Sudden onset misfire mystery

[Laird Scooby]

Originally Posted by Foeux

I did slug back a lot of tea and have some quiet time in the garage.

Your pattern spotting is bob-on. Sooty plugs 1, 2 and 4. New distributor cap has made it better but it is still rough on idle.

The exhaust is totally airtight now.

Sooty plugs = unburned fuel or a load of oil sneaking in...no?

What do you recommend as next steps? Head off? Leakdown test? Buy another engine? Buy another Volvo?

Tea heavily laced with Napoleon or JD i hope?

I have a theory that injectors 1, 2 and 4 are leaky - here's why i think this.

On a cold start, the engine runs open loop (no control over mixture from the Lambda sensor, uses preset values for a reasonably good cold performance/economy until the engine and Lambda sensor warm up) so all 4 plugs (assuming non-leaky injectors) should run pretty evenly.

Once hot, the Lambda sensor will have warmed up and be giving a signal to the ECU to control the mixture within the design parameters. Again, assuming non-leaky injectors, all 4 cylinders will run evenly.

While cold, cyl #3 runs as it should while the other cyls (with potentially leaky injectors) are over-fueling - hence getting sooty from unburned fuel.

Once hot and the ECU goes closed loop and uses the Lambda sensor signal to adjust the mixture, cyl #3 runs on little or no fuel as the ECU has reduced the injector time to all cylinders. As #3 isn't leaking (supposedly according to my theory) this results in the other 3 cylinders running as they should (give or take a bit) and #3 barely getting any fuel so gives no output using the damp manifold test.

This is a variation on my earlier theory about the FPR going rogue and providing excess fuel pressure.

I would suggest you check the fuel delivery from the injectors, there should be a method in a proper Haynes manual but if not, the basic method is to remove the bolts holding the fuel rail down, lift it up with all 4 injectors attached, use 4 jam jars or similar to catch the fuel and place one under each injector then crank the engine for a few seconds. I would guess from the info so far that 1, 2 and 4 will deliver more than #3 and probably still be dribbling when you stop cranking to inspect the jam jars but #3 won't be dribbling.

[Foeux]

Originally Posted by Laird Scooby

Tea heavily laced with Napoleon or JD i hope?

I have a theory that injectors 1, 2 and 4 are leaky - here's why i think this.

On a cold start, the engine runs open loop (no control over mixture from the Lambda sensor, uses preset values for a reasonably good cold performance/economy until the engine and Lambda sensor warm up) so all 4 plugs (assuming non-leaky injectors) should run pretty evenly.

Once hot, the Lambda sensor will have warmed up and be giving a signal to the ECU to control the mixture within the design parameters. Again, assuming non-leaky injectors, all 4 cylinders will run evenly.

While cold, cyl #3 runs as it should while the other cyls (with potentially leaky injectors) are over-fueling - hence getting sooty from unburned fuel.

Once hot and the ECU goes closed loop and uses the Lambda sensor signal to adjust the mixture, cyl #3 runs on little or no fuel as the ECU has reduced the injector time to all cylinders. As #3 isn't leaking (supposedly according to my theory) this results in the other 3 cylinders running as they should (give or take a bit) and #3 barely getting any fuel so gives no output using the damp manifold test.

This is a variation on my earlier theory about the FPR going rogue and providing excess fuel pressure.

I would suggest you check the fuel delivery from the injectors, there should be a method in a proper Haynes manual but if not, the basic method is to remove the bolts holding the fuel rail down, lift it up with all 4 injectors attached, use 4 jam jars or similar to catch the fuel and place one under each injector then crank the engine for a few seconds. I would guess from the info so far that 1, 2 and 4 will deliver more than #3 and probably still be dribbling when you stop cranking to inspect the jam jars but #3 won't be dribbling.

Thanks very much for helping me out.

I can make an apparatus to catch fuel from the injectors. I will get on to that tomorrow.

I know things are allowed to break, especially old ones but, I am a bit puzzled as to how a failure that started in a snap of fingers could be tri-pronged. I am also thrown by the idea that the distributor cap made it better but didn't fix totally...

As an extra bonus, I've left the battery disconnected - a 'turn it off and turn it back on' special move.

I hope the car isn't sentient as I have just "checked" to see if there are some 940s for sale...there are.

That said, it was a cheap car and has been semi-reliable: better the devil you know.

[Laird Scooby]

Originally Posted by Foeux

Thanks very much for helping me out.

I can make an apparatus to catch fuel from the injectors. I will get on to that tomorrow.

