New here sorry if someone has already asked so.ething similar. I have a mk4 polo 1.4tdi BNM JHG 3 door. It has a rough idle and dirty emissions at idle. I cant figure out what's doing it. I've changed the coolant temp sensor and thermostat. I've checked the injector harness(removed, inspected, con checked etc.). I've removed and cleaned the egr valve, it also appears to be holding a vacuum. I have zero fault codes when scanned with VCDS. Is done a resistance check on the glowplug and they appear fine.

Other than throwing random parts at it in the Hope's something will fix it I'm not sure what else to look at.

Please share your wisdom

Water accumulate in the diesel fuel filter?

 

EGR valve might be sticky, even if clean.

Does it make the pop noise of closing when shut off engine?

 

Turbo vanes might be sticky (if the EGR was full of crap).

Mass airflow sensor might be dodgy.

 

Timing might be slightly off.

Does it start easily from cold with good battery?

So I'll change the fuel filter at my asap just in case. I did it 4 months ago but I dont think it hurt to do it again

 

The egr does open and close. I havent ever heard a pop from it though. I've manually exercised it while out and when I push it open and release with my thumb over the vacuum connector it holds a vacuum(scientific testing method) how can I know for sure the egr is operating correctly?

I've pulled the manifold off ot the solenoid block with the engine running and it evens out momentarily then goes back to how it was. 

 

What's the best method of checking the turbovanes?

 

I'm getting live data from the mass air flow sensor, the reading fluctuates rapidly is that normal? When I unplug it I get a fault code.

 

It was running fine and then started doing this. Could something have changed to put the timing out without work being done on the car?

 

Battery is good and so is starter, I fitted a new starter last week and it turns the engine really nicely and she fires up.

 

Sometimes she starts and seems to be fine and then as she sits the roughness starts.

 

Thanks for you suggestions because I'm really lost as to what I should try

You can usually drain the fuel filter of water from below - lift it up stick a tube on it, undo the plastic thing. If only 4 months old it won't be that, unless someone's been running it on chip shop oil.

 

It starts fine, so it's not timing - the diesel injection pump is aligned to TDC with 3 quite small bolts (and the cambelt). If someone had worked on that and not re-tightened those 3 bolts properly, it would lose timing by a fraction.

 

Turbo vanes, er,

take off the turbo and look?

 

It would have shown as reduced power mid range, maybe even limp mode at high power attempt until engine restarted. And would very likely have an engine code.

 

Or leave it fitted and you can spray EGR + Turbo cleaner down the throttle inlet - I used Lucas 41019 spray on a 265,000 mile 1.9TDI (at EGR, take off pipe from intercooler) while revving it gently.

https://www.lucasoil.co.uk/products/problem-solvers-utility/diesel-egr-turbo-cleaner

 

It's now done nearly 275,000 miles.

Or if feeling brave water spray (must be very tiny amounts like from a perfume bottle). 

 

Or remove turbo and use Mr Muscle foaming oven or barbecue cleaner on the dirty side impeller blades overnight in a plastic bag to remove the soot, rinse, refit, blow it out by starting and revving. You might want to disconnect the cat convertor, fit a second hand backbox to that pipe, or use ear plugs . 

 

Yes, signal will fluctuate as each cylinder descends with open inlet valve but each suck should be the same. 

 

If the airflow signal is dropping every 4th beat, it might be a burnt valve (usually exhaust), worn cam lobes or lost compression from too little oil and worn rings. But at mega miles if well serviced.

 

The 1.9 TDI idles at absolute steady 900rpm (15 revs per second) according to the rev counter.

 

Compression test down the glowplug holes? If one cylinder is way different that's the one that's the problem. Test procedure might need a helper to crank with fuel disconnected while you push a rubber tube down the hole with compression gauge attached.

 

Or shell out for a glowplug adaptor tube. I didn't find a cheap one of these. Think its M9 threaded tube.

 

Another idea: collapsed catalytic convertor, if you drove through deep water the thermal shock can break the core.  

Edited 29 January, 20233 yr by dvderlm

So definitely no chip oil has been near it lol.

Theres no loss of power. I drive to work and back every day down the motorway and it easily gets to the speed limit would do more it I wanted but I always stick to the speed limit or below.

So maybe I'll look at the turbo then and I'll order a compression tester too so I can check that.

I've had bad experience with garages ripping me off so I want to fix this myself also so i know the car and know it's done right. I'm keeping this car for as long as possible so I'll be doing whatever is needed to make it right. I just dont want to spend money on the wrong stuff.

Any chance it could have a dodgy injector and give these symptoms?

 

Thanks again for the advice.

Pretty sure you'd have fault codes in range 17671.. 17678 if it was an injector fault.

 

I think Mk4 is pre Pumpedüse anyway. What year and engine code?

Any oily seepage around the injectors? 

 

You could always put some Redex or similar diesel injector cleaner snake oil in a 1/4 tank of fuel.

 

Be careful removing injectors, alloy head is quite soft. Needs new seals.

 

Avoid Silverline compression tester.

I found Draper to be reliable. 

Mechanic mate says rough idle can be the pneumatic control valve solenoid that operates the EGR unit, vac tubes from that can leak if rubbed against engine, electrical contacts or the valve itself can become faulty. 

 

N18 ?