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VW Lupo 1.4 misfire - JohnM{P}
Daughter's 1.416v Lupo: apparantly a few days ago the EPC warning light (" Engine Management System" in the manual) lit up but went away when the engine restarted. Nothing seemed to be amiss.
A couple of days later, the engine symbol lit up (" Exhaust Emission warning lamp"); the engine is now missing on a cylinder. It has only been driven a couple of miles like that.

I have put new leads on the car, and have also tried changing the coil pack with one that was known to be ok (from our other 1.0 Lupo - same coil pack on both engines), but the problem persists.
I've not yet checked the plugs (tools and car in different places) but as they were changed at the last service (Bosch) and are only 2000 miles old, they shouldn't be the problem.

Any other ideas I could easily check, before I have to take it to a
garage?

Thanks, JohnM
VW Lupo 1.4 misfire - Screwloose
JohnM

Can we have year: mileage; engine BHP and engine code if possible. If it's a 100bhp one; what octane rating of petrol has it been run on? While you have the plugs out, check the compressions.
VW Lupo 1.4 misfire - JohnM{P}
April 2001, 62k, it's an S, so 73bhp I think (certainly not the 100bhp Sport), engine code AUA...
VW Lupo 1.4 misfire - Big John
Is it just missing on one cylinder or is it two?
VW Lupo 1.4 misfire - JohnM{P}
"Is it just missing on one cylinder or is it two?
I'd say 1 - I'd expect 2 cylinders awol to cause it to run a lot rougher and slower than it does.
Could a fault with the emissions sensor (lamda probe - showing my age, remembering the Volvo badges...) cause a similar problem?
VW Lupo 1.4 misfire - Big John
Good news if its 1 only, it rules out the CPU being fried by a dicky wasted spark coil pack. The coil pack consists of two coils each driving two plugs (hence "wasted spark").
Spark plugs next? could be a faulty new one!
How about the injector solenoid for that cylinder?

I doubt if its the lambda probe as it would affect all cylinders.

VW Lupo 1.4 misfire - JohnM{P}
Changed the fouled plug with a new one - still misfiring. Coil pack, plugs and leads all checked so I've got the car booked in at VW specialist next week. Will let you know the outcome...
VW Lupo 1.4 misfire - Screwloose
JohnM

Were you getting a good spark at this plug? Did you carry out that compression test; minimum of 10 bar present?
VW Lupo 1.4 misfire - JohnM{P}
I haven't got a compression tester; with the new plug out of the engine it was sparking. It was a Bosch with twin electrodes so nothing like the plugs from my youth (20 odd cars and bikes in the '70s, pre company car and morgage...) so cannot judge if it was ok (but with working coil pack and new leads it should be).
VW Lupo 1.4 misfire - Big John
After running does the plug smell of petrol?
VW Lupo 1.4 misfire - JohnM{P}
Watching my son in the pouring rain last Sunday play football in a sea of mud meant that I didn't get to look at any further. However, the car has been at my trusted VW Independant today, resulting in much head-scratching.
There's a good spark, normal compression, fuel getting to the cylinders - yet it still is missing. Removing the plug lead or injector on the offending cylinder makes no difference to its lumpy running. When I queried the amount of fuel getting to the cylinder, they replied that they'd compared injectors and were getting the same flow of fuel from both/all.

However, the engine computer is repeatedly reporting a (manifold) air leak - there is no apparant leak/crack/etc, so they are starting to remove the manifold...
VW Lupo 1.4 misfire - Screwloose
JohnM

Curious... I wonder if the EGR valve is stuck open and is affecting one cylinder more than the others? There's no code specifically for a manifold air leak; it must be generally low vacuum that the MAP sensor is reporting. An intake air leak wouldn't affect one cylinder; they're all just sucking in air.

If it were an 8 valve, I'd think about a missing cam lobe; but on a 16v how do you lose two? Might be a broken valve spring or sticky valve doing something weird. Get your garage to check the compression with it running.

Failed timing belt tensioners/jumped belts are common on these; but why it should cut off just one cylinder....?
VW Lupo 1.4 misfire - JohnM{P}
Next installment...

The EGR been checked, as has the breather (known weakness). The plugs, leads and lnjectors from the offending cylinder have been swapped around - but still the same cylinder misfires.

