Well, I thought I had this problem fixed, but apparently not. My engine is still misfiring, and It's having me tearing my hair out at this point trying to figure out why.

This problem seems intermittent. When the engine is first started if left sitting overnight, it will run rough for the first few minutes, then smooth out with a healthy exhaust note. After you drive the car around for awhile, if you listen to the exhaust note at idle you can hear what sound like "foomps" or muffled pops. Sometimes it will sound like a muffled pop and other time it will be more subtle and sound like breaks in the exhaust note. RPM fluctuations can be heard along with this. It seems inconsistent too. Sometimes it will idle like this and other times it will idle fine. It goes like this:

glug glug glug glug glug *foomp* glug glug *foomp* *foomp* glug glug glug glug glug glug glug *foomp* glug *foomp*...

Spark plugs are NGK copper heat range 7 gapped at .040 with an accel supercoil. I inspected my wires are could not find any cracks or breaks in them. The wires do touch the valvecovers and the t-bolts in certain areas but do NOT touch each other and are evenly spaced with spreaders. Driving around town at stoplights, sometimes the car will idle healthy and other times it can be felt randomly misfiring. I also noticed that it likes to misfire during decel, or if you rev it up a little in neutral then let off the gas.

ALL the sensors on this car except for the knock sensor are brand spanking new. The longblock has under 20k miles and I just replaced the fuel pressure regulator with a holley unit after I found out the stock diaphragm was blown. Fuel pressure is currently set at 44PSI a little higher than stock but not too radical. When I replaced the regulator, I found out that all 8 spark plugs AND the O2 sensor had been fouled as a result. I upgraded the O2 sensor to a 3 wire heated bosch unit while I was down there. I checked my base timing and discovered that teh shop that installed the longblock had it at 4* so I advanced the distributor to 6* BTDC. I am using a JET chip which adds 2* at WOT using 89 octane and am not getting any detonation at WOT.

I pulled plugs 1, 3, 5 and 2 after I got home from work to check. admittedly my spark plug reading skills are not up to par, but here's what I found. Seems to be black deposits around the outer ring, with the insulator nose still retaining a white color. One side of the insulator nose does have a dark brown spot just below the electrode. I noticed plug 3 seemed to look worse than the others, with the black deposits around the insulator nose and the brown spot was larger/darker than the rest.

I'm wondering if It's the injectors that could be causing this. They're pretty much the only remaining critical wear and tear item I haven't replaced yet. I ran a can of seafoam through the tank but I know this is like putting a band-aid on a gunshot wound. Just hoped it would help somewhat until I get new injectors. One thing to note, is that when I removed the stock diaphragm, I saw tiny particles of the orange rubber that was breaking away from the diaphragm in the fuel rails. It is to my understanding this can further clog the injectors. Also, my fuel pressure seems to bleed off fairly fast. Within about 10 - 20 minutes of turning the engine off It's down to 12 ~ 15 PSI, then drops to zero from there within another hour. Might it be worth mentioning, that this car seems to struggle a tad bit with hot restarts.

The cap and rotor are fairly new and have about as many miles on them as the new longblock. The fuel filter was replaced almost 2 years ago but has under 10k miles on it. It's approaching the time for a new one but I don't believe it is what is causing the problem at this time.

What do you think guys? Please let me know if you need anymore information provided. Thanks.

 

Im getting misfires in my 85 also. Sometimes the car will run good, other times it will misfire from what seems like 1 cylinder. I ran some gumout cleaner thru my tank and the problem got better, but not perfect.

I ordered a set of Bosch III injectors from Southbay and I will let you know if it fixes my misfire.

 

Please do. I really would like to put a set of silver tip spark plugs in here but I'm not spending that kind of cash if they're just going to get ruined.

 

Any spark plug besides regular coppers are just a waste of money and can actually make the car run worse.

I will update you on the fuel injector swap when I get it done.

 

99% positive the injectors are shot. With the engine cold, I did key on then ran around and watched the fuel pressure gauge. I watched it drop from the high 30's to about 15PSI in under 2 minutes. You could watch it gradually falling off and every few PSI it would have a sudden 1 - 2 PSI drop almost like watching the second hand on a watch.

I then repeated the test after the engine was warmed up. It took much longer for the pressure to bleed off but you could still stick around and watch the gauge drop.

This correlates with my rough idle and hesitation when cold pretty well. I'm calling the injectors out on this one.

 

I'm not a buyer of gimmick spark plugs at all (Bosch +4, E3 etc...), however I do believe in a set of good platinums due to their increased service life. Doing spark plug changes on my engine with headers of doable without having to get under the car but definitely not a job I look forward to especially when dealing with cylinders 6 and 8.

I was thinking of the ACDelco rapidfires however this may impose a porblem since I have headers and these plugs do not have a shorty design.

 

Just wanted to update you like I said I would. The injectors did not fix my issue. Although I have sharper throttle response I still have a miss and the engine has a hard time running when cold. You can read my whole thread in the TPI section. But there are some other things in the ignition that still needs to be gone through.

Good Luck.

 

Bringing this back from the dead I know. OP did you ever get this problem figured out? What about you Motown?