Strange hesitation/power loss under 2000rpm

[ninjag]

I have this problem for quite a while, it's a little annoying and I've found others online with the same problem with various ideas on the solution, so I've learnt to drive around it for the time being.

When under 2000rpm if I accelerate with light to moderate power it seems to lose power for a split second, almost as if the fuel has been cut off and you get a slight jerk. Sometimes it's just once and other times it can be a double jerk, all very fast though. Happens in Drive, Sport and Manual modes and pretty much all gears. If I accelerate very lightly or put my foot to the floor it doesn't happen. It also doesn't seem to happen when the car is cold. Over 2000rpm it never happens.

• I've have a sporadic PCV code thrown up maybe twice in the last year but it clears and doesn't come back for months.
• No other codes are showing from the engine, transmission or any other module.
• I shoved in some Wynns Dry Fuel and I thought it worked but I think it was more a placebo effect.
• I use my local Esso which is very busy and I've also tried different fuel stations as well with no difference.
• Spark plugs were changed last year although I used Denso Iridium instead of NGK, could this be an issue? I've checked the gaps twice and they are good and they aren't fouled.
• Fuel trims are perfect.
• Oil is changed every 6 months
• DSG fluids changed, gearbox/diff/transfer case fluids all changed.
• Supercharger and aux belts all changed.

I'm wondering about the coil packs, but then wouldn't that also impact hard acceleration, cold engine, over 2000rpm etc? Has anyone experienced this and fixed it or have any ideas where to look next as I'm stumped.

 

[ninjag]

Just done a double check, have I got the wrong spark plugs in my car (S4 CREC): Denso Iridium IK24?

 

[ninjag]

So far it looks like it was the plugs, the Denso ones looked a little over heated. NGK ones now in, injectors have quietened as well; perhaps somehow related.

 

[g00se]

Based on your description of the problem, my first thought was plugs... relatively easy fix in the grand scheme

I dont think ive used anything but NGK for circa 15 yrs now

Glad you got it sorted

goose

 

[ninjag]

Based on your description of the problem, my first thought was plugs... relatively easy fix in the grand scheme

I dont think ive used anything but NGK for circa 15 yrs now

Glad you got it sorted

goose

Cheers. Aye, the more I thought about it the more I realised that it didn't have the issue when I bought the car and wasn't really until after I had changed the plugs. I just couldn't figure out how they could be causing such an issue within such specific parameters. I think I ended up overthinking it rather than just using deduction logic.

That'll teach me to blindly trust Euro Car Parts and the results returned against my reg.

I don't know why I had a problem finding the NGK ones before, perhaps just new to the car but recently I came across the ECSTuning listings which had the OEM ones (101905611G) that matched the online Audi parts site I use, but I wasn't 100% sure if they were NGK so I looked into the actual NGK listing ones that had a number 94460 which corresponded to the Opie Oils results against my reg and these also showed the part number PFR8S8EG and this corresponded with the actual NGK website results for my car. Ok, maybe I can see how I got a little confused the first time round! haha

To summarise, just in case anyone is reading this and wondering themselves what plugs to get for stock engine:

VW OEM plugs: 101905611G
NKG plugs: PFR8S8EG (94460)

 

[Dippy]

Any ideas why the Denso plugs caused this? On paper I cannot see any reason. Were the gaps correct?

 

[ninjag]

Any ideas why the Denso plugs caused this? On paper I cannot see any reason. Were the gaps correct?

I'm not sure but I think they were running hot. I'll take a photo of them and post to see what you reckon. Checked the gaps many times and I also tried slightly bigger and smaller. They are iridium and not platinum, if that would make any difference?

 

[ninjag]

Here's some photos of one of the plugs (they all looked the same). Don't look quite as bad as I remember. What do you reckon?

 

[Dippy]

As far as I know iridium is supposed to have benefits over platinum with the only con being that they are more expensive.
I'm no expert on spark plugs. I can't quite see but is there any pitting as I think that's an indication of overheating. But if it was overheating and this was due to timing or fuelling then I'd expect there to be DTCs.
Re-reading your post I see that you have had intermittent PCV codes? Maybe that's something that needs more investigation.

 

[ninjag]

Aye I thought iridium was primarily just down to better wear. It was a CAT mechanic at work who saw them and thought they looked a little overheated, but when I look at them now they don't seem too bad. I had been monitoring the fuel trims as well and they were perfect. No DTC's either. The PCV one is very intermittent, I think I've seen it like twice since having the car. It's really weird but I guess the main thing is that it's fixed.

