[Mk2] [Turbo] Hesitation/Bucking at 3k when warm

[Maverick9999]

Hi there,

I've recently had problems with my car (rev2 turbo) when it's warmed up (temp gauge at half). When I try to boot it and the revs rise to near 3k the engine starts to hesitate and if I keep my foot on the throttle the car bucks quite violently.

I have recently fitted NGK spark plugs which are a grade colder and have been using normal unleaded rather than super. The distributor rotor and cap have also been changed. Would the symptoms be caused by knocking or should I be looking for another culprit? A search on the forum says to look at a number of things such as

air leaks in the system (I have checked all the hoses to the best of my abilities and can't find a leak)
coil
cold temp sensor
alternator (?)

If anyone could advise me on the order in which I need to check these things, or simply tell me that I should be using super unleaded (stupid) I would be very grateful!

[djpkins]

use propper fuel mate...

[Maverick9999]

simple as that eh? Tanked it up a few weeks ago, will try when I next get the chance. I guess it's very important with turbo charged engine, even if it's standard boost.

[Harold]

Why the hell are you using normal unleaded? That really isn't good for the car, the turbos are designed to run on japanese petrol which is higher ron than our normal unleaded, so you have to use super to compensate.

[Maverick9999]

Didn't realise it was critical to be honest, but I get the message. Will try super at the next chance...

[cvega666]

apart from using the right fuel (DUH), worth checking error codes (AFM ?) and are you using a BOV that wents to atmosphere?

[Maverick9999]

Will try the RIGHT fuel as soon as I can, no error codes and the BOV is the standard one so I presume it vents back to the exhaust (?)

[Freebird]

I had a similar thing happen turned out to be a crack in the coil, worth checking if the fuel doesn't sort it.

[djpkins]

The fuel thing is a deffo good move, but as suggested, this prob may be ignition related...no error codes? maybe pull the EFI fuse anyways, see if it helps any...AFM cars are very intolerant of any kind of post afm air leak, wouldn't hurt to give the hoses etc a onceover.

[cvega666]

Ok, haha, sorry about going on about fuel again

if no error codes i'd start with ignition. coil pack/ignitor. Does it start easily when hot ?

[g.lewarne]

dodgy king lead - common on turbos

[Maverick9999]

A cheers for the pointers guys. I'll start with the fuel, then check the coil / king lead.

BTW what does the EFI fuse do and what's AFM?

...and yes, it starts fine when hot but I still get hesitation when I boot it...

[cvega666]

AFM is the air flow meter (the box after the airbox, before the turbo) They're a common reason for any hesitation / weird behaviour in rev1-2's. Always worth giving it a clean inside with some carb cleaner.

[Maverick9999]

Will have a look at the AFM. When they say to remove the EFI, it is a remove and replace job to reset something?

[djpkins]

just pull the EFI fuse....engine bay fusebox....for about 10 seconds, this will reset any error codes stored in the ecu.

[Maverick9999]

ah! gotcha! constantly learning!

[ryan]

I had poor running issues as boost came in. I changed the coil for a new one and it was better but still a bit lumpy. Changing the 12k mile old NGK copper plugs got it running even smoother, the difference was very noticeable!
My 14yr old coil was rustier than Bin Ladens fillings, the screws had rusted into the bracket as the ignitor screws were

[Danbob]

I now have this exact same problem, but running on 99Ron fuel.

Symptons:

Starts and Idles absolutely fine every time.
Under any throttle (even OFF BOOST) engine violently bucks - Cuts on off on off
The rythym of the pulses is very steady and uniform.
Problem came in very gradually throughout a track day, in very hot weather, more and more until eventually it was undrivable and had to be towed home
Cannot find any air leaks
NO FAULT CODES


Can you tell me if your original issue was resolved and how you fixed it?


Cheers
Dan

[psummers]

Sounds like there is something wrong with your ignition components. You'll need to be looking at the spark plugs, ht leads, distributor cap/rotor arm, king lead, coil and igniter.

Check if there is anything obviously wrong with any of those and replace as necessary.

[Danbob]

FYI, I found this post on a different forum and I am 95% this is the problem;

Have you guys checked the AFM? It sounds like their could be a dead spot in the door sweep signal, so that when the door sweep movement in between certain points in the AFM gets a bad reading. That would cause it to run lean for that second, which would cause the bucking and hesitation you guys are talking about between the RPM's which correspond to the dead spots. You can check the door movement with an oscilloscope (a high end autoranging multimeter my work too), but if you know someone with a known good AFM it may be easiest to install that and see if it works.


Makes perfect sense why it works fine at idle and bucks through the rev range, also explains why it only happens when hot, engine runs off lambda in closed loop not AFM when cold and in idle.
95% this is the problem so try it if you still are having trouble!