bent spark plug tips 1986 328gts

I pulled the plugs on #7 & #8 after it started missing. No real strange noises but runs rough. The tips on both plugs for these cylinders are bent to contact. The plugs in all the other cylinders are fine. I assume that I have chipped valves in both cylinders and will be pulling the head off. I am going to run a compression check first but won't start it any more. Belts were done by Rocky Mountain Sportscar about 30k and 4 years ago with about 4.5k per year driven. It ran perfect until this happened. Shouldn't all 4 on the back be bent or chipped if it was a tooth jump. Will it run if it jumped a tooth. #5 & # 6 were still firing fine? Any thoughts would be appreciated.

 

I would think that if it only jumped 1 or 2 teeth, then the timing on all of the valves would be off, but some would still not hit the plug tips. If the timing belt broke, then all the tips on that bank would be damaged very quickly.

 

Hi.........

Very strange problem !?

Perhaps the pistons are rising higher in those two cylinders because of some problem with clearances at the rod journals or the piston pins are bent and/or not held securely either in the pistons and/or the rods.

Or there is a subtle problem with either the block deck height or the geometry of the cylinder head.

I would not run it; do you really need a compression test now ??

Frank.......23005
www.masiarz.net/bb_resource

 

Do the compression test for sure before tearing in to it, might be fine. I had my car on the dyno a few years ago and it dropped a cylinder at 6600 rpm. I pulled the plug and found exactly what you describe, the electrode bent and contacting/shorting the plug. I did a compression test right there on the dyno and it was fine. I regapped the plug and it sparked fine. But the cylinder didn't run. Repalced the fuel injector (mine is converted to EFI) and everything was fine. Over the next month, I lost 2 more cylinders while driving at full power and about 6600 rpm. I changed fuel injector brands and the problem was gone forever. For some reason something was breaking inside the injector and flying through the engine and bending the plug before disappearing. Anyway, you may just have had something come loose somewhere and everything will be fine when you regap or replace the plug.

If it were the cam belt,you would expect all 4 cylinders to have problems and it would have been running more than a little rough. Good luck.

 

I have a strong feeling that this is what has happened with mine also. I haven't changed any of the factory components but it sure seems like what you are describing. Unfortunately this has been the week and 1/2 from hell in my company and I haven't had time to do any further diagnosis. Thanks for the input.

 

I have had this happen on a 4 cylinder ford engine (high compression). Put the plug under a microscope and saw no fracture, and no evidence that the tip had been hit by a piston. Re-adjusted the gap and it never happened again. Is it possbile under extreme stress that a plug tip can be bent by the force of detonation?

 

I finally was able to do a compression test after buying a 14 to 12mm adapter for my tester. All cylinders checked 150psi. Air filter looks good and I haven't checked the injectors yet. I will inspect the cylinders with a borescope next.

 

I finally broke down and bought a borescope with a right angle attachment and checked out 1 good cylinder and the 2 which had the bent plugs. Nothing in the cylinders, valves look good and no significant major markings but many imperfections in close proximity which I would call small dimples(a few thousands of an inch deep and looks like it was made by somthing 15/1000's of an inch in diameter) on top of the 2 non firing pistons. It must have injested something as previously suggested(no carbon in any of the cylinders and all the plugs color are excellent. I will now continue to take things apart and see where the offending contaminate came from. The air filter was perfect. The only other symptoms I can think of before this happened are, 1-there was a rattling sound when it was just started and 2-the idle was searching or hunting when it was warmed up. It ran perfectly at speed.

 

Roger,
If it's any consolation, the occasional rattling sound is considered normal. It's the accessory belts hitting the inside of the cover. Usually lasts a few minutes upon initial startup then quiets down. The idle speed "hunting" is also normal for K-Jetronic, fuel injected cars.