Intermittent Non-Start Issue...Any Opinions?

[tim04USPa4]

Alright, so I apologize for the long winded question in advance.

I am having an intermittent non-start issue. The car will turn over perfectly fine, however wont start. I have plenty of fuel, and pressure, at the fuel rail (checked off the schrader bleed on the rail), and the car has new (less than 500 miles) spark plugs. Coils check out OK as well, and I have Zero ignition or fuel related DTC/MIL's.

Now all of that being said, my short research has turned up three possible gremlins..

- Crank and/or Cam position sensor. Could be going out, not firing when it needs to to start? But shows no VAG-COM errors.
- Another error in the start-up cycle...i.e fuse and/or relay?
- And the crazy one...coolant sensor.

So from what I've read, the lower coolant sensor triggers the fan, and also controls the 'cold start' vs 'warm start' mode (i.e. choke?), so is it possible the car is thinking its a cold start, and dumping to much fuel to start? I DO have a MIL for low voltage to the lower coolant sensor...

Any help would be greatly appreciated. This is starting to get annoying, lol. I'm going to change the coolant sensor tomorrow in hopes that that helps, but I'd like to not have to buy $200-300 in sensors for the Crank and Cam if I do not have to.

Cheers,
Tim

[boostedAvant]

Replace the CTS and then the Crank sensor.

[Myst420]

Replace the CTS and then the Crank sensor.

^This is where you want to start. I would replace the CTS on the flange first.

[tim04USPa4]

Rear flange and sensor were replaced less than a year ago. To my understanding, the rear flange and (upper) sensor, is just for the Gauge Cluster. The lower sensor is the one that controls most ECU and engine functions? I.e. cooling fans, cold vs warm start etc.

I will switch that lower temp sensor either tonight or tomorrow and cross my fingers.

Thanks for the suggestions guys.

[ECS Tuning-Audi]

Have the exact same issue with my car now. Coolant temp sensor acts as a two way switch, one side sends a signal to the gauge cluster for temp reading the other side sends a signal for starting conditions to the ECU.

If the ECU is not getting the right signal for cold start you will have a hell of a time trying to start the car.

My car is currently in the shop getting fixed as we speak, ended up being a coolant temp sensor. I would have changed it myself but I'd rather pay someone to do it since it's a PITA to get to and it's freezing outside.

Jason

[tim04USPa4]

Have the exact same issue with my car now. Coolant temp sensor acts as a two way switch, one side sends a signal to the gauge cluster for temp reading the other side sends a signal for starting conditions to the ECU.

If the ECU is not getting the right signal for cold start you will have a hell of a time trying to start the car.

My car is currently in the shop getting fixed as we speak, ended up being a coolant temp sensor. I would have changed it myself but I'd rather pay someone to do it since it's a PITA to get to and it's freezing outside.

Jason

Jason - thank you for the heads up! Luckily I got both sensors (from ECS to boot ) way back when I did my rear flange...just got lazy and never changed it . Luckily we have some decent weather right now down south, just need to knock it out tonight.

Thanks again to everyone for the help!

[ECS Tuning-Audi]

Jason - thank you for the heads up! Luckily I got both sensors (from ECS to boot ) way back when I did my rear flange...just got lazy and never changed it . Luckily we have some decent weather right now down south, just need to knock it out tonight.

Thanks again to everyone for the help!

Sounds good! Thanks for the order! All of us up north are jealous of your weather haha! When I go to pick my car up tonight they will be really prying that money out of my hands as it really is tough to accept paying someone for something you're 100% capable of doing, but due to the climate it's a no go.

Jason

[B6_Dolphin]

On my '03 1.8T, the cold starts were fine, but warm starts > 30 minutes of sitting would cause a no-start condition on the 1st turn, and then it would start on the 2nd turn every time.

I changed the crank sensor, despite no codes. It fixed the issue.

[ECS Tuning-Audi]

On my '03 1.8T, the cold starts were fine, but warm starts > 30 minutes of sitting would cause a no-start condition on the 1st turn, and then it would start on the 2nd turn every time.

I changed the crank sensor, despite no codes. It fixed the issue.

That's usually the difference between the CTS and the CPS. CTS will often cause cold starting issues, where as the CPS will cause the warm start issues. Not saying its always that black and white you should still pull codes regardless.

Jason