Excuse the lengthy post, but I want to give the repair history in the correct order because I’m looking for mechanical/diagnostic opinions, not guesses about fault.
YEAR: 2009
MAKE: Pontiac
MODEL: G6
SUBMODEL: GT Convertible
ENGINE: 3.5L
STATE YOUR ISSUE:
My car has repeatedly mixed coolant and transmission fluid even after a radiator replacement, coolant flush, engine flush, replacement transmission/cooler lines, and two replacement transmissions.
Here is the timeline as accurately as I can document it:
March 17: I took the vehicle to the first shop because coolant and transmission fluid were mixing. The shop replaced the radiator and performed a coolant flush, along with other unrelated repairs. When I picked the car up that day, I was specifically told that the coolant and transmission fluid were no longer mixing.
March 19: Two days later, the vehicle overheated.
March 20: I returned it to the same shop. I was told that a line had been left disconnected from the radiator during the March 17 repair. They corrected it and I picked the car back up.
April 10: The transmission failed.
I personally have not driven the vehicle since April 10. From that point forward, it has remained in the repair process and has been towed whenever it needed to be transferred between shops.
The vehicle went to a second mechanic, who diagnosed the transmission failure. During that diagnosis, he told me the transmission/cooler lines had not been flushed during the original radiator repair.
Before replacing the transmission, I specifically asked whether the engine was okay. He told me he had flushed it, checked that nothing was clumped up, and that the engine was solid throughout.
During the transmission repair, the transmission/cooler lines were replaced with new lines.
May 22: The first replacement transmission was being completed. The mechanic told me that if there were any problems with the transmission, the warranty would be handled through him.
May 26: The car was still in his possession, and he told me he had not fully driven/tested it yet.
June 4: While the car was still with the second mechanic, he contacted me and told me coolant and transmission fluid had mixed again. He also told me the replacement transmission was being covered under warranty and that he had figured out why the fluids were mixing.
It was after that first transmission replacement and the repeated mixing that he identified the issue as involving the internal transmission cooler inside the radiator.
A second replacement transmission was then ordered under warranty.
June 9: The second replacement transmission was out for delivery.
June 11–15: The second transmission was installed and GM came out to program it.
After programming, the second mechanic drove the vehicle himself as part of the repair/testing process. I still had not received or driven the car.
He later told me the vehicle drove great for a day or two, but while he was driving it, it started whining again and coolant and transmission fluid began mixing again.
He then told me the previous radiator repair/part had:
“Failed again and done the same thing.”
June 23: The second mechanic contacted the first shop that replaced the radiator regarding the repeated failure. The first shop directed him to contact the parts/corporate supplier regarding the radiator.
He later told me he spoke with the supplier and that they were contacting corporate regarding the issue.
Around June 26: I personally contacted the first shop regarding the continuing coolant/transmission-fluid mixing. They told me to bring the vehicle back on Monday, June 29.
June 29: The vehicle was returned to the original shop that performed the March 17 radiator replacement. That day, the first shop told me the supplier would come out on July 7 to test the part/radiator.
July 7: This was the date I was told the supplier was supposed to come out and test the part.
June 29 to present: The vehicle has remained continuously in the first shop’s possession. The first shop has not contacted me at all since June 29.
I have not been told whether the supplier actually came out on July 7, what testing was performed, what the results were, whether the radiator/internal transmission cooler was confirmed defective, or what the repair plan is. I also have not been provided a claim/case/reference number.
So, in short:
March 17 — Radiator replaced + coolant flushed; told fluids were no longer mixing
March 19 — Vehicle overheated
March 20 — Returned to first shop; told a radiator line had been left disconnected
April 10 — Transmission failed
April 10 forward — I personally have not driven the vehicle; towed whenever transferred
First transmission repair — Engine flushed + transmission/cooler lines replaced
June 4 — Coolant/transmission fluid mixing again after first replacement transmission
Second transmission installed under warranty
After second transmission — Mechanic drove/tested it; it ran well briefly, then began whining and mixing again
June 23 — Second mechanic contacted first radiator shop; first shop directed him to parts/corporate supplier
Around June 26 — I contacted first shop and was told to bring the car back June 29
June 29 — Car returned to first shop; told supplier would come July 7 to test the part
July 7 — Scheduled supplier test date
June 29 to present — Car remains at first shop; no communication initiated by that shop since June 29
My questions for the mechanics here are:
1. If coolant and transmission fluid were originally mixing, the radiator was replaced, and I was told on March 17 that the fluids were no longer mixing, but the same contamination returned later, what would you suspect or test?
