its compression stroke. To do so, start by loosening the Number One spark plug. Don’t remove it completely; leave it loose enough so that air can escape from the cylinder around the threads. Using a ratchet with appropriately sized socket to turn the engine over by hand, rotate the engine and listen for the sound of air hissing. Once you hear air hissing past the threads of the spark plug, you will know that the Number One cylinder is starting upward travel on the compression or exhaust stroke. Once the compression stroke is started, remove the spark plug and insert a decent size screwdriver or similarly shaped tool. Rotate the engine slowly. You will notice the shank of the screwdriver emerging from the hole. At top dead center, the majority of the screwdriver will remain outside the spark plug hole. If you keep going, the piston will start to drop during the compression stroke and the screwdriver will sink into the hole along with it. Top dead center is that high point right before the screwdriver recedes into the cylinder.