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Discussion Starter #1
From my '78 351M,

Here are all 8 plugs



Here is a close up of #8. It's a bit rusty.


Do these look fairly normal except maybe for #8? What do you think?
 

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1978 Bronco Ranger XLT, 400/C6 92 K documented miles &1994 Bronco,XLT 85K original miles
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The business ends look too rich and some look oil fouled. Look them up on the web. Should be white/tanisha. Good electrodes would be nice and sharp. Worn would be rounded like your little finger.
 

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No way are those plugs original from 78, they'd look way worse. Also they would have been motorcraft not autolite.

I concur with above. Those look rich and some oil fouled. Given the rust I'd throw in some new plugs and run them a while and check again but doesn't look that good.
 

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What they said. Run new plugs a bit and check them. Rust may mean a coolant leak in to the cylinder, oil can be anything from rings to valve seals.

Sent from my Pixel 3 XL using Tapatalk
 

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1979 Bronco Ranger XLT, 400m engine, C6 trans, D44 front with Dick Cepek manual locking hubs
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No way are those plugs original from 78, they'd look way worse. Also they would have been motorcraft not autolite.

I concur with above. Those look rich and some oil fouled. Given the rust I'd throw in some new plugs and run them a while and check again but doesn't look that good.
how much oil you're engine is consuming?
If not a lot, put in new plugs (I do not like the rust at the top connection) and potentially replace wires as well.
Then have a look after 500 - 1000 miles and see how they are.

if oil consumption is not too high, leave it all together until it really starts making problems.

I learned that if its running, leave it until it stops or creating huge hassle.
 

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Discussion Starter #10
Thanks for all the input on the plugs. Oil issues makes sense with them based on what else I am seeing. I've been trying to diagnose rough start and and idling as well as fuel in the oil. After pulling the plugs, I did a compression test on all 8 cylinders. This is what I got:

1: negligible
2: 150
3: 155
4: negligible
5: 150
6: 155
7: negligible
8: 145 top pressure but it did leak down quickly

I also re-tried number 1 wet and it didn't seem to make any difference in the pressure so I was hoping it was a valve issue.

Then, I hooked up an air compressor to #1. I could hear and feel air coming out of the oil filler cap (shown below while closed).


So, with some fuel in the oil, and compressed air coming out of the oil filler, are we talking about bad rings? Not really what I wanted to diagnose.
 

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1978 Bronco Ranger XLT, 400/C6 92 K documented miles &1994 Bronco,XLT 85K original miles
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Did you open the throttle body when cranking? Where all 8 plugs out? Was the engine 50% warmed up?

It makes a difference.
 

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Discussion Starter #12
Did you open the throttle body when cranking? Where all 8 plugs out? Was the engine 50% warmed up?

It makes a difference.
I actually forgot to open the throttle the first time on plugs 1-4 and I got this instead:
#1 - 125
#2 - 150
#3 - 150
#4 - 90

Then I re-did them holding this (see red arrow) back toward the firewall with one hand while I cranked the remote starter in the other hand.


That's when I got:

#1 - nothing
#2 - 150
#3 - 155
#4 - nothing

And yes, all 8 plugs were out. However, the engine was cold as the truck is very difficult to start and won't idle long without dying. Is there hope it's something else?
 

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1978 Bronco Ranger XLT, 400/C6 92 K documented miles &1994 Bronco,XLT 85K original miles
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Ok. 1 and 4, nothing, try repeating everything BUT squirt 3-4 from an old school oil ( just a mind pix) in the dead cylinders. If there's no change, likely the valves not sealing. If you get something, its rings. The oil helps to create a seal. Odd you would have "0". That's catastrophic failure.
 

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Discussion Starter #14
Ok. 1 and 4, nothing, try repeating everything BUT squirt 3-4 from an old school oil ( just a mind pix) in the dead cylinders. If there's no change, likely the valves not sealing. If you get something, its rings. The oil helps to create a seal. Odd you would have "0". That's catastrophic failure.
Hey DB,

Yes, I re-did #1 wet and still came back with no pressure which is why I had hoped it was valves. But then I ran compressed air into #1 and could hear and feel air coming out the oil filler cap but not out of the exhaust or carb. That's why I was thinking it was probably rings.
 

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that is a bummer. out of curiosity...how well does (did?) that thing run? if you're down multiple cylinders i wouldn't think it would run well at all.

i hope you're able to rebuild it and make it better than ever.
 

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Discussion Starter #17
that is a bummer. out of curiosity...how well does (did?) that thing run? if you're down multiple cylinders i wouldn't think it would run well at all.

i hope you're able to rebuild it and make it better than ever.
Bummer, yes I agree.

It ran/runs real rough. It's all I can do to coax it to start and run a bit. Maybe an upgrade to 400 is in the cards!😀
 
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