2. Could the internal transmission cooler inside a replacement radiator fail and allow coolant and transmission fluid to cross-contaminate?
3. Could a replacement radiator be defective from the beginning?
4. Could an incorrect radiator, installation problem, damaged fitting, cooler-line connection issue, or another problem cause this type of repeated cross-contamination?
5. How significant is it that the vehicle overheated two days after the radiator replacement after a radiator line had been left disconnected?
6. How significant is it that the second mechanic said the transmission/cooler lines had not been flushed during the original radiator repair?
7. Could contamination left in those lines contribute to the original transmission failing several weeks later?
8. Once the engine had been flushed, the transmission/cooler lines had been replaced, and a replacement transmission installed, would coolant and transmission fluid mixing again point toward an active cross-leak rather than old residual contamination?
9. If the internal transmission cooler inside the radiator was actively leaking, could it contaminate or damage multiple replacement transmissions if that problem remained unresolved?
10. Is it mechanically significant that the second replacement transmission reportedly drove normally for a day or two and then started whining and mixing again?
11. What specific test would you perform to conclusively confirm or rule out a leak in the internal transmission cooler inside the radiator?
12. Can the radiator/internal cooler be isolated and pressure-tested independently?
13. Can testing distinguish old residual contamination from fresh coolant/transmission fluid actively crossing between the two systems?
14. If a supplier came out to test a suspected defective radiator/internal cooler, what type of testing or documentation would normally be produced that I should be asking the shop for?
15. Based on this repair history, what would you insist be inspected and tested before another transmission, radiator, or major component is replaced?
I’m not asking anyone here to determine legal responsibility, and I’m intentionally leaving the shops unnamed. I’m trying to understand what mechanically makes sense and what should actually be tested because I do not want another major part thrown at this car without the actual source of the repeated cross-contamination being identified.
Thank you to anyone who took the time to read all of this and can offer some professional mechanical insight.
YEAR: 2009
MAKE: Pontiac
MODEL: G6
SUBMODEL: GT Convertible
ENGINE: 3.5L
STATE YOUR ISSUE:
My car has repeatedly mixed coolant and transmission fluid even after a radiator replacement, coolant flush, engine flush, replacement transmission/cooler lines, and two replacement transmissions.
Here is the timeline as accurately as I can document it:
March 17: I took the vehicle to the first shop because coolant and transmission fluid were mixing. The shop replaced the radiator and performed a coolant flush, along with other unrelated repairs. When I picked the car up that day, I was specifically told that the coolant and transmission fluid were no longer mixing.
March 19: Two days later, the vehicle overheated.
March 20: I returned it to the same shop. I was told that a line had been left disconnected from the radiator during the March 17 repair. They corrected it and I picked the car back up.
April 10: The transmission failed.
I personally have not driven the vehicle since April 10. From that point forward, it has remained in the repair process and has been towed whenever it needed to be transferred between shops.
The vehicle went to a second mechanic, who diagnosed the transmission failure. During that diagnosis, he told me the transmission/cooler lines had not been flushed during the original radiator repair.
Before replacing the transmission, I specifically asked whether the engine was okay. He told me he had flushed it, checked that nothing was clumped up, and that the engine was solid throughout.
During the transmission repair, the transmission/cooler lines were replaced with new lines.
May 22: The first replacement transmission was being completed. The mechanic told me that if there were any problems with the transmission, the warranty would be handled through him.
May 26: The car was still in his possession, and he told me he had not fully driven/tested it yet.
June 4: While the car was still with the second mechanic, he contacted me and told me coolant and transmission fluid had mixed again. He also told me the replacement transmission was being covered under warranty and that he had figured out why the fluids were mixing.
It was after that first transmission replacement and the repeated mixing that he identified the issue as involving the internal transmission cooler inside the radiator.
A second replacement transmission was then ordered under warranty.
June 9: The second replacement transmission was out for delivery.
June 11–15: The second transmission was installed and GM came out to program it.
