Missing, dead cylinders after tune up

[SuperCab]

Well Duncan is going to have a fit but oh well

I have acquired another non ford product recently. 1995 GMC Subby, 454/auto.
They just seem to follow me home But in my defense I'm probably not keeping this one long... Might pull an Austin on it


It had a little roughness/miss to it and the ignition parts were obviously dated, so I got new cap/rotor/plugs/wires for it.

Now it runs like crap. Worse than typical GM crap

Tach bounces from 1000 to ~1400 rpm, engine is shaking and missing, smells like unburned fuel from the exhaust.

Swapped all parts back to the originals one at a time to check for defective new parts and all seem ok. Through pulling wires off the dist one by one (and zapping myself in the process ) I've narrowed it down to cyl 6 & 7 bein effectively dead. Haven't done a compression test but it was pretty nicely drivable when I got it home... Can't imagine what has happened.

Thinking coil maybe? But you can tell most of the cyls are hitting ok... Kinda stumped here.

Thanks for any help. And much deserved taunting no doubt

 

[DNFXDLI]

That is simply the Ford Gods planting a triple coiler on you

You are getting spark on 6 and 7?....and I know this sounds elementary, but did you get the wires back on the same places and did the distributor possibly move?

 

[SuperCab]

Checked the firing order and even put a timing light on it to make sure the distributor hadn't moved...

There is spark leaving the cap for 6&7... I'm tempted to pull a plug and crank it once to be 100% whether it's getting spark to those cyls or not.

 

[fatherdoug]

That is simply the Ford Gods planting a triple coiler on you

You are getting spark on 6 and 7?....and I know this sounds elementary, but did you get the wires back on the same places and did the distributor possibly move?

Overlapped spark plug wires?

 

[DNFXDLI]

I'd pull the plug lead off and stick a screwdriver in it and ground it....or in your case, stand in a puddle of water barefoot and hold on to it

 

[SuperCab]

I'm gonna go try that if I can find motivation... I thought for what I paid for it it may make a decent winter beater as it is 4wd. But I am having second thoughts.

 

[5.0Flareside]

Its a Chevy what ya expect.

 

[DNFXDLI]

 

[SuperCab]

Indeed.

 

[m78]

If they're straight boots on the wires, you could have separated the carbon from the brass terminal inside.
Go with Duncans puddle of water test.

 

[SuperCab]

Well I checked a few plug wires with the screwdriver... All have spark but it is a weak orange spark which surprised me. Leads me to possibly coil again? Not sure.

 

[DNFXDLI]

Well I checked a few plug wires with the screwdriver... All have spark but it is a weak orange spark which surprised me. Leads me to possibly coil again? Not sure.

Could be....are you getting full voltage to the coil?

 

[SuperCab]

Not sure but as I think about it the spark is hot as the devils $$$ at the cap. Will jump to the firewall. Or to your hand from a considerable distance.

 

[DNFXDLI]

How did you find that out?

That is odd....points to the wires.

 

[SuperCab]

Seems that way to me. I'm tempted to buy another wire set from a different parts house tomorrow and see if there is a difference. I have had bad new parts before.

 

[DNFXDLI]

It seems pretty weird that plug wires would be pooched out of the box.

 

[john112deere]

Good spark at the distributor, weak spark at all eight plugs, correct?

It ran with the old wires, right?

What happens if you put all the old wires back on?


*I don't know nothin' about Chevy ignitions, but wasn't there some kinda TFI module or something in the distributors around that era that's famous for crapping out? Maybe it got bumped and isn't OK any more?

 

[DNFXDLI]

Ian might be right on that...It's been years since I worked on one of those and I was kinda thinking the same thing....isn't it a small module with 4 spade pins?

I kinda remember doing a similar type tuneup and having the same thing happen.

 

[Fellro]

Weak coil would affect all cylinders, not one or two. If it has good spark from the cap, but not at the plug, then the wires are bad. If you can, resistance check them. The way they do the wires for the crimps are jenky, kinda like the cheapo jumper cables. The resistance of all the wires should be in the same general range, so if you have higher resistance on those two than is on the rest, then you need different wires. Since it ran better before you changed the parts, then it is most likely the parts you have that caused the current issue.

 

[Sparky83]

just curious.. what brand ignition parts did you buy?? GM's tend to get finikey with their ignition parts...