I've had a quick look at the specs between the two and noticed that the Denso resistance is 5kΩ whereas the NGK resistance is 1kΩ. I know almost nothing about spark plugs so did a quick bit of reading and came across this:

"Lower resistance means more of the ignition coils energy is going towards bridging the gap or the spark plug. That hotter spark can create a more even burn of the fuel/air mixture in the cylinder, and thus, more power output from the engine."

Does this imply that the Denso ones could have been throttling power? But then why were they ok when I'd mash my foot to the floor?

Also, when looking up the NGK's recommended for tuned S4's (spark plug BKR9EIX) the resistance for these is the same as the Denso at 5kΩ (although the heat range is 9 instead of the NGK's PFR8S8EG heat range of 8).

 

[Dippy]

I don't know. However I do know that Audi list NGK and Bosch as approved spark plugs for this engine - no mention of Denso.

 

[whojammyflip]

I've experienced a loss of power, as if the turbo is not firing up, whilst away on holiday in Snowdonia. The annoying thing is its intermittent, so driving back home it was a problem for the second half of the journey, but having just popped out to the petrol station, its back to full power. I've booked into the garage and will try to remember to update this thread with the results. As far as I can see, common causes for this are the diverter valve, fuel cam follower, spark plugs, fuel filter. Ours is a BGB engine with 94k on the clock and is serviced on schedule. I've just changed the diverter valve, so it seems unlikely to be that.

 

[audiwaterpump]

Probably one or more faulty plugs, even thou they were new.
Or just the wrong spec of plugs.
I had a similar issue with "new" plugs on a Jap car.
When plugs were changed again the problem was sorted.

 

[whojammyflip]

In my car, the cam follower was worn through, which was then causing insufficient fuel to be pumped to the engine. I have had both the pump and the cam follower replaced, and the car is running better. They said that it was the worst cam follower they have seen (top of it is missing) and is likely to have been caused by either by oil not being up to standard, or long service intervals. Maybe a sensible service interval would be half the mileage taken to get to this stage (94k), so checking maybe every 40k or 50k miles. Garage that fixed this was APS, in Brackley, highly recommended.

 

[audiwaterpump]

View attachment 259487
In my car, the cam follower was worn through, which was then causing insufficient fuel to be pumped to the engine. I have had both the pump and the cam follower replaced, and the car is running better. They said that it was the worst cam follower they have seen (top of it is missing) and is likely to have been caused by either by oil not being up to standard, or long service intervals. Maybe a sensible service interval would be half the mileage taken to get to this stage (94k), so checking maybe every 40k or 50k miles. Garage that fixed this was APS, in Brackley, highly recommended.

the worrying part would be where has all that missing metal gone?

 

[ninjag]

The hesitation thing has returned, but under much more precise throttle conditions so very rarely happens. When it does it's very subtle and far less pronounced, but it is there. No where near like before though. Will keep my eye on it.

 

[whojammyflip]

So I continued to have hesitation in my wife's A4, if I accelerated hard, after about 70mph.

This has now definitely been fixed by changing the fuel filter, and it is once again a bit of a demon sleeper.

I'd recommend anyone experiencing a drop in power at high RPM/heavy acceleration to look into their fuel filter. Ours was apparently stamped 2005, and had black gunk coming out of it when they changed it. The amazing thing is that it cost about 10 quid.

 

[whojammyflip]

OK, not fixed, going back into limp mode after doing some hard acceleration on a climb on the motorway. Maybe the low pressure fuel pump? The trouble with this A4 is it sat for 3 years in a barn (really, in Oxfordshire countryside after the previous owner divorced her husband), so I wonder whether the fuel tank and lines didnt like it.

 

[ninjag]

Finally changed my ignition coils. The originals were E revision, but the latest and the ones that I fitted are G revision.

I also double checked all of my spark plugs and noted that I must have used too much dielectric grease as it was on the tips and probably not doing it any favours. Cleaned them all up, checked gaps and refitted. New coils come pre greased so didn't add any extra.

The hesitation seems to have finally gone and I definitely feel like I have a little more power and economy. I don't think the grease was the cause of the hesitation because it was happening before I had changed them, so my best guess is that it was the ignition coils.