Yet Another VR6 Running Rough Thread

[Petros 0]

Hi Folks,

 

Having a problem with my VR running poorly just now. I've been through a load of other threads, but can't find one that exactly describes my problem.

 

In the middle of a longish motorway cruise, the engine started struggling under load at low engine speeds. Accelerating from below 2000 rpm in any gear produces a very nasty judder and spluttering noise. The engine mostly pulls fine above 2-3000 rpm, but doesn't feel quite as smooth as usual.

 

The only thing that has changed is the fuel, I normally run Optimax, but am running 97RON from Sainsburys (it was too cheap, leave me alone!). I'd been running on that for about 2 days before the problem surfaced.

 

I've had someone follow me while its doing this, and they couldn't see any real signs of smoke or flames from the exhaust.

 

Scanning for error codes with VAG COM showed an intermittent battery voltage > 16 volts fault. I cleared that and it hasn't returned in the several days since.

 

I tried running without the MAF connected, the engine ran but wouldn't idle. With some perseverance I managed to do a proper road test in this state, and the problem was still there. (Should the engine idle without the MAF? is this another problem, or maybe related?)

 

I know the Lamda probe is often blamed in situations like this, but mine was changed less than 1 year ago, so ought to be OK. No fault codes logged for it anyway.

 

I checked the operation of the Throttle Position Sensor using VAG COM, and I suspect it may be faulty. When I open the throttle slowly & uniformly, as instructed in the Bentley manual, the TPS display jumps in increments of 5-10 degrees. I can force it to produce a continuous reading by opening the throttle ridiculously slowly - over 1 minute between closed & WOT - and smoothly, but I'm not convinced that was what the manual intended. At WOT the display reads between 78 and 84 degrees. I understand it should read over 94 when wide open.

 

Could this be related or a red-herring? It kinda makes sense in my head; TPS looses accuracy, at low engine speeds ECU doesn't know how much air is getting in and hence gets the mixture wrong, and viola engine doesn't pull smoothly.

 

Can anyone offer any suggestions on whether I'm on the right track with the TPS, or are there any other tests & checks I can try?

[Mike Edwards 0]

The car will run without a MAF, or without a TPS, or without an ISV, or without a lambda probe, or without a cam position sensor, or without an inlet air temperature sensor, or without and oil temperature sensor, or some combination of the above*.

 

* which can be mimicked by a broken wire to any of the above....

[dr_mat 0]

.. but it won't run very well.

 

It makes perfect sense that if the engine isn't seeing your small throttle increments smoothly then it won't fuel correctly or do ignition timing correctly.

Replace the TPS - you KNOW it's faulty.

[Petros 0]

.. but it won't run very well.

 

It makes perfect sense that if the engine isn't seeing your small throttle increments smoothly then it won't fuel correctly or do ignition timing correctly.

Replace the TPS - you KNOW it's faulty.

 

Thanks Doc, I've ordered one up from VW, should arrive saturday.

 

Just wanted my diagnosis sanity checked really... while I knew it was faulty, I didn't know if it was faulty enough to cause these symptoms.

[Petros 0]

The car will run without a MAF

 

Thanks for the info Mike. My engine does run without the MAF, but won't idle. It doesn't seem to abruptly stall or cut out - closing the throttle results in the engine speed dropping smoothly all the way to zero. This obviously isn't normal, could it relate to my faulty TPS?

 

I'll check if this changes after the TPS is repaced, but if it doesn't then I guess I've got another problem to track down.

[dr_mat 0]

Replace the TPS, reset the ECU and try it again.

If the ECU doesn't have reliable signals from the TPS OR the MAF, it won't run predictably at all, since they are both used as proxies for the other in the event of a failure...

[Petros 0]

I've now replaced the TPS, reset the ECU and run Basic Settings.

 

The problem was still apparent during the first 15 minutes of driving after resetting the ECU, accelerating from 25mph in 4th gear caused a fair bit of engine judder until it reached about 30 mph.

 

Then it got a whole lot worse. The engine is now struggling at all engine speeds under acceleration, at part and full throttle.

 

During Basic Settings the O₂ sensor was reading 0.8 - 0.9 at idle. The ignition timing was fluctuating between 4.5 and 6.8 degrees BTDC.

 

Using the measuring blocks I checked the O2 value with the engine running at ~2000 rpms, and the reading was much nearer 1.0.

 

From reading the KB article on resetting the ECU, it looks like these values are borderline and outwith the allowed ranges respectively.

 

Checking the measuring blocks for the TPS show that it's still only reading 80 degrees at WOT. The progression from closed to open is much smoother though.

 

Really not sure what to do/try next. I'm thinking it's time to get it into a garage.

[Petros 0]

I took the car to a garage this morning and let them have a look at it. They checked the plugs and found they were badly oiled up.

 

Having replaced these, the problem has almost gone away. The engine isn't struggling and is running much, much smoother. However, at high engine speeds there is still a discernable, but not dramatic, loss of power. The engine note sounds slightly off, and there's a noticeable vibration that wasn't there before all this started.

