Forgive me if any of you see this posted in multiple places, but I can't figure this out, and I need to, because I need to have the car emissions tested again soon and it won't pass in its current state.
The car
1992 Mustang, 5.0 T5 A9L as it came from the factory, with a Supertune tune in it, been there for at least 6 years without trouble
Pro-Flow 75mm MAF
Brand new Bosch O2 sensors
Vacuum lines have all been checked, replaced as necessary.
EGR is fairly new
FP is 42lbs with vacuum off
The problem
One day last year, with no warning and no change to anything I can think of that would cause such a thing, it started bucking violently at part throttle cruise a few minutes after startup. When this is happening, it will barely idle, smells pig rich, gets about 10mpg tops. When I say it bucks violently it's as if somebody cuts the ignition momentarily, gives it gas, and turns the ignition back on. Violent. Still runs OK at WOT, but soft on power. In other words, it's running pig rich when this is happening. Yet the O2 sensors are still switching.
It will do this as long as the O2 sensors are hooked up, with the O2 sensors disconnected, the car runs perfectly fine. I'd be happy to leave it that way, but when they're disconnected, it eventually throws a CEL, and I won't be able to pass emissions with the CEL showing. I'd just disconnect the CEL but the emissions tester will check to make sure it's on when the ignition is turned on.
Diagnostic attempts
I've pulled codes and all I get are a KAM problem (it's shown that ever since I had it tuned years ago), and TAB/TAD (no air pump). Cylinder balance test came back clean.
I installed a breakout box, and checked the following:
VREF is 4.98v
MAF checks out good, 1.05v at idle, 4.3v+ at WOT
TPS checks out good, 0.84 at idle, 4.6 at WOT, no dropouts when it's swept slowly
ECT checks out good, 0.78v at about 160 degrees
ACT checks out good, 1.18v at about 50 degrees
EVP checks out good, 0.5v at idle and WOT, 0.8v at 70mph cruise, 1.3v at 80 mph cruise, 2.8v at gentle acceleration
HEGOGround shows 0.9 ohms with the ignition off, but resistance goes up to 33 ohms when ignition is on, and goes up as accessories are turned on.
O2 heater circuits both show 11.98v, so theyr'e not open or shorted
HEGOs both switch between 0.01 and 0.9v. I tested both of them before I installed them by heating the element until voltage got to 0.85+, then took heat away, they both dropped to 0.01v or less within 3 seconds.
In short, everything I can think of that affects the EEC feedback loop is working fine and dandy, but the car runs like .
So now what the
Again, the car runs beautifully when the O2's are disconnected, but when they're connected, it runs like total ass shortly after it's started. I'm at a complete loss and I'm all ears.
The car
1992 Mustang, 5.0 T5 A9L as it came from the factory, with a Supertune tune in it, been there for at least 6 years without trouble
Pro-Flow 75mm MAF
Brand new Bosch O2 sensors
Vacuum lines have all been checked, replaced as necessary.
EGR is fairly new
FP is 42lbs with vacuum off
The problem
One day last year, with no warning and no change to anything I can think of that would cause such a thing, it started bucking violently at part throttle cruise a few minutes after startup. When this is happening, it will barely idle, smells pig rich, gets about 10mpg tops. When I say it bucks violently it's as if somebody cuts the ignition momentarily, gives it gas, and turns the ignition back on. Violent. Still runs OK at WOT, but soft on power. In other words, it's running pig rich when this is happening. Yet the O2 sensors are still switching.
It will do this as long as the O2 sensors are hooked up, with the O2 sensors disconnected, the car runs perfectly fine. I'd be happy to leave it that way, but when they're disconnected, it eventually throws a CEL, and I won't be able to pass emissions with the CEL showing. I'd just disconnect the CEL but the emissions tester will check to make sure it's on when the ignition is turned on.
Diagnostic attempts
I've pulled codes and all I get are a KAM problem (it's shown that ever since I had it tuned years ago), and TAB/TAD (no air pump). Cylinder balance test came back clean.
I installed a breakout box, and checked the following:
VREF is 4.98v
MAF checks out good, 1.05v at idle, 4.3v+ at WOT
TPS checks out good, 0.84 at idle, 4.6 at WOT, no dropouts when it's swept slowly
ECT checks out good, 0.78v at about 160 degrees
ACT checks out good, 1.18v at about 50 degrees
EVP checks out good, 0.5v at idle and WOT, 0.8v at 70mph cruise, 1.3v at 80 mph cruise, 2.8v at gentle acceleration
HEGOGround shows 0.9 ohms with the ignition off, but resistance goes up to 33 ohms when ignition is on, and goes up as accessories are turned on.
O2 heater circuits both show 11.98v, so theyr'e not open or shorted
HEGOs both switch between 0.01 and 0.9v. I tested both of them before I installed them by heating the element until voltage got to 0.85+, then took heat away, they both dropped to 0.01v or less within 3 seconds.
In short, everything I can think of that affects the EEC feedback loop is working fine and dandy, but the car runs like .
So now what the
Again, the car runs beautifully when the O2's are disconnected, but when they're connected, it runs like total ass shortly after it's started. I'm at a complete loss and I'm all ears.