Can spark plug gap affect how the car starts?

phils89sc

Registered User
When I put my Autolite 5144 plugs in when I did my head gaskets, I gapped them to .040. I had planned on running more boost, but decided not to until i rebuild my engine.

So, my question is, since they are gapped smaller than stock, can that affect cold starts or idle quality? I think the factory gap is like .052 or .054. I know everyone says vacuum leaks cause idle issues, and I agree, however, I have used a smoke machine on my entire intake tract and could find none.

I think I am going to change the plugs again and go back with the Autolite Double Platinums and gap them back up to .054. Currently, I only make 10 pounds of boost, probably due to the ported exhaust manifolds, free flowing true dual exhaust and factory pulley on the SC. At idle, I have about 18 inches of vacuum.

Thoughts? Experiences?
 
When I put my Autolite 5144 plugs in when I did my head gaskets, I gapped them to .040. I had planned on running more boost, but decided not to until i rebuild my engine.

So, my question is, since they are gapped smaller than stock, can that affect cold starts or idle quality? I think the factory gap is like .052 or .054. I know everyone says vacuum leaks cause idle issues, and I agree, however, I have used a smoke machine on my entire intake tract and could find none.

I think I am going to change the plugs again and go back with the Autolite Double Platinums and gap them back up to .054. Currently, I only make 10 pounds of boost, probably due to the ported exhaust manifolds, free flowing true dual exhaust and factory pulley on the SC. At idle, I have about 18 inches of vacuum.

Thoughts? Experiences?

Plug gap can have an impact on starting. However, if you have hard starting it is more likely that you have a tune issue. If you don't have a vacuum leak, it is still possible that for some reason your MAF is not reading 100% accurately. For example, it might be dirty. Regardless, if your MAF function is "off" a little that will contribute to harder starting than it should be. Even with .035" gap a finely tuned engine will start within the first rotation.

Another important factor that is often overlooked is that if the starter is starting to drag, the vehicle won't start well. This is because a dragging starter will turn the engine slower than it should, and then also it will draw more current than it should leaving less voltage for firing the plugs. This combined with a weaker than optimal battery will result in crappy starting. I know this sounds dumb, but you'd be surprised how much difference it makes. I have this issue right now with one of my cars. The starter is slow making the thing crank more than it should when starting.

Also, did you put in new 02 sensors when you did the headgaskets? If you did not, then it is also very possible that they are causing the AFR trims to be off resulting in an incorrect mixture.

Ideally vacuum should be about 19-20", but there are many factors that contribute to the final value. Do not assume that your vacuum reading is a direct reflection of the quality of seal that your intake system has. The gauge itself could be way off on top of everything else.
 
Plug gap can have an impact on starting. However, if you have hard starting it is more likely that you have a tune issue. If you don't have a vacuum leak, it is still possible that for some reason your MAF is not reading 100% accurately. For example, it might be dirty. Regardless, if your MAF function is "off" a little that will contribute to harder starting than it should be. Even with .035" gap a finely tuned engine will start within the first rotation.

Another important factor that is often overlooked is that if the starter is starting to drag, the vehicle won't start well. This is because a dragging starter will turn the engine slower than it should, and then also it will draw more current than it should leaving less voltage for firing the plugs. This combined with a weaker than optimal battery will result in crappy starting. I know this sounds dumb, but you'd be surprised how much difference it makes. I have this issue right now with one of my cars. The starter is slow making the thing crank more than it should when starting.

Also, did you put in new 02 sensors when you did the headgaskets? If you did not, then it is also very possible that they are causing the AFR trims to be off resulting in an incorrect mixture.

Ideally vacuum should be about 19-20", but there are many factors that contribute to the final value. Do not assume that your vacuum reading is a direct reflection of the quality of seal that your intake system has. The gauge itself could be way off on top of everything else.

Thanks Dave...

What it has been doing more and more of lately is taking longer to start and when it does start, especially when cold, it will, about 75% of the time, will sputter for a second or two, then it's like it catches and idles up, then back down to normal, like a misfire on startup that goes away shortly after. I did not change the O2's after the HG swap, probably ought to because they are the factory sensors. I have tried another MAF, no change. I am also seriously thinking about plugging off the EGR valve. I think it still works, but just so I can eliminate a possible future problem.

About the starter dragging, it doesn't seem to. It will always crank fine, but sometimes, it cranks for like 6-10 times before it will finally fire up!
 
Change those O2 sensors before you do anything else. They are toast now because of the HG failure.

Fraser
 
Change those O2 sensors before you do anything else. They are toast now because of the HG failure.

Fraser

Not that I doubt that the oxy sensors are now bad because of the HG failure, but would that not toss a CEL or at least a code of some kind?
 
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