Judder on constant throttle

[TomPaine]

Did a 150-mile round trip yesterday, mostly motorway. On the way I had an at-points pretty worrying judder on constant throttle; it felt like driving into a strong head wind, or fuel starvation or something. It wasn't there in town driving, or under acceleration. While waiting for a CEL which never came, I was thinking ...

vacuum hoses?
throttle body?
failing DI? [though wouldn't a misfire throw a CEL?]

One diagnostic clue: this seemed to start, on the way, when I engaged cruise. The car was unhappy, so I switched cruise off; but constant m-way speeds still generated the same symptoms. However, on the way back (engine had therefore been re-started), duly wary, I didn't engage cruise - and the car felt completely normal.

Any thoughts?

[sgould]

A misfire of that magnitude caused by a DI fault would surely generate a warning light?  The only bad judder that I experienced that resembles your issue was on the V6 Carlsson.  That started juddering badly on the M40 a year or so back.  At first I thought that I had got a puncture.  But I did get an engine light.

It was tracked to a  faulty coil.  They are separate ones on the V6.  I happened to have my Tech2 with me and cleared the code and the car ran normally.  As it was the middle of winter, I bought replacement coils and plugs but didn't fit them immediately.  The car was OK for  a month or so, but then it happened again and I couldn't recover it.  So i limped home and changed the plugs and coils.

If you have a code reader, or and app on your phone, like "Torque" or "Dash Command", with a dongle for the OBD socket, you may be able to locate a "Pending code" which could guide you to the problem.  A pending code is set when there is a condition which is normal, but becomes a problem if it occurs too often.  In this case the manufacturer sets various limits.  The final one is when the engine light comes on, but a lower limit sets a pending code as a warning to be found by someone looking for it.

[carrera]

If it only happens then on cruise, then it seems that there may be something to do with the speed detection (unlikely) or throttle adjustment to maintain speed

Try cruise again & see if the problem returns

[Audax]

I'd put the spare DI from the boot onto the car and see what happened, you do have a spare in the boot right?   

[TomPaine]

A misfire of that magnitude caused by a DI fault would surely generate a warning light?

That's what I thought yes. I was a combination of puzzled and relieved that it didn't come on. Code reading is beyond me, but ...

If it only happens then on cruise, then it seems that there may be something to do with the speed detection (unlikely) or throttle adjustment to maintain speed / Try cruise again & see if the problem returns

It didn't only happen in cruise but it happened only after I'd had cruise on. It's due its annual service and MOT in under a fortnight so I'll perhaps wait and try it again en route, in case an AA truck needs to get me there ... And I can ask them to check any stored codes.

I'd put the spare DI from the boot onto the car and see what happened, you do have a spare in the boot right? 

I don't at the moment but I am intending to replace it at service time as it happens, as preventative maintenance. It's 8 years old. I was going to do it at service time last year but Saabtech were out of stock.

Thanks gents and will keep you posted.

[TomPaine]

Sorry not to have reported back before now. I haven't dared engage cruise again as I didn't want to trigger the issue when I was some way from home. But the behaviour has started to be odd at idle - surging a bit, and snatchy when pulling away. I'm convining myself it's the throttle body. Service next week which will hopefully shed some light.

[sgould]

I wonder if you have an issue with the throttle body itself?  As they age, the wiring inside the control box under the plastic cover, loses its insulation.

Carrera had a similar issue that was sorted by replacing the throttle body.  See his post earlier today. 

[carrera]

Now there's a coincidence

I have had a little issue with the green car recently. It's been going occasionally into limp mode (error 15 on Opensid) and not into limp mode with error 10

DTC's seen were 1251 & sometimes 1231

Working through the DI & plugs I noticed the engine was hunting a little on idle & thanks to a borrowed Tech 2 I saw some misfires across all cylinders. Opensid error 10 came back a few times, but performance was unaffected & the limp mode was not activated. Finally P1231 popped up again.

WIS said it was a problem on the potentiometer in the throttle body. TB swapped, no misfires, no limp mode errors on opensid.

Strange things aren't they sometimes

I drove home & the car was running smooth again, especially when transitioning from overrun to producing a little power again

[TomPaine]

MOT and service this week.

In my case as I thought, throttle body.

So: passed MOT, and I had new DI [8 years old] and plugs, and battery [5 years old], all as preventative maintenance.

But Tech II pointed to the TB re the issues I'd been experiencing; they could see it was misbehaving even at idle. They also picked up an ABS ring fault. I was hoping that might have been the cause, since it would be cheaper, but apparently not.

TB (and ABS ring) will be replaced next week.

(I hope that the occasional juddering and poor running in the interim doesn't damage my nice new DI. That hadn't occurred to me till I was driving home. I assume not ...)

[Audax]

Expensive but worth it, at least having that good DI in the boot as a spare will see you good!

[carrera]

Tech II's are good diagnostic units in the right hands

Lots of deep menus though

Glad you got it sorted

[TomPaine]

Just to footnote this one - new throttle body and all well (except of course for wallet).

Thanks to those who posted.

[sgould]

Glad it's sorted.

My sympathies to your wallet...