Motors Engine juddering/bucking under acceleration when warm - ideas?

The bank holiday weekend seems as good a time as any to try and get some ideas as what this annoying problem could be.

Ever since I bought my car (2002 Seat Leon, 1.4 16v petrol - engine code BCA) it's juddered/bucked slightly when accelerating, particularly up inclines - even minor ones. I've put up with and driven around it until now, but the problem seems to have got slightly worse since I had an interim service with the MOT last week - so I've decided to get it sorted.

The problem happens in 2nd gear and above, from 1,000rpm upwards. It'll be fine at constant throttle, but the slightest increase in demand usually sets it jumping, sometimes just once or twice until changing gear, but up to about 1-2 bucks/second at worst. It's usually fine above 2,000rpm on the flat, but will buck above this up hills and even occasionally on the motorway at 3,500rpm+. The strange thing is that it runs absolutely perfectly when cold, before the coolant temp gauge levels out at 90.

So far I've changed the coolant temp sender, run through some Wynn's injector cleaner and EGR3 cleaner, and manually cleaned the throttle body (which was filthy). I've taken it to my local garage (who did the MOT & service), who plugged in to the OBD and found no fault codes. They also reset the throttle body after my clean, but this hasn't made a difference.

I've booked it in to garage again on Thursday, but wanted to get an idea of what it could be before then. The temperature related nature seems to rule out transmission problems to me, and now I'm thinking it could be the EGR, or somewhere in the ignition system?

Any ideas or suggestions of things I could do today or tomorrow appreciated.

 

Guessing you have checked the spark plugs and ignition coils?

 

It had new plugs with the interim service, and with its major service last August; been the same throughout.

It has coil on plugs so there's 4 coils and no HT leads, is there an easy way I can check these myself?

 

If it is anything like a VW engine, they will be above the spark plugs and simply pull out (part of getting to the plugs). With my GTi some genius had jet washed the engine and caused all four coils to rust, leading to a similar issue to yours.

 

Sounds like fuel starvation, so crap in the tank, or the fuel filter is due for replacement. I was thinking vac leak, but since it's fine above 2K where vac drops off is not that.

Test a theory for me by parking on a hill, steep as possible handbrake on as hard as possible, and lift the clutch to near bite point to load the engine. Then repeat on the flat surface. If it only coughs on the hill it's fuel starvation due to incline.

Some fuel tanks can be pretty bad with sediment, but my money is on a fuel filter being plugged.

 

Not sure whether it's the same engine, but I have a 2002 1.4 Golf. Used to have a problem with a varying idle, and every so often surging at idle (swinging between normal idle up to around 2500RPM). Never quite got to the bottom of what caused it but new spark plugs and coil packs seemed to do it.

 

It is indeed a VW engine, they also fitted the BCA to Golfs, Polos and Beetles I think. The coils are just sitting on plugs from what I can tell. I'll whip them off sometime and have a go with the multi meter, there seems to be a test procedure in ELSAWin so I'll have a look - unless there's a generic test that you know of?

Pretty sure the fuel filter has been changed in one or both services, it does pull perfectly up hills when cold though - would it do that with a fuel problem?

Might be the same engine, there seem to be a handful of different codes for the Golf but they'll all be similar. Idle was a bit wandering (+- 50rpm ish) and it did surge slightly at constant throttle position (+- 2mph or so), but that seems to have improved a bit with the throttle body clean.

Since the new plugs went in the idle feels a bit rougher when warm, with the odd very occasional lurch, almost like one bad combustion every now and then - is that a misfire? This is my first car so I'm not experienced with misfiring. It also seems to splutter to death when you turn off the ignition, rather than smoothly shutting down like it did before. Dunno if that's any help from a diagnostic viewpoint.

Thanks for the replies so far, all the better to arm myself with as much as possible before it goes back to the garage.

 

Ooo TPS comes to mind. When cold a faulty TPS sticking open isn't a problem as it runs a bit richer to help warm up, but once warm he extra fuel can cause pinking/detenation which is lumpy and causes power surges.

Throttle position sensor, have a look for known issues.

 

Coilpack I would say, I've had the same issue with one on mine. It didn't matter whether it was uphill etc, but the amount of throttle opened - you needed a wide opening to introduce it, presumably a weak spark that didn't get good ignition on the higher gas flow.

ETA: A coil-on-plug/head-mounted coil pack gets very warm once the engine heats up - the constant heat cycling tends to fatigue the electronics, leading to failure. You can get them on eBay for decent prices, carparts4less also have pattern ones from £18. If you can, however, I'd recommend converting to a coilpack+lead setup, although the way the loom is done may make it impossible.

