Help needed. Engine wont run

NineNails

NineNails

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Hi.
Had a problem last night. Engine cut out at a juction and woudnt re-start. Fuel pump relay wouldnt energise and pump wouldnt prime. Sat wondering what to do with ignition on and after about 5 mins relay clicked and pump primed and car started. Cut out again the same after about 2 mins and then started and run for 5 mins then cut out again. After goint through this for about 15 mins it started and ran fine and i got home.
This morning it did the same but would run ok if i keep revs above 1000 otherwise it just stalls.
Any ideas would be great lads please. The fuel pump relay clicked in and out a couple of times while just sat with ignition on so should I be looking at that? or fuel pump? or something else?. Bit stumped on this!
Si
 
id swap the relay see if that works 1st
then take it from there
 
Is the relay under the knee panel on drivers side? And which one is it? Sorry for the questions but I'm in work and dont have time to search the net for the info right now.
Cheers
 
Ordered a new relay to try as its the cheapest fix first. The guy says he'll take it back if its not the fix, so nowt lost is there.
 
Hmm, right but there were no fault codes stored, would this throw a code?
 
Maybe, but then maybe not.

If the signal drops out the ECU doesnt think theres a fault, it just thinks the engine has stopped, so shuts off the fuel and spark.

I've heard of almost identical symptoms on the A3 forum, and it was caused by the crank sensor.
 
Fuel pump relay fitted and not the problem so I'll try crank sensor next then aragorn. Is it located on the back of the head somewhere? The car is dumped in works car park at the moment so I cant go look. Is it a fairly easy unit to fit with basic tools and any idea on price. Could really do without this in the most pennyless month of the year lol. Thanks for the help
Si.
 
Just to add... When I start the car the revs are bouncing at idle between about 300/900 for a few seconds then it will settle, then it will either cut out or let me get a few miles up the road before trying to throw me out the windscreen with engine stop/starts. I know its very hard for people to try to help diagnose these faults through forums so Im just trying to give as much info as I can (soz for waffling on)
Si
 
Is the crank sensor also referred to as a cam shaft sensor or are they two seperate things? I'm trying some searches for info and I'm getting confused
 
Seperate, cam sensor wouldnt stop the engine running, crank will.

crank sensor is in the side of the block, kinda underneath the oil filter, near the bellhousing flange.

Its held in by a single 10mm bolt (and 11years of grime) iirc
 
Thanks aragorn, this is sounding more & more likely now. It was in the garage a couple of weeks ago for rear coil springs and I asked them to do oil/filter change while it was in the air so I'm wondering if the guy has maybe slipped while removing the filter and gave the sensor a knock or something, seems a bit of a coincidence doesn't it.
Do you know if the crank sensor would effect revs bouncing at start up? and does the sensor connect up near the coolant tank?
Thanks again pal
Si.
 
Just been speaking to a guy who seems quite clued up on these engines and he thinks that it might not be the crank sensor because he says that the fuel pump should still prime when the ignition is first turned on even if the sensor is faulty. One of the symptoms of my problem is that the fuel pump wont always prime when the ignition is turned on, sometimes it will, sometimes it wont. When it primes it starts fine then will run ok for a random amount of time then I hear the relay click and the engine stops.
I just dunno what to do now...
Si
 
The fuel pump doesnt always prime anyway i dont think, only does it if the engines been off for a certain amount of time.

Any fault codes?
 
Ok, crank sensor replaced and fault still the same. Really really doing my head in now!!!!
Suppose now I'm looking for a wiring fault or possibly fuel pump. Cam sensor has been mentioned to me now but I just dont know what to look at next really. Close to throwing the towel in but cant get it to a garage anyway...
 
cam sensor wouldnt stop it running.

You need to work out if the ECU is turning the fuel pump off, or if something else is at play.

I dont have the diagram to hand, but you want to find the ECU fuel pump output pin and see what its doing.

When the engine is cranking but not starting, do you see any RPM's shown on vagcom?
 
Thanks for the continued help. This is all being made more difficult as the fault is still somewhat intermittent. Would there be any problem with me running a 12v feed straight to the pump so I can rule that out?
I really could do with the pin outs from the ecu if you can find them aragorn. I'm trying to fit all this in whilst in work and dont have internet at home at the moment.
****** ball ache this is!
 
Running power straight to the pump might help, but if the ECU is turning the relay off (you say you can hear it click) then it will also be shutting down the spark and fuel injectors too.
 
Yeah agreed but I just want to prove the pump is not the fault, cheers. Do you have a link or any info where I can find the ecu pin outs? I've very limited internet use at the moment so don't have the time to search.
 
a mates just had a very similar thing on a vw t4, after 2 years he finally managed to sort it, no. 30 relay that powers the ecu. no fault codes but on vagcom the battery voltage was going from 4 - 18v and anywhere inbetween, changed the relay and he's now a very happy man
 
Ok Sam, thanks for the info mate, I've an assortment of vag relays so I'll have a look at that coz there's not much left for me to check, although I've not noticed any voltage fluctuations, ya never know.
I'm getting close to handing the car over to an auto electrician because this fault really has me fairly stumped now. HATE paying for someone else to fix the car but needs must.... I'll have a few more fiddles with it yet though first.
I've ran a live feed to the pump and all seems fine there.
 
