Spark Plugs & leads resistance readings

[dom14]

Hi Guys,
I took the readings off the spark plugs and leads.
Vehicle is RB30 Patrol carby on dual fuel.
The reason is to find the reason for occasional misfiring and sometime backfiring as well
Appears to be worse in cold climate, but intermittent kinda issue.

These are the readings. Numbers correspond to the cylinder number.
Reading are in ohms.

Leads
Coil lead 1400 ohms
1)2000 ohms
2)5200 ohms
3)2500 ohms
4)3000 ohms
5)2800 ohms
6)2900 ohms

Plugs
1)4900
2)4900
3)4700
4)4800
5)4500
6)4600

Do they look ok?

I've attached few photos of the spark plugs as well.

Four of the spark plugs show some light reddish dis-colouration of the spark base which looks like from excessive heat.

[threedogs]

Hi Dom a few reasons why a dual fuel will back fire, 1= leads shorting out keep them clean. lift the bonnet at night and in the
dark put it under load [rev it ] and look for sparks arcing. 2= is an air leak from the air filter, make sure all joins are air tight
check the airbox seal is in good condition.
and 3= is an exhaust leak more so at the motor end than near the back of your Patrol, Timing should be advanced, on petrol
it should just ping then back off the timing a degree or two. Check dizzy cap for cracks and moisture in the dizzy itself.
All this will cause a back fire.

[4bye4]

G'day Dom - a bit hard to do diagnostics based on pictures. How long have the plugs been in there? 1 and 2 look like they may have a bit of oil burnt on them but probably not a problem. The ones with red on them may be a problem but depends how liong they have been in there. You can spend hours measuring ohms etc and still not find your problem. Might be a better idea just to change all the plugs and leads. If your pockets aren't deep enough as most of ours aren't, get one or two plugs and change them and see is the backfire goes away. Keep moving the plugs untill you get a result or you have changed all cylinders.
Your lead resistance readings are not much help as they would be done on a multimeter at maybe 1.5 volts. The ignition works at 12000 volts plus and resistance at that voltage would be different. its also AC and you are measuring DC. Also insulation breakdown or arcing will not show up on a multimeter. Again get a new lead, the longest you need and change it one by one through the cylinders. Or buy a new set altogether. May be a dizzy cap as well or the lead from the dizzy to the coil.
Try the in the dark method suggested by TD as well. the other thing is the "button" in the dizzy cap might be worn or jumping making bad contact. You can usually see this on the rotor arm by arcing "burn marks" where the \"button" contacts the arm

[dom14]

Hi Dom a few reasons why a dual fuel will back fire, 1= leads shorting out keep them clean. lift the bonnet at night and in the
dark put it under load [rev it ] and look for sparks arcing. 2= is an air leak from the air filter, make sure all joins are air tight
check the airbox seal is in good condition.
and 3= is an exhaust leak more so at the motor end than near the back of your Patrol, Timing should be advanced, on petrol
it should just ping then back off the timing a degree or two. Check dizzy cap for cracks and moisture in the dizzy itself.
All this will cause a back fire.

Hi mate,
I think when I left the car outside all day & night in freezing raining weather, the moisture inside the dizzy and other ignition parts must have had a built up, hence could be the reason for non stop misfiring and back firing..
If that's the case I need to find a solution, I reckon.

There's a slight exhaust leak that needs fixing, so that obviously didn't help.

I'll wait for to night and look for sparks while running it.

What is your opinion on the funny colours on some of the spark plugs and the resistance readings?

Thanx

[dom14]

G'day Dom - a bit hard to do diagnostics based on pictures. How long have the plugs been in there? 1 and 2 look like they may have a bit of oil burnt on them but probably not a problem. The ones with red on them may be a problem but depends how liong they have been in there. You can spend hours measuring ohms etc and still not find your problem. Might be a better idea just to change all the plugs and leads. If your pockets aren't deep enough as most of ours aren't, get one or two plugs and change them and see is the backfire goes away. Keep moving the plugs untill you get a result or you have changed all cylinders.
Your lead resistance readings are not much help as they would be done on a multimeter at maybe 1.5 volts. The ignition works at 12000 volts plus and resistance at that voltage would be different. its also AC and you are measuring DC. Also insulation breakdown or arcing will not show up on a multimeter. Again get a new lead, the longest you need and change it one by one through the cylinders. Or buy a new set altogether. May be a dizzy cap as well or the lead from the dizzy to the coil.
Try the in the dark method suggested by TD as well. the other thing is the "button" in the dizzy cap might be worn or jumping making bad contact. You can usually see this on the rotor arm by arcing "burn marks" where the \"button" contacts the arm

Hi mate,
Plugs have been there for just over six months. I changed them, just 'cos I had them. The old spark plugs weren't causing any issues as far I could remember, but I thought change them since I had the old ones for about two years.
I still got the old plugs, which were still reading ok as far as multimeter test was telling me.
The dizzy cap looked pretty clean when I opened it last week and couldn't see any visible signs of arcing.

I was particularly concerned about the lead no 2 showing twice as much resistance as the others, while it's relatively shorter than the rest of them, other than the No 1 lead and the coil lead. I was also wiggling and bending the leads and lead ends to find any breakages which came out as ok.

