98 S70, missing

[empomire]

Not been on here for a long time, seem to have been lucky. My last Volvo, 2003 V70 D5, the only one of the eleven I've had since 1993 to have failed completely. At 270k miles, the cam belt tension pulley mounting broke off the engine.
Any old way, trouble with the petrol S70, 1998, gave my daughter. 160k miles. It has started missing on one cylinder. I got her to use premium fuel and Redex for the last few weeks, thought it was getting better. It is sometimes fine, mostly when cruising the M way, but then can struggle low revs/up hills. Two days ago for a few hours absolutely fine, low speed, up hill, loaded, couldn't get it to miss. Then missing again. Went for a diagnostic. Garage changed the plugs, despite fact they were (allegedly, they don't look good) changed in April by someone else. Charged £129 for diag, new plugs, and a test drive. No different, sometimes good, mostly at high revs light throttle. Watched her drive away this minute, soon as she opened up, started missing with puffs of smoke from exhaust in time with the misfire.
Any ideas would be very much appreciated.

[volvo again]

Ideally a compression test...
Does it have a distributor cap & leads or coil packs?

[empomire]

Oh yes, leads not pencil coils. Leads and dist cap changed in April, when the plugs were or weren't, she's going back to the garage in Bristol with the old ones. And the bill.

[empomire]

Wonder if I need to check over the new leads.......

[capt jack]

Does the car have the notorious throttle body problem? Depends upon the age and the exact engine, but it could be the root of all kinds of evils - just as you describe.

Jack

[Luxobarge]

Originally Posted by capt jack

Does the car have the notorious throttle body problem? Depends upon the age and the exact engine, but it could be the root of all kinds of evils - just as you describe.

Jack

Do you mean ETM (Electronic Throttle Module)? If the OP's car has a conventional distributor and plug leads then it won't have an ETM, just a normal cable operated throttle body, and I'm not aware of any particular problems with those.

Sounds to be like dizzy cap, rotor or leads, although I'd give the injector wiring a good inspection too, to check there's no chafing or broken wires.

[empomire]

Would the diagnostic have picked up an injector wiring problem? Maybe not if intermittent?
Yes the throttle is cable.
I was thinking injector hence the fuel cleaning attempt.

[empomire]

Has all the hallmarks of a wiring /ignition problem. Bonnet open, ticks over fine, but open the throttle hard, or second gear and slip the clutch, it starts missing.
Checked the leads, seem OK. Didn't have the time to swap them round to see if the misfire follows a lead or a cylinder.

[Luxobarge]

It does indeed.

Try the night-time test - take the spark plug cover off, and run the engine in the dark. Have a very careful look around all the HT components, i.e. leads and dizzy cap, and see if you can see any stray sparks. They are invisible in daylight, but do show up to the naked eye when it's properly dark. If there are any sparks, then that's a likely culprit.

[ASt85]

Originally Posted by Luxobarge

It does indeed.

Try the night-time test - take the spark plug cover off, and run the engine in the dark. Have a very careful look around all the HT components, i.e. leads and dizzy cap, and see if you can see any stray sparks. They are invisible in daylight, but do show up to the naked eye when it's properly dark. If there are any sparks, then that's a likely culprit.

Those symptoms can turn up when an ignition condenser is failing under load - I used to get this happening on a frequent basis in an older conventional coil & distributor, spark plug lead system.. If it the '93 S70 system has a contact breaker check too that the gap is correct and that the contact breaker's nylon heel that runs on the distributor cam isn't warn - if it is, the spark will not be advanced sufficiently under load which can cause misfires due to the increasingly larger volume air/petrol mix under accleration/load not fully burning.