Hot starting

[gezzerS4]

Has anybody had any hot start issues, occasionally if I want to start after stopping for between 10-40 mins it can take a bit of churning to get going.

Starting straight away after a good run is no problem, as is cold start, it's only after being left a while, leave it longer and it's ok again.

It's not every time though, worse when ambient is higher, like today for instance, and the guy from AMD mentioned after the Dyno run, feels like a heat soak problem, car runs normal temps (90), and does not throw anything unusual from Vag-Com.

Any thoughts ??

[Nige_RS4]

Difficult to pinpoint, but I have heard of hot starting issues that were caused by an intermittantly faulty coolant temp sender (G62) - this a common problem on VAG cars. You can log the sensor with vag-com - if you log over a period of time, you will see it going to -21deg (or something around there). This is it's lowest value and is an indication that it's gone open circuit. It's a cheap part, so might be worthwhile replacing it anyway?

[gezzerS4]

Just logged Coolant Temp via Vag-Com was showing 72 degrees (car was left for about 45 mins after a fast (ish) run, took a bit of churning to get going, other thing I notice is that when it does this you have to push accelerator to get it to fire, almost like a car running on carbs.
Runs fine once started.

No other faults in vag-com, like you say Nige could be difficult to track down

[simple1]

Check and make sure that recirc pump is running after switch off........... petrol will boil at 38 degrees and induce air into system, usually purged at start-up by fuel pump, could be that fuel prssure is starting to fall off or accumulator is failing..........which usually points to dirty fuel filter or failing pump.

[wozza]

Got exactly the same problem with my rs4 - i.e fine when hot, fine when cold - crap when warm.

I've been chasing this problem for a number of months and maybe a step closer to sorting . Have had an oil leak at the back of both banks on the rocker covers leaking onto the turbos - which was progressively getting worsre and posing a bit of a risk. So I've gotten both rocker cover gaskets replaced.

And as my mechanic says - 'it was p*ssing oil out' and 'oil out equals air' so and we're suspecting that it may be something to do with the vacuum circuit as we've also has problems with the oil breather circuit with tubes fracturing due to the heat coming off the turbos. Once hose was completely screwed and we've encased in heat shrink (it holds up just when hot) for the moment until the replacement pipe arrives from germany.

I'm wondering if this has something to do with the fuel vapour recirc circuit and obviously the rebeather enters the inlet side of the engine after the MAF so could be screwing with the measurements.

So far no starting problems - but will keep you posted.

[wozza]

forgot to say - as part of the diagnosis (aka we've run out of ideas so swap out likely candidates) coolant temp sensor replaced before any of this work commenced - but it didnt solve the problem. Whilst down there though we first noticed the cracked oil re-breather hose - comes off the left and right banks and joins one from the right that comes off those silver valves at back end of the banks and then joins the distribution piece.

[s4woody]

also check ya vacumm hose for ya fuel pressure regulator..if its split or not on it wont hold pressure in the fuel rail..although its more for cold start it can cause a hot start prob..

[Golich]

Did you get to the bottom of this?

[wozza]

Forgot to report that I sorted the hot start issue with my motor.

Issue was a leaky injector.

Diagnosis:

So you can actually pull the fuel rail off complete if you detach the pipes with the green non return vaccum valves on and undo all the allen bolts etc. Once it's off you can flip it upside down. I could easily see the leaking injector as fuel built up in the hole around the needle valve and could also see bubble appearing.

I did this immediately after a run - and left for 30 mins - so a little heat soak also expands the fuel which increased rail pressue.

If you dont fancy doing this - then nuther v.easy way to test is to run the car until 90 degrees hot. Stop the engine, pull the fuel pump fuse and the run the car until is conks out which will drain the rail. Leave for 30 mins and then put the fuse back in, as you turn the ignition the pump will prime the rail and she should start up with in a couple of cranks. Obviously no fuel in the rail means no leak and no pressure build up.

If you have a leaky injector and a hot start issue the car will over crank (i.e more than 2 seconds cranking), and you'll see a puff of black smoke out of the exhaust immediately after it finally catches.

Causes:

Might also be worth checking the fuel pressure regulator seal as in my case the seal had degraded and perished, the strainers on the regulator and the injectors should catch any bits (within reason) but I might have had fragments getting into the injectors. Other theory is that immediately after getting the injectors ultrasonically cleaned and pressure tested my car developed the hot start issue - so could be dirt introduced by the garage or failure brought on by the original cleaning (they were cleaned to diagnose/eliminate a rough idle issue but possibly caused a totally different one)

Remedy:

Decided to renew the whole set of injectors rather than do just the two leaky ones or get em cleaned (again).

Hope this helps.