General: 850: 2.5 10v auto misfire under even slight load

[mattytowel]

A friend phoned and asked my advice. His wife has a Volvo 850 estate 2.5 10v with auto gearbox. Under even slight load it misfires, whether driving or setting off. Put it on the drive in neutral or park and it revs smoothly when the pedal is pressed.

I shrugged, which didn't work so well on the phone, and said I'd start a post on here. I will also send him a link to it, as I'm going out for a couple of hours.

The car in question is from 1995, and unlike my wife's car gets serviced by professionals. My friend (and his wife) are qualified engineers and also know one end of a screwdriver from the other, but they're only a mile away, so I can go and look myself if needed. There is a possibility of swapping parts between our cars to trace the problem, if it's going to be quick. If it's not going to be quick I suspect professionals will be engaged.

Thank you in advance.

Matthew

[capt jack]

In order of likelihod: Plugs, leads, rotor, dissy cap, fuel filter, throttle cable rusty or linkage gunked up, throttle body dirty, air filter clogged.

Start with the simple old fashioned stuff electrcs-wise and work your way through.

The leads, rotor and dissy are not service items but they do wear out and need to be changed - about 100,000 miles or so.

Cheers

Jack

[mattytowel]

My friend Paul phoned back- it's fine when cold and gets worse as it warms up. I suggested HT leads and distributor cap as first check, and then check here, and find Captain Jack thinks likewise.

[mattytowel]

Good plug

Bad plug

Good lead cap

Bad lead cap


Paul will be phoning Rufe tomorrow, and comparing prices with partsforvolvos.

[Blackcamo]

I know it’s been 12 years since, but what was it? I’ve got exactly same problem, changed plugs, checked leads and distributor cap- cleaned contacts inside. The problem occurred kind of suddenly- one day was fine, two slight misfiring issues the next day and undrivable day after.
Thanks in advance if anyone reads this

[SteveSarre]

Hi Blackcamo.

What car (model, year etc) do you have?

Steve

[capt jack]

The joy of these cars is their relative simplicity. Misfires are usually electrical - plugs, rotor, distributor, plug leads.

Having owned a couple of 70-series cars to a combined mileage of almost 600,000 I found that plug leads, distributor cap and rotor need to be changed every 100,000 miles or so.

Also, spark plugs can look OK, but you need to check the gap, and that the car has been fitted with correct plugs.

After that the culprits are fuel and air - something amiss with the delivery of one or the other.

Things to check - air filter and housing, fuel filter (underneath the car at the back, and prone to rusting out), idle air control valve.

Another thing to look out for would be perished / damaged vacuum tube connections.

After that, if it has the electronic throttle module, possibly there's a problem there.

Do you know exactly which engine and engine management system is fitted to your car?

Are there any warning lights?

Have you tried a code reader?

Jack