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Junior Charter Member
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Discussion Starter #1
I need your collective expertise to figure out why I lost a cylinder when I was on the track this weekend (NJMP Moe HPDE).
302 Explorer short block – stock internals. AFR 165 heads. 1:1.7 Crane roller rockers. Stock EFI with SCT chip. FMS Cobra intake.

This is what I have checked so far:
At the track I tried the following
  • Measuring the temperature on the exhaust headers, it appears that Cylinder 8 does not fire. It stays cool, while the other 7 warms up.
  • Tried swapping ignition wires (on distributor and plugs)
  • Took valve cover off to inspect rockers/springs/push rods – all appear fine (although I did not measure total lift)
  • Swapped injector w. Cylinder 7-8
  • Checked that there is spark in Cylinder 8 plug.
When I got home
  • Measured compression on 6, 7 and 8 and they are all in the 200-210 PSI range
  • Swapped different injector in 8.
All of this with the same result. Sounds like it is not firing on all cylinders and engine is vibrating, and exhaust on Cyl 8 stays cold.

Now I am scratching my head – what to try next?
 

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I would check the wire to the injector on the #8 cyl.
You can move the #8 wire to the #7 cyl and see if the miss moves.
If you have not, put a new spark plug in #8 and make sure the wire is not burned through somewhere. You can do a check in the dark, for arcing.
 

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Did you pop off the distributor cap and check the contacts all look good?

Number 8 plug dry or wet?
 

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Junior Charter Member
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Discussion Starter #4
All good suggestions - I will tinker a bit with tonight and let you know how it goes!:detective:
 

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Charter Member
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It would help a lot if you looked at the no. 8 plug
 

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Do you have a mechanics stethoscope? If you do, try listening to the injectors one at a time to see if #8 sounds different. Should hear a click as the computer activates the injector. If you don’t check the connections and wire for injector all the way back to the computer. Hook an analyzer up to the computer and see if it tells you anything. If that all checks out good then pull the valve cover again and slowly rotate the engine and watch the valves on #8.

I am out of suggestions for now.

HTH

Norm
 

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Senior Member
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You could disconnect the injector and put a test light across the contacts. You should get a light if a signal goes to the injector. Also check the salt/pepper plugs.
 

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Discussion Starter #8 (Edited)
Double post
 

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Discussion Starter #9
Thank you for all the suggestions!

Here is what I tried tonight:

Swapped injector wire 7 and 8
Swapped plug 6 and 8
Fired it up and exhaust on 8 was cold

Checked the plug on 8 and it looked wet.

Then got a different plug wire for 8 and checked distributor cap. Tried that. Same result.

Took upper intake and valve cover off again and turned the engine to see if both valves open and they did (I guess if the pushrod was bent and valve stayed close there could still be good compression).

Very confusing.

So if I have compression and fuel, it is ignition. If Cap, wire and plug are fine. I am back to square one.
Next I think I will re-do the compression test and get a different cap for it. Also will try the stethoscope test of the injector if I can get to it (8 buried sits under the intake). Any other suggestions?
 

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Discussion Starter #10
Plug on 8 after running a minute
 

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Discussion Starter #12
Update:

Tried to put a LED w resistor in the injector plug on 8, and it flashed when cranking
Did compression test again - 200 psi
Did run it in the dark to check for "leaking" plug wires - all good
Took plug 8 out and grounded it to check for spark. There was a spark, but it did look weak - almost with a red color.

Think next move is to put another coil on it and see if the long wire to 8 is a little higher resistance for a bad coil to get through.
 

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Update:

Tried to put a LED w resistor in the injector plug on 8, and it flashed when cranking
Did compression test again - 200 psi
Did run it in the dark to check for "leaking" plug wires - all good
Took plug 8 out and grounded it to check for spark. There was a spark, but it did look weak - almost with a red color.

Think next move is to put another coil on it and see if the long wire to 8 is a little higher resistance for a bad coil to get through.
Right there is your answer, bad plug. Just replace it and see what happens.
 

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that was my next suggestion - have you actually tried a new plug in # 8

or just swap with another hole and see if it follows
 

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More...

What kind of plug wires to you have? I have heard of some that had a carbon core that could crack and use up spark energy trying to jump the gap.

Regards, Rick.
 

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It's not the injector, the plug is wet

change both the wire and plug. Make sure the plug gap is correct. The coil works since the other cylinders are firing.
 

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Junior Charter Member
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Discussion Starter #17
Update:

Tried a different coil, distributor cap and plug wires last night. Same result.

Will go out and get new plugs today. Autolite 3924s is what AFR recommends. Will keep you updated.
 

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Is the #8 plug wet with fuel or might it be coolant. Cylinders 1,4,5 & 8 are beside the coolant crossover ports in the intake manifold and these areas most common for intake gasket failure.

Hope the new plugs fix it but, I see that you swapped plugs from a different cylinder and the problem stayed at 8, so I am doubtful.

Good Luck

Norm
 

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Discussion Starter #19
#8 does get wet and it smells like fuel. Good idea about the head gasket tough - if the plugs do not make a difference I will try to investigate coolant/head gasket :detective:
 

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FFCobra Craftsman
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A day or two ago you said this;
"Swapped injector wire 7 and 8"
Exactly what do you mean here? Just pulled the wires off the injectors and installed them on the other injector? That won't work since the injectors shoot gas at different times. It's been a long time since I worked on one of these, so I am trying to understand if you mean you swapped the entire wire, but don't the injector wires go into the harness and run all the way to the ecu?
 
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