Bad running after new head gasket - cause?

[FullEnglish]

I wondered how on earth El Mecco managed to run the engine on three cylinders and direct the flow of the third injector into a yoghurt pot - the injector rail won't allow it. I would however work on a K-Jet system, which had a separate fuel line to each injector.

Why did the head gasket go in the first place? Head gaskets don't just fail because the engine is left ticking over.

Banging around in a field - The bottom of the fuel tank could have taken a big knock, has this upset the in-tank pump? The fuel lines sit under the car - has something happened here? Return fuel line damaged, for example? Unlikely to ever have been checked. Cat destroyed internally? They don't like knocks, apparently, although I've never had a problem with one.

Could a blocked cat generate enough back pressure and extra heat to overheat the engine on tickover and cook the head gasket in the first place?

I remember you gave me some useful pointers in my epic Caravan Brake Balancing thread from several years ago, this thread reminds me of mine. When you finally get it sorted it's wildly satisfying...

[Dirty Rooster]

There's yer problem ;

Originally Posted by rogerthechorister

... since she has new cat and both new sensors (lambda and the other oxygen sensor) ...

Too many sensors.

(What engine is this?)

[rogerthechorister]

As per plate that's what she is. A 940 Sport Edition on an N plate. I have not PERSONALLY looked, but mecco assures me, there are two of the said sensors.

[rogerthechorister]

As above, hoping for a spring fix!

[rogerthechorister]

Tank is plastic my 95/6 so it cannot be dented - either a virgin or a big 'ole!

[Laird Scooby]

Really weird thought - have you tried it with the fuel filler cap removed? If the vent in it isn't working properly you might create a partial vacuum in the tank causing lack of fuel.

I know it's a long shot but surely worth a try as it's probably about the only thing that hasn't been tried!

[ovlov236]

Hi
Have You Tried The Ignition Coil If Its Dirty Give It A Clean. The Ignition Module Tends To Suffer This Is An External Amplifier In Addition To The EZ UNIT CONTROL UNIT which boosts output to coil unfortunately this vital component is postioned at the front of the car just behind ns headlamp with its socket facing skywards and regularly gets watered from bonnet edge and wing water gets into socket causing corrosion of terminals always remove and inspect take the rubber boot off clean terminals up thoroughly pack it with Vaseline
Clean out the rotary idle control also is the exhaust manifold ok? even slight leaks lead to problems the oxygen sensor reacts to extra air defines it as a lean mixture and sends a go rich signal to ecu result overfuelling simply because of a air leak its a closed system only way to clean a amm is ultrasonically even 0.25v with throw the amm output value off
Has the coolant sensor been fitted with incorrect one I think yours should be a two wire one containing two resistive wires if its the wrong one the signals sent to ecu resistive values will be incorrect the effect on the ignition is not too serious but the fuelling side will generate incorrect injector values so durations will be wrong

Hope That Helps sorry if I have gone over what other people have said regards

[ovlov236]

Forgot to mention the pins of the coolant sensor should read when sensor removed 3.500ohms when cold between block and each of its pins and 300ohms when hot if not suspect wrong sensor has been fitted

regards

[Basil]

If you had brought it to my workshop I could not have brought myself to charge for what appears to be zero diagnosis and a plethora of randomly changed sensors and gizmos. Any garage with a `scope, a multi meter and a basic understanding of how this very basic management system works should have diagnosed it within 3 hours and fixed it in probably less time that that.

I hate e-mail diagnosis but the symptoms would draw my initial attention to the air flow metering unit and its connection to the ECU. If a modern garage does not have a portable, multi channel oscilloscope and the knowledge as to how to effectively use it, I would choose a different one. Whilst late model OBD codes are useful and often pinpoint the issue, it's vital a technician understands HOW all this stuff interacts and to know what are correct and incorrect scope patterns from all the relevant sensors.

[rogerthechorister]

Whitchurch appears to be near Wales. It'd be worth a drive if you can fix it.

However, this vehicle does not have an Air Flow meter, it has an Air Mass Meter - and that has been changed. No difference.

Further, this vehicle does not have true OBD2. It has the Volvo blinking lights - but in this car the signal for the blinking lights comes out of what looks like an OBD2 port in the central cubby.

Following your train of thought, do you think that a rolling road would at least be able to tell me whether the problem is fuel or sparks?

Ovlov

She has had a coil. She has had an ignition amplifier. Rotary Idle Control? I think this car has an idle air bleed and if so it's been done. Inlet manifold was obviously off when the head gasket was done and went back on with proper Volvo gasket, no cracks observed (rare in the inlet manifold, very common in the 90+ exhaust manifold - and that's been done too). She has had a swapped AMM. No difference. She has had coolant sensor (aren't there two at this age?) and the ones going on were the same as the ones coming off. She does it hot or cold (I think, might try again to check).


I'm still guessing loom.