'93 mustang breaking up bad under 3000 rpm's

89-LX

89-LX

Forum Member
So a little background on the car. Last year, I blew the HG but the var ran great and strong. So in the fall, I stripped the upper half down and had the heads decked. The engine sat all winter, until I got it together this spring. Its a standard 302, with an on3 single turbo, h/c/i. MSD Digital 6 plus, stock distributor, MSD cap/rotor, FMS wires. Tuned by Lidio.

So I got it all together, and it had a misfire. After decking the heads, only thing that was changed was the spark plugs, going from champion to autolite. Free revving, it revs just fine. But as soon as you begin to let off the clutch, even just a little, it breaks up bad with a misfire and no power until 3000 RPM's, then it's perfect at part and full throttle. Sometimes it doesn't do it at all, but it does it more so than not doing it. I did notice that from the boost, I get some oil coming out of the breather on the valve cover which seems to be leaking down on the spark plug, but it did this last year as well with no misfiring.

I'm at a loss as to what it could be or where to begin looking.
 
Why did you change over to the auto-lite brand? Are the auto-lites same temp and protruding style as the champions? What gap settings were the champions and what are the lites current gap?
 
Beigg said:
Why did you change over to the auto-lite brand? Are the auto-lites same temp and protruding style as the champions? What gap settings were the champions and what are the lites current gap?
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Was just going to replace the plugs anyways, and it seemed that most on corral recommended the autolites. Went from a Champion RC9YC to the Autolite 3923, I believe that they are a direct swap for each other. Gap I believe I set at .030 or .032, can't remember what they were on the champions.
 
1BADAIR said:
Did you check compression? Valves adjusted correctly?
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Don't think it would be that. Valves are adjusted properly. It's random. At times, it runs perfect. Other times, a small misfire, then others a big misfire where it has trouble accelerating. But with no load, it will rev just fine.
 
89-LX said:
Was just going to replace the plugs anyways, and it seemed that most on corral recommended the autolites. Went from a Champion RC9YC to the Autolite 3923, I believe that they are a direct swap for each other. Gap I believe I set at .030 or .032, can't remember what they were on the champions.
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Current condition (picture) of all the plugs burning/firing 'equal'?
 
The Man said:
Lidio tuned it, then it developed a misfire?

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Nope, the tune was perfect after Lidio touched it. The misfire started after I did the heads over the winter. I think I may have found the issue by trial and error. I read on corral that someone unpluged their o2 sensors, and it ran perfect, but acted up again when they plugged them back in. So I tried that, and it seemed to run perfect. But as soon as I plugged them back in, it started to act up again with a very slight misfire. I'm suspecting bad o2 sensors. They sat in a bag all winter since the exhaust was out being ceramic coated, but I seem to remember at one point that they came out of the bag and were laying on the garage floor. May have gotten damaged or something at that point and it making it do weird things.
 
89-LX said:
Nope, the tune was perfect after Lidio touched it. The misfire started after I did the heads over the winter. I think I may have found the issue by trial and error. I read on corral that someone unpluged their o2 sensors, and it ran perfect, but acted up again when they plugged them back in. So I tried that, and it seemed to run perfect. But as soon as I plugged them back in, it started to act up again with a very slight misfire. I'm suspecting bad o2 sensors. They sat in a bag all winter since the exhaust was out being ceramic coated, but I seem to remember at one point that they came out of the bag and were laying on the garage floor. May have gotten damaged or something at that point and it making it do weird things.
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any physical shock therapy applied to the oxygen sensors will more than likely render them junk. Especially if they've been heat cycled and exposed to carbonification conditions.

Some scenarios to which (And not limited to) could be considered shock therapy are: dropped on the floor, tossed at a friend with butterfingers syndrome, rolls off the bench on to a jackstand.
 
I'm sure with the tune he shut off the o2 sensors. He tuned my blower car and said the o2 don't do anything anymore so I could delete them if I wanted too
 
punnisher said:
I'm sure with the tune he shut off the o2 sensors. He tuned my blower car and said the o2 don't do anything anymore so I could delete them if I wanted too
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Wouldn't explain why it runs like crap with the o2's plugged in, but runs fine with them unplugged though. I wish I could have asked him, but he wasn't in for the weekend. Ordered 2 new ones from amazon for $20 each. I'll be able to do more testing this weekend, but I drove it for close to 50 miles yesterday to really see if it runs great unplugged, and it did.
 
I don't think he would delete the o2's without telling you tho....it would then be closed loop, instead of open loop....or the other way around.

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I cant see how turning off the upstream O2s would help anything. How is the car supposed to correct A/F ratio? Shutting off the downstream O2s is common for deleting a catalyst.

I have heard of people unplugging an O2 sensor to put the vehicle into Open Loop after adding a blower or turbo as a cheap way out of paying for a tune. This will run the vehicle in rich (warm-up) fuel trims to avoid a lean condition under boost. Not the best idea for long term drivability.
 
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