Spark Plug Diagnosis

[jprevat]

My 89 2.9 was running rough and I pulled some wires and found two cylinders that were not firing. Pulled the plugs on those two and they looked really rough.

What could cause them to be this bad?

 

[pentode]

Those look just about how I'd expect non-firing plugs to look. To my eye they're wet with unburned fuel and they've got a decent layer of carbon, either from partially burned fuel or oil burning. Have you pulled the other plugs to check? That might give some good clues. Also, which 2 cylinders were those? That might key someone in on something 2.9-specific. Good luck!

 

[19Walt93]

The plug gap looks too wide, if the wires are questionable that might cause problems.

 

[lil_Blue_Ford]

Also, it’s usually recommended to stick with Motorcraft or Autolite plugs, double Platinum is good.

What is the condition of the cap and rotor? I’ve seen posts get carboned up or damaged and cause weak/no spark.

 

[Cees Klumper]

X2 on the too wide gap. Are they the correct heat range? Engine burning oil?

 

[jprevat]

Thank yall for the ideas. These two plugs were cylinders 4 and 5. They were denso plugs but I replaced them with MC. Ill be pulling the others soon to see what is going on with the other holes. My wires are dense but a couple have been chewed on so new wires are in order too. My cap and rotor probably don't have 500 miles on them so they should be ok.

Ill report back after plugs and wires but it's no longer stumbling so hard under load. I may have to run some

 

[jprevat]

X2 on the too wide gap. Are they the correct heat range? Engine burning oil?

They were the standard range denso platinum plug but I'm going back with MC now. I'm not smoking or smelling burning oil so I don't think I'm burning any. They were gapped correctly when I installed them.

 

[mr_manny]

If you have access to a compression gauge, It might be a good idea to document the compression numbers.

I am kinda surprised how affordable they've become (chinese).

 

[jprevat]

I have one I've been holding onto for a few years. I'll break it out this weekend while I'm changing the rest of the plugs.

 

[19Walt93]

They were the standard range denso platinum plug but I'm going back with MC now. I'm not smoking or smelling burning oil so I don't think I'm burning any. They were gapped correctly when I installed them.

There is no 'standard range" for plugs and often aftermarket plugs approximate what manufacturers spec so they can use the same plug for different applications with no way to tell if they're actually the correct heat range. Many early 70's GM's would surge on the highway if Champions were used and the surge disappeared when AC's were installed. Whatever plug you buy, verify the gap is correct, the ones you removed were way too wide.

 

[jprevat]

There is no 'standard range" for plugs and often aftermarket plugs approximate what manufacturers spec so they can use the same plug for different applications with no way to tell if they're actually the correct heat range. Many early 70's GM's would surge on the highway if Champions were used and the surge disappeared when AC's were installed. Whatever plug you buy, verify the gap is correct, the ones you removed were way too wide.

I meant stock range not standard but that's a fair reasoning for switching to MC this time around. What plug exactly should I be using?

I gapped every plug before installing with feeler gauges so my best bet is that something went terribly wrong and compromised the strap or electrodes integrity.

 

[jprevat]

These are the MC plugs that are gping in.

 

[jprevat]

Pulled plug on cyl 6 today and it looked pretty rough too. Electrode didn't look right and I had dark oil wet on the plug in the threads. Put the compression guage on and it looks to be at about 120. I'll post pics tomorrow.

 

[jprevat]

So my lineup so far. 4 and 5 look close to the same condition but 6 is degraded substantially andwas heavily oiled down.

 

[jprevat]

So 6 was an iridium plug. Definitely not right. Will be changing everything out soon.