 

Had similar issue with my 2002 Golf (1.6 16v, BCB engine code)
Fixed it by adding a new fuel filter, followed by cleaning throttle body and readapting it using vagcom.

 

It's drive by wire so has sensors on the pedal and inside the throttle body I think, will try to have a quick look at both and see if there's anything obviously wrong.

Unfortunately I think that may be slightly beyond my current skills! I'm just about to go and test the coils with the multi meter, haven't been able to find specification values yet but I guess if one of them is different it will be a dud.

I'll look in to the fuel filter again, the throttle body is much cleaner now and it has been reset by the garage but doesn't seem to have made any difference.

After a bit more research I'm thinking it could also be a MAF/MAP sensor. I don't think mine has a MAF but does have an "Intake manifold pressure sender with intake air temperature sender", which I'm guessing is the MAP. I'm considering unplugging this and going for a drive to see what happens, and the same with the EGR valve after I've tested it.

 

The TPS sit's on the side on the intake, it's basically an automatic choke. When they get coked up they stick open and basically enrich the fuelling during cold start.

 

Have a look at how the coils are connected - there are two types of coils listed on various parts websites. One is a separate pack that uses leads, the other is coil-on-plug. If both types were fitted, you might find that the coil-on-plug type use a small sub-loom to adapt it that can be connected up to the single coil type.

Chances are you won't be able to discern anything with a multimeter - unless you have a constant misfire the coil is working most of the time, only in extreme situations are you challenging it, when it will show its weakness.

My suggestion is buy a single replacement, then swap them about - start with one cylinder and see if you can still provoke it.

Honestly, try it, a quick Google shows a few people having the same thing. I've had exactly how you describe it, and could even repeat it if I dug that coil pack out of the garage.

If you unplug the MAP sensor chances are the engine will run in limp mode - it's a crucial part of the FI system, unplugging it means an important sensor in this type of system is completely missing.

 

Any issue with the air system would show up as an error code. As has already been said, disabling any of the sensors will put the engine into self protection mode.

 

I did wonder why there were COP coils and single coil packs and HT leads all available for my registration on GSF and Eurocarparts, maybe they are semi interchangeable.

I've just had all the coils off and they all gave the same resistance, unsurprising given they were unloaded as you said. I'll see if I can get hold of one to swap in and out before it goes in to the garage on Thursday.

 

Sounds daft, but be careful if you're swapping coils around when hot - very easy to burn yourself on the head if it's just been running up to temp.

 

And it seems the winners of the e-cookie are...bodkin and Krikkit - you'll have to share I'm afraid.

The one new coil I ordered finally arrived at my office at 4:55 this afternoon, so I swapped it out for the old one on plug 1 for the drive home...no difference, still bucking when warm but perfect when cold. I then put it on to plug 2 and put 1 back in, quick drive around the block and still no change.

But then, with the new coil on plug 3, all was well. No juddering, no jolting, just smooth acceleration, even when doing daft things like flooring it in 4th at 20mph. It doesn't go anywhere fast (having only 75HP & 90lb/ft...when new), but it does so without turning in to a shuddering mess.

Thanks for all the suggestions everyone, that's no doubt saved me a fair few quid

I avoided burning myself on the cam cover and head, but gave myself a bit of a surprise on the coil itself - no wonder they can eventually go bad with that much heat cycling.

 

Glad to hear it worked, I feared I might have egg on my face when you updated the thread!

Coilpacks that sit close to the plug are really abused, not surprising they fail after a 10+ year duty!

I had a nasty moment once, I was removing the coilpack and cam cover, on my car that means about 20 bolts. Of course I was doing it about 20 mins after I got back, it was only once I'd pulled the bolts and shoved them in my pocket that I noticed they were rather toasty!

 

Awesome, glad you got it sorted. Might be worth doing the rest depending on price. I changed all of mine and my long term average went from 32mpg to 36mpg. Though to be fair they had all rusted.

Removing engine covers/parts while it still cooling! Madness!

 

I'm not sure if any of mine have been in there for quite that long; looking back through the few receipts I have I did see coil #1 was changed about 3 years ago. They're also all different, with only #4 having VAG markings on the top and the others having different markings on the barrel - at least I know what to look for when one inevitably goes again.

They all looked in pretty decent nick but I'll see how the fuel economy goes, been getting ~40mpg so far which seems about right. Cleaning the throttle body out might have made a difference as well, it'll need filling again in the next few days so I'll work it out.