I was wondering, whether low resistance leads like mine has the tendency to arc more as the insulation wears out over time.
Leads are over two years old, so I'm not sure whether I should change them regardless of multimeter giving ok readings.

I'm buying a new set of leads anyway, but taking few opinions before I go ahead. I've opened a different thread for that in the forum.

I'll post a picture of the dizzy cap shortly. I'll also wait for the night to come so I can observe any sparks in dark.

Thanx for all the tips.

P.S. It's really good to know the multimeter test on plugs & leads isn't a conclusive reliable indication of their serviceability. I learn something new everyday. Thanx

[threedogs]

Spray inside the dizzy with some WD40 or CRC to disperse any moisture
Dont the Petrol motors use those centre fire spark plugs?

[dom14]

These are the photos of dizzy cap & rotor, taken today.

[dom14]

These are the photos of dizzy cap & rotor, taken today.


Spray inside the dizzy with some WD40 or CRC to disperse any moisture
Dont the Petrol motors use those centre fire spark plugs?

Not sure mate. I usually go with the Nissan recommended spark plugs(Bosch ones) or compatible NGK ones.
There's sticker on the rocker cover which says the part number of the spark plug.

I don't adjust the gap either. I leave it with the stock gap, which is around 0.8mm.
I just make sure they are all the same gap. I don't think the spark plugs are bad, but can't say for 100%. Multimeter test came out ok, but as you guys pointed out to me, multimeter test doesn't give 100% insurance.
I'll check any cross spark shorts to night.

[billyj]

lead no2 is probly stuffed, and causing a possible weak spark under load. my manual states leads should be under 3000ohm resistance

[dom14]

lead no2 is probly stuffed, and causing a possible weak spark under load. my manual states leads should be under 3000ohm resistance

Thanx mate. I suspected the lead no 2 as well, 'cos it's twice the resistance of the others, given it's the third shortest lead.
I couldn't buy a single lead to test it, so I have to buy the whole lead set, which is my idea.
But, I wanna make sure I get a good set of lead after listening to the experienced opinions here.
There's a very small exhaust leak from the flange gasket as well, but I thought that couldn't cause all these misfires and backfires. I will fix it tomorrow anyway.

[dom14]

just did the pitch dark ignition leak inspection.
I couldn't see any cross sparks at all.
I'm hoping to fix the exhaust leak & flush the LPG plumbing tomorrow.
The reason I think it's not LPG fuel related, 'cos it's doing the misfiring/backfiring on both fuels, even though in petrol it is probably not as bad as LPG.

[threedogs]

Oh and another reason they back fire is cause they are running lean on gas,
I high recommend you change any "T" for brass "Y" pieces this will aid flow and stop
cavitation. Dont forget to crack the hose at the converter ,air will get trapped there
http://www.ebay.com.au/itm/BRASS-Y-PIECE-FITTING-3-8-HOSE-BARB-ENZ-P75-06-/251595381398?pt=LH_DefaultDomain_15&hash=item3a9440da96

[dom14]

These are the photos of dizzy cap & rotor, taken today.


Oh and another reason they back fire is cause they are running lean on gas,
I high recommend you change any "T" for brass "Y" pieces this will aid flow and stop
cavitation. Dont forget to crack the hose at the converter ,air will get trapped there
http://www.ebay.com.au/itm/BRASS-Y-PIECE-FITTING-3-8-HOSE-BARB-ENZ-P75-06-/251595381398?pt=LH_DefaultDomain_15&hash=item3a9440da96

Thanx mate.

I found out a leaking welsh plug as well.
It's probably the time for head and the gasket to get it done as well.
I'm hoping to get the radiator reconditioned as well.

Converter water hoses are 5/8". I found it following your link above.
I should've done it ages ago, when you pointed out that to me.
But, I'm doing it today, definitely. Replacing all T pieces with Y's.

Are PVC pieces any good?
I don't think there' any saving between PVC and Brass anyway.
It's better to put the best stuff in the first place, isn't it?


Cheers

[4bye4]

Don't use PVC for LP gas mate. Use brass.

[dom14]

Don't use PVC for LP gas mate. Use brass.

Hi mate,
It's for the water line.
I wouldn't wanna replaceLPG gas joints until I know what I'm doing.
I don't think there's many(or any) T-joints in the LPG fuel line other than the hydro T-valve, which I don't have
a choice but leaving it alone.
Yeah, I should stick with a metal Y-piece anyway

[4bye4]

No worries mate.

[dom14]

After fixing small exhaust leaks & sticky taping the top "vent" of the Thermal Vacuum Valve(TVV), the engine appears to be running fine.
No misfiring or backfiring during the test runs today.

[mudnut]

Glad to read you are getting
on top of the problems, Dom.

[dom14]

Glad to read you are getting
on top of the problems, Dom.

Hey Craig,
Try blocking the top of the TVV with sticky tape.
As we discussed earlier, it's not suppose to vent in to the atmosphere.
Let me know what happens when you do that.
My guess is your engine will run smoother and perhaps bit more boost once the engine's warmed up?!
Cheers
Dom

[dom14]

I ordered a replacement dizzy(used one) just in case. It looks like I might not need it after all.
But, it's good to have a spare one anyway.
RB30 has electronic/magnetic breaker system, which usually when it fails, it fails in a snap of a finger. :D