How to address a complex intermittent coolant system issue
Vehicle affected: 2008 Chevrolet Tahoe LT
Concern: Intermittent engine overheating
Tools used:
- Personal Safety Equipment
- Refractometer or Coolant Test Strips
- Vehicle Repair Information
- Diagnostic Scan Tool
- Radiator Pressure Tester
- DMM with Temperature Probe
- Lab Scope with Pressure Transducer
- Borescope
- Various Hand Tools
- Matco Flat Clamp Pliers
- Vacuum Lift Tool
At first glance, engine cooling systems may seem very basic. For the most part, they have not changed significantly for many years. But when you consider some of the technology in today’s vehicles and add in an intermittent issue, problems in a cooling system may no longer be so basic.
Every time a vehicle comes into the shop with a customer concern, the service advisor should obtain enough information from the customer so the technician is able to duplicate the issue. When the concern is intermittent, it is necessary to get as many details as possible to duplicate the issue in order to save time in the diagnostic process. After speaking with the customer, we determined that the vehicle was running hotter than normal at various times. It would make no difference whether the air conditioning was on or off, and it would happen in traffic and on the highway. Since there was no specific information from the customer indicating a pattern, it was necessary to inspect the complete system as well as perform a lengthy road test to confirm the issue.
Step 1: Inspect the system
There were no indications of overheating during the road test in normal driving, but upon hard acceleration on a hill the engine temperature started to rise fairly quickly. Since the issue seemed to be somewhat intermittent, or at least related to high load conditions, a thorough inspection of the cooling system was necessary.
Start with looking at the recovery bottle to determine the level, color and condition of the coolant. Keep in mind, the coolant in the recovery bottle may be slightly different from what is in the radiator, but the level and color may give clues to what is happening in the system.
Squeeze the hoses to not only determine their condition, but to see how much pressure is in the system and if it is safe to remove the radiator cap. Please remember to use all of your personal safety gear, including protective eyewear, gloves and, since the cooling system may be hot, a heat-resistant apron.
After you remove the cap, use a test strip or refractometer to obtain an actual reading of the quality of the coolant. It is important to understand how the cooling system operates and how both the quality of the coolant and pressure work together to keep the engine from overheating.
Coolant in the correct proportion (50 percent) provides rust protection, lubrication and raises the boiling point of the cooling system. When pressure raises in the system and is properly controlled by the radiator cap, the boiling point also increases. In most cases, a properly filled cooling system with a 50/50 mix of coolant and water, as well as a functioning radiator cap keeping the pressure at approximately 14 lbs, will raise the boiling point from 212 to 270 degrees F.
Our inspection of the Tahoe cooling system revealed that the system was full, but the test strip showed the pH level was too high (See Fig. 1). Additionally, the hoses were original and appeared soft. Based on the initial inspection, we felt that the hoses should be replaced and the cooling system serviced with new coolant, but those items would most likely not cure an intermittent overheating issue.
The next step in diagnosing almost any issue on a late-model vehicle will require using vehicle information to determine if there are any service bulletins related to the issue. In this case, there were none.
Additionally, it is necessary to use a scan tool almost every time to determine if there are any codes or communication issues. Having the capability to use bi-directional commands to operate components like cooling fans and observe data from sensors such as the coolant temp sensor is also necessary. In the case of our Tahoe, there were no fault codes and we were able to use the scan tool to operate the cooling fans at both a low and a high speed.
Since the customer reported that the overheat was occurring with the A/C on and off, we decided that it would not be necessary to go into a deep diagnosis of the air conditioning system, but we did perform a system check to make sure it was operating properly.
Additional items to be checked with the scan tool are the CTS (Coolant Temperature Sensor), Fuel Trims which can cause an engine to overheat if it is too lean, and the Knock Sensor (if equipped) which can retard the timing and cause an overheat. The rear oxygen sensor and intake air temperature sensor also should be checked, as they can indicate a problem with the catalytic converter, allowing an exhaust restriction to keep heat in the engine.
Step 2: Perform deeper diagnostic process
So far all of the typical tests have not revealed any issue that could cause overheating, so it is time to be a little creative in the diagnostic process. Many newer vehicles have remote plastic reservoirs that use a screw-on radiator cap. Quite a few of these vehicles are difficult to pressure test the system and the cap because it seems as if you never have the correct adapters. We use the Snap-on Tools pressure kit for pressure testing the system as well as the radiator cap (see Fig. 2).