I know things are allowed to break, especially old ones but, I am a bit puzzled as to how a failure that started in a snap of fingers could be tri-pronged. I am also thrown by the idea that the distributor cap made it better but didn't fix totally...

As an extra bonus, I've left the battery disconnected - a 'turn it off and turn it back on' special move.

I hope the car isn't sentient as I have just "checked" to see if there are some 940s for sale...there are.

That said, it was a cheap car and has been semi-reliable: better the devil you know.

A better spark will help fire an extreme of mixtures, rich or lean so there was obviously a weakness in the cap that you couldn't see.

I'd hazard a guess (assuming my theory on the injectors is correct) that a bit of dirt happened to split between those 4 injectors and it was just luck #3 didn't get it too.

There is another possible theory about the electrical supply to #3 but that theory doesn't quite fit the symptoms.

[Foeux]

I am pleased with the test rig I made.

4 pots of equal size, hole big enough for injector to poke into top of lid, small relief hole in lid.

Cranked engine for 3x 10 seconds. Pots numbered. Pots removed and measured against a steel rule in a vice.

Injector 1 - 8mm
Injector 2 - 8mm
Injector 3 - 3mm
Injector 4 - 8mm


Either 1) injector 3 is running at 37.5% efficiency of the other 3 or

2) injectors 1, 2 and 4 are running at approx 266.6% 'over' injector 3.

Neighbour Nigel also suggested 'the card trick'. Place brown cardboard in front of the row of injectors and study the mess they leave. As it was, the pot-rig was abundantly demonstrative as to what is happening.

As a precaution, I cleaned all contacts for each injector and also the circular terminals that the fuel rail bolts go through. No difference.

Battery left disconnected overnight for a good old fashioned 'hard reset' too.

What are the next steps? Crank sensor worth doing as a preventative or not yet?

Thanks for your sticking with me.

F

[Laird Scooby]

Originally Posted by Foeux

I am pleased with the test rig I made.

4 pots of equal size, hole big enough for injector to poke into top of lid, small relief hole in lid.

Cranked engine for 3x 10 seconds. Pots numbered. Pots removed and measured against a steel rule in a vice.

Injector 1 - 8mm
Injector 2 - 8mm
Injector 3 - 3mm
Injector 4 - 8mm


Either 1) injector 3 is running at 37.5% efficiency of the other 3 or

2) injectors 1, 2 and 4 are running at approx 266.6% 'over' injector 3.

Neighbour Nigel also suggested 'the card trick'. Place brown cardboard in front of the row of injectors and study the mess they leave. As it was, the pot-rig was abundantly demonstrative as to what is happening.

As a precaution, I cleaned all contacts for each injector and also the circular terminals that the fuel rail bolts go through. No difference.

Battery left disconnected overnight for a good old fashioned 'hard reset' too.

What are the next steps? Crank sensor worth doing as a preventative or not yet?

Thanks for your sticking with me.

F

That makes things much clearer and explains a lot!

Next step, swap injectors #2 and #3 (they're held onto the fuel rail by a C- Clip that has a tab at 90deg to make it easy to remove/refit and an "O" ring, put s mear of silicone grease onto the "O" rings to make life esier and ensure they seal) to confirm the fault moves from #3 to #2 cylinder, if it does, either get a replacement injector for the duff one or get a complete replacement set.

There should be a number on the injector, 10 digits long, if memory serves it should start 0 280 and then 6 more digits.
With that number, replacements more modern could be found that will give a better spray pattern giving more economy and performance.

[Laird Scooby]

Originally Posted by Foeux

Neighbour Nigel also suggested 'the card trick'. Place brown cardboard in front of the row of injectors and study the mess they leave. As it was, the pot-rig was abundantly demonstrative as to what is happening.


What are the next steps? Crank sensor worth doing as a preventative or not yet?

Thanks for your sticking with me.

F

Forgot to answer these two points in my post ^^^^^

The pots to meaasure the amount of fuel is the "upgrade" of the card trick and gives you something you can measure - the benefits are obvious here!

The crank sensor is working at the moment or the injectors wouldn't fire, worth a visual inspection of the cable from the CPS to ensure it's in good condition, if not order a replacement (from ~£20 on fleabay) and fit it once the injector problem is resolved.

The two tabs on the fuel rail are just brackets, no electrical purpose but i think on the back end some cables terminated in a ring terminal bolt on, can't remember what they're for though. The injectors get a +ve feed from ignition through the RSR (Radio Suppression Relay) and they are switched on the -ve side by the ECU to fire them, it's full-group/batch-fire injection so all 4 fire at the same time so it's just one signal from the ECU to earth the injectors all together to fire them.

