Flickering headlights and misfire.

dikidera · Post by **dikidera** » 29 Oct 2024, 09:22

My S60 2.4i is factory-fitted with a CNG installation. I was chasing a serious idle misfire when running CNG. CNG is such a fuel that it will expose any issues with an aging ignition system.

I also have a slight misfire when I cold start on petrol, 1st cylinder is usually the suspect with most events, but goes away when hot. But when I switch to CNG, I get many more misfires(but no codes) regardless of engine temp.

I have replaced:
All spark plugs with 0.8mm gapped ones for CNG.
4 of my coils are brand new(one from a few days ago) to around a year or two old(it's a mix and match tbh) and only one is a genuine super old Volvo one that I will replace soon but it's on Cyl #4.

I am starting to suspect a bad ground. Why? Almost all coils are new and yet I also get a misfire on other cylinders as well(running CNG), and my headlights are flickering on idle. However it's not the regulator, because i have replaced it a year ago.

I need some advice on how and where to test. Is it the alternator itself not producing a stable voltage causing the misfire OR a bad ground and the coils don't get grounded correctly. The wiring harness around the coils is super hard and the insulation in certain places is cracked

I have heard that it's possible to test for a rectifier by measuring for AC voltage across the battery terminals. But as for testing for bad grounds, I am not certain what is the best method. The battery itself is in the trunk so my multimeter cannot possibly reach that far between engine bay and rear. Not to mention I don't have any rule of thumb resistances to compare to.

Lastly it could be a failing ETM, I have noticed that the avg difference between TPS1 and TPS2 is 1 degree of difference(and sometimes slightly more).

lrock · Post by **lrock** » 29 Oct 2024, 09:53

The jumping points in the engine bay may be used as a proxy for the battery, in my experience.

Post by **volvolugnut** » 29 Oct 2024, 13:49

I will add my 2 cents. Misfire and headlights are very separate electric systems but have ground in common. Check for a bad/loose/corroded ground.
volvolugnut

dikidera · Post by **dikidera** » 29 Oct 2024, 15:58

They are, but it does perhaps tell me something is wrong with it, and may be contributing to the misfire when running fuels like LPG/CNG since they are harder to ignite.

Post by **vtl** » 29 Oct 2024, 18:26

dikidera wrote: ↑29 Oct 2024, 15:58 They are, but it does perhaps tell me something is wrong with it, and may be contributing to the misfire when running fuels like LPG/CNG since they are harder to ignite.

I had one cylinder rejecting to ignite (burnt valve), drove back home 250 miles at night, headlights were not flickering. I smell electric issue.

dikidera · Post by **dikidera** » 30 Oct 2024, 12:39

I will be testing the alternator by checking for AC ripple current with a True RMS meter.

dikidera · Post by **dikidera** » 06 Nov 2024, 14:58

I want to give an update:

I tested the car battery with a True RMS multimeter and it showed no AC voltage at all. I should also mention that not only do headlights flicker, but also interior lights and I think the DIM as well(I have not confirmed this). The flickering of both interior and headlights does need some slight squinting but its there if you stare long enough.

The misfires are present when I cold start on petrol and quite obvious, so it's not an issue with CNG fueling like I had thought. The behaviour is exactly the same especially now that temps are cold, but it kind of normalizes on petrol once the engine warms up, not so on CNG. So if it's fuel agnostic - it's not the fuel.

I have narrowed down the potential issues, but not really helpful at all.

ETM ? Maybe an inconsistent air mass in cylinders? If I hold RPM steady at 1500 RPM during driving or when in Park, there is stronger indication of misfires, more pronounced.
Brake booster seal - the seal on my particular variant does tend to fail and causes a rock hard pedal minutes after shutting car off. Maybe there is unmetered air entering there more than I ever assumed?
Alternator? I've already replaced the voltage regulator one year ago. But maybe it requires a new freewheel or new diode block?
A generic grounding issue, somewhere.

I'm no closer to solving this than when I started, and throwing parts at it just makes it a bigger money pit.

Post by **vtl** » 06 Nov 2024, 15:35

On way or another, you will fix it. Maybe after it takes out some other module.

I was rebuilding alternators, until my latest rebuild failed in short time. Bad part, I guess. Got a Bosch reman alternator, with lifetime warranty.

Post by **jonesg** » 06 Nov 2024, 17:30

dikidera wrote: ↑29 Oct 2024, 09:22 My S60 2.4i is factory-fitted with a CNG installation. I was chasing a serious idle misfire when running CNG. CNG is such a fuel that it will expose any issues with an aging ignition system.

I also have a slight misfire when I cold start on petrol, 1st cylinder is usually the suspect with most events, but goes away when hot. But when I switch to CNG, I get many more misfires(but no codes) regardless of engine temp.

I have replaced:
All spark plugs with 0.8mm gapped ones for CNG.
4 of my coils are brand new(one from a few days ago) to around a year or two old(it's a mix and match tbh) and only one is a genuine super old Volvo one that I will replace soon but it's on Cyl #4.

I am starting to suspect a bad ground. Why? Almost all coils are new and yet I also get a misfire on other cylinders as well(running CNG), and my headlights are flickering on idle. However it's not the regulator, because i have replaced it a year ago.

I need some advice on how and where to test. Is it the alternator itself not producing a stable voltage causing the misfire OR a bad ground and the coils don't get grounded correctly. The wiring harness around the coils is super hard and the insulation in certain places is cracked

I have heard that it's possible to test for a rectifier by measuring for AC voltage across the battery terminals. But as for testing for bad grounds, I am not certain what is the best method. The battery itself is in the trunk so my multimeter cannot possibly reach that far between engine bay and rear. Not to mention I don't have any rule of thumb resistances to compare to.

Lastly it could be a failing ETM, I have noticed that the avg difference between TPS1 and TPS2 is 1 degree of difference(and sometimes slightly more).

dikidera · Post by **dikidera** » 07 Nov 2024, 07:15

Thanks jonesg. Honestly, I admit I did not measure my battery voltage. The battery itself is 2.5 years old, however there was a brief period where I had not charged it for around 4 to 5 months. So I will check it with all terminals disconnected.

Anyway, today I got underneath the dashboard and sprayed starter fluid on and around the brake pedal seal, I saw no change in RPM or fuel trims. This doesn't necessarily mean everything is okay, the seal itself is leaking when I press the brakes, but the idea was to see if it affected RPM or misfires on idle.

What is now left is electrical issues(grounds or alternator) or ETM trouble. The MAF itself is also 2.5 years old. The trouble is finding a known good ETM.