There was no problem with the inlet manifold and gasket. After a compression test, the offending cylinder was 'approx 10 down, compared to the others; 140 vs 150". (I reckon that's a lot more than 10 bar...). With some oil in the bores, compression was equal for all 4 at 170. They've had the cam (s?) out to check them and the valves; valves not leaking either.

Puzzlingly, the exhaust emissions, although out, are not wildly out, despite the misfire.

I found a reference (on a Lupo forum) to the brake light switch having caused a problem. The EPC light had come on and gone a couple of days before the misfire started; the manual says this is for the engine management system, yet a poster on the forum is insistant that this is a mistake as it stands for the Electronic Pedal Control?

Until the next episode... any further ideas welcomed!
VW Lupo 1.4 misfire - Screwloose
JohnM

10 bar is approx 145 psi. Anything over 70 psi will usually fire. Did they try the compression with it running? Sounds a weird test; but sometimes things like sticky valves etc. don't show up at 300RPM cranking speed. It's not the actual pressure [often not much different to the cranking one] that you're looking for; it's if it's varying or drops to below 70.

To fire; your missing cylinder needs a compressed, burnable mixture ignited by a spark at the correct time. It's obviously lacking one of those things, so the task is to find out which. Your garage appears to have eliminated most of them; so what's left.... a blockage in the exhaust manifold?
VW Lupo 1.4 misfire - JohnM{P}
SL: as you say, seemingly enough sparks and fuel at right time - so what is it?
Blocked exhaust manifold branch is interesting suggestion - but what could block it enough to cause a misfire?
I'll ask again tomorrow as to how they exactly they have checked for a sticking valve - if the ecu was reporting an air leak (or low inlet pressure), I guess that could that be due to an exhaust valve not shutting completely, or to an inlet not opening fully...
VW Lupo 1.4 misfire - Screwloose
John

I'd love to know that code number that keeps coming up. Low inlet vacuum [or absolute pressure, if you prefer] can be a lack of suck or excess air by-passing the throttle plate. [Or usually, on these, jumped cam timing.] It can also be compressed mixture from the current or previous charge being forced back into the inlet manifold. I can't see how an exhaust manifold blocks-up either, unless it was a faulty casting from new; but as you've - apparently - eliminated all the reasonable things...... You can never say never with a car.

I did once see an inlet manifold on a 4-y.o. car [that had never run right from new] that was partially blocked by a sand casting-core. It had even had a new engine [and everything else!] when under warranty - but they'd swapped the manifolds over from the old unit.....
VW Lupo 1.4 misfire - JohnM{P}
Next step: Garage spoke to their local engineering company, who said they'd seen valve guide and associated problems with that engine
Garage took head off - and discovered burnt exhaust valve seat. Local eng companies don't want to touch the head as 'they've never managed to repair that head successfully'. VW exchange is lottsa money.
Search on internet hasn't found many VW head reconditioners; the one that had a list of VW heads they'd recon did not include the 1.4; the price for a 1.8 16v was not much less than the VW exchange.
Looks like I'm going to have to pay out...
VW Lupo 1.4 misfire - Screwloose
John

I've seen low compression/head faults on lots of the 100bhp versions that have been run on 95 octane. Never seen it on a 75bhp one..... yet. In every case the compression on the affected cylinder was intermittantly below 60 psi [c.4 bar] and the fault developed from bad cold starting into a misfire. If there really was 140 psi in that cylinder, then it would have run - certainly at higher revs. It also shouldn't have shown the "low vacuum" code; inlet valve, yes: exhaust, no. Was that seat burned because the valve-spring had broken? Something's not quite right here....

VW Lupo 1.4 misfire - JohnM{P}
Ah-ha - it did seem to me that the engine sounded better at higher revs, but I thought it could just be that the higher revs 'smothered' the miss as it still seemed down on power though. No reason for the previous owner, or my daughter, to have run it on > 95 octane - it's not a performance or FSi engine.

My garage guy couldn't explain why the compression test (cranking, not running) didn't show a greater loss and is just as puzzled as you and I, but is adamant that the seat is damaged and is the cause (perhaps I got the inlet/exhaust muddled - I took that call on the hands-free this morning).

We will all see in the New Year, when the exchange head is fitted...

Thanks for your comments, will update in due course!
VW Lupo 1.4 misfire - JohnM{P}
OK! Car has had the exchange VW head fitted and is running smoothly again now. I saw the old head before it was sent back, but couldn't see the offending seat as the spring was still on it. Garage and I are relieved - it's cost both of us time and money!
Happy New Year to all!
JohnM
 

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