After programming, the second mechanic drove the vehicle himself as part of the repair/testing process. I still had not received or driven the car.
He later told me the vehicle drove great for a day or two, but while he was driving it, it started whining again and coolant and transmission fluid began mixing again.
He then told me the previous radiator repair/part had:
“Failed again and done the same thing.”
June 23: The second mechanic contacted the first shop that replaced the radiator regarding the repeated failure. The first shop directed him to contact the parts/corporate supplier regarding the radiator.
He later told me he spoke with the supplier and that they were contacting corporate regarding the issue.
Around June 26: I personally contacted the first shop regarding the continuing coolant/transmission-fluid mixing. They told me to bring the vehicle back on Monday, June 29.
June 29: The vehicle was returned to the original shop that performed the March 17 radiator replacement. That day, the first shop told me the supplier would come out on July 7 to test the part/radiator.
July 7: This was the date I was told the supplier was supposed to come out and test the part.
June 29 to present: The vehicle has remained continuously in the first shop’s possession. The first shop has not contacted me at all since June 29.
I have not been told whether the supplier actually came out on July 7, what testing was performed, what the results were, whether the radiator/internal transmission cooler was confirmed defective, or what the repair plan is. I also have not been provided a claim/case/reference number.
So, in short:
March 17 — Radiator replaced + coolant flushed; told fluids were no longer mixing
March 19 — Vehicle overheated
March 20 — Returned to first shop; told a radiator line had been left disconnected
April 10 — Transmission failed
April 10 forward — I personally have not driven the vehicle; towed whenever transferred
First transmission repair — Engine flushed + transmission/cooler lines replaced
June 4 — Coolant/transmission fluid mixing again after first replacement transmission
Second transmission installed under warranty
After second transmission — Mechanic drove/tested it; it ran well briefly, then began whining and mixing again
June 23 — Second mechanic contacted first radiator shop; first shop directed him to parts/corporate supplier
Around June 26 — I contacted first shop and was told to bring the car back June 29
June 29 — Car returned to first shop; told supplier would come July 7 to test the part
July 7 — Scheduled supplier test date
June 29 to present — Car remains at first shop; no communication initiated by that shop since June 29
My questions for the mechanics here are:
1. If coolant and transmission fluid were originally mixing, the radiator was replaced, and I was told on March 17 that the fluids were no longer mixing, but the same contamination returned later, what would you suspect or test?
2. Could the internal transmission cooler inside a replacement radiator fail and allow coolant and transmission fluid to cross-contaminate?
3. Could a replacement radiator be defective from the beginning?
4. Could an incorrect radiator, installation problem, damaged fitting, cooler-line connection issue, or another problem cause this type of repeated cross-contamination?
5. How significant is it that the vehicle overheated two days after the radiator replacement after a radiator line had been left disconnected?
6. How significant is it that the second mechanic said the transmission/cooler lines had not been flushed during the original radiator repair?
7. Could contamination left in those lines contribute to the original transmission failing several weeks later?
8. Once the engine had been flushed, the transmission/cooler lines had been replaced, and a replacement transmission installed, would coolant and transmission fluid mixing again point toward an active cross-leak rather than old residual contamination?
9. If the internal transmission cooler inside the radiator was actively leaking, could it contaminate or damage multiple replacement transmissions if that problem remained unresolved?
10. Is it mechanically significant that the second replacement transmission reportedly drove normally for a day or two and then started whining and mixing again?
11. What specific test would you perform to conclusively confirm or rule out a leak in the internal transmission cooler inside the radiator?
12. Can the radiator/internal cooler be isolated and pressure-tested independently?
13. Can testing distinguish old residual contamination from fresh coolant/transmission fluid actively crossing between the two systems?
14. If a supplier came out to test a suspected defective radiator/internal cooler, what type of testing or documentation would normally be produced that I should be asking the shop for?
15. Based on this repair history, what would you insist be inspected and tested before another transmission, radiator, or major component is replaced?
I’m not asking anyone here to determine legal responsibility, and I’m intentionally leaving the shops unnamed. I’m trying to understand what mechanically makes sense and what should actually be tested because I do not want another major part thrown at this car without the actual source of the repeated cross-contamination being identified.
Thank you to anyone who took the time to read all of this and can offer some professional mechanical insight.
Aug 12, 2026 at 4:08 PM
