Juddering on acceleration

[Sheena]

A 1991 1.7 Turbo Auto. I have two problems although they may be connected, engine on idle will surge ( hunting ) and car has started juddering when accelerating from a slow speed or from stopping at for example a roundabout. Seems to run fine when cold. Could this be cold star sensor? The car has recently had a full service.
I have had this at my local garage but they said they didn't know what it was and I had a feeling they didn't want to know.
I have trawled through the posts and can find similar problems but not exact. Any help much appreciated. Thank you.

[Ade]

Could be a tricky one to trace or it might be something really simple, hard to say just based on a description of symptoms I don't know how mechanically minded you are but ruling out basics shouldn't be too hard. First thing to do is check for vacuum leaks, if there's any unmetered air finding its way into the engine it can cause all sorts of weird and wonderful problems. It could be a gasket or a fitting or something that's only letting air in once everything has expanded when the engine is up to temperature. I know someone on here had a throttle body gasket that only leaked air in when hot. If it is a vac leak, that could possibly account for both symptoms.

If the problems are unrelated, bad idle is almost certainly the idle valve, try cleaning it out either with carb cleaner or in a bath of petrol. If cleaning it doesn't solve it then you will need to source a replacement. I changed mine about 3 years ago when suffering a similar idle issue (no acceleration problems though) and it cured it like magic. I had difficulty finding one available from a motor factor however one from a Saab of the same era will do the trick and these are available from factors, you'll have to modify your plumbing slightly though as the hose tails on the volvo one are at 180 degrees and the Saab one's are at 90, they are otherwise identical though and I can confirm from personal experience that it will work just fine. The B18FT lump in our Volvo's shares the same Bosch LH2.2 Jetronic fuel system as the earlier Saab 9000 2.0 16v and 2.0T. If you do decide to buy a Saab one, to make life easy, here's a reg to quote to the supplier- H215 CNB, Saab 9000 Carlsson Turbo. This is my Saab and was the very car that temporarily donated its idle valve to my turbo to see if cured it, which it did and hence I went and bought another identical one as this was available and the volvo one was not.

Another thing to bear in mind is that, in my experience, the turbo engine is VERY fussy about spark plugs. I've had mine for a good while now and done a hell of a lot of miles in it, hence I have changed the plugs several times and never again will I fit anything other than NGK plugs. On Bosch and Denso plugs I had all sorts of strange, seemingly temperature related but also kind of random running/idle issues that I could never trace but since going back to NGK's, never had a problem. An interesting point to note on this subject is that, some time after the fact, I started working for Ilmor engineering, a race engine manufacturer, and in some of our spark plug trials in various WRX/indy/F1 engines on the dynos, the NGK competition plugs always consistently outperform the bosch equivalents both in terms of quality/consistency of combustion and longevity. In short, NGK all the way! For the sake of £5-£10, I'd stick a set of new NGK plugs in it, it might not cure it but it certainly won't hurt.

I'm not saying that your problems are definitely any of the things I've mentioned, there are lots of other possibilities and I'm sure others will add their own valuable suggestions (there is a huge wealth of 480 knowledge on this forum!) but at least you have some food for thought and a couple of cheap and simple things to try. I wish you the best of luck with it and hope you get it sorted

ps. If you're anywhere near Northampton, I'd be happy to take a look at it for you if you wanted to swing by my workshop one evening or weekend, touch wood, I haven't encountered a car problem yet that has escaped my diagnosis! Always happy to help a classic car owner in need, I wouldn't expect any payment to cast my eye over it, maybe just a couple of nice cold beers! You could try my idle valve as well as a test before parting with any £££

[Sheena]

Thank you Ade for you lengthy and detailed reply. Sadly my machanical knowledge is not much more than basic however I do have a Haynes to keep me right and have sorted a few things myself.
The engine recently had a core plug that had rusted through replaced and I am wondering if this has anything to do with it as a lot of parts/ cables etc removed for access.
I will try your suggestions and luckily have a spare car I can salvage parts from. I had read about spark plugs on the forums a while back and as far as I know the garage did not change them during the service but I will double check this.
Perhaps Scotland is a tad to far to pop round but thank you for your offer

[glasgowjim]

Sheena are you based in Scotland ?

[Sheena]

Yes Jim, in the Borders. Not too far from yourself.

[Sheena]

If it was an air leak wouldn't it be juddering just the same whilst in reverse?