Misfire on cyl 5 & 6 after doing TFI upgrade

[rollguy]

I installed one of GRONK's TFI ignition upgrades a few months ago.  The car was running and starting great up to a couple weeks ago (I maybe drive it once a week).   Today I pulled the wires one at a time while it was idling, and found # 5 & 6 make no difference in how it runs.   If I pull any of the other wires, it stumbles so bad it almost dies.   I pulled the plugs on 5 & 6 (properly gaped NEW plugs that came with the kit), and checked the compression.  Both had 150 lbs.  I switched the 2 wires with others, and no change.  I put a spark plug on one of the wires while it was running, and it has spark.   I can only assume that the problem is ignition related because the fuel/air mixture can't decide not to go in those two cyls, and 150 # compression means that the valves are good. I can't imagine the coil being bad either, because it can't choose when to fire and not (the same every revolution of the dist).   What would cause this?   Anyone know what I can try?

[ammachine390]

So the wires from 5 and 6 are sparking correct? If so, that sounds very strange, and IIRC somebody else once had the same problem, and I believe they said when they replaced the timing chain and the symptoms went away.

[txjeeptx]

Since #5 & #6 draw from the same intake runner, you might check to see if the intake gasket has failed and allows a vacuum leak.

[rollguy]

Since #5 & #6 draw from the same intake runner, you might check to see if the intake gasket has failed and allows a vacuum leak.

Dude!! You might be on to something there!   I will check that tomorrow.   However, I did notice the smell of fuel when I was checking the compression.  On second thought, it does miss a little at speed, so maybe that may rule out the intake gasket?   Even though it is getting spark through the wire, could I have gotten 2 bad spark plugs out of the box?

[BenM]

There are a couple of big vacuum ports over the 5-6 runner running accessories. Plug those and see if it makes a difference. I think 5-6 run cooler too, last time I changed plugs those two were fowled, I was surprised that 6 ran at all, the gap was bridged with ash.

[rollguy]

There are a couple of big vacuum ports over the 5-6 runner running accessories. Plug those and see if it makes a difference.

Dude!  you are the bomb!! (whatever that means).   That was exactly the problem.  There was a large vacuum line that was off the fitting just above the 5&6 intake runner.   I am usually not stumped by something that simple, but this time I was.  Thanks to all in this community (the 'Nest) that help make it the best automobile related forum on the WWW.

[jim]

There are a couple of big vacuum ports over the 5-6 runner running accessories. Plug those and see if it makes a difference. I think 5-6 run cooler too, last time I changed plugs those two were fowled, I was surprised that 6 ran at all, the gap was bridged with ash.

My 88 has a maor vacuum leak that I haven't found so far.  I've sprayed ether around all the usual suspect places. 
Can someone furnish more detail about locating these ports, or even a picture?

[carnuck]

Stuck open EGR valve?

[BenM]

There are a couple of big vacuum ports over the 5-6 runner running accessories. Plug those and see if it makes a difference. I think 5-6 run cooler too, last time I changed plugs those two were fowled, I was surprised that 6 ran at all, the gap was bridged with ash.

My 88 has a maor vacuum leak that I haven't found so far.  I've sprayed ether around all the usual suspect places. 
Can someone furnish more detail about locating these ports, or even a picture?

It varies from year to year, but it's a screw in vacuum connector with 1-3 ports that sits on top of the intake almost at the back. Mostly it's for 4wd and the heater head unit, but some years there were extra takeoffs for spark-control or other operations. Sometimes it had a larger then 5/16 hose port, but if it's loose or leaky, or anything connected to it is, then there's a lean idle misfire in 6 and maybe 5.