940 poor running and jerking

[Delta66]

Originally Posted by Willthevolvoaddict

....

I have given it a full service, all filters, new plugs and HT leads, ....
Will

Which HT leads did you go for? Mine appear to original (the spark plug cap connector is blue), so at 23 years old, maybe worth a change?

[Burntbananas]

Originally Posted by Delta66

Which HT leads did you go for? Mine appear to original (the spark plug cap connector is blue), so at 23 years old, maybe worth a change?

This is on my mind currently also Delta66.

Anybodies opinion would be greatly valued!
Picked up my first 940 last weekend and doing some general service items this weekend including plugs. The Dist. Cap looks new however my coil and HT leads both look like original 1992 vintage. Could this be contributing to some intermittent hesitation i'm experiencing?

[Laird Scooby]

Originally Posted by Burntbananas

This is on my mind currently also Delta66.

Anybodies opinion would be greatly valued!
Picked up my first 940 last weekend and doing some general service items this weekend including plugs. The Dist. Cap looks new however my coil and HT leads both look like original 1992 vintage. Could this be contributing to some intermittent hesitation i'm experiencing?

Probably not - usually with an ignition related misfire/hesitation it's always there.

This has been covered elsewhere on the forum in great depth so i'll give you the "potted version" here.

Check for loose, split or leaking vacuum hoses, clean the PCV system, check for leaks and security of the hose between the MAF and the throttle body including those that feed the idle control valve, plug gaps (should be 0.65 - 0.70mm) and also check for water condensate in the fuel. Easiest way is wait until you're down to about 2 gallons of fuel and add 2L of meths to the tank followed immediately by no more than 2 gallons (9L) of petrol. The big thing here is the concentration, about 10% works best and in 4 gallons (19L) the concentration of 2L of meths is 10% so will emulsify with the water/condensate in your tank and carry it through to be burned off.

https://www.toolstation.com/shop/Met...+Spirit/p99550

Best price i've found for 2L of meths!

Other things that can cause occasional misfires, hesitation, flat spots etc are old/worn Lambda sensors. If the car has covered 100k+ and the Lambda looks original, chances are changing it would bring immediate benefits, certainly to your fuel economy.

Obviously also check the air filter, fuel filter, plugs, HT leads, dizzy cap, rotor arm and also check the carbon brush in the inside of the dizzy cap.

[Rversteeg]

From personal experience: HT leads can cause intermittent hesitation or even worse. I have been towed from the motorway twice in a period of several weeks before I found the culprit, a bad HT lead between coil and dizzy cap. Funny thing it was that it only played up after running several miles in rainy weather. As long as the engine and lead were still cold you could spray water on it and the engine kept running fine. Only once it was warmed up the lead failed.

[Burntbananas]

Originally Posted by Laird Scooby

Probably not - usually with an ignition related misfire/hesitation it's always there.

This has been covered elsewhere on the forum in great depth so i'll give you the "potted version" here.

Check for loose, split or leaking vacuum hoses, clean the PCV system, check for leaks and security of the hose between the MAF and the throttle body including those that feed the idle control valve, plug gaps (should be 0.65 - 0.70mm) and also check for water condensate in the fuel. Easiest way is wait until you're down to about 2 gallons of fuel and add 2L of meths to the tank followed immediately by no more than 2 gallons (9L) of petrol. The big thing here is the concentration, about 10% works best and in 4 gallons (19L) the concentration of 2L of meths is 10% so will emulsify with the water/condensate in your tank and carry it through to be burned off.

https://www.toolstation.com/shop/Met...+Spirit/p99550

Best price i've found for 2L of meths!

Other things that can cause occasional misfires, hesitation, flat spots etc are old/worn Lambda sensors. If the car has covered 100k+ and the Lambda looks original, chances are changing it would bring immediate benefits, certainly to your fuel economy.

Obviously also check the air filter, fuel filter, plugs, HT leads, dizzy cap, rotor arm and also check the carbon brush in the inside of the dizzy cap.

Many thanks for compressing that down for me Dave , good job i was already planning on removing and cleaning the whole induction system! Luckily i have some meths already so thanks for providing those dilution instructions. I'll take a look at the Lambda sensors, car has done 105k and i highly doubt they've been done.

Originally Posted by Rversteeg

From personal experience: HT leads can cause intermittent hesitation or even worse. I have been towed from the motorway twice in a period of several weeks before I found the culprit, a bad HT lead between coil and dizzy cap. Funny thing it was that it only played up after running several miles in rainy weather. As long as the engine and lead were still cold you could spray water on it and the engine kept running fine. Only once it was warmed up the lead failed.

Rversteeg thanks for that, was gonna check my leads with a multi-meter but if they can quickly cause an issue i'll look in to replacing them, any ideas whether i need a specific B200 set or ? (Sorry my first car with a Distributor )

[Rversteeg]

What I meant to say is that I checked the HT leads several times on a cold engine, sprayed them with water, not a single misfire. You would expect them to fail on starting up, not after running for several miles.
I finally managed to recreate the failure by spraying water over it on a hot engine. I do not think you will be able to measure a weak insulation of the cable with a multimeter.
Not sure which particular set I used for replacing, but they looked the same as original.

[aardvarkash10]

don't test - just replace them. If they are carbon track leads (almost all are these days) they degrade over time. Eventually, the spark finds it easier to track along the outside of the leads than the inside, and it then misses its proper destination.

With a meter, they can test ok (5k per foot +/- 50%) and still do this.

[Laird Scooby]

I've just remembered a strange one i had with my last 740 - the top hose developed a pinhole leak that only leaked when hot and under pressure, and then not all the time. This caused an occasional misfire because when it leaked, it shot a fine jet of water/coolant straight onto the cooling fan.
This was then blown all over the plugs, leads and dizzy cap but after it had caused the misfire, because it was such a fine spray, evaporated off before i could actually find it!

In the end i went out down a dark lane one night to see if i could see where the HT was arcing out. I had one of those high power LED head torches that could be dimmed. On the dim setting the light is actually pulsed, a bit like a timing light. This had the effect of not only illuminating the droplets in the pinhole jet but "freezing" them so i could see them!
Obviously i saw no HT arcing out because i saw the cause of the problem.

[aardvarkash10]

Love that story Dave! The Interconnectedness of Component Failure

[Laird Scooby]

It was all the wrong symptoms for a coolant leak though Ash! Wasn't even dropping anything on the level it was that fine but just enough to wreak havoc on the HT side of things!

Took ages to find the problem as well!