1.4 16v (AUA) jerks and bucks when cold + coolant light flashing randomly

[jaffabaffa]

Hello everyone,


My name is Dennis, and I recently became the owner of a 2001 Volkswagen Caddy Mk2 with the 1.4 16v petrol engine (the AUA engine). The reason I ask my question here, is because the AUA engine is more commonly found in the Lupo and Polo than in the Caddy. The car has less than 85.000 km (53.000 miles) on the odometer, but unfortunately it has an engine problem.

When I start the car cold, it runs rough and doesn't want to hold a steady idle. The RPMs jump around between 1250 and 500 rpm, and eventually the engine shuts off completely.
When I quickly drive off before allowing the engine to shut off, my car jerks, bucks and doesn't want to accelerate. As the engine warms up (usually after 3 or 4 minutes or so), the bucking decreases in severity, and I can actually drive the car normally without much hassle. However, the problem never goes away completely. When I drive at 80+ km/h (50+ mph), or when I accelerate at full throttle, the car runs like a charm. The top speed is also as it should be. The problem mainly occurs at low revs and with a cold engine. Once the car is at operating temperature, it runs much better. However, the problem never disappears completely. It only decreases in severity.

Furthermore, my coolant temperature light keeps starting to flash at purely random occasions.
Sometimes, the light starts flashing in less than one minute after starting the car. Other times, I can drive for well over an hour without the light coming on. The car doesn't actually overheat, and the coolant level is good as well. I already replaced the green coolant temperature sensor, but that didn't solve the issue… Additionally, I also replaced the spark plugs and spark plug cables/caps, but to no avail.

I also had my local garage check the car for any fault codes. The only code than come up was a faulty lambda probe sensor, but according to the mechanic, that couldn't possibly be the cause of the problem I'm experiencing.

My gut feeling is leaning towards a faulty ECU, but I really don't know…

Any help, advise or suggestions would be immensely apricated!


Dennis

[Crasher]

Replace the coolant temperature sensor, VAG part number 059 919 501 A. This is likely to be stored as a code in the ECU but your garage may be using a generic code reader which will only retrieve emissions related codes such as lambda sensor errors and that code could also be due to the temperature sensor being faulty BUT on these engines, thermostat housing top section failure is common and people remove the thermostat which cause the engine to over cool and run rich so causing a lambda error. ECU failure is extremely rare, always the last resort.

[jaffabaffa]

Replace the coolant temperature sensor, VAG part number 059 919 501 A. This is likely to be stored as a code in the ECU but your garage may be using a generic code reader which will only retrieve emissions related codes such as lambda sensor errors and that code could also be due to the temperature sensor being faulty BUT on these engines, thermostat housing top section failure is common and people remove the thermostat which cause the engine to over cool and run rich so causing a lambda error. ECU failure is extremely rare, always the last resort.

Hey Crasher,


Thank you very much for your comment!

Like I mentioned in my post, I already replaced that exact sensor, and unfortunately it didn't make a difference...
Or are you saying I need to replace that sensor AND delete the codes immediately after?

Regarding your statement on ECU failures being extremely rare; that's actually not the case with AUA engines.
I've read multiple stories online in which the ECU was actually the issue. Apparently the ECU is quite prone to water damage, and covering the ECU in a plastic bag is actually a popular "mod" with these engines haha.


Any other suggestions?
Anything would be much help!

[Crasher]

Generally across all VAG models I have replaced very few ECU's over the last 35 years, I don't remember ever changing one for an AUA. You could do with knowing the actual value of the temp sender as the ECU sees it from cold right through to hot but you would need a diagnostic system with live data to view this.

[mickeybo]

The coolant temp sensor on this caddy is tied into the air conditioning on this caddy. I would recommend at room temp measure resistance & with it submerged in hot water take the resistance if the resistance does not change change replace sensor

[jaffabaffa]

The coolant temp sensor on this caddy is tied into the air conditioning on this caddy. I would recommend at room temp measure resistance & with it submerged in hot water take the resistance if the resistance does not change change replace sensor

Thanks for your comment, Mickey!

I will try measuring the resistance��

How likely would it be that my newly bought coolant temperature sensor is also faulty?
I admittedly bought a sensor from a "budget brand" (Stark) because all the renowned stuff was sold out and unavailable...

[Crasher]

I would want to see the values the sender is producing in the instrument panel and engine ECU live data outputs, saves guessing. If you go to start it from cold, say 10 centigrade at the moment but the ECU sasy the coolant is at 80 centigrade, you know the issue is likely to be the sensor.