Additional tests were performed by using a DMM with a temperature probe to determine if the radiator was doing its job by cooling the flowing coolant down. In this case, the temperature measured 225 degrees F at the top of the radiator (see Fig. 3) and 200 degrees F near the bottom (see Fig. 4). At the ambient air temperature of the day and the fact the cooling fan was not running at the time of the test, the temps were what we expected to see.
For this vehicle, we decided to use our lab scope with a pressure transducer because we wanted to eliminate the possibility of a leaking head gasket as well as observe the pressure waveform, checking for cavitation of the water pump. We made an adapter for the pressure transducer that can be installed into either the heater hose inlet to the heater (see Fig. 5), or in some cases, into the hose from the radiator to the coolant reservoir.
When you start a cold engine, normal pressure readings with a lab scope should show no pressure followed by a slight vacuum as the thermostat opens. Once the thermostat opens, there should be a steady increase of pressure with no spikes and no significant pressure pulses (see Fig. 6).
High-pressure pulsations will be an indicator of high pressure leaking into the cooling system, likely due to a leaking head gasket or cracked cylinder head. Steady pressure waves could be an indicator of a bent or slipping water pump impeller. With these results, there are slight pulsations indicating an issue with the water pump (see Fig. 7).
Our next step would be to inspect the water pump impeller using a boroscope. If the water pump does not have a backing plate, it is possible to inspect the impeller blades by inserting a boroscope into either the heater hose fitting or where the lower radiator hose connects to the water pump.
In the case of this Tahoe, there was not a clear passage to inspect the impeller. Information gained from using the lab scope confirmed there was an issue with the water pump. In this case, it was much faster to use this tool to confirm our suspicion rather than removing the pump for a physical inspection.
Step 3: The repair
Once we determined what the overheating issue was, resolving it by replacing the water pump was a relatively straightforward job. Since we noted some maintenance items needed attention, we obtained authorization from the customer to replace the cooling system hoses, the thermostat, belt and radiator cap, as well as clean and refill the cooling system.
Since many newer vehicles have reverted to using a flat band squeeze clamp on the hoses, we have been using the Matco HCP1710 clamp pliers to remove them. This tool allows easy removal without the hazard of regular pliers slipping and causing injury (see Fig. 8).
As part of our cooling system repair and maintenance procedure we have been in the habit of cleaning the front of the radiator and air conditioning condenser to make sure there is proper air flow through the radiator. Since there are many different types of coolant, it is imperative you look to your repair information source to determine which coolant is required by the manufacturer.
For the most part, late-model vehicles have remote reservoirs for coolant overflow. Due to underhood space restrictions, the fill may be near the same level as the top of the radiator or upper radiator hose connection at the engine. Because of this, it is sometimes difficult to remove all of the air from the cooling system after a repair. We have used the Air Lift Tool to do this and have found that not only does it allow you to remove all of the air from the system, but it performs a vacuum test to determine if there are leaks in the system (see Fig. 9).
After replacing the hoses, thermostat and hose clamps, place the vacuum lift tool on the radiator or reservoir fill neck. You will then use shop air connected to the air lift tool to create a vacuum in the cooling system by a venturi effect of the shop air passing through the tool (see Fig. 10). Once a system vacuum of approximately 25” is achieved, disconnect the shop air and note the vacuum reading on the tool’s gauge.
Let the system sit for 10 to 15 minutes. If the gauge on the air lift tool does not show a loss of vacuum, there is not a leak in the system. If there are no leaks, place the clear hose of the tool into a container of new 50/50 coolant-water mix and open the valve on the tool. Since there is a vacuum in the cooling system, the coolant will flow from the container of coolant into the radiator and engine and completely fill the system, not allowing air to enter the system. Filling the system in this manner saves a lot of time and effort because the technician in most cases will not need to bleed the air from the system (see Fig. 11).
In order to assure the cooling system is full, start the engine and run until it reaches operating temperature. Then, shut the engine off. Let it sit for about 15 minutes or so, then slowly remove the radiator cap and top off the reservoir if necessary.
Step 4: Test drive and confirm repair
Since we had lab scope readings that indicated pressure pulses in the cooling system, we checked the system after the repair to assure the pulses were not present. The lab scope readings were normal and during a lengthy test drive under the same conditions that caused the engine to overheat, there were no other issues. Taking a few extra steps both during the diagnostic procedures ensured we made a complete repair. Replacing the hoses and other items as well as servicing the system completely will assure the customer has many trouble-free miles without an additional cooling system issue.
