AVOIDING HOT WATER WITH COOLING SYSTEM DTCs BY PAUL WEISSLER Photo: Thinkstock Meeting strict emissions targets requires tight cooling system control. Additional cooling system DTCs can aid your diagnostic efforts, but can t be taken at face value. 18 Thermal.indd 1 7/18/16 4:59 PM
Photo: Thinkstock Remember when the cooling system had the simple jobs of keeping the engine in an approximate temperature range and giving up enough heat through the heater core to keep the passengers comfortable in winter and the windshield defogged? These still are the basic jobs, but now the cooling system has a more precise range in which it has to operate, and it gets a number of other tasks related to emissions control and engine performance. These additional jobs have a high priority, too. Here s a graphic example: The Check Engine light comes on. Most technicians actually like this situation, because it means there s an emissions control issue, and the OE service information systems should have a lot of diagnostic details to help isolate the problem. And the motorist wants the MIL to go out, of course. So the technician connects his scan tool and looks for a diagnostic trouble code. In this case, there s a powertrain DTC, P0128. A look at the trouble code list reveals something about thermostat rationality. Or, at General Motors, there ll be an engine coolant temperature below thermostat regulating temperature message. Although we focus on Ford, Fiat Chrysler and General Motors for examples in this report, you ll find DTC P0128 is across the board on import nameplates, too. What s Irrational? Can a thermostat be irrational? Sure, just as a coolant temperature sensor can be. Basically, what irrational means is that the operation of the thermostat or reading from the sensor is significantly different from what the powertrain control module (PCM) thinks it should be, based on computer modeling plus readings from other sensors that seem to align. This makes the irrational one the anomaly. Why is this even meaningful? The MIL is the most visually obvious. But going deeper, the on-board diagnostics want to ensure that the engine warms up both quickly and sufficiently enough after a cold start to put the engine into closed-loop emissions control ASAP. And it takes a long train of logic to figure that out. If there s one thing you shouldn t do, it s simply replace parts in this case, the thermostat. For many years you could assume a code pointed to a specific circuit and take it from there. But that s not necessarily the case with an increasing number of codes, which are proliferating as the carmakers look to improve the potential diagnostics. By now, experienced technicians have learned that the part named on a code is not necessarily the replace with known good part at the end of a trouble tree. A most recent example we observed was the Ford DTC P0191 for a fuel pressure sensor that was caused by a defective PCM. Cooling system codes are not excluded from this scenario. Causing the Codes to Set If you check your service information, you ll find the following under the breakdowns of the trouble codes: Conditions to run the algorithm IAT MAF ECT PCM for the code: Which sensors and switches the computer is monitoring, and which minimums (particularly battery voltage, engine off time, ambient temperature and coolant temperature) are required and, of course, which electronic control module(s) is/are involved. What conditions caused the code to set. What are the prevailing conditions with the code logged? These are particularly important in the search for a cause. For example, the a/c compressor might be commanded off, a warning light may turn on or observable engine performance limits may occur. If this were a driveability issue, most technicians would take a systematic approach. It s almost automatic, however, that when it comes to cooling systems, there s virtually a compulsion among many technicians to start out by installing a new thermostat. When it comes to a DTC P0128, the urge is nearly irresistible as an opening approach. First, let s understand why this code turns on the MIL. It means the emissions will be higher after all, most engine emissions come right after a cold start. Depending on the parameters monitored, the on-board CHT The Ford thermostat algorithm for rationality monitors five inputs: intake air temperature, mass airflow, engine coolant temperature, cylinder head temperature and vehicle speed. Inputs report to the powertrain control module, which may turn on the MIL. MIL VS 19
Avoiding Hot Water With Cooling System DTCs PENNZOIL PLATINUM HE OIL FILTER Synthetic-blended 2-ply gradient density media has a 99% efficiency* Heater Terminal The presence of a heated General Motors thermostat can be confirmed by the electrical connector on the thermostat housing, with reference to the wiring color-coding. Greater capacity compared to economy oil filters helps reduce engine wear and helps promote longer engine life Pennzoil Platinum HE oil filters outperform economy oil filters when it comes to efficiency and capacity. So your customers get maximized performance and you get a maximized bottom line. Contact your local Shell sales representative about new Platinum HE oil filters. *For particle sizes 30 microns. SOPUS Products 2016. All rights reserved. CS10444-08 diagnostics on Ford and Fiat Chrysler products also may log the more familiar DTC P0125 (coolant temperature too low for closed-loop), along with P0128. General Motors does not use P0125 in its diagnostic architecture. What s Behind DTC P0128? The algorithm for DTC P0128 runs once per drive cycle, of course, but varies according to manufacturer. So never make any assumptions. On GM and Fiat Chrysler cars, for example, the MIL goes on only if the code is detected during a second ignition cycle. And the DTC set parameters are not always the same even among models of