[Foeux]

Originally Posted by Laird Scooby

Forgot to answer these two points in my post ^^^^^

The pots to meaasure the amount of fuel is the "upgrade" of the card trick and gives you something you can measure - the benefits are obvious here!

The crank sensor is working at the moment or the injectors wouldn't fire, worth a visual inspection of the cable from the CPS to ensure it's in good condition, if not order a replacement (from ~£20 on fleabay) and fit it once the injector problem is resolved.

The two tabs on the fuel rail are just brackets, no electrical purpose but i think on the back end some cables terminated in a ring terminal bolt on, can't remember what they're for though. The injectors get a +ve feed from ignition through the RSR (Radio Suppression Relay) and they are switched on the -ve side by the ECU to fire them, it's full-group/batch-fire injection so all 4 fire at the same time so it's just one signal from the ECU to earth the injectors all together to fire them.

Curiouser and curiouser...

I am really pleased with the simple '4 pot' setup. It is abundantly clear to see what everything is doing and it is pretty simple to make/use/reuse.

This is where it gets interesting. I switched injectors 2 and 3. Thanks for the tip. This would then show whether I have a faulty injector or faulty wiring (if the problem follows injector 3, then the injector is at fault, if the problem sits on cylinder 3 then the wiring is at fault).

Boyed by my earlier, clearly displayed diagnosis, I thought I'd at least be able to tell what sort of problem I had, even if not able to fix it there and then. One thing I didn't account for was all 4 test pots having exactly the same amount of fuel in each time. well, guess what? Each time I ran the test, all 4 pots had the same amount of fuel in each time. This included running the car in between 2 of the tests in case something, anything funny was happening in between running cycles...

The only clue I had was that when I removed the injectors from their seats in the inlet manifold after running, cylinders 1, 3 and 4 all had small wisps of whitish smoke coming from them, 2 did not. This means that cyl 2 became the odd one out with the injector moved in terms of the 'does some whitish smoke come from the holes into which the injectors slot' test. This means the 'odd one out' syndrome has maybe migrated to cylinder 2, maybe - but perhaps the inlet valves were just open on that cylinder etc...

The injectors themselves:

The injectors in there are Bosch 0 280 150 762. They are yellow plastic in colour. They are no longer made. You can get them reconditioned and 'matched' etc.

I found this useful website that told me Bosch 0 280 150 762 has now been usurped by Bosch 0 280 156 045

For about £45 each is it worth trying a set of 4 of these?

Just before I go slopping new injectors after the problem - is there anything to suggest that these might also become blocked as soon as I fit them or was I just unlucky?

Cheers for persisting.

F

[Rversteeg]

Do I understand correctly that by fiddling with the injectors you now have four injectors injecting the same amount of fuel? And the engine is therefore running fine again?
I can't think of a connection just now, but it is a strange coincidence that both the ignition and fuel injection problem are both on cylinder no. 3.

[Foeux]

Originally Posted by Rversteeg

Do I understand correctly that by fiddling with the injectors you now have four injectors injecting the same amount of fuel? And the engine is therefore running fine again?
I can't think of a connection just now, but it is a strange coincidence that both the ignition and fuel injection problem are both on cylinder no. 3.

The engine isn't running fine again. The misfire is still there but it is much better since I changed cap, arm, plugs, leads, resealed exhaust.

Problem isn't necessarily cyl 3 specific. Moving the injector it still misfires but it is hard to tell where or why now as swapping injectors yielded same amount of fuel from each... Despite that, still lumpy idle and struggles to get off idle and rev up.

[Laird Scooby]

Originally Posted by Foeux

The engine isn't running fine again. The misfire is still there but it is much better since I changed cap, arm, plugs, leads, resealed exhaust.

Problem isn't necessarily cyl 3 specific. Moving the injector it still misfires but it is hard to tell where or why now as swapping injectors yielded same amount of fuel from each... Despite that, still lumpy idle and struggles to get off idle and rev up.

Have you tried spraying the exhaust manifold with water again to see if it's now #2 not firing?

Seems very odd that all injectors suddenly provide the same amount of fuel when there was a clear problem earlier. Would suggest a tiny bit of grit (maybe microscopic tiny) is in the injector and moving about so it gives enough sometimes but not all the time.

Extending the theory somewhat, if you remove all injectors from the fuel rail and use the pots to catch the fuel, does anything strange end up in the pots? I've known sometimes fur sludge to form in fuel and cause a jelly-like lump, if one of those happened and was blocking #3 and #3 is still misfiring this might be the cause. It's a very long shot though but given you've checked everything and now have all 4 injectors delivering the same amount, long shots become the place to look.