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I'm trying to avoid a molgrips Passasesque saga here, but I fear a long post...
Mrs njee20-to-be has a 2004 1.2 Polo (little 3 cylinder jobby), had it 6 years, it's now done 100k miles, mainly her shuttling 17 miles to/from work - single carriageway A-road, not stop start, but certainly not motorway speeds.
About 3 years ago she pulled up to a junction on her route, the ECU light came on, the car started running really rough, had zero power etc etc. She pulled over, called AA, who diagnosed (as I'm sure folk are shouting at the screen) dodgy coil. Local (trusted) garage replaced all 3, plus spark plugs (IIRC), car's been fine since.
A couple of weeks ago, it did it again, exactly the same place. She called the AA, he plugged it in and said "error code says a misfire in cylinder 1, aah yes, coils", changed coil. Problem sorted. For about 46 seconds. Whilst AA man was still there and car was still at idle it did it again, same cylinder, with the new coil. He removed it and reinstalled original one. Car restarted, no lights, running fine. Drove home, no issues, drove it around a bit, no issues.
Drove to work yesterday, got to the same junction - light comes on, running rough, no power. Turns engine off in traffic, restarts, it's fine. Same this morning, although now the ECU light has stayed on, although the car's returned to running fine.
The fact it's happened 4 times at exactly the same place must be a complete coincidence, right!? The fact that turning the car on/off usually resets it (lately) suggests to me electrical rather than mechanical(?). A bit of Googling suggests coolant temperature sensor, I've ordered another one, for £8 it seems daft not to try it!
We're looking to replace the car, but ideally not for 6 months or so, it's worth virtually nothing, and with an intermittent fault we can only replicate at the junction of the A24/A272 (where sadly there's no VW dealer) means that it could be a very costly exercise in fault finding!
Any ideas...?
Edit: bloody hell, that is long, sorry!
That light isn't the ECU light, by the way, it's the Malfunction Indicator Light. So you most likely do not have an ECU problem as such.
Sounds like a wiring fault to me - corroded, frayed wires or water. Although you woudlnt' be seeing it now if it were water since your car would be pretty dry 🙂 You may have other codes that come up later as the car gets more of an idea what's going on. It's a shame I'm not down your way currently as I'd bring my VCDS and have a look.
If you've not got a fault reader, buy a basic one of ebay, will more than pay for itself (<£10).
If it's showing a misfire on cyl 1, change the coils for cyl 2 or 3 - does same fault re-occur?
If yes, change spark plugs, etc., etc.; just work logically, methodically and you'll fix it for a lot less than a trip to the dealers.
PS same engine, different car, I changed the coolant temp sensor (identified by fault code reader) and also changed the 3 coils; both faults caused misfiring/rough running.
If it's showing a misfire on cyl 1, change the coils for cyl 2 or 3 - does same fault re-occur?
I believe so - but it was AA man who did the coils, and I wasn't there, I'm pretty sure it's not the coils though - surely they'd generally stay broken anyway if they'd gone? Replacing with brand new didn't change the fault. It appeared to, but it seems it was actually the restarting of the car which solved the fault, not the fact the part had been changed, it still gave the same fault code.
In usual STW style I am no car expert but will post anyway ...
The fact you are speaking of temperature sensors and that the problem occurs in the same place suggests to me it is temperature related, ie the car has warmed up to a specific temperature after being driven a certain distance. The fact you are slowing down at the junction too probably is a factor as it changes the loads on the engine.
That was what I wondered about, it's just really odd that it's happened at the same place in both directions! On the way into work it's the first place she stops after the car's up to temperature, but it's not on the way home.
I had a problem with cyl 1 misfiring. I swapped coil 1 for coil 2.
Much to my surprise, all was fine; however, the next day the engine was misfiring, fault code reader now showed a misfire with cyl 2 - coil now on cyl 2 replaced.
First stop has to be to get fault code reader.
Yes, that does make sense re fault reader, but as per your example, we've eliminated that.
Fault reader says misfire cylinder one
Fit brand new coil to cylinder one
Fault reader continues to say misfire cylinder one and problem persists.
Not coil related...?
IF that's still the fault code that's being generated, I'd swap the plugs over next, clear the codes and take it from there
Before you spend thousands randomly swapping parts around, you need to properly understand and investigate.
The problem is obviously intermittent, which makes diagnosis harder, but if you can repeatably get it to occur by driving the same route or in the same fashion, that is good.
Now, a "misfire" is one of four things:
1) Incorrect amount of fuel
2) Incorrect spark event
3) either of those being "poorly" timed
4) A mechanical fault, like a failing lifter or low compression etc
First, you need to be ABSOLUTELY sure the issue is with the ignition side of things. To do this, you can swap fuel injectors between cylinders, and even swap the injector harness plugs between cylinders (it may run a little rougher with the wrong injector timing, but it will still run). If moving injection components makes absolutely no difference you know it must be a issue with the ignition.
You could do worse than do a compression test on the engine too, to check that it is mechanically fine. These days, people immediate blame the "computer" and change the extremely robust and hardly ever failing "ECU" when it's just about never at fault.
It's fairly common for ignition coils to fail or breakdown at a specific temp, so they work when cold, but not when hot etc. However, it sounds like you have swapped enough coils to be relatively sure that this is not the case here.
It's very unlikely to be a sensor issue, such as with a coolant temp sensor, as this would effect ALL cylinders equally, and also the dash display of coolant temp would be wrong (assuming ecu drives dash display of engine temp, which it should on this age of car). Ditto, with MAF sensors, crank sensors, cam sensors etc.
Most likely, is a loom fault, or power distribution fault to the coils, so look for cracked wiring, chafing, loose/dirty/damaged loom plugs.
Dissemble the loom, spray contact cleaner into all plugs etc Check all engine and loom earth points too!
It's also worth checking that the fault code reader you have reports the correct cylinder by pulling off an injector plug at idle, which should return faults for the cylinder in question!
If you are really getting stuck, then you're going to need to start to carry out what i would call "invasive" investigation, for example, splice a cable and indicator led into the coil pack power feed, mount led where it can be seen, drive car, and when fault occurs, see if LED flashes (ie, the coil IS loosing it's power supply etc)
You can also use this LED on the coil trigger wire, to check the ecu is still triggering the coil, although using an oscilloscope obviously makes this easier to see what's going on.
It's worth pulling the ecu out of the ebox (think it's under the windscreen scuttle on these cars) and check for water damge / ingress / dodgy plugs etc (iirc, this can occur on polos??)
Awesome, thanks maxtorque.
Turned out to be burnt out valves in cylinder one, which given the age/mileage/condition of the car was terminal!
Currently on eBay and generating a surprising amount of interest, I understand it's not a hugely expensive job for parts, so presumably someone with a garage can sort it relatively easily.
Not ideal timing, hadn't planned to sell it for 6 months or so, but c'est la vie.