Any Ford EEC-IV experts in the house? I need serious diagnostic help.

I can't figure this out, and I need to, because I need to have the car emissions tested again soon and it I think it'll fail on a technicality in its current state.

[B][U]The car[/U][/B] 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, bench tested beforehand, and installed before the last check ride that led to this post. Vacuum lines have all been checked, replaced as necessary. EGR is fairly new FP is 42lbs with vacuum off

[B][U]The problem[/U][/B] 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 totally happy to leave it that way, but when they're disconnected, it eventually throws a CEL, and I'm afraid it won't be able to pass emissions with the CEL showing.

I'd just disconnect the CEL but the emissions tester will fail the car if the CEL doesn't illuminate when the ignition is turned on.

[B][U]Diagnostic attempts[/U][/B] 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 installed). Cylinder balance test came back clean.

I installed a breakout box, and checked the following: [B]VREF[/B] is 4.98v [B]MAF[/B] checks out good, 1.05v at idle, 4.3v+ at WOT [B]TPS [/B] checks out good, 0.84 at idle, 4.6 at WOT, no dropouts when it's swept slowly [B]ECT[/B] checks out good, 0.78v at about 160 degrees [B]ACT[/B] checks out good, 1.18v at about 50 degrees [B]EVP[/B] 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 [B]HEGOGround[/B] checks out good, 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. Normal? The orange ground wire is intact and securely mounted to the back of the drivers side cylinder head like it's always been. [B]O2 heater circuits[/B] both show 11.98v, so theyr'e not open or shorted [B]HEGOs[/B] 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 E36 M3.

[B][u]So now what the hell?[/u][/B]

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.

Throw another EEC on it. I would suspect that the O2 drivers are kaput. So it doesn't respond to the feedback provided by the O2's. You are forcing open loop with the O2's disconnected.

After rereading your post, grounds, grounds, and grounds. It is a few billion years old in car years. I know Chrysler specs ground circuits at less then 5 ohms. Ford should be similar.

Btw, is this is the same MFE from the Corral?

In reply to Ranger50:

Yes, it's me. I'm confident in the grounds, I went all through them not long ago when I was chasing a high-rpm, high-load misfire. I just cleaned/improved the little set of grounds near the coil on the fender, and I also I just finished a voltage drop test between the battery and the timing cover next to the main ground and only got .05v less than post-to-post on the battery with the lights and radiator fan on.

Have you hooked up any kind of scan tool for the data stream?

Also to the grounds, have you checked the ground there near the EEC in the footwell, IIRC? I really think you have a ground problem with the EEC itself.

Ignoring everything else in your post, the bucking at part throttle would have a new TPS installed first.

No particularly good reason for the o2 sensors to mask that problem, though.

Just a point of advice, never try to check resistance with power on. It just does not work that way, any numbers you see are lying to you. You also risk damage to either your meter, or the device under test.

TPS checks out OK, short of putting an analog meter on it and watching the sweep, I can't see any dropouts or irregular voltages as I slowly sweep it. I watched it during the bucking, same thing. As for the EEC ground, I'm assuming this would be what Ford calls the "Case Ground", and it too checked out at 0.9 ohms.

You said it was tuned... Have you attempted to reseat the chip? Or verify that you can run correctly with it removed, which would bring back if you had an EEC problem or not?

I remember a bunch of problems with the plug in chips through the J9 port from varying port heights. I just wonder if yours has just vibrated loose from that port height problem or just a chip that finally died.

Streetwiseguy wrote: Ignoring everything else in your post, the bucking at part throttle would have a new TPS installed first. No particularly good reason for the o2 sensors to mask that problem, though.

Yea, I had that bucking issue with my beater Ranger. It did, however, throw a code for the TPS so it made the troubleshooting fairly painless.

Good luck, Fraser.