Just to add... also identified the fuel pump live wire from the relay and bypassed the loom to the pump completly and that checks out ok but like you say aragorn it is cutting power to the ignition system too so it was unlikely to be the fix anyway. I'm sure its deffo the ecu shutting things down.
 
find and swap/bridge the 75x relay?

Are you sure the fuel pump relay you fitted is working? Afaik its a bit of a monster on these engines and supplies power to a lot more than just the fuel pump.

PM me your email, and i'll see if i can PDF you a copy of the wiring diagram.
 
Thanks aragorn, yeah I bridged the relay with a switch and the pump ran every time. I've found the power to the ecu and there is a permanent live and a switched live, both check out at 13.8v and seem steady and constant. I've thrown in the towel now and an auto electrician is picking it up this afternoon. Bit gutted to be having to pay someone to sort this but I've really had enough now and its beaten me :(
Thanks for the offer of the pdf, I'll let you know pal.
 
Still not sorted this yet. Been badly let down by the sparky who messed me about all week then said he couldnt look at it so its still with me.
Its looking more like the ecu is loosing its feed ,aybe. I know there is a permanent 12v and a switched 12v to the ecu but where does the 12v come from? does it get its supply through a relay? if so, this maybe my problem. I've had it on vagcom last night again and for some reason it lost comms with the ecu which its never done before so this is whats making me think the ecu is lossing power
I feel quite alone with all this now and i'm getting lost as to what to do! so any help or ideas very much appreciated lads
 
audi upgrades
514 page views...Has no one got any ideas at all? :crying: All suggestions welcome
 
The fuel pump relay is a bit of a beast on these engines, Its a big multi contact thing with 3 different outputs running thru three different fuses...

Try feeding 12v directly into pins marked 87A and 87F on the relay, this will provide constant power to the injectors, fuel pump and all the engines sensors.

Beyond that we're getting into the relams of dry solder joints in the ECU or similar.
 
Hmm, well, if I did this and everything sprang into life would this mean that the relay is faulty??? I just replaced this with a new one but I suppose it could be a faulty unit as you mentioned before
Also do you think it would be ok to run the car like this temporarily??
 
Yup car will run fine like that, so long as you use nice thick wire and make the connections good so nothing overheats.

You can take a feed from the pin labelled 30 in the bus bar below the relays, thats permenant live.
 
Latest update
Replaced crank sensor again and another fuel pump relay just in case previous replacements were faulty. No change. Starts up sometimes with rough bouncy idle, sometimes starts perfectly. Got a couple of throttle body fault codes witch i cleared and they didnt return. Then started up like it had dropped a cylinder and ran like a bag of spanners. Did a hard reset by leaving batt disconnected for half an hr or so and it started and ran fine and i went for a drive, seemed ok. Sat idleing and fuel pump relay clicked out again and engine stopped. It seems better but still the fault persists as in that sometimes you turn ignition on and fuel relay wont engage so no start and if you leave ignition on after a random amount of time from a few seconds to a few minutes the relay engages, pump primes and it will start.
The only other code logged was power supply voltage terminal 30 too low. I know you can get this after batt disconection but got this before i disconnected it.
I'm baffled and started to loose sleep over this now. I so need my car
 
If we assume the relay is good, we can say that either the ECU itself WANTS to stop the engine (for instance because it thinks it has stopped), and so turns the relay off, or the ECU is faulty in some way, which makes it unintentionally shut the power off.

I would patch a couple of test lamps into the ECU's power feeds, and sellotape them to the dashboard or similar, to rule out a power supply issue, Also worth checking ALL the earths, battery to chassis, chassis to engine, interior harness to chassis, to rule those out.

If after all of that its still acting odd, it does seem to be pointing at a fault with the ECU.
 
audi upgrades
Had a similar problem with my missus pug 306, totally different car I know but the ecu was goosed. The ecu on the 306 basically provided the earth for the fuel pump when the ignition was turned on and immob deactivated etc, sometimes it would start sometimes not lol.

Could try earthing the wire that the ecu uses to turn the pump on?
 
Last edited:
The ECU provides the earth path for lots more than just the fuel relay. It fires each injector by earthing them at the right time, similarly with the coil etc etc. So you cant really just replace it with one permenant earth.
 
I'm really leaning towards a power supply issue to the ecu now especially after what you've said above aragorn, its helped me focus a bit. I think it may be possible the ecu is loosing one of its feeds. I'm assuming it has a perm supply from batt and a switched live fed from fuel pump relay??
You have a PM aragorn
Si.
 
The fuel pump relay is controlled by the ECU. The ECU sources its power directly from Terminal 30, and Terminal 15, ie the battery and the ignition switch.
 
Thanks aragorn, just waiting for those diags off ya.
Todays thoughts.....
Cant really do much now as its dark and below zero when i get home from work and I'm working in the road outside my house and no lights so its awkward lol. There seems to be a slight pattern emerging as in it always seems to start ok when cold and the fault only seems to kick in as its getting warm or when hot (had it sat idling fine for 10 mins last night then 'click') When its in its no start state vagcom drops comms with ecu and wont reconnect untill it decides to engage the fuel pump relay again. Magaged to do a full scan in between relay clicking in and out and got no engine codes at all. The only code coming up is with climate control and thats 0779 - Outside Air Temperature Sensor (G17) 29-10 - Short to Ground intermittent.
Mean anything??
 
not had time to get them yet pal, having to sneak onto the net in work when i can, only get a few mins. Will have to leave it till luinch to check. Do you think that temp sensor short has any bearing on anything?
 
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