Lawn Mower Syndrome
It shouldn't be a car killer and quite how common it is I don't know (except I've had it happen to an 850 and a V70)
The basic scenario is that a car that hasn't had any other problems is started for a short time, such as moving it out of the garage to get the lawnmower out from behind it, and then stopped. Next time you go to start the car it spins over OK but won't fire up. There's a couple of theories on the actual cause, I tend towards the one that the hydraulic tappets on older carts get a bit crudded up and then the cold oil doesn't flow so well leaving the valves slightly open which means there's no compression. The way to avoid it is to be sure the engine is warm before you turn it off again. When it happened to me I pulled all the plugs, put a squirt of oil in each cylinder, turned it over a couple of time with the plugs out , put the plugs back and it then fired up in a cloud of blue smoke.

Snot on the dipstick
The engine tends to have a lot of blow by gases in the crankcase, if the rubber rings on the dipstick have hardened and don't give a good seal the gases try to escape up the dipstick tube, when they condense in the tube they leave a load of oil/water emulsion snot on the dipstick. it can lead people to think the head gasket has blown but changing the rubber O ring on the dipstick does the job* and maybe sorting out the Positive Crankcase Ventilation PCV system to allow the gasses to be sucked back up to the manifold and burnt off in the cylinders.

*a new dipstick and tube isn't too expensive from Volvo.

And we're off...

If the gases are trying to escape up the dipsick tube, the very last thing you'd do is seal it... if there really are crudded-up lifters, flush.

PCV This is good
https://www.youtube.com/watch?v=hwCzPPiBxk8

https://www.youtube.com/watch?v=AucWm2zFewo

Yes PCV is the important check, but the snot up the dipstick can occur even when the PCV is good. I'm not talking about a lot of gases trying to escape like steam from the spout of a boiling kettle (that would suggest blocked PCV), more like the steam rising gently from a hot cuppa. The dipstick tube is quite long outside of the engine block so it never warms up hence the condensation especially in winter. I had it on mine when I bought it, changed the dipstick and the tube as the top of the stick was also broken and no problems since. The replacement dipstick now has two O rings rather than one.
https://www.ipdusa.com/uploads/image...ontent/823.jpg

This is not how the PCV system works. These O-rings are there to ensure the -ve crankcase pressure stays negative. Not to prevent gases escaping up but quite the opposite, to prevent induction of air down the dipstick tube, killing hte vacuum.
There is still misunderstanding here. If it is 'rising gently from a hot cuppa', the intensity should not be the question, more we should ask why? Evidence of PCV issues, likely the factory fit is not up to the job. It should be inducted 'gently' down the dipstick tube, if the O rings leak. Again you will be asking yourself why is there snot on the tube? There will be no condensation unless the car is not 'driven', it will be drawn down the tube, (if it leaks) and any moisture will be removed as steam when the engine gets hot. Only by short runs will this problem persist.

On older cars, even with 'good' PCV the system is not up to the job, hence insufficient vacuum. This already covered in the listed hack.

https://www.matthewsvolvosite.com/fo...hp?f=1&t=53448

The value for clean crankcase ventilation is -0.2 kPa or lower.

Correct fluid - Pentosin CHF 11S
As previously stated, steering box can be a weak point, later cars msut be running proper Pentosin CHF 11S in there. Earlier cars less picky. Known fragility to the steering, means a 3/8" Magnefine is a good idea. Wrong fluid soon rots the seals and at the cost of new racks, this is a P80 killer. Later car? Don't put ATF in there. Many do.

Tow bars & Strut braces on the C70
These cars are known to be a bit wobbly. You might not want such a thing on your rag-top, but word is, a tow bar stops a lot of the 'wobble' not just because of adding rigidity but the pendulum-effect damps down and hence slwos down, the sweet-spot at which the body will start to shake.

C70: 2001-2003 suffer from very fragile boxes
Flush now. Known issue, has same awful box as P2 cars in these years. A aux cooler is a must, and the only cure if you catch it in time. Left too long and a cooler or a flush likely won't cure it, to the crusher it goes. See previous points in this thread

3/8" Magnefine (and 1/2"?)
I like these in autos & steering for all. Buy two 3/8" , one for the your autobox and one for the steering. Fit when you flush. Exception: 1/2" fits the four speed,

One-Swipe Delay wiper upgrade - Programmable Front Wiper Relay
Love this mod. Cheap to do. uses a VW relay 99 easliy and cheaply swapped. I hated the fact that my perfectly clear windscreen would be made smeary again by the extra sweep of the wipers. I loathed the non-adjustable one-setting delay and all the various niggles you have using the stock delay. If you nearly live in the thing, and do 1000s of miles as I do, very worthwhile. Bit of a 'knuckle-grazer' to get at, but as I said, a must if you actaully drive the thing...

http://www.volvoforums.org.uk/showthread.php?t=51041
https://www.matthewsvolvosite.com/fo...ic.php?t=26474

Yup, done for a tenner, paid for itself in wasted screen concentrate alone. When you want one swipe, you get one swipe. Not two. Brilliant.

My 2000 20v (227k, no history, passed MoT last week!) has just started doing this. P0300/P0505 fault codes (plus a couple of others). Will give this a go before it gets dark! :thumbs_up:

Some idiot faults...

No Seat Belt Warning?
I had this issue from the day I got my car. I was surprised to find Volvo with all the 'safety shizzle' they spin, had no idiot-lights for seat-belt warning. It's a Volvo right?

Turns out it did, you can even hear the relay clicking that fires the bulbs. 2 x mini-bulbs had gone in the overhead interior-light cluster. A £3.00 fix.

Head-Restraints Locked
For a two years I thought they were locked. My owners manual doesn't detail this, so it was only by luck that I ever found how to ease the locked-up release. If like me you got your car as a P80 newby, third or fourth-hand, you might find the head restraints appear locked solid. So you think they are fixed which is annoying. Incorrect. It's lack of use. Jiggle and play about a bit and you'll find they flop forward to allow your seats to fold flat.

Antenna immobiliser-ring: Your Volvo P80 C70 / S70 / V70 starts then immediately dies/ stalls
A classic symptom for 1999 and 2000 models. Not sure if earlier models have this part. Can be the fuel-pump relay but nine times out of ten, it's the antenna ring for the immobiliser system.

The ring around where your key inserts. Take the 3 torx screws out of the steering column, pop the clamshell off, and remove the antenna ring. Clean up the contacts on the plug and reinstall and see if it works again. No? Order a antenna immobiliser-ring.

A proper sized can of Deoxit D5 or Inox MX3 is nearly as useful as WD40. For old contacts. Unfortunately not as cheap, but don't bother with the incy-wincy 5 & 10ml tubes. They are a false economy. Buy one large can, it will last you a lifetime.

Without Deoxit or Inox the fault will return. [Great on Scalextric or your model railway BTW]

Not Volvo related, but a very good demo how effective these products are:

https://www.youtube.com/watch?v=NnnSjyXyhrU

It looks faked. It isn't.

Deoxit D5 or Inox MX3, both are identical. Thye work as a dielectric, and prevent reoxidisation. I swear by them. Use on hi-fi, your laptop/mobile too. Works with your driver door window-switch cluster issues too. On cars of this age many plugs , contacts and connections go iffy. Used it on corroded bulb fittings for my trailer and on my dishwasher only last week.

Deoxit D5 or Inox MX3? 300ml. Get whichever is cheapest, but never the tiny tubes.