So I finally got around to firing up my engine that has been sitting for a couple months to do some tuning on it as I finish up the restoration. The only problem is that when I'm pulling the wires off the distributor cap, they all change the idle when pulled, which is good, expect cylinder 8. when pulled it has absolutely no change on the idle. I've replaced the wire, spark plug, even the dist cap and core. I'm clueless as to what the problem could be. Any help would be great. Thanks.
Pulling the plug to find it has little affect on the engine only narrows down your problem to that cylinder. It could still be air, fuel or compression. Checking to see if the plug does fire will cancel that area. Do a compression check and see what shape that cylinder is in. All of the gas, air and spark in the world won't make up for a bad valve seat or overly worn rings. Not to scare you, but I recently saw an exhaust valve with a hole clean (well, not really that clean) through it. That truck could hardly idle though. You probably would have know you had a pretty serious problem if it was this bad.
Just in case you need a way to see if the plug is firing. Here's my favorite. Invite a friend over who isn't very car savy. Ask him to "hold this" real quick while handing him the plug in question. Note: Plug is plugged into plug wire and looking for a sucker, I mean ground. Walk over to the drivers side of the car and hit the ignition switch for one or two rotations. If your friend screams like a little girl the plug is firing.
I think the use of a simple timing light will tell you if a cylinder is coming along with the others when you rev the engine up....... :Smarty:
Too Funny!!!!!!!!!! Remind me to be careful around you!! :laugh: The first thing I would do is start the car and pull that wire off the plug while the engine is running and ground the wire close to the head to see if you're getting a spark to the plug. If you're getting a spark to that plug do the same thing with a plug in the wire to make sure you're getting a spark there. If you have a strong spark that far you've got a stuck intake valve or a blockage that's keeping air or fuel to that hole. The plug should tell you if it firing proper, I don't think that pulling the plug and seeing if the rpms drop is the best way to see if you've got a dead hole.
I would do a compression test if the compression test is around 130 or so you should be ok. Then check for spark, if you get spark then check to see if the porcelean on the plug is not cracked. What ignition are you runing HEI / points.
The #8 plug when pulled looks clean whereas the other are somewhat dirty. The compression, checked twice, all the way around ranges from 113-120. Is that too low? Also the spark is getting all the way through the spark plug (tested this by having the wire with the plug in it pulled while the engine was running). So does pulling the wire and not having the idle affected really mean the cylinder isn't firing because I'm not sure what the deal is? I'm going to give the valves and rocker arms a look next.
Disregard the 130 compression deal. It appears that the spark plug is not getting fire I would check the wire again for continuity and replace if needed. double check your firing order also
You have spark at the plug. Stop looking at the ignition system. You have compression. Stop looking at valves 'n' rockers. That pretty much leaves fuel. I'd be looking for a vacuum leak that's turned your idle fuel mixture to that cylinider so lean it won't burn. Look for a leak at a fitting on the intake manifold runner that feeds that cylinder.
Which would be the transmission vacuum modulator line right behind the carb on the passenger side. Make sure it's plugged. Frank
bad hole id like to hear what the problem was when you find it the other guys are right on ///if you have compresion dont tear into eng if you have spark and plug and wires are good stop looking there with eng running spray starting either around manifold /carb vacumn hoses etc if eng rpms increase you found your problem luck pete
What has been done to this engine since it ran properly? If you just had the intake off you might have a vacuum leak. If not I doubt that's it. You said the plug was clean. Was it clean and wet or dry? If it's wet you have a ignition problem. I check for a cylinder firing with a timing light. If it's dry your intake valve is not opening enough and that can be a few of things like a collapsed lifter, a flat cam lobe, a bent pushrod, worn rocker shaft or misadjusted rocker or pushrod of you had them. All of the above will make a tappet noise with the exception of a flat cam lobe which will be silent. When I had my cam lobes go flat I started the engine with the valve cover off and it was obvious which rockers were barely moving.
The plug was wet when it was pulled. But you guys have a good point in regards to what not to tear apart. The timing light does indicate the cylinder is firing through the rpm range. While check the rockers I ran the engine without the valve cover and the rockers were moving smoothly like all the rest. So Andy, I'm hoping it's not a flat cam lobe and it's not making other wierd noises. I'm going to take a look at the possible vacuum leaks next. Thanks. :TU:
I don't see how a vacuum leak would cause one cylinder to misfire and foul the plug. At this point the first thing I'd do is install a new plug. The #8 hole is tough to get at if you have A/C and you can crack a plug fairly easily (yes, I've done it). A cracked plug will do what you are describing. If the plug is still wet it's either not firing or it's firing at the wrong time. I'd double check the firing order 18436572 clockwise. If that's right you might have a cracked or dirty distributor cap causing spark to jumping from another cylinder. But, if either of those things were happening i would suspect more than just #8 would be misfiring. Can you hear the misfire at the exhaust? I'm putting my money on a bad spark plug!
After much looking around we found out that the transmission has a vacuum line that is tapped off the intake manifold. The line was connected on the intake manifold but, as I finally took a look, it wasn't connected on the trasmission. I reconnected it and bam, the engine was running smooth with all 8 cylinders firing. Thanks again for all you help guys!