the same make. Check your service information system. It s also important to understand that the algorithm is based on a computer model for that particular engine and its state of tune, plus a full range of operating conditions. They include ambient and coolant temperatures, even the type of fuel (with a flex-fuel vehicle, ethanol in the tank means lower energy content than gasoline and that affects engine power and, therefore, warm-up characteristics) and items you might not even think would affect coolant temperatures. Based on all of these things and perhaps others hidden in software code, the PCM will predict how long it should take for the coolant to reach a specific temperature. Let s look at some examples, being cognizant of the fact that all temperatures and ranges cited in this report are strictly that examples. Also, be aware that the service information systems don t provide all of the subconditions evaluated by software algorithms in the PCM. At Fiat Chrysler, the typical P0128 diagnostic will run as long as the coolant temperature on a cold-engine start is below 122 F, the ambient is between 18 and 122 F, the engine is running and the vehicle is being driven at over 10 mph until coolant temperature reaches 185 F. DTC P0128 (and likely P0125, too) logs when the coolant temperature does not reach a predicted target, depending on start-up coolant temperature (34 F or higher) and the ambient temp. For lower coolant temperatures, more time is allowed. Circle #12 20 Thermal.indd 3 7/18/16 4:59 PM
Avoiding Hot Water With Cooling System DTCs Left: This scan tool screen shows engine-off readings, including a coolant temperature of 132 F and the thermostat heater set for 10%. Right: This screen capture was taken from the same vehicle with the engine running. The thermostat heater is set for 100% and the coolant temperature is rising. Screen captures: Paul Weissler At Ford, a typical precondition for the thermostat monitor is that the start-up air temperature (as measured from the intake air temperature sensor) must be between 20 F and a target temperature determined by the PCM, after a minimum of two hours of engine-off time. The monitor begins when vehicle speed is over 15 mph and engine load is over 30%. The Ford PCM, as noted, also is likely to set DTC P0125 (insufficient coolant temperature for closed-loop operation). This is an example where code-logging criteria are closely related, including both the thermostat and the cylinder head temperature sensor and/or if the engine has a conventional coolant temperature sensor, that, too. Ford does have a level of complexity from use of the cylinder head temperature sensor, which sits in a well in the head and never actually touches coolant. That sensor may coexist with a conventional in-coolant sensor. GM Heated Thermostat The greatest complexity, however, is at General Motors, where some engines have a heated thermostat, which you can identify by the twowire connector on the thermostat housing. On the Cruze, for example, the color codes are red/white and blue or violet/blue and blue. Many European vehicles also use a heated stat, but unlike GM, they don t have the possibility of either in the same vehicle. On the Cruze, for example, the 1.8L 4-cylinder has just a conventional stat, and the 1.4L turbo may or may not have the heated type. A cylinder head sensor similar to Ford s also is used on some GM engines. Happily, there s a lot of information in the description of the P0128 algorithm. The diagnostics look at everything that can affect this, starting with the fact that the engine typically must be off for at least 30 minutes, intake air temperature must be above 14 F and the coolant temperature sensor reading must be between 14 and 167 F. Next are the sensors that check coolant temperature and how they compare with the algorithm. With the 1.8L, for example, the PCM first checks to determine that a long string of sensor (and other) readings that affect coolant and air temperatures are not set, specifically: P0101 (mass airflow/intake air temperature), P0102 (mass airflow low voltage), P0103 (mass airflow high voltage), P0111 (intake air temperature sensor 1 performance), P0112 (intake air temperature sensor 1 voltage low), P0113 (intake air temperature sensor 1 voltage high), P0116 (coolant temperature performance), P0117 (coolant temperature voltage low), P0118 (coolant temperature voltage high), P0119 (coolant temperature voltage intermittent), P0122 (TPS sensor 2 voltage low), P0123 (TPS sensor 2 voltage high), P0221 (sensor 2 performance), P0222 (TPS sensor 2 low voltage) and P0223 (TPS sensor 2 high voltage). If you find that the DTC lists contain some codes that don t apply to the particular engine in front of you, just ignore them. This kind of information helps guide a technician s diagnostic path. You ll also notice that DTC P0128 is accompanied by the fans being commanded on, the coolant temperature gauge is off and the a/c compressor will not engage. GM also has a simple algorithm to find a thermostat stuck open. If you get DTC P2181 (engine cooling system performance), it really means that all of the coolant sensor readings are okay, the engine run time is under 30 minutes (but more than 70 seconds), intake air temperature is 22
