'96 Cobra, Cylinder 8 misfire

[cobraman1024]

Ok, after a search I've found that most people are suggesting a compression test and a leakdown test. Since I don't want to go that deep into the motor diagnostics yet, I'm taking any other suggestions. OBDII code for cylinder 8 misfire.

What has been done:
1. swapped coils side to side-----cured the problem for one test drive then it came back.
2. Thinking maybe a bad ground, I removed the coils and cleaned up the metal grounding surfaces of each coil pack and the metal bracketry where it contacted for mounting. It didn't help at all.
3. checked spark plugs-----cylinder 8 plug appears darker and possibly more fuel soaked than the other plugs. plug 7 looks normal. The plugs are NGK TR6's gapped to .035". I know this gap is reserved for power adder cars, but this smaller gap should not have anything to do with a misfire. I have since changed the gap back to .054".
4. swapped places with #8 plug wire for #7 plug wire. After erasing code for cylinder 8 misfire, restarted the car and the misfire was still definitely there, code for cylinder 8 misfire came back.
5. At night, individually removed each wire from the coil pack to "draw" a spark from the coil. Each coil pack terminal seems very strong. the plastic terminals are not cracked. The spark plug wires are Ford Racing less than 2 years old. Cylinder 8 plug wire shows 22k ohms of resistance on my Fluke meter which doesn't seem high to me.

My next step is to take it to a dealership for a cylinder balance test but I won't if I don't have to.

Whenever anyone here had a misfire, what finally ended up being your culprit??

 

[PSUCOBRA96]

so the misfire is always in cylinder 8 no matter what you change correct?

 

[cobraman1024]

so the misfire is always in cylinder 8 no matter what you change correct?

correct. I've tried to get it to move to a different cylinder in the hopes that the problem could be the coil packs, the spark plug wire or spark plug but the problem always comes back to #8. Maybe the injector for the cylinder is leaking which could explain the darker look of the spark plug? but I don't see exactly how that would cause a misfire.

 

[PSUCOBRA96]

take out the fuel injector and move it to another cylinder and see if it follows, if its leaking it could easily cause the car to run rich, make the plug look more fouled and cause the misfire, hopefully thats all thats wrong

 

[cobraman1024]

ok, well I know its unbelieveably simple to swap injectors on a pushrod 5.0 engine but looking at the 4v motor it seems that you have to remove the upper intake just to gain access to the injectors wiring harness. Any suggestions on easily swapping two of the injectors on the drivers side?

 

[SVTPower97]

thats it bro...removing the upper with an air gun it can be done in 5 minutes tho...gasket is rubber and re usable if in good shape...i would just unbolt the intake tub from throttle body and leave everything else attached...im not sure about torque i just get em tight and go across from the one i am tightening and have never had problems before...its by no means a hard task it can be done literally in just a few minutes with air tools

 

[PSUCOBRA96]

I think you can unbolt the fuel rails from the upper intake without taking it off from what I can remember, then its just a case of popping out the individual fuel injecotrs you wnat to switch, hopefully someone else chimes in and knows for sure if it can be done

 

[SVTPower97]

on my 01 motor i can take the driver side off without taking the intake off but passenger i can't...but that doesn't mean anything being that our motors are different

 

[cobraman1024]

After doing a search, I found a complete procedure on replacing all of the injectors on the DOHC motor. Now my next question is, can I use a 24lb. injector from a pushrod motor? I'm pretty sure the same injectors were carried over to the modular engines but I just need to know for sure. A buddy of mine has a bunch of spare 24lb. injectors that were pulled from a pushrod motor. He said if they fit, I could try them on my engine.

 

[cobraman1024]

A buddy of mine had a spare set of 24lb. injectors from his fox-body; I popped one of those in and the misfire is gone. The original #8 injector read an "open" on my Fluke multimeter. All other injectors read 14.9 to 15.3 ohms. The idea here is that if your spark plug is dark in a misfiring cylinder, check the injector.

 

[PSUCOBRA96]

glad it got fixed, sounds like it was dumping tons of fuel in the cylinder at all rpms

 

[cobraman1024]

glad it got fixed, sounds like it was dumping tons of fuel in the cylinder at all rpms

Yeah, I guess that was what was happening. The misfire was pretty bad at idle, the whole car was shaking. Once the engine revs rose, it seemed to smooth out.

 

[busta]

I'm having a similar problem I'm going to have to check my injectors. My car is definitely missing at idle but it hasn't thrown a code.

 

[cobraman1024]

I'm having a similar problem I'm going to have to check my injectors. My car is definitely missing at idle but it hasn't thrown a code.

mine did not throw a code all the time either. I didn't even get a fuel injector related CEL------only a "cylinder 8 misfire detected".

 

[busta]

How did you pull that code?

 

[cobraman1024]

How did you pull that code?

I had a CEL and checked it with my Actron Scan tool. The code that came up was a P0308, cylinder #8 misfire detected.

 

[busta]

I had a CEL and checked it with my Actron Scan tool. The code that came up was a P0308, cylinder #8 misfire detected.

I misread that you said 'at the time' lol. My car is only slightly missing... A little shake nothing major and the plugs are new. The spark plug wires are 2 years old and have seen a lot so I ordered a new set hopefully that solves it.

 

[cobraman1024]

I misread that you said 'at the time' lol. My car is only slightly missing... A little shake nothing major and the plugs are new. The spark plug wires are 2 years old and have seen a lot so I ordered a new set hopefully that solves it.

From what I've been reading on here lately, its almost "normal" to have a very slight tiny miss in these engines because of age or whatever. I wouldn't worry about it unless you have really low mileage.

 

[busta]

From what I've been reading on here lately, its almost "normal" to have a very slight tiny miss in these engines because of age or whatever. I wouldn't worry about it unless you have really low mileage.

The motor has about 2,500 miles on it now :lol:

 

[cobraman1024]

The motor has about 2,500 miles on it now :lol:

wow, sorry to hear that lol......I actually have a matched/flowed set of 24lb. injectors that a friend of mine let me borrow for troubleshooting. If I install those and have brand new spark plugs and my engine still has a very slight misfire then I won't be worried. What injectors, coil packs, wires and plugs w/ gap are you using?