Rough cold starts

[Alfred97]

Anyone could help here that would be appreciated I’ve been dealing with this issue for over a year and can’t find a forum about it.
First off it a 2004 A4 1.8T Quattro
So basically what the car does is on only cold starts (seems it gets worse over multiple days sitting) it starts and then wants to die, and pushing the accelerator to keep it from dying actually makes it worst and causes it to die, Unless you floor it. When the car finally evens out after the cold start and eventual warm starts, it drives perfectly normal.
So far, I’ve replaced the following:
All injectors
All spark plugs
Fuel filter
I replaced the crankcase sensor a couple years ago as well, not sure how often those should be looked at, but when that was bad the car would die well driving and I’d get an EPC
I originally thought it could be the fuel pump, but I created the issue with a gauge attached to the fuel rail and the PSI stays stable at 58.

[willemml]

Try unplugging the MAF and then starting it. You might have a boost leak somewhere or bad MAF. Not sure but worth a shot.

[Cybersombosis]

Do you have VCDS? If so, take a look at your coolant temperature with the car cold. It should read ambient. If it’s something odd like -40 or 160 degrees then your temp sensor is done.

Other issues could be coolant in your cylinders due to a cracked head. Take the spark plugs out of cylinder 2 and 3 and if you have a bore scope, take a look to see if the tops of the pistons are clean or wet when the car is cold. Example of a cracked head.

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[EuroxS4]

If you're loosing coolant with no external leaks your issue is a cracked head especially if its a coscast head.

[Turbo510]

Sounds like a classic coolant temp sensor problem. I had a 2.8 B5 that acted exactly like that when the sensor was shot.

[Alfred97]

I don’t have VCDS or a scope, but when I changed the spark plugs I let it sit and checked the tops of the cylinders to see if they were wet, coolant level hasn’t gotten low.
I will have to take it in somewhere to test the coolant sensor. You know where it’s located? If not I’m sure I can find a forum on replacing that.

[Turbo510]

If you've never changed that sensor, I'd just change it instead of paying someone to tell you what the thing is doing. And if un-plugging the sensor's connector improves starting, that would imply a very incorrect signal from the sensor. A new sensor with O-ring and retaining clip is less than $8 at autohausaz for instance.

[Mike Tries]

There are two sensors. One is at the bottom of the radiator where the hose attaches. The other is at the back of the cylinder head, sticking out the top of the plastic coolant flange thingy. Someone chime in... is the one he needs to change the one at the back of the head?

[Alfred97]

Yeah I’ve never changed it I’ll try unplugging it and seeing if it improves starting as well as the MAF. Is the coolant sensor the one off the lower radiator hose or at the back of the engine on the coolant flange? Or could they both be effecting starting?

[Mike Tries]

I thought they were both coolant sensors. But I could be wrong--one could just be a switch. I think the one the ECU uses is at the back of the head. Try that one first. I believe the one on the bottom near the radiator pipe is used by the fans--that's the one that might be a thermal switch. Note that if you pull that lower unit out (as opposed to just unplugging the harness), you'll be dumping all of the contents of the radiator and the expansion tank onto the floor of your garage. You'll lose about a gallon of coolant.

[BumblebVR6]

I thought they were both coolant sensors. But I could be wrong--one could just be a switch. I think the one the ECU uses is at the back of the head. Try that one first. I believe the one on the bottom near the radiator pipe is used by the fans--that's the one that might be a thermal switch. Note that if you pull that lower unit out (as opposed to just unplugging the harness), you'll be dumping all of the contents of the radiator and the expansion tank onto the floor of your garage. You'll lose about a gallon of coolant.

The 2-pin temperature sensor located in lower radiator hose provides information to the engine control unit about the temperature of the coolant. This allows the engine control unit to relay information back to the driver, turn on the engine cooling fan, and many other functions that rely on temperature readings. Failed sensors can cause erratic or incorrect gauge displays and result in several undesirable performance issues.



The green 4 pin sensor in the coolant flange controls ECU function of fuel and also controls cluster temperature gauge & serves numerous functions. For one, it sends the signal to the display in your vehicle to give you the temperature of your coolant. This lets you keep an eye on engine temps to prevent overheating and damage. Another function is to deliver the coolant temperature to the ECU. The ECU decides when the vehicle has warmed up after starting and adjusts the fuel curve to compensate for engine temperature. If this sensor fails, you will have no heads-up display for coolant temperatures, and you can experience poor performance. Hard idling, rough starts, and poor warm engine performance are all symptoms of a failed sensor.

[Alfred97]

Thanks for the information! I’ve got the green one at the coolant flange on order from ECS right now. I don’t notice anything wrong with the temp gauge in the car, but obviously have a bunch of rough starting problems when cold. Hoping this solves my problem.

[Turbo510]

Let us know if the sensor fixes it. That four-pin unit has two independent sensor devices inside, so the one serving the ECU could go bad without anything abnormal on the dash.

[BumblebVR6]

Thanks for the information! I’ve got the green one at the coolant flange on order from ECS right now. I don’t notice anything wrong with the temp gauge in the car, but obviously have a bunch of rough starting problems when cold. Hoping this solves my problem.

No problem. I checked mine the other day and the temperature was 29 degrees Celsius in the morning before I started my car so I think that rules that out being the issue on mine. I think I did notice a hiss coming from somewhere though that seemed louder than before. Going to pressurize the system again one day and see if I can locate it.

[Alfred97]

Man that horseshoe clip is a pain, but got the new sensor in and the car has been starting like new! Did it a couple days ago and let it sit because normally it got worse over time and even after 2 days it fired up great. Never thought that this issue would have been just a temp sensor.

[Mike Tries]

Ha! Awesome! Good job, and thanks for updating us to close the loop on this!