Bad Spark Plug??????

[USMC302]

On my 88GT, i just got done building it, which is most likely what is wrong with it. We built it from the piston rings up. Anyway, it has a light pop during idle like a miss but its not. It almost sounds like it is a valve not seating properly, they are remanufactured heads with a valve job, new springs, etc.. ANyway, under hard acceleration, it seems to miss badly but does get up and go, almost like it has 7 cylinders. The reason I think it is a spark plugs is because when getting the motor to run, it ran very lean, then very very rich, then we sand blasted the plugs to clean then and opened the gap to .060 I believe. There are also two different temp plugs in right now. The wires are 9mm, new, the cap and rotor have about 15K miles on them, and a MSD coil. What am I missing? What could it be? These plugs have about 500 miles on them, could one have gone bad already?

[LX5liter]

When you tightened your rockers did you check to see if you needed any shims under any of the pedestals? It sounds like you may have a valve hanging open. It could be a bad plug as well, just warm it up and spray water on the header tubes to find which cylinder is colder and try switching that plug out. Check to make sure there is spark to the plug as well. If it isn't a plug or wire my money would be on a valve hanging open.

[USMC302]

The rockers are stock, but you think because of the valve job and everything they could still need them? Also I will do the header check, that is awesome, I never thought of that, thanks. How much are shims, and do you think I would have to shim them all since I have no idea which one it would be?

[7000rpmisheaven]

Here's a test to help isolate where it is coming from. Start the car let it warm up. Adjust the idle a little faster than normal between 1500 and 2000. Unplug wire #1 and observe the drop in rpm. Then replace the wire, then move on to the next one. What you should see is that the rpm drop is about the same for all the cylinders. If you unplug one wire and there is no drop or drop is much less than the others than you know there is a problem in that cylinder.

[USMC302]

Awesome thanks, I will do it as soon as I get home.

[Mr 5 0]

You might want to check your plug wire routing. You could have switched wires around when putting the engine together. It happens. Easy enough to check.

[USMC302]

Thanx. After 7 mustangs I have never had this problem before, and I would probably kill myself if I would put the wires on wrong for the first time. I did check a while ago just to make sure and they are all good, the thing is, it doesn't really missfire during idle or light acceleration around town, only does it start acting up when under hard acceleration. There is a very faint pop in the exhaust during idle though, but not like a missfire you know, I have seen friends mix the wires before and man alive was it obvious but this is really really faint. I think the shims idea makes perfect sense and I am going to make sure it's not a spark plug first then check the cylinders and start shimming.

I DO have another prob though, the car when cold or hot starting will not start if while turning over to start it I let the key back too early. Almost like it is flooded. I don't give it gas during starting and the BBK FPR is set to 38lbs. I have been told this could be the coil going bad, but have just put a MSD coil on yesterday and the problem persists. Even after the car has been running a while, it won't start up again unless I let it cool for a while or sit for a few minutes. ANy ideas on that?

[PKRWUD]

Your last problem could be the TFI module. Your first problem could be in the wires, too. I agree that the heads are the likely problem, but if wires #5 & #6 are touching at all, a missfire/crossfire situation can occur. The better the wire quality, the higher the rpms will be before a problem is detected. Those two cylinders are sequential in the firing order, and are next to each other in the engine, which is one of Fords few designing flaws. Honestly, that problem tends to present itself more as a pinging, or detonation problem, than a backfire problem, but it's still worth noting.

Gppd luck!

Take care,
-Chris

[DRASTiK]

Shawn,
Remember that "hot starting" problem I had? Sounds kinda similar to the second problem you're having. I'd try the TFI first, and if that doesn't solve it, use a good DMM to test the Pickup inside the dizzy. Mine would blink out and come right back to life, both within one second. That was my problem. Just a thought.

Dave

[USMC302]

Thanks Dave but how do i test the pickup? What Is a DMM? The car is running much better now, I haven't done anything to it, go figure. The wires are 9mm and have seperators so hopefully the 5 & 6 wires aren't crossing, I have heard of this often happening too. The car is running good now with no pops but still is slow in reaction when the gas is mashed and sometimes backfires when let off of the gas....

[DRASTiK]

Shawn,
I'd Replace the TFI first if the problem persists, and then if that doesn' t fix it, use a DMM (Digital Multi-Meter) and while the car's running check the volts to the wires that connect to the TFI. If they suddenly go to 0 and then come back, then the pickup is on the fritz. At least that's how we figured out mine was bad.

[USMC302]

awesome thanks,
on the way home for lunch today one of the rockers started knocking on the valvecover. Great just when I thought it couldn't get worse. I will be pulling the valve cover tomorrow to see how bad it is. Thanks for everything everyone! Thanks Dave!

[DRASTiK]

you know, that could've been your problem all along. If one of the valves wasn't opening and closing all the way, then it could make it run like crap. Then when the rocker started loosening up, then valve was operating correctly, and now it just loosened the rest of the way.

Check for bent pushrods, but it's most likely that a rocker just came off and the pushrod is just being slung up and hitting your valve cover if the noise is very loud. If it's not too loud, then it's probably just a loose rocker.