I need some serious EEC-IV diagnostic help

[MFE92]

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.

 

[jrichker]

"HEGOGround shows 31.3k ohms, so it's not open or shorted"

The HEGO Ground is an orange wire that comes out of the passenger side injector harness: it should read less that 1.5 Ohm to battery ground. It is supposed to bolt to either the intake manifold or the back of the cylinder head.

 

[Blown88GT]

...
The car
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.
...

What is FP with vacuum on? Your symptoms sound just like mine, except mine would idle all day long with normal fuel pressure. You've got a blockage in the fuel system and/or a bad fuel pump. This was my fuel pump sock. When the sock was removed from the pump, found a piece of it at the pump inlet. Probably acting as a valve, i.e. on & off with the flow. Also found a partial blockage in the return line hose right at the tank. Blow out all the fuel lines with compressed air.

 

[stang&2Birds]

It will do this as long as the O2 sensors are hooked up, with the O2 sensors disconnected, the car runs perfectly fine.

That's the key. With the O2 sensors disconnected, the car runs fine in open loop.

For the EEC-IV, the ECT and ACT are there for "a little fluff, better smog, better gas mileage, a little more power, and to prevent knocks". So, don't worry about those.

So....
That leaves the MAF and O2 sensors.
I agree that there should be very little resistance between the O2 ground wire and ground. But, a lot of people have problems getting accurate measurements (including myself!) for many reasons.
1) Check the ground wires from the O2 sensors.
2) Make sure your body to engine ground straps are fine.
3) Check for the ECC ground.
4) Get some extra wire, hook it up between the battery ground terminal (heck use (a lot) tape to hold it in), and the O2 and EEC ground wires. This is for testing only!

Good Luck!

 

[MFE92]

I re-checked the HEGO Ground at the breakout box (pins 49 and 60), and it 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. Normal? The orange ground wire is intact and securely mounted to the back of the drivers side cylinder head like it's always been.

 

[jrichker]

I re-checked the HEGO Ground at the breakout box (pins 49 and 60), and it 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. Normal? The orange ground wire is intact and securely mounted to the back of the drivers side cylinder head like it's always been.

That's typical of a bad engine to body ground. With an ohms measurement, you measure the voltage drop across a connection or circuit. With the ignition turned on, the current through the engine to body ground increases. As the current increases across bad connections, so does the voltage drop. Clean all the ground and especially the engine to body ground on the firewall to the back of then cylinder head or intake manifold. All grounds need to be clean and shiny, so a little fine sandpaper or drill mounted wire bush is a good thing to use.

I recommend an extra 4 gauge ground from the battery block ground to the body common grond by the windshield washer filler neck (see picture below). This is a must if you have a 3G or other high output alternator.


Grounds

Revised 26-Feb-2012 to add testing of voltage drops with maximum load on the circuit or connection under test.

Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.

1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.


2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges.

Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972

Correct negative battery ground cable.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.


4.) All the sensors have a common separate signal ground. This includes the TPS, ACT, EGR, BAP, & VSS sensors.

5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.

6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself. The TFI module ground terminates inside the computer.

7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.


See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .

Extra grounds are like the reserve parachute for a sky diver. If the main one fails, there is always your reserve.

The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop.

 

[stang&2Birds]

Like jrichker said, that indicates that there is a bad/poor ground somewhere - common with Foxes!
The different metals in the connector and body react after a while - many factors. So, after a while, there's a poor ground connection. It could be in a bunch of places.

1): Make sure the (moronic) ground cable connection to the timing cover is good.

2) Make sure the battery ground terminal to body wire has a good tight connection. It's a good idea to take it off, wire brush both ends, throw on dielectric grease, reconnect.

3) Check the engine to body ground wire, remove, clean, reinstall (yea, my *ss!, that's a huge PITA in my car).

4) Add another BEEFY ground strap from the engine to a good solid body connection. Clean the areas, use dielectric grease.


FWIW, I changed the moronic (actually, a low-cost solution) battery ground connection and added a very beefy (the biggest/widest I could find) from the engine to the body. I mounted my battery cable ground to the AC bracket.

Good Luck!

 

[MFE92]

Thanks guys, I appreciate you taking the time. I went through the grounds pretty thoroughly a while back when I was chasing a high-RPM, high-heat, high-load misfire. However I did clean up the computer ground connection on the fenderwell, then I was advised by an electrical engineer friend of mine that one shouldn't try to take resistance readings on a live circuit.. So I ran a voltage-drop test and only got .05v difference in the reading from batt to timing cover and the reading from post to post on the batt, with the lights and cooling fan on.

 

[stang&2Birds]

then I was advised by an electrical engineer friend of mine that one shouldn't try to take resistance readings on a live circuit..

I breezed right past that in your post above. I was thinking of measuring voltage drops.
Yea, you can not measure resistance in a live circuit. A resistance measurement is really a voltage measurement done by the VOM/DVM. The new DVMs use a low voltage and low current to measure resistance.

What's important are the voltage drops/changes between the O2 ground, EEC ground, and battery ground.
It's also possible that the connections are borderline, or there is some (local) noise on the ground. However, even an ~0.05V difference should only make the engine run a little rich/lean.

 

[MFE92]

Pulled the supertune chip...still did it. Installed a known good computer, still did it. I said to hell with it, no care as long as it runs right and passes smog. So I disconnected the O2's, rigged up some ghetto-ass dryer-duct side pipes, took it to the testing station, and passed emissions. I'm good for another 2 years. Problem not solved, not a single given anymore.

 

[MFE92]

Just wanted to post a resolution to this: It was either the ACT sensor or a bad pin in the female side of the main O2 harness connector on the passenger fender, because while I had the car down for new valve springs, I changed the ACT and made sure the one pin that looked funky had a good connection. It's been running fine with the O2's connected ever since. Frankly, I think it was the ACT because years ago I had an '83 T-bird with a CFI 5.0 in it and the same thing happened, with damn near the same symptoms, and it was the ACT sensor in that case too. Strange thing is, this one was producing voltage according to the breakout box, so I guess it's one of those sensors where it doesn't take much of a variance to screw the EEC's pooch, yet won't throw a goddamned code.