Sudden onset misfire mystery

[Foeux]

Dear chums,

I have mostly just been using the 232k mile 2.3 non turbo Volvo 940 estate we have and enjoying it. As is often the case though, I've not come here to say everything is fine. I am not cross or giving up with the car by any means but I am mystified so far...

What happened:

Using car on motorway. Doing normal person motorway speed. Stopped at end of M32 to go on M4 back to Wiltshire. Idling evenly at traffic lights. First depression if accelerator to move away from lights, car began to misfire violently. Very little power but I wasn't going to stop on a motorway on a Sunday night - i'd never have made it home before Christmas! The car just about did 60mph but up hill, far less.

Didn't overheat but I suspected head gasket all the way home...

New day, keen to solve the puzzle I checked all 4 plugs. All 4 light tan and only a year old -no answers there. HT leads one looks ropey but again, bougicord items only a year or so old.

Misted manifold with water before starting to do the old evaporation trick (if a branch of a manifold stays wet and cool, then nothing is happening in the corresponding cylinder). Cyl 3 stayed wet and cold.

Crawled it onto the drive like a dying rat and again began to pull plugs to do compression tests. All 4 plugs now pitch black and smell of fuel...weird.

Compression test unexpected cylinder 3 to be f***ed. All 4 cylinders had 12.5 bar or 180 psi! Really weird.

Inside cap, the rotor arm and cap are clean and uncorrupted being less than a year old

Tomorrow, piece meal, so I can tell what makes a difference, I will be replacing the leads, then plugs. If neither of these help, I will replace the coil.

I've had the fuel rail off and injectors out. Good fuel pressure.

Anyone had similar to this?

I am hoping to report back with the fix to help others out with similar symptoms.

[Volvo_Utrecht]

Please use a code reader to check for easy fixes. I had an ignition problem, reader told me it was the sensor that needed fixing

[classicswede]

Check the dizzy cap for oil. It is common for the distributor to leak oil onto the cap causing cylinders to loose spark

[Rversteeg]

Does it run "nicely" on three cylinders or is it running lumpy and uneven?
I had a similar issue not too long ago. Very lumpy idling, no power. What puzzled me is that the engine actually ran somewhat smoother when I took no. 3 HT lead off. It turned out to be a barely visible crack in the distributor cap. I guess there was a short-cut in the cap that divided the spark over two cylinders, thus weakening it. With one lead disconnected the other cylinder got a full spark again thus running better on three cylinders thanon all four.
If it is smoothly running on three cylinders I would not suspect anything upstream of the distributor cap.
My luck is that it is running on LPG, so I can usually rule out several causes by switching over between LPG and petrol.

[Laird Scooby]

Originally Posted by Foeux

Crawled it onto the drive like a dying rat and again began to pull plugs to do compression tests. All 4 plugs now pitch black and smell of fuel...weird.

Check the vacuum pipe on the fuel pressure regulator (the small bore one) for cracks and then with the engine idling (if you can get it idling) pull the vac hose off the FPR and plug the hose with your thumb and observe the vac stub on the FPR - if fuel comes out, the FPR is shot - it could be that it flodded cyl #3 first and now the rest are running excessively rich.

This is in addition to your other diagnostic steps outlined, not instead of! It's probably fine but Sods Law being what it is, if you don't check it, that's where the fault will be!

[Foeux]

Thanks for all the suggestions.

Cap has been off.

No cracks, no oil inside it.

FPR hose not perished or cracked - no fuel out of FPR vac hose and no fuel smells either. There is vacuum at the little brass spigot coming from the inlet manifold.

Just about to bung new plugs and leads on.

If that doesn't work, coil.

If that doesn't work I am not sure where to go next...

I am definitely not giving up but am very puzzled.

A friend suggested a 'hall sensor'...?

[Laird Scooby]

Originally Posted by Foeux

Thanks for all the suggestions.

Cap has been off.

No cracks, no oil inside it.

FPR hose not perished or cracked - no fuel out of FPR vac hose and no fuel smells either. There is vacuum at the little brass spigot coming from the inlet manifold.

Just about to bung new plugs and leads on.

If that doesn't work, coil.

If that doesn't work I am not sure where to go next...

I am definitely not giving up but am very puzzled.

A friend suggested a 'hall sensor'...?

Firstly, a failing coil won't cause a failure purely on #3 cylinder, all 4 will cut out or you will have random misfiring rather than regular. Yours is regular as you've isolated it to #3 cylinder.

Ditto on the Hall Sensor except yours should have a CPS (Crank Position Sensor) instead and not only will this cut your ignition but your fuel supply too so you wouldn't have wet plugs.

Have you checked the injector wiring on #3 injector? If the ignition is ok (and assuming the valve clearances are ok on #3) then the only other thing likely to cause that sort of a problem has to be fuel. You could also swap #2 and #3 injectors and see if the fault moves in case the injector has failed - again not likely but still a possibility.

Also worth double checking the cam timing hasn't altered, this could be because of a lost tooth on the timing belt, it's a fairly quick and simple check.

[griston64]

Originally Posted by Foeux

Thanks for all the suggestions.

Cap has been off.

No cracks, no oil inside it.

FPR hose not perished or cracked - no fuel out of FPR vac hose and no fuel smells either. There is vacuum at the little brass spigot coming from the inlet manifold.

Just about to bung new plugs and leads on.

If that doesn't work, coil.

If that doesn't work I am not sure where to go next...

I am definitely not giving up but am very puzzled.

A friend suggested a 'hall sensor'...?

very unusual for a 940 coil to fail. If leads are original it could well be them as my 940 was misfiring due to knackered leads. Get Bougicord ones if you can

[Foeux]

Thanks for sticking with me and having ideas to share.

New plugs on their own did nothing.

New leads on their own did nothing.

New plugs and leads together did nothing.

Disconnecting cyl 3 makes it run much nicer - still not perfect but just a regular old misfire, not 'throwing a running washing machine off a cliff' misfire.

Spark on all 4 plugs. King lead replaced too. Outgoing ht leads are bougicord...

Based on what others have said, the closest thing I have to someone else's findings is a buggered distributor cap - it is less than 1 year old and visually looks new inside still. Despite this, I am not against changing it.

As for now, car is still out of action but can be started to run very roughly/moved a small distance.

Thanks for clarification on crank sensor and hall sensor.

[tofufi]

For what it's worth, I had a failed distributor cap on my 340 turbo (using a 940 turbo engine, 2.3l, same as your redblock).

I'd parked at motorway services, and on restarting it would barely run at all. When the engine had cooled, it would run fine for a few miles, until the engine got up towards operating temperature and it would run poorly again.

Dizzy cap had no visible signs of failure, but a new cap cured it...

These days when changing service parts, I keep the old ones in the boot just 'in case'.