Technical Misfiring problems

[Gilbey121]

Hi, I'm the owner of a Fiat Ducato 40 MAXI 160 M-JET 2009 Motorhome 3 ltr. In the last six months it's started to misfire occasionally as you are driving along, when this happens the coil light flickers and the power dips then pulls away again back to normal. I've had the computer on and the only problem showing was a faulty air conditioning sensor. It starts well and tick over is ok. This problem is driving me mad and stopping me using the Motorhome, any advice would be appreciated thanks.

 

[Communicator]

Hi Gilbey121,

Welcome to the forum. I doubt that I can help much, as there is not much to build any hypothesis on.

I assume that you are not suffering from water in fuel. and that main battery connections are secure.

Here are some possible causes that come to mind, which you may consider worth eliminating.

1. What is the condition of the engine block earth strap?
2, Have you checked for "rusty junction syndrome? See attached photo which was originally posted by @Reg 65 . It is worth checking there, even if all OK Access is by removing the LHS headlamp.
3. A failing crankshaft sensor can cause problems, without leaving a fault code. This is more likely with a hot engine, but if it were to occur, I would expect the engine to stop immediately, and allow restarting after a few minutes. This is not what you have described.

@jackwhoo ? @bugsymike ? @Mike1alike ?

 

[Mike1alike]

That’s a very sensible approach
You’ve covered the key basics first and ruled out the usual suspects. Nothing obvious to add at this stage without more symptoms or codes.

 

[bugsymike]

Hi Gilbey121,

Welcome to the forum. I doubt that I can help much, as there is not much to build any hypothesis on.

I assume that you are not suffering from water in fuel. and that main battery connections are secure.

Here are some possible causes that come to mind, which you may consider worth eliminating.

1. What is the condition of the engine block earth strap?
2, Have you checked for "rusty junction syndrome? See attached photo which was originally posted by @Reg 65 . It is worth checking there, even if all OK Access is by removing the LHS headlamp.
3. A failing crankshaft sensor can cause problems, without leaving a fault code. This is more likely with a hot engine, but if it were to occur, I would expect the engine to stop immediately, and allow restarting after a few minutes. This is not what you have described.

@jackwhoo ? @bugsymike ? @Mike1alike ?

As you say, not a lot to go on.
OP mentions it has been on computer? Was it a garage professional one or at least one giving specific Fiat capabilities, MES etc? Basic scanner will not give enough detail.
Ideally you need to be able to read "Live Data" as driving along when fault occurs.
From description it could be electrical issue, does rev counter drop to zero?
Not one I have ever worked on.

 

[Gilbey121]

Hi Gilbey121,

Welcome to the forum. I doubt that I can help much, as there is not much to build any hypothesis on.

I assume that you are not suffering from water in fuel. and that main battery connections are secure.

Here are some possible causes that come to mind, which you may consider worth eliminating.

1. What is the condition of the engine block earth strap?
2, Have you checked for "rusty junction syndrome? See attached photo which was originally posted by @Reg 65 . It is worth checking there, even if all OK Access is by removing the LHS headlamp.
3. A failing crankshaft sensor can cause problems, without leaving a fault code. This is more likely with a hot engine, but if it were to occur, I would expect the engine to stop immediately, and allow restarting after a few minutes. This is not what you have described.

@jackwhoo ? @bugsymike ? @Mike1alike ?

Thanks for the response, we've checked all earths and they are ok. A friend has a mate who is a auto electrician and said that he thinks the air conditioning sensor ,it is sending false codes to the ecu and causing the engine to misfire. What do you think thanks again

 

[bugsymike]

Thanks for the response, we've checked all earths and they are ok. A friend has a mate who is a auto electrician and said that he thinks the air conditioning sensor ,it is sending false codes to the ecu and causing the engine to misfire. What do you think thanks again

Never heard of aircon causing engine misfire, not saying impossible, but pretty unlikely in my view.
So what diagnostics have been used?

