curious hesitation around 3500rpms

[andyoleary]

Hi all,

I have probably discussed this kind of thing before on here and I reckon it points to the inlet manifold but thought I would throw it out to the PH massive anyway for opinions....

I have always thought my Tuscan was a little "lumpy" when town driving - very light throttle and low/constant rev's, say 2200-2500rpm's - and have regularly had the TB's checked and balanced etc. Recently, I had both throttle pots changed for new too and the inlet fully balanced and set up etc. All fine, no problems.

Anyway, just in the last few hundred miles or so, I have noticed a considerable "hesitance" when motorway cruising at around 3400/3600rpm's. Acceleration is fine, idling is fine, nothing else untoward at all. Had the TB's checked and balanced yesterday as a "first step" - 2 of them were showing "5" (can't recall the units) and the others showing "6" on the vacuum test. No major issue, balanced and took it for a spin. Same thing happened when motorway cruising. It doesn't feel quite the same as it does when driving on very light throttle in town so I am wondering if it could be fueling-related? Would a fueling issue only be apparent at one particular part of the map though?

Of course, if it is the inlet manifold just having too much "slop" in it and causing erratic readings from the pots to the ECU then I will look again and changing it out for a replacement unit such as Dreadnought's - which I have been considering for a while anyway but don't want to spend £500 if it is not going to make a difference.

Any thoughts people?

Ta

Andy

[Tuska]

My engineer says the original throttle bodies are 'pants'. Big grey ones.
Mine does the same as yours, despite balancing and i am going to change for dreadnought TB's.
I haven't done it this year as i'm waiting for the next service.

http://www.dreadnoughtgarage.co.uk/mg_zt260_perfor...

[andyoleary]

Hi Martin,

Yeah, I reckon your engineer is right. Brian in Dreadnought told me that they offer a re-furbed set with improved bushes/spindle assembly etc for around £500 +VAT and you need to make sure that they get the old set back within a month to get back a deposit they require. A lot cheaper than getting a completely new set though....

I think I might need to go down this road after all...carbon airbox might help too...

Andy

[Tuska]

[Whitey]

yep sounds like your throttle body bushes etc.

[stillgrinning]

I had a similar problem with that sort of rev level when cruising. It went in for a service and they found a corroded plug or coil lead. Changed the whole set for about £30 and it's been as right as rain ever since.

[andyoleary]

Thanks Whitey - I know we discussed this before and came up with the same solution, it's just that this particular behaviour at this particular rev's/speed is something new. I do agree though about the TB's still.

Hi Ian - did you notice any "spluttering" under acceleration/load or was this just a subtle hesitation when at a specific load/rev's? The leads set and plugs were all changed around 3.5K miles ago in the last major service. We checked the cam cover and plug leads for water ingress or corrosion but there were no signs. I did wonder about the coil-pack but the problem doesn't manifest itself at any other load or part of the rev range really (apart from very low and constant rev's)

Didn't actually take the plugs out though so might check them out better first.

Thanks

Andy

[Whitey]

I always just lived with the same fault(which was the throttle body bushes and spindles) until the factory rebuilt the inlet manifold for me when doing the other usual work last year.

Depends how badly it's annoying you I suppose...assuming that is your fault! When I have had problems with the leads it was always low rpm stutter, fixed by replacing the leads, now I always replace them each service.

[stillgrinning]

It just used to miss occasionally. Not much but enough to send your tummy turning with the thought of the bill !

[Oilyboy]

I had a similar effect at 3200 to 3500 rpm on long motorway runs. A pair of lamba sensors and its as good as new . The effect wasn't a miss fire just an intermittent hesitation

[andyoleary]

Thanks for the reply guys,

The Lambda's look to be ok on the software when connected up. It doesn't feel like a mis-fire, just a constant lumpiness - as if the front wheel track is loose and there is too much play. A bit like it is not running on all 6 cylinders.

Does anyone know when the lambda's are removed from the feedback loop as far as the ECU is concerned - would this be around 3500rpms?

Cheers

Andy

[Paul V]

Mine was doing this, then started to get worse until is was badly misfiring under load between 2500-3500rpm, fitted a new coil pack and it's been fine since.

[andyoleary]

Hi Paul,

I am still leaning towards the TB's needing re-furbing but I am going to change the coil-pack anyway as it is a quick and (relatively) inexpensive option. The existing coil-pack is the original one so has done over 30K miles now and could well benefit from being changed anyway.

If that doesn't do it then I shall be looking at getting Power to re-furb the spindles etc. Will report back when done....

Ta

Andy

[Paul V]

They are well worth trying, do your throttle pots stay accurate all the way up the scale? If the spindle is worn will they fluctuate to more than a few percent difference?

[andyoleary]

Hi all,

After some other discussions with those in the business, throttle-body spindles etc were still suspected of being worn so I gave Dom a call in Power to discuss a refurb (so I could compare costs/advantages against the Dreadnought option) and he suggested changing the coil-pack as the symptoms were identical to another vehicle he had in which was sorted by fitting a new coil-pack. They shipped me a new pack and I just fitted it today. Took the car for a spin, when nice and warm, up the motorway and tried to simulate what I was getting before.

To be honest, it was difficult as the amount of wind on the motorway was shocking but I did feel that there is an improvement. I think I could still feel the hesitation at 3500rpms in 5th but certainly not as severe as before. I'll give it a while before deciding if there has been a permanent improvement and may well go down the TB refurb/replacement route anyway but will probably take it to a dyno first to see if we can find out what's happening. I'll try to get a laptop plugged in whilst driving (or on a dyno) to see what the pots are doing.

Cheers for the help guys,

Andy

[pchirons]

Sounds like your guys know what your talking about,had my tuscan 3 years now,but on first playing about with the bonnet off, knocked off flimsey breather pipes off air box front and back,with out realising ,ran fine,but under heavy right foot hesitated and kicked back abit.Repaired air box with hydrollic pipe connectors from Pirtek.

Edited by pchirons on Sunday 30th September 09:39

[andyoleary]

Well I wouldn't be doing anything too complicated - just the simple stuff like removing the airbox (coilpack is easy to get to then), some electrics, bulbs, plugs etc. Anything else and it is straight to the garage...

Andy