Hot/warm start issue

fb1 · 22-12-15, 09:06 AM

Apologies if this has been covered before but i cant get the search bar to work (posted in ca website problems).
On saturday morning i picked up a 1990, 2.3 audi 100 c3. Started ok (engine cold), on collection and drives lovely but with a slight fluctuation on idle when warm and quite erratic idle when just started.
I jumped in saturday evening to fill up with petrol (engine cold), started with no problems, but then wouldnt start after filling up (engine warm). Turns over fine but not starting. I thought maybe it was the fuel pump as the car hasnt been used much in the last few years. I had to leave it there overnight and arranged to collect in the morning. When i got back in the morning i gave it a try (engine cold), only for it to start straightaway! I took it for a 20 minute drive, turned it off then tried to start (engine hot), and guess what?? It wouldnt start. Left it a few hours and it started again no problem.
I can only assume i have a hot start problem and im hoping by changing a temperature sensor it will be ok. BUT ive no idea where or which sensor to start with. Ive googled it but answers seem a bit vague. Im hoping that one of the experts (?)

on here can help with location and part number if possible. Im using the car as my daily driver but have to plan my journeys with enough time to let it cool down!
Thanks in advance.

mikes2 · 22-12-15, 01:47 PM

Strong possibility of a fuel pressure problem - leaking injectors or the fuel pump check valve.

you could try unplugging the cold start valve to see if it improves. blue plug injector on the inlet manifold

fb1 · 22-12-15, 07:41 PM

What would the cold start valve have to do with not starting when hot? Sorry hope thats not a dumb question!

Colin Aitchison · 22-12-15, 08:10 PM

Fb1 need to agree with mkie2, this type of fault it is normally fuel pressure problems, but I did give a bit of advice to a Classic Audi member a few month ago, his user name on this site is ELTEE and this worked for him, if the car has been standing for some time the fuses in the fuse box get corrosion on them and do not pass voltage and erratic idle is one of the common problems, now this won't take you long and is a free fix apart for the time it takes you (5 mins), Remove the fuse box lid and remove all the fuses one by one and replace them in the same location as you pulled them out from, you may have to use long noise pliers if they are tight, got this solution from a state Audi forum and I said at the time that this won't work but it did and I was shocked, give it a try and let us know how you get on.

fb1 · 22-12-15, 11:38 PM

I'll try it tomorrow on my lunch break. I'll try the cold start valve as well. Cheers.

fb1 · 23-12-15, 10:47 AM

Rightyo. I removed and replaced all the fuses. All looked fine. No corrosion or damage. Made no difference to erratic idle or hot starting. Removed cold start connector when cold, wouldnt start. Forgot to try again when warm! Will try that after work.

fb1 · 24-12-15, 06:06 PM

Removed cold start connector when egine hot and engine turned off. Same as before, wouldnt restart. Any ideas of anything else i can try before i start spending money i dont have??

Colin Aitchison · 24-12-15, 06:38 PM

Sorry I may have picked you up wrong, the engine was running and at normal running temp, you then unplugged the cold starting connector and the engine cut out, is this correct? also may need photos to see what you are doing or even better post YouTube vid. Don't spend any money yet well not for the next few days, can you confirm the engine code NG, 7A , there is others. May find it on V5 under engine number.

fb1 · 24-12-15, 07:05 PM

Ah no sorry, it was warmed up as id just got to work (10 miles). I turned it off, unplugged coldstart then tried to restart. Wouldnt start, turned over fine though. I'll have to try again boxing day

moonwatcher · 24-12-15, 07:32 PM

This happened to me some time ago, started fine from cold, drove about 5-600 yards pulled into a lay-by to answer the phone and afterwards it wouldn't start, called out the AA breakdown, the guy knew exactly what to do, he went into the box of relays under the bonnet, pulled one out (sorry but I didn't ask which one) and it fired up straight away. He then put the relay back in and I've had no problem since.

22-12-15, 01:47 PM	#2
mikes2 4 ring whore! Classic Audi Club Member Join Date: Nov 2007 Location: Polmont Posts: 2,617	Strong possibility of a fuel pressure problem - leaking injectors or the fuel pump check valve. you could try unplugging the cold start valve to see if it improves. blue plug injector on the inlet manifold __________________ 1991 90 quattro 20v sport 1991 90 quattro

24-12-15, 07:32 PM	#10
moonwatcher 4 ring whore! Classic Audi Club Member Join Date: Jul 2008 Location: Barrow in Furness, Cumbria Posts: 2,049	This happened to me some time ago, started fine from cold, drove about 5-600 yards pulled into a lay-by to answer the phone and afterwards it wouldn't start, called out the AA breakdown, the guy knew exactly what to do, he went into the box of relays under the bonnet, pulled one out (sorry but I didn't ask which one) and it fired up straight away. He then put the relay back in and I've had no problem since. __________________ Coupe Quattro 1985

22-12-15, 07:41 PM	#3
fb1 Grown up member Classic Audi Club Member Join Date: May 2012 Posts: 98	What would the cold start valve have to do with not starting when hot? Sorry hope thats not a dumb question!

22-12-15, 08:10 PM	#4
Colin Aitchison Senior Member Classic Audi Club Member Join Date: Dec 2013 Location: Back home in Tillicoultry Posts: 1,308	Fb1 need to agree with mkie2, this type of fault it is normally fuel pressure problems, but I did give a bit of advice to a Classic Audi member a few month ago, his user name on this site is ELTEE and this worked for him, if the car has been standing for some time the fuses in the fuse box get corrosion on them and do not pass voltage and erratic idle is one of the common problems, now this won't take you long and is a free fix apart for the time it takes you (5 mins), Remove the fuse box lid and remove all the fuses one by one and replace them in the same location as you pulled them out from, you may have to use long noise pliers if they are tight, got this solution from a state Audi forum and I said at the time that this won't work but it did and I was shocked, give it a try and let us know how you get on.

22-12-15, 11:38 PM	#5
fb1 Grown up member Classic Audi Club Member Join Date: May 2012 Posts: 98	I'll try it tomorrow on my lunch break. I'll try the cold start valve as well. Cheers.

23-12-15, 10:47 AM	#6
fb1 Grown up member Classic Audi Club Member Join Date: May 2012 Posts: 98	Rightyo. I removed and replaced all the fuses. All looked fine. No corrosion or damage. Made no difference to erratic idle or hot starting. Removed cold start connector when cold, wouldnt start. Forgot to try again when warm! Will try that after work.

24-12-15, 06:06 PM	#7
fb1 Grown up member Classic Audi Club Member Join Date: May 2012 Posts: 98	Removed cold start connector when egine hot and engine turned off. Same as before, wouldnt restart. Any ideas of anything else i can try before i start spending money i dont have??

24-12-15, 06:38 PM	#8
Colin Aitchison Senior Member Classic Audi Club Member Join Date: Dec 2013 Location: Back home in Tillicoultry Posts: 1,308	Sorry I may have picked you up wrong, the engine was running and at normal running temp, you then unplugged the cold starting connector and the engine cut out, is this correct? also may need photos to see what you are doing or even better post YouTube vid. Don't spend any money yet well not for the next few days, can you confirm the engine code NG, 7A , there is others. May find it on V5 under engine number.

24-12-15, 07:05 PM	#9
fb1 Grown up member Classic Audi Club Member Join Date: May 2012 Posts: 98	Ah no sorry, it was warmed up as id just got to work (10 miles). I turned it off, unplugged coldstart then tried to restart. Wouldnt start, turned over fine though. I'll have to try again boxing day