 

[Gilbey121]

As you say, not a lot to go on.
OP mentions it has been on computer? Was it a garage professional one or at least one giving specific Fiat capabilities, MES etc? Basic scanner will not give enough detail.
Ideally you need to be able to read "Live Data" as driving along when fault occurs.
From description it could be electrical issue, does rev counter drop to zero?
Not one I have ever worked on.

Thanks yes it was a professional one and he's done a thorough check and only the one fault showed up, when it misfired Speedo and rev counter goes off

 

[Mike1alike]

An A/C pressure sensor cannot cause an engine misfire on its own.

The A/C system can request the ECU to adjust idle speed or reduce engine load, but it does not control ignition or fueling in a way that would create a misfire. If the A/C pressure sensor were faulty, you’d expect issues like the compressor not engaging, fans behaving oddly, or an A/C fault code - not misfiring.

If the engine is genuinely misfiring, focus should stay on:
ignition (coils, plugs, wiring)
crankshaft sensor signal
injector operation
intake air leaks
voltage stability

The key thing now is to confirm whether this is a true misfire or a driveability issue being interpreted as one, and to read proper fault codes with Fiat-capable diagnostics before changing parts.

 

[bugsymike]

Thanks yes it was a professional one and he's done a thorough check and only the one fault showed up, when it misfired Speedo and rev counter goes off

So that would a suggest a main power failure from ignition switch onwards is where I would start.
Crank position sensor can cause rev counter to drop but I can't see it affecting the speedo also.
I would trace wiring from ignition switch to ECU, did the auto electrician do that or just advise your mate?

 

[Mike1alike]

Thanks yes it was a professional one and he's done a thorough check and only the one fault showed up, when it misfired Speedo and rev counter goes off

That’s a very important detail. If the speedo and rev counter both drop out when it misfires, this strongly points to a loss of engine speed/vehicle speed signal, not an A/C-related issue.
On these cars the rev counter and speed signal both depend on data coming from the ECU. If the ECU momentarily loses the crankshaft sensor signal, power supply, or CAN communication, you’ll see exactly that:
engine stumbles or cuts
rev counter drops to zero
speedo drops out
then everything comes back
A faulty A/C pressure sensor would not cause the instruments to drop out.
Much more likely causes now are:
Crankshaft position sensor (even without a stored fault)
Intermittent ECU power or ground issue
CAN wiring/junction issue (rusty junction syndrome area worth rechecking)
Ignition switch voltage drop

This doesn’t sound like a true combustion misfire, it sounds like the ECU is momentarily losing critical input or power.

 

[Gilbey121]

An A/C pressure sensor cannot cause an engine misfire on its own.

The A/C system can request the ECU to adjust idle speed or reduce engine load, but it does not control ignition or fueling in a way that would create a misfire. If the A/C pressure sensor were faulty, you’d expect issues like the compressor not engaging, fans behaving oddly, or an A/C fault code - not misfiring.

If the engine is genuinely misfiring, focus should stay on:
ignition (coils, plugs, wiring)
crankshaft sensor signal
injector operation
intake air leaks
voltage stability

The key thing now is to confirm whether this is a true misfire or a driveability issue being interpreted as one, and to read proper fault codes with Fiat-capable diagnostics before changing parts.

It sounds like I need to do the check when the vehicle is running to see if any faults show up then

 

[bugsymike]

It sounds like I need to do the check when the vehicle is running to see if any faults show up then

Ideally if it is a good diagnostic tool then try a road test with it connected and being read by the passenger with live data and ideally data logging.

 

[Gilbey121]

Thanks will get my mechanic to check this out thanks

 

[Communicator]

Thanks for the response, we've checked all earths and they are ok. A friend has a mate who is a auto electrician and said that he thinks the air conditioning sensor ,it is sending false codes to the ecu and causing the engine to misfire. What do you think thanks again

As @bugsymike rather sceptical.