18 August 2016

BY PAUL WEISSLER

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.

AVOIDING HOT WATER WITH COOLING SYSTEM DTCs

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19August 2016

AVOIDING HOT WATER WITH COOLING SYSTEM DTCs

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 passen-gers 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 perfor-mance. 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 in-formation 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 some-thing 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 sen-sor can be. Basically, what irrational means is that the operation of the thermostat or reading from the sensor is signifi cantly different from what the powertrain control module (PCM) thinks it should be, based on comput-er modeling plus readings from other sensors that seem to align. This makes the “irrational” one the anomaly.

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for the code: Which sensors and switches the computer is monitor-ing, and which minimums (particu-larly 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 condi-tions 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 sys-tematic approach. It’s almost auto-matic, however, that when it comes to cooling systems, there’s virtually a compulsion among many techni-cians 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

Why is this even meaningful? The MIL is the most visually obvious. But going deeper, the on-board diagnos-tics want to ensure that the engine warms up both quickly and suffi cient-ly enough after a cold start to put the engine into closed-loop emissions control ASAP. And it takes a long train of logic to fi gure 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 specifi c 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 trou-ble 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 SetIf you check your service informa-tion, you’ll fi nd the following under the breakdowns of the trouble codes:

•Conditions to run the algorithm

IAT MAF ECT

MIL

CHT

PCM

VS

The Ford thermostat algorithm for rationality monitors fi ve inputs: intake air temperature, mass airfl ow, engine coolant temperature, cylinder head temperature and vehicle speed. Inputs report to the powertrain control module, which may turn on the MIL.

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20 August 2016

diagnostics on Ford and Fiat Chrysler products also may log the more famil-iar 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 under-stand 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 con-tent than gasoline and that affects en-gine 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 cool-ant to reach a specifi c temperature.

Let’s look at some examples, being cognizant of the fact that all tempera-tures and ranges cited in this report are strictly that—examples. Also, be aware that the service information systems don’t provide all of the “sub-conditions” evaluated by software al-gorithms in the PCM.

At Fiat Chrysler, the typical P0128 diagnostic will run as long as the cool-ant 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 tem-perature (34°F or higher) and the ambient temp. For lower coolant temperatures, more time is allowed.

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Circle #12

Avoiding Hot Water With Cooling System DTCs

Heater Terminal

The presence of a heated General Motors thermostat can be confi rmed by the electrical con-nector on the thermostat housing, with reference to the wiring color-coding.

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Avoiding Hot Water With Cooling System DTCs

housing. On the Cruze, for exam-ple, 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 exam-ple, the 1.8L 4-cylinder has just a con-ventional 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 informa-tion in the description of the P0128 algorithm. The diagnostics look at everything that can affect this, starting with the fact that the en-gine 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 tem-perature 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 tem-perature), P0102 (mass airflow low voltage), P0103 (mass airflow high

voltage), P0111 (intake air tempera-ture sensor 1 performance), P0112 (intake air temperature sensor 1 volt-age low), P0113 (intake air tempera-ture sensor 1 voltage high), P0116 (coolant temperature performance), P0117 (coolant temperature voltage low), P0118 (coolant temperature volt-age high), P0119 (coolant temperature voltage intermittent), P0122 (TPS sen-sor 2 voltage low), P0123 (TPS sensor 2 voltage high), P0221 (sensor 2 per-formance), 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 com-manded 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 read-ings are okay, the engine run time is under 30 minutes (but more than 70 seconds), intake air temperature is

At Ford, a typical precondition for the thermostat monitor is that the start-up air temperature (as mea-sured from the intake air tempera-ture 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 moni-tor begins when vehicle speed is over 15 mph and engine load is over 30%. The Ford PCM, as noted, also is like-ly to set DTC P0125 (insufficient coolant temperature for closed-loop operation). This is an example where code-logging criteria are closely relat-ed, including both the thermostat and the cylinder head temperature sen-sor—and/or if the engine has a con-ventional coolant temperature sensor, that, too.

Ford does have a level of complexi-ty from use of the cylinder head tem-perature sensor, which sits in a well in the head and never actually touch-es coolant. That sensor may coexist with a conventional in-coolant sensor.

GM Heated ThermostatThe greatest complexity, however, is at General Motors, where some engines have a heated thermostat, which you can identify by the two-wire connector on the thermostat

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.

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14° to 138°F and mass airfl ow is up to 100 grams/second (g/S). On vehicles with both radiator and engine coolant temperature sensors, the PCM sim-ply 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 heat-ed thermostat, there is electronic control of coolant fl ow. 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 specifi c for the 1.4L turbo: P0116, P0B3 and P0B6. DTC P0116 is an ECT perfor-mance 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

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Circle #14

Avoiding Hot Water With Cooling System DTCs

24 August 2016

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.

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Avoiding Hot Water With Cooling System DTCs

no problem in that sensor. That’s true, to some extent. So the fi rst in-spection step is to check the cool-ant 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 con-nections, corrosion, etc., at a sensor increase resistance, which provides a false indication of a cold tempera-ture. Another problem is when the sensing circuitry provides an irratio-nal coolant-warmer signal, such as from a defective sensor or voltage feed through a damaged harness.

There are three codes for GM heat-ed 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 test-ing. 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 fl ow is another pos-sible problem. Electric water pump circuits can log trouble codes for this. But there also is an algorithm for GM

engines with a mechani-cal pump and that heated stat. The logic checks for a straightforward validation of the radiator and engine coolant sensors, then com-pares the sensor readings vs. any changes in ther-mostat duty cycle. But it’s a gross test only, as cool-ant temperature has to be more than 176°F. There must also be a difference of 104°F between the ra-diator sensor and the en-gine 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 cool-ant 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 cool-ant fl ow is transferring a lot of heat to the cabin and lowering coolant tem-perature, this would be an unexpect-ed 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 moni-tors 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 com-puter will log DTC B104D (blend-air 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.

be commanded on and the compressor will be com-manded off.

DTCs P0B3 (voltage low) and P0B6 (voltage high) are for the radiator coolant tem-perature sensor—an addi-tional sensor. Is another cool-ant sensor really useful? Keep reading for an example.

The heated thermostat comes into the picture un-der the following conditions for running the P0128 al-gorithm: engine off for at least 30 minutes, ECT read-ing 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 com-manding 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 be-tween 50° and 126°F; the second model says the target is 138°F but coolant temperature is measured at under 55°F, with ambient tempera-ture between 19° and 50°F.

That Cruze heated stat also has tar-gets based on reaching 194° to 212°F in 18 minutes or less at 80°F am-bient, 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 hous-ing and a connector with wiring that matches the color-coding.

Diagnostic ApproachesIt 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

This article can be found online at www.motormagazine.com.

Most Ford engines and some GM engines—that use the engine pro-tection 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 fi nd a reason to remove this sensor, it’s a discard; do not reuse it.

Cylinder Head Temperature Sensor

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