one cylinder running colder than the rest.....
 

About a month ago I had my car at a new mechanic to see if he could get the car to idle at a normal speed (it had been idling around 1000 rpm; previous mechanic claimed the car had a vacuum leak)
Anyway, he was able to get it to idle around 750 but, told me that one of the cylinders was running about 100 degrees cooler than the rest.
Would this affect the idle characteristics of the car, what could cause this and how badly would it affect the cars performance?

Re: one cylinder running colder than the rest.....
 

Sounds like a broken ring. I would do a leakdown and check all your cylinders.

sean

Re: one cylinder running colder than the rest.....
 

------It could be a ring, but wouldnt that plug be oiled up???.......Bill S

Re: one cylinder running colder than the rest.....
 

Could it be a bad plug wire or spark plug?
James

Re: one cylinder running colder than the rest.....
 

What intake and carb?

Assuming this was 100° at idle?

How does the plug look on the cool cylinder?

Re: one cylinder running colder than the rest.....
 

If there are other problems, such as a miss, exhuast smoke, ??? then you may have a problem, but 100 degrees does not sound like much to me. The reason I say this, the amount of fuel or lack of it each cylinder receives affects the temp, which means for all cylinders to be the same temp, the carb/intake would have to be suppling the same amount to all cylinders, which is is ideal, but nearly impossible, especially at idle.

Besides fuel, the other thing that can easily affect temps is valve settings.


We check temps, but are looking for a "cold" pipe, which means a cylinder is not firing. As long as they are all heating up, and no other problems, good to go.

Re: one cylinder running colder than the rest.....
 

[ QUOTE ]
If there are other problems, such as a miss, exhuast smoke, ??? then you may have a problem, but 100 degrees does not sound like much to me. The reason I say this, the amount of fuel or lack of it each cylinder receives affects the temp, which means for all cylinders to be the same
temp, the carb/intake would have to be suppling the same amount to all cylinders, which is is ideal, but nearly impossible, especially at idle.Besides fuel, the other thing that can easily affect temps is valve settings.


We check temps, but are looking for a "cold" pipe, which means a cylinder is not firing. As long as they are all heating up, and no other problems, good to go.

[/ QUOTE ]

I agree with this post!
If you were racing it and wanted to run right on the egde of lean,and that 100 degrees brings you over the top.then i would investigate.

If you just cruise and the car runs great with no other apparent problems i think your good to go. 100 degrees doesnt seem alarming....

Re: one cylinder running colder than the rest.....
 

Ditto..
Motion would occationally use a colder or hotter plug in different cylinders to try and even things out...when racing...

Re: one cylinder running colder than the rest.....
 

Yep..forgot about plug heat ranges affecting things. I learned that with single cylinder 4 stroke go-kart racing.

Re: one cylinder running colder than the rest.....
 

Thanks for all of the replies; the car runs fine I just wanted to make sure this wasn't something I should address right away.
For those that asked; it has the stock cast iron intake and Quadrajet carb.
The plugs have just been changed but, I am not so sure about the wires or distributor cap.

Re: one cylinder running colder than the rest.....
 

[ QUOTE ]
Yep..forgot about plug heat ranges affecting things. I learned that with single cylinder 4 stroke go-kart racing.

[/ QUOTE ]

Tom,
I doubt the plug heat rating will affect the combustion temp. It will affect how long it lasts before melting or fouling. I think that changing a plug heat rating in a cylinder is just a compensation for the heat of that cylinder; not a method of changing the heat in that cylinder.
JMO
Verne https://www.yenko.net/ubbthreads/imag...iggthumpup.gif

Re: one cylinder running colder than the rest.....
 

Usually if a motor is running cooler on a cylinder it is fat I.E. blow by or to much fuel. I would still do a leakdown better safe then sorry but I agree 100 degree temp is not alarming. On a bbc it is hard to tune each specific cylinder eqaully. 8-5 are the most difficult.

Sean

Re: one cylinder running colder than the rest.....
 

[ QUOTE ]

Tom,
I doubt the plug heat rating will affect the combustion temp. It will affect how long it lasts before melting or fouling. I think that changing a plug heat rating in a cylinder is just a compensation for the heat of that cylinder; not a method of changing the heat in that cylinder.
JMO
Verne https://www.yenko.net/ubbthreads/imag...iggthumpup.gif

[/ QUOTE ]
Verne: a spark plug is doing exactly that, ie, helping regulate chamber (combustion) temperature by dispersing (or not) heat...the amount dissipated has to do with insulator length. I can't remember the exact numbers off the top of my head, but the % of cylinder temp that is transfered via the spark plug is surprisingly (alarmingly!) high...over twice the amount cooled by the charge, I believe?

Stagger jetting and plenum design/alteration can help cover up the distribution problem, but I'm gonna bet Eric has some great info/numbers on this subject...

Re: one cylinder running colder than the rest.....
 

Actually Rob, you guys already nailed it.

https://www.yenko.net/ubbthreads/imag...ouguysrock.gif

Re: one cylinder running colder than the rest.....
 

Rob,

I know that I'm paddling against the current with my perceptions, but I still believe that the heat range of the plug should be selected based on the combustion temp that is is present so that it does not foul or heat to the point of melting or causing a "hot spot" that will cause pre-ignition. Race cars make more power with more heat in the combustion chamber (heat is energy), but race cars always require colder plugs than a street car. At low rpm a hotter plug is better because it will keep the electrode cleaner (not fouling) but race cars operate at high rpm which brings in more cool fuel which produces more energy when it's ignited. The transition from cold to hot is what produces more power. Cold fuel mixture is denser, thereby allowing more molecules of fuel in a given mass of air. Power is derived when that fuel-air mixture is ignited by a spark. I don't think that if the electrode of the plug is hotter or colder, it is going to increase the heat produced by the ignition of that mixture. I believe the spark itself ignites the mixture and the mixture produces it's own heat, regardless of how hot the plug electrode is.
If a hotter plug produced more combustion heat (which equals power) everyone would be running hotter plugs, but race cars require colder plugs. The best way to tune a race car's plugs is to start with the coldest plug you can find, then gradually increase the heat rating until the porclain is nearly white, without a trace of aluminum spots on it.
As I said, I know I'm rowing upstream on this debate, but that's how I see it. https://www.yenko.net/ubbthreads/imag...iggthumpup.gif

Verne https://www.yenko.net/ubbthreads/imag...mlins/wink.gif

Re: one cylinder running colder than the rest.....
 

Hey guys,My brother used to work at a engine shop and I watched them run a alcohol sprint car motor on the dyno.When they were warming it up he had it running at about 1500rpm and a couple of the cylinders were running cherry red.So I said something to him and he buzzed the motor a couple of times and then set it at a different rpm and then all the pipes were the same.Like a couple of the cylinders were running a little lean at the first rpm and fattened up at the other rpm.Just an observation. https://www.yenko.net/ubbthreads/imag...iggthumpup.gif

Kendall