Avoiding Hot Water With Cooling System DTCs With the thermostat heater off or inoperative, the thermostat has a conventional mechanical backup with a wax pellet that will open the valve and control coolant warm-up once the coolant temp reaches 176 F or if it becomes inoperative. 14 to 138 F and mass airflow is up to 100 grams/second (g/s). On vehicles with both radiator and engine coolant temperature sensors, the PCM simply compares the two readings during a cold start. If they re very close for 2½ minutes when the stat should be closed, that says the stat is stuck open. Here again, the radiator sensor may not be used, in which case the algorithm may not be available. Where the 1.4L turbo has the heated thermostat, there is electronic control of coolant flow. DTC P0128 also checks to see that the PCM did not log, specifically, the following codes: P0101, P0102, P0103, P0111, P0112, P0113, P0117 or P0118, or three that on the Cruze are specific for the 1.4L turbo: P0116, P0B3 and P0B6. DTC P0116 is an ECT performance code that logs after 8 hours of ignition off if it sees a large difference between coolant temperature and air temperature (in one calibration, ECT must be 68 F greater than intake air temperature). Once again, fans will Replace a Jiffy-tite WITH a Jiffy-tite 500 million THE WORLD LEADER IN OIL COOLER CONNECTORS That s how many Jiffy-tite parts are in service worldwide today. In fact, chances are whether repairing an automatic transmission, radiator, or auxiliary cooler, the OEM used Jiffy-tite fluid fittings. Replace a Jiffy-tite with a Jiffy-tite to maintain premium quality. 888.605.7788 aftermarket.jiffy-tite.com Circle #14 24 Thermal.indd 5 7/18/16 5:00 PM
Avoiding Hot Water With Cooling System DTCs be commanded on and the compressor will be commanded off. DTCs P0B3 (voltage low) and P0B6 (voltage high) are for the radiator coolant temperature sensor an additional sensor. Is another coolant sensor really useful? Keep reading for an example. The heated thermostat comes into the picture under the following conditions for running the P0128 algorithm: engine off for at least 30 minutes, ECT reading under 165 F, engine run time over 30 seconds (and under 30 minutes) and more than.6 mile of vehicle travel, ethanol in the fuel less than 87% and PCM commanding lower than 20% duty cycle of the thermostat. All of the data is matched against two engine performance models. One model says the temperature should be at a specifi c target (in this case, 174 F), but according to the ECT it s no more than about 127 F when start-up ambient temperature is between 50 and 126 F; the second model says the target is 138 F but coolant temperature is measured at under 55 F, with ambient temperature between 19 and 50 F. That Cruze heated stat also has targets based on reaching 194 to 212 F in 18 minutes or less at 80 F ambient, in 26 minutes or less at 50 F ambient or in 22 minutes or less at 68 F ambient. The important caveat here: Not all versions of the Cruze 1.4L turbo have that stat; there has to be a wiring terminal on the stat housing and a connector with wiring that matches the color-coding. Diagnostic Approaches It might seem that with so many codes not set, diagnostics should be straightforward, particularly when if a code is not set, it means that there s no problem in that sensor. That s true, to some extent. So the first inspection step is to check the coolant level, or air or exhaust gas in the coolant. In the case of the heated stat, GM recommends running the engine for 15 minutes with the a/c off, raising engine rpm to 3000 and commanding the stat to zero. If you don t get at least 174 F, the stat is defective; if you do, the cause of the code is within the circuit. Bad connections, corrosion, etc., at a sensor increase resistance, which provides a false indication of a cold temperature. Another problem is when the sensing circuitry provides an irrational coolant-warmer signal, such as from a defective sensor or voltage feed through a damaged harness. There are three codes for GM heated stat failures: P0597 (heater circuit), P0598 (circuit low voltage) and P0599 (circuit high voltage). But those are for total failures, and you may have better diagnostic results from duty cycle testing. That stat heater failure is unlikely to cause overheating, as the stat has a mechanical backup, so it starts to open at 176 F. Poor coolant flow is another possible problem. Electric water pump circuits can log trouble codes for this. But there also is an algorithm for GM Cylinder Head Temperature Sensor Most Ford engines and some GM engines that use the engine protection algorithm to prevent engine damage when there s a major loss of coolant have a sensor resting in a cylinder head well, where it senses the temperature of the metal instead of the coolant. This sensor also is used for a variety of trouble code algorithms. If you find a reason to remove this sensor, it s a discard; do not reuse it. engines with a mechanical pump and that heated stat. The logic checks for a straightforward validation of the radiator and engine coolant sensors, then compares the sensor readings vs. any changes in thermostat duty cycle. But it s a gross test only, as coolant temperature has to be more than 176 F. There must also be a difference of 104 F between the radiator sensor and the engine sensor readings, with the latter at over 248 F. With any DTC P0128 diagnosis, you always have to keep in mind that anything that affects coolant temperature must be monitored by algorithms, even if it s not in any troubleshooting charts. For example, if the HVAC blend-air door is stuck in the Warm position but the control is in the Cold position, it s the Cold position that s the signal to the PCM from the body computer. If the coolant flow is transferring a lot of heat to the cabin and lowering coolant temperature, this would be an unexpected result if the blend-air door truly was in the Cold position. So always check to be sure the door operation responds normally to the control on the center stack. Fiat Chrysler monitors blend-air door position, but bases coolant temperature calculations on the setting. Not all makes cover this issue. If there s a short to ground on a Fiat Chrysler product, the body computer will log DTC B104D (blendair control circuit low voltage), but a physical break in a mechanical circuit (or a vehicle without the control logic for this condition) could mimic the problem without the code. This article can be found online at www.motormagazine.com. 26