ViribusUnits

Na man. It doesn't work like that.

For all of our intents, Mass in equals Mass out. If the exaust isn't getting out, there is less mass going through the MAF, and it reduces to compensate for the mixture.

Therefor you problems are in the fuel system, somehow it's not compensateing like it's suppost to.

Check the values out of the Temp II sensor, but I dought their causeing your problems.

Brain time?

MBMB

But if a cat is clogged it could be letting the same mass out, but less volume. Water through a hose, instead of steam.

ViribusUnits

Doesn't matter.

Fuel and air will burn completely at a specific molar ratio. Basicly, a certain number of fuel molecules will burn with a certain number of air molecules. The mass of the air molecules is well known from their type and composition, as is the mass of the fuel molecules.

Since the MAF measures the mass of air put into the engine, the computer take the mass ratio, and calculates the required fuel mass. This is then turned into a plus length.

Anyways, point being, there is no way to throw the fuel mixture off by a cloged pipe down stream. It just can't happen.

If need be, I can provide all the relevent chemistry and engineering information, but I dought you really need it, and it would lose something to the medium.

My bet is you've eighter got a bad injector, or a bad ECU if you ruled the MAF and the Temp II out.

SharkSkin

Mark, try Here.

Easy test.

MBMB

VU --

I haven't ruled out bad MAF or Temp II (replaced MAF, and Temp II tests fine when cool), but bad injector is not likely because the MPG gauge shows progressively lower as the failure occurs, and the denominator is derived from the number of injector pulses (so the ECU is telling the injectors to fire more than they should). Do you know any local electronics hobbyists (EE majors at UH?) who would like to take on a little contract fabrication job for me?

Thanks, Dave.

ViribusUnits

With the fuel milage meter going down like that, the MAF rulled out, your limited to one of 3 componenets. Bad Temp II, bad brain, or bad wireing.

I might, but not right now. Finals are kinda takeing their toll.

worf928

MB, did you do your Temp II test at both the 35-pin LH and EZK harness connections to rule out bad wiring? Nice clean connections at the sensor?

Did you check Temp II resistance at two different temperatures on both circuits?

If you know that you have good continuity to the ECUs, then you should be able to see resistance change on both T-II circuits by simply warming the body of the sensor with your hand... hmmm... nope. Not in Houston. So, get a bit of ice and cool the sensor down and check linearity.

Everything sure sounds like a bad T-II. Perhaps one that needs to be warm for a while before going non-linear on you.

ViribusUnits

The more I think about it, the more I'm suspecting the wireing.

It can easily take 30-90 minutes to heat soak everything in the engine bay untill all of the connections are hot. Once that happens, if a connection is ditry, and it gets hot, the resistance could easily build up.

The MAF and Temp II inputs are resistance based, IIRC. On the Temp II, the higher the resistance, the colder the engine. So if a bit of wireing goes nuts and increases it's resistance, the car goes to full rich, and that may not be so good.

The only other suspishion would be the brain, IMHO.

MBMB

Worf -- no, I haven't done the complete Temp II testing. It's on my TTD list. When I get my LH test box put together, I'll be able to take measurements at the brain immediately after the problem begins.

VU -- I think wiring is a plausible explanation. My box will tell me which circuit is haywire; then I can trace the wire back.

I haven't seen a convincing explanation for a brain failing only after the engine has been warmed up and driven for an hour or so.

Ben Allison

Have you cleaned your grounds recently? Heat soaking increases resistance of the ground straps. Bad ground = bad spark. I've 'resurrected' more than my fair share of galvanized Porsches by a simple ground cleaning.

Also - I'm with Randy on the coil theory. The testing procedure doesn't really confirm that a coil is good (i.e. you can get a false positive reading), but it's worth a shot. Test them when cold and after the problem starts.

MBMB

Ben -

I've cleaned grounds as I've come upon them. I've also replaced both coils (as well as plug wires), but will have to get back on the road to see if it helped.

SharkSkin

MBMB, that warming up and dying thing is a VERY common thing on Ford ignitions. It's possible that something inside the brain is heating up then causing a connection to go haywire. Look for any cold solder joints or cracked joints on the PCB... also look for heat marks on the PCB.

MBMB

Dave,

Is the Ford ECU inside the cockpit or the engine compartment? Is it heat generated by the ECU that is causing the trouble, or external heat?

Mt PCB looked pristine on last inspection.

Earl Gillstrom

Mark,
I am still fighting a "check engine light" problem on my '91GT. The failure sensed by the ECU is EZK Temp sensor 2 out of range. It only fails after hours of running. Never cold or before an hour of running. When it fails the car seems to run normally, so seems like a false error. I have not observed the MPG monitor when it is failing, so it may be using more fuel. I also have an air fuel meter, so will try to observe next time it fails.
The temp sensor 2 was replaced during rebuild after the fire and I figured it was new defictive and replaced it again with no effect on the failure rate. Then I replaced the 75 degree thermostat with a stock 83 degree thermostat which seemed to reduce the failure rate. Now to my point: To test the temp sensor 2 in circuit, since I don't have Bosch break out boxes, I soldered a wire to the back of the connector on the EZK and attached a digital volt meter, so I could monitor while driving. I found that the cold voltage is 2.3 volts. After 1 minute running 1.5 volts. Fully warm ~300 mv. After 1 hour drive on the way to the Hershey swap meet, it slowly went up to 1 volt and the check engine light came on. On the way home, after a traffic jam in Hershey The check engine light came on and the VOM jumped from 700mv to 300mv. I replaced the temp sensor with the original sensor, trying to eliminate the possibilty of two new defective sensors. The next possibility is the new engine harness has a bad wire. One thing I found curious, when I replaced the sensor this time the engine was cold and I did not drain any antifreeze. I removed the sensor and noted the AF level about 1/2" below the sensor hole. Do I have an airlock? I plan to test the old sensor and if it fails will loosen the sensor while hot to let the air out and try again. I don't mean to hijack your thread, just point out how I am testing the temp sensor 2.

SharkSkin

Mark, the Ford ECUs would generate their own heat and fail. Usually they lived on the inner fenderwell, but it's a well known problem on the 80's cars. They tended to fail quicker with the hood closed than open when troubleshooting. No, I've never owned a Ford, BTW... ugh.

Anyway, next time you're in there, try re-seating the ROM and any internal connections, and inspect the solder joints closely.

Earl makes a good point about the potential for an air bubble under the sensor. Seems to me I've read about that somewhere in my research.