Spark plug problem?

[Lee Goodall]

I replaced my spark plugs last night with NGK PFR6Q plugs but now have a lumpy idle which I assume is down to the plugs causing a slight missfire.

I gapped them as close to .32 as possible although my feeler gauge only has .30 and a .05 so I went with .30 with a bit of movement in.

What should I try?

Cheers.

 

[Broken Byzan]

drop them to 028 mate. The miss go away when the car warms up?

I have had that with plats before, i am trying copper plugs this time to see if they are any better

 

[Lee Goodall]

Yeah.. I'll try a smaller gap.

No the miss is more obvious when the car is idling at running temperature?

Cheers mate.

 

[dieselfitter251]

Yeah, I'd double check the plug gap, can you run the car with one of them out? Check the spark?
...no help this bit... if all else fails, Buy a diesel! ;O)

 

[Lee Goodall]

Yeah I can check the spark so I think I'll gap them to 0.28 and see how she runs if no improvement I'll check the spark at each plug individually.

..then the glow plugs would fail on me too knowing my luck lol

 

[Lee Goodall]

Can anybody explain this..

So I took the spark plugs out again and gapped them to .28 put them back in and the misfire was worse/more frequent?

I took them out again gapped them at .40 put them back in and she was running fine?

What determines the gap size for each application? Could this point to other problems? Mixture etc?

 

[dappadan786]

interesting you say that mate, i get a slight misfire with the plats as well gapped at 0.28!

ive got a set of new NGK coppers the BRK7E waiting to go in but havent gapped em yet might just throw them in and see what happens!

 

[Lee Goodall]

I think I've found the problem...

[FONT=Verdana, Arial, Helvetica, sans-serif]Technical Information[/FONT]​

[FONT=Verdana, Arial, Helvetica, sans-serif]Spark Plug Gap Settings[/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]The table below shows metric to imperial gap setting equivalents.[/FONT]

mm

0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.4
1.5
1.6
1.7
1.8
1.9
2.0
ins
.016
.020
.024
.028
.032
.035
.039
.055
.059
.063
.067
.071
.075
.079​

when .32 is refered to it actually means 0.8mm and should be .032 ins.

I gapped mine at .32mm which is below .016 ins. So I really need 0.8mm which is the gap the plugs are supplied with.

 

[aragorn]

ive never understood people representing it as 0.28

it's 28thou or 0.028"

which as you've discovered is 0.71mm not 0.28mm!

 

[Lee Goodall]

Yeah.. I did think the gap was tiny but have never gapped a plug for a petrol turbo before so just got on with it!! Ha the simplest things always seem to cause the most trouble.

Better whip 'em out agian and make the gap even bigger.

 

[dappadan786]

Yeah.. I did think the gap was tiny but have never gapped a plug for a petrol turbo before so just got on with it!! Ha the simplest things always seem to cause the most trouble.

Better whip 'em out agian and make the gap even bigger.

howd you get on mate? all sorted?

 

[Lee Goodall]

Yes mate all sorted now cheers.

Gone back up to 0.7mm / 0.028ins and firing well

 

[auroan]

Plugs are supplied pre gapped nowadays, so why all this messing around ?

 

[Lee Goodall]

With tuned/chipped/remapped cars it's recommended that you use a smaller gap so there's a reason.

Not all are pre gapped anyway are they?

 

[auroan]

Every set I've bought has been pre-gapped. I don't understand the reasoning by decreasing the size of the gap. Could someone point me to decent description why ?

Surly a smaller gap would decrease the surface area the spark produces making it harder to ignite the fuel air mix.

And from what I understand from physics the distance a spark travels doesn't lesson it's "power".

 

[Lee Goodall]

Since the gap size has a direct affect on the spark plug's tip temperature
and on the voltage necessary to ionize (light) the air/fuel mixture, careful attention is required. While it is a popular misconception that plugs are pre-gapped from the factory, the fact remains that the gap must be adjusted for the vehicle that the spark plug is intended for. Those with modified engines must remember that a modified engine with higher compression or forced induction will typically require a smaller gap settings (to ensure ignitability
in these denser air/fuel mixtures). As a rule, the more power you are making, the smaller the gap you will need.

 

[auroan]

source ?

 

[Broken Byzan]

For which bit?

 

[dappadan786]

put my BRK7Es today and its kinda sorted out the idling issue and they came PRE-gapped at 0.7mm! saved me messing around with them!

thanks for the thread Lee helped me out alot!

 

[aragorn]

I must admit i never checked the gaps on mine when i fitted them, but then with the engine being standard its not as much of a big deal. I also have a feeling i fitted BKR6EK's which have twin electrodes, but i cant remember too well and it may well have been bkr6e's

As lee said its mainly to do with spark strength, the more fuel/air in the cylinder the harder it is to ignite. Smaller gap ensures a more powerful spark. On some older cars, (i'm thinking cosworth YB and vauxhall LET here as an example) high boost levels or running a lot of nitrous requires a change from the standard coil/dizzy setup to a direct ignition system. They dont use coil-on-plug like audi, usually use two coils in a wasted spark setup, but without the added resistance of the distributor/rotorarm etc it provides a much more powerful spark.

 

[auroan]

As lee said its mainly to do with spark strength, the more fuel/air in the cylinder the harder it is to ignite. Smaller gap ensures a more powerful spark.

Hmmmm someone one point me to where there getting this info from. I'd like to know how a small gap produces a more powerful spark.

 

[Lee Goodall]

Hmmmm someone one point me to where there getting this info from. I'd like to know how a small gap produces a more powerful spark.

Google it mate.. we're not making it up.

Spark duration is also linked to gap size which obviously has an influence on ignition timing. Different air/fuel ratios require different spark characteristics and a high performance engine requires the spark produced by a smaller gapped plug. It's true honest.

 

[Lee Goodall]

Hmmmm someone one point me to where there getting this info from. I'd like to know how a small gap produces a more powerful spark.

It's not necessarily the 'spark strength' but the characteristic of the spark that is required by each application that varies.

 

[auroan]

Thing is.... gapping the plugs smaller made the car run worse. Gapping it to audi specs and it was fine ......

Information on the internet isn't always 100% correct.

 

[Lee Goodall]

I'm at 0.7mm Audi Spec 0.8mm.. seems to be smoother under boost now than it's ever been before too.

The problem was the conversion from imperial to metric (and me being a bit thick).

Yeah I do def agree that you shouldn't always rely on information directly from the internet without any other investigation.

 

[Lee Goodall]

put my BRK7Es today and its kinda sorted out the idling issue and they came PRE-gapped at 0.7mm! saved me messing around with them!

thanks for the thread Lee helped me out alot!

Nice one.. glad it helped.