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Lakers$$$
02-13-2015, 03:09 PM
Already replaced my crank sensor twice, still have intermittent starting issues. If I am unable to start, if I wait a few hours, the car starts up fine. My local mechanic already replaced the crank sensor twice and cleaned out the spark plugs and engine bay from excess gas. Any recommendations?

2001 A4, 1.8T 114k miles

Jungleboon
02-13-2015, 04:09 PM
Have you checked whether you're getting any spark

Lakers$$$
02-13-2015, 04:58 PM
Have you checked whether you're getting any spark

yeah, i had all the spark plugs replaced too, it starts 80% of the time, then once in a while it just cranks, so strange, it's been in the shopped 4 times now and they still have not really figured it out

Jungleboon
02-13-2015, 06:48 PM
I had an issue like this it ended up being relay 30 in the ecu box burning out, ended up completely burnt out and wouldn't start. I would check it out cause it's easy to get to, sounds like a relay or wire harness short since it's intermittent. Do you have any codes?

99jimse
02-14-2015, 07:40 AM
You need Spark, fuel, & air for ignition. When it's just cranking, check for spark & fuel pressure. Definitely check all related fuses for ECU & fuel.

MarkAnto
06-10-2015, 03:05 PM
Check the coolant temp sensor.

walky_talky20
06-10-2015, 03:47 PM
Step 0: Check for codes with VCDS. Not a generic tool, it must be VCDS or a VW/Audi Dealer-level scan tool. Generic tools routinely miss the pending and/or manufacturer specific codes. Doesn't matter if the CEL is on or off or flashing or whatever. Doesn't matter if the AutoZone tool or your mechanics generic tool said there are no codes. Check the codes with VCDS anyway. If you complete this step, there is a very good chance it will identify your issue with no further work. Only skip Step 0 if you absolutely cannot complete it.

Step 1: I would begin with checking for spark. I'll bet that you are completely missing spark whenever it refuses to start.

First provoke the no-start issue. It is important that the issue is presenting at the time you are doing your tests.

Check for spark. If none, you need to determine what you are missing at the coil packs that is preventing that. There are 4 wires:

12v+ (from ECU power supply relay, should be 12v+ with key in RUN position)
Coil Ground (from valve cover)
5V square wave trigger from ECU (should be present during cranking)
ECU ground reference (from firewall ground point)

You need to find out which one is missing. The square wave is the only one that is a little tricky to test. You can't really use a meter or a conventional light bulb test light. It should only be tested with an oscilloscope or an LED-based test light.

Step 2: If there is spark, then it is likely a fuel issue. Checking fuel pressure would be nice, but requires some messing around (obtaining a gauge, etc). I would rather just confirm that the fuel pump is running during cranking. Have somebody put their ear to the trunk floor (USA passenger side) while another person cranks the engine briefly. The pump should give a nice "whir".

You can check fuel delivery to the combustion chamber by removing a spark plug after a bit of cranking. If they are wet, then you are definitely getting fuel. So with spark and fuel, the lack of combustion is probably due to an incorrect mixture. I would be suspect of the coolant temp sensor. This would be a good time to check the value that it is reporting the ECU. You can do this using VCDS or a generic scan tool. If the car hasn't run in several hours, it should be reporting nearly ambient temp.

That should be enought to get you started and identify most of the common things that could be causing your intermittent no-start.