Weird fault, almost like a misfire

[dsolds]

A bit perplexed by this one chaps, wondering if anyone can shed some light on it.

Superb, 1.9PD, 130bhp. 2006 registered, 208k miles

 

So, when the revs are between idle and 1500 it feels like there is a slight misfire. This is made worse buy opening the throttle wider. But once the revs are over 1500 it all smooths out again and pulls strongly through the gears.

 

Most noticeable in 4th or 5th, especially if I let the revs drop when slowing down. It always used to pull smoothly from about 1100 rpm but this is becoming more noticeable.

 

2 things which have come to mind are injectors or perhaps the dual mass flywheel. I know on one of my Land Rovers the injectors fell out of spec over time and they had to be refurbished and then re-calibrated to the car (Discovery 3) but that was more about exhaust smoke than lumpy running. No smoke issues on the Superb unless wide open throttle in 3rd and up around 70mph such as accelerating onto a motorway and this looks more like soot being expelled from the exhaust rather than unburnt fuel. As for the DMF, no rattles, no jerkiness through the gears so I'm not sure what to look for there.

 

Or it might be something completely different. 

 

Any suggestions gratefully received. Thanks.

[rotodiesel]

Most likely the injector loom in the engine has become intermittent. A bit of a wangle to change - warm the new one before fitting.

 

rotodiesel.

[dsolds]

Thanks Roto.

Assuming i've got this correct, this is the long loom inside the cam box. I buzzed it out with a Fluke meter over the weekend. Nothing untoward, max value I got was 5 ohms. Do they go intermittent with the heating/cooling of the engine perhaps? Should I just change the thing and be done?

Thanks

Dom

 

EDIT. This is the one I checked.

http://www.ebay.co.uk/itm/Genuine-Audi-VW-Injector-Wiring-Loom-For-1-9-TDI-PD-Diesel-Engines-038971600-/172177412634?hash=item281692d21a:g:BWwAAOSwwE5Waagu

Edited 8 August, 2016 by dsolds

[Tdi1.9]

....or this one for a little bit less   http://www.darksidedevelopments.co.uk/products/genuine-vw-injector-wiring-loom-for-vw-1-9-2-0-8v-tdi-pd-engines-038-971-600.html#

[octyal]

I would say either injectors or possibly turbo vanes sticking (if it's VNT of course) DMF won't cause a misfire, even when worn. PD's do suffer with injector issues, especially where they have had the incorrect oil used for servicing, and at that mileage I would certainly consider having them refurbished. Best option to test them on the car would be a leak off test. Basically you connect a pipe to the leak off connection and run it into a calibrated container. Then run the car at idle for 30 seconds, repeat for all 4 and see what you get, more than 10% between cylinders would indicate a flow rate issue.

[rotodiesel]

The connectors to the injector solenoids lose their tension with long term engine vibration and thermal cycling. This gives an intermittent connection when the engine runs at certain critical speeds (resonance) which will not generally be revealed by buzzing out the loom with a Fluke, which passes negligible current through the connectors and has a slow readout response.

 

One way to find a bad loom is to disconnect the large circular connector, apply a low voltage at constant current to an injector and monitor the current draw with a current probe and a storage 'scope on a slow timebase speed. Flicking the injector connector bodies with a biro will often reveal a bad one.

 

I just change the loom these days. After about 150k miles, the loom is on borrowed time.

 

The (Bosch) injectors are dead reliable if the fuel filter maintenance has been up to scratch.

 

rotodiesel.

Edited 8 August, 2016 by rotodiesel

[Tech1e]

I would say either injectors or possibly turbo vanes sticking (if it's VNT of course) DMF won't cause a misfire, even when worn. PD's do suffer with injector issues, especially where they have had the incorrect oil used for servicing, and at that mileage I would certainly consider having them refurbished. Best option to test them on the car would be a leak off test. Basically you connect a pipe to the leak off connection and run it into a calibrated container. Then run the car at idle for 30 seconds, repeat for all 4 and see what you get, more than 10% between cylinders would indicate a flow rate issue.

No but a faulty DMF will cause a vibration especially at idle. I've seen them before where number 4 injector starts to compensate for it.

[octyal]

No but a faulty DMF will cause a vibration especially at idle. I've seen them before where number 4 injector starts to compensate for it.

 

True, you will get a vibration at idle, however, you need to look at why the DMF has failed, rather than just replacing it. The DMF is there to balance the torsional vibrations generated during the combustion process, alter those characteristics with say low compression, blocked injectors, faulty TVD etc, and the DMF will be trying to absorb vibrations that are greater than it's design permits, this leads to premature failure. We see too many times (on an almost daily basis) whereby garages have replaced DMF's only to have the car come back a month or two later with the same symptoms that it was originally presented for repair with - in some cases they only call us after the 3rd or 4th DMF has gone in!

[Speedy46]

On 8/8/2016 at 18:25, Tech1e said:

No but a faulty DMF will cause a vibration especially at idle. I've seen them before where number 4 injector starts to compensate for it.

 

Mine 1.9 TDI 85KW PD suffer from vibration and booming noise between 1400-1800 rpm.

 

Injector number 4 reading is close to 1.0. Sum of other three injector’s readings is close to minus 1. So, ECU is trying to equalize injection. In my opinion ideal situation will be where sum of readings from two injectors are equal to sum of values from other two injectors – two positive against two negative readings.

 

Also, after removing the engine cover I spotted worn exhaust camshaft’s lobe on 4th cylinder. Could this be related to injector’s readings on injector number 4?

From my point of view you can expect DMF to fail from problem(s) originating to engine – in other way, you cannot expect DMF to make faulty injector or engine.

Edited 16 July, 2017 by Speedy46