I am having some frustrating issues with my automatic transmission ever since performing some routine maintenance and repairs.
The Background:
When I first bought the car (1.6L Petrol running on LPG), the transmission shifted perfectly. However, I noticed the engine wasn’t reaching its ideal operating temperature. The previous owner had been using plain water instead of proper coolant, which caused a massive amount of rust and left the engine thermostat stuck completely open. I flushed the cooling system as thoroughly as possible, replaced the engine thermostat (which opens at 91°C), and filled it with the proper coolant.
The Fluid Mix-Up:
On the exact same day, I decided to service the transmission. I wanted to replace the internal oil filter, but my mechanic realized too late that changing the filter on this gearbox requires dropping the transmission and splitting the case. So, we stuck to a simple fluid drain and fill.
Unfortunately, the shop made a critical mistake and used Castrol Transmax ATF Dexron VI instead of the required Mobil ATF LT 71141 spec. Right after this mix-up, the trouble started. Once the car got hot in traffic, the transmission started slipping and kicking hard when trying to move from a dead stop, requiring me to rev the engine past 3,000 RPM just to get the car to engage and drive.
What I’ve Done So Far:
Realizing the fluid mistake, I immediately did a flush using Liqui Moly Top Tec 1200 (the closest matching spec I could find locally at the time). The shifting improved significantly, but the hard kick still happens whenever the car gets very hot. It feels like the oil thins out too much under high heat, losing the pressure required for proper engagement.
To fix this properly, I have sourced Petro-Canada DuraDrive MV Synthetic ATF here in Albania, which officially covers the LT 71141 spec. My current plan is to:
- Replace the hydraulic valve body (solenoid block).
- Perform two more complete fluid flushes with the Petro-Canada ATF to completely get rid of any remaining wrong fluid.
- Swap the engine thermostat out for a cooler 83°C or 87° (would be perfect if i can find) Copening unit because the car runs hot on LPG during our intense Albanian summers, and the factory transmission heat exchanger relies entirely on engine coolant temperatures.
Has anyone successfully experimented with a custom external cooling system on the AL4 / DP0 gearbox?
While a basic external air radiator is great for the summer, it can over-cool the transmission and be detrimental in the winter. I’ve done some research and found external 4-port fluid thermostats like the Derale 25792 Fluid Control Thermostat Kit or the Mishimoto Transmission Bypass Valve.
My idea is simple with a thermostat bypass valve system:
- When the car is cold: The thermostat remains closed. The fluid bypasses the external radiator and runs through a liquid-to-liquid heat exchanger so the engine coolant warms the transmission up quickly. (As it is originally)
- When the fluid hits 75°C – 80°C: The thermostat opens and routes the fluid out to a small auxiliary air radiator mounted behind the front bumper to keep the transmission at its optimal temperature.
Has anyone done this specific modification on an AL4 gearbox? If so, what exact adapter plates, inline water coolers, and fittings did you use to build the kit?
Any advice or part recommendations would be highly appreciated!