 

I also checked the engine's behaviour with the MAF disconnected (following the earlier TPS replacement). This remains the same, the engine will start and run with the throttle open, but will not idle; it just stalls as before.

 

Does anyone have any ideas why the TPS still isn't recognising WOT?

 

And next, is it time to start considering a(n) head/engine rebuid? (Or 24v conversion :-) )

 

I was also recomended to relace the HT leads - I'm running on non-std 'ones atm - with genuine VW parts. Obviously that won't help the oiling up, but might this help with the slight power drop/vibration at high rpms?

[Petros 0]

The low-speed/high-load struggling started to show up again with the new plugs.

 

I replaced the plug leads with the genuine VW ones, and that seems to have cured the problems. The engine doesn't struggle at any speed/load, and the lost power at high rpms seems to have disappeared (although I've got a wobbly wheel bearing that needs sorted ASAP, so I haven't been really pushing the car). Idle seems smoother, and the engine pulls more readily at low speeds.

 

And into the bargain the engine bay looks much cooler with bright red leads :smile:

 

Hopefully that'll be the end of the problems for now. Thanks again to all those who helped!

[dr_mat 0]

Oh well, OE equipment wins again .. Pity it's so damned expensive!

[Petros 0]

Oh well, OE equipment wins again .. Pity it's so damned expensive!

 

Makes me wonder if I should start un-doing all the other mods the car's had :wink:

[-Neil- 0]

so you rplaced the leads and it seems ok now?

How much did the set of leads and plugs set you back, i think i need to change mine.

 

and the TPS, where is it and how much are they and how easy are they to replace?

[Petros 0]

Yup, changing the leads seems to have fixed things. The OE leads are a much tighter fit over the plug ends than the aftermarket ones I had on. I guess they must be making a 'cleaner' spark; the old leads were so loose when I removed them it's a wonder some were sparking at all!

 

The leads were about £98 from VW. The plugs were about £30 from an independant specialist.

 

The TPS costs about £50 from VW. It is mounted on the throttle body, directly opposite where the throttle cable is attached. Look for a roughly rhombus-shaped black plastic thingy with wires plugged into the top of it.

 

It only took 10 minutes to replace this. You'll probably need to disconnect the breather pipe from the cam-shaft cover to get at the bolts, but this is only held on with a hose clip. Also, the shape of the TPS has changed, and the original bolts are too long for the new one - you'll need 2x 15mm M4 bolts (IIRC) + washers.

 

And, according to the KB article, you need to do an ECU reset+Basic Settings after replacing this.

[-Neil- 0]

damn, £100 for leads! i kinda really need to make sure it them before i go spending £100! Think ill try n investigate more..

[Petros 0]

Yeah, deffo worth being sure before you potentially throw 100 bucks away.

 

Have a look at the leads you've got on, there will be a VW & AUDI logo embossed in the metal plug end if they are genuine. You probably shouldn't be able to pull the lead off the plug without using the tool - if it does come off easily by hand, like mine did, it may be worth chaning them (or at least closing the ends a little with some pliers).

 

And check the condition of the leads too, as any damage to the insulation could well cause problems (AFAIK).

 

What kind of probs are you having that makes you think they need changed?

[Kevin Bacon 5]

Glad it's sorted. You might want to put some pennies away for a head and chain job though. Oiled plugs is an unfortunate sign of impending ££££s....

[Petros 0]

Thanks Kev. What's your stance on head re-build vs. replacement with new?

[Kevin Bacon 5]

Might aswell get a rebuild. Fresh guides and valves etc etc. New heads are OK providing they haven't been sat around for donkeys years....which you can never tell on Ebay!

[-Neil- 0]

i just think that mine doesnt pull as good as it should, idle is little lumpy, judders whole car.

But its alot of money.

[dr_mat 0]

Try another MAF too ...

[Petros 0]

Is there a test to check whether a MAF sensor is working well or not?

 

At the risk of sounding tight, it isn't a cheap part so I don't fancy replacing one that might be ok!

[-Neil- 0]

what difference would the MAF make and where woudl i find it?

[Petros 0]

The MAF sits just upstream from the throttle body on the air intake. It's the black box that the air pipe passes through, with an electrical connector plugged into the top of it.

 

It measures the volume of air flowing into the engine, and reports it to the ECU to help it get the air/fuel ratio and ignition timing correct.

[-Neil- 0]

i could of sworn this was an AFM, Air Flow Meter?

[dr_mat 0]

"Mass Air Flow meter, MAF."

 

There's no easy test, you have to use a test rig and an oscilloscope or accurate voltmeter to see if it's within calibrated range at each fluid flow rate.

 

But they often show up on ebay from cars that are being broken. Get one cheap. It doesn't really matter if it's 100% or not, just *any* improvement in behaviour tells you your original one was probably not working right.