Okay guys so I'm having a strange issue with my truck. First off, its a 1988 F-250 4x4 ZF5 460. I was having major issues these last few months and after a long and stressful process, came to find out that I had a bad PIP signal coming from the distributor....swapped a new one in and bam, issue fixed.....


Now I'm noticing a weird rich condition that seems to be completely random. I have an Air/Fuel ratio gauge hooked up in the truck that uses the factory HEGO signal to provide its readings. Lately, the truck will randomly start running extremely rich and fuel mileage can be measured in gallons per mile it seems I haven't been able to correlate this rich condition with anything. Yesterday I drove home from Greenville (about an hour and a half drive) and the rich condition never occurred and the truck was running good. I got home, sat around the house for a bit, and then drove over to a buddy's house. Pretty much as soon as I pulled out of the driveway, my AFR gauge pinged rich (the gauge goes down to 10:1 and it was flashing that it was less than 10:1) and stayed that rich no matter how I drove it. I attempted to pull over and cut the truck off and back on to see if it would change anything and nothing....truck was still pegged >10:1.......


Does this sound like a stuck open injector? I don't have a stethoscope so I will have to rent one to see if I can hear the injectors clicking but I'm hoping it will be something else so I don't have to pull the fuel rail etc...


Could this be a failing HEGO? I'm not sure if HEGOs tend to act like this or fail altogether.


Just a little background: while trying to figure out my major issue before this rich condition came about, I ended up replacing all of the sensors (ECT, ACT, TPS, MAP, HEGO) so they are all fairly new.


Thanks in advance!

 

Does it run any different? Sluggish? Idle ok? Smoke?

If nothing is apparent, then I would question the gauge or sensor.

 

It seems to have slightly less power than normal conditions and I haven't noticed any smoke (I also wasn't looking for it). I did happen to be sitting in the car wash the other day while it started really running rich and I did notice the rpms dropped about 50-100 rpms or so and sounded like it bogged down somewhat

 

How does your A/F gauge connect with the HEGO sensor? Condition triggering any codes?

 

When you say HEGO, do you mean the O2 sensor? Without pulling codes it does sound like an O2 sensor or maybe an injector issue.

 

I haven't had a chance to pull codes yet but I'm hoping to be able to today.


The gauge gets its readings directly from the signal wire of the HEGO and correlates that low-voltage signal (0.1-1.0vdc) into an A/F ratio (20:1-10:1). I have the gauge wire spiced into the signal wire of the HEGO

 

Yes the heated oxygen sensor

 

I would inspect the spliced wires.

 

Well I didn't get a chance yesterday to pull codes, had to stay pretty late at work (big project going on) but I should get a chance today to check it out. I will also inspect my wiring for the A/F gauge and update the post when I can.


If my wiring for the gauge checks out with no issues, does this possibly sound like it could be a faulty O2? The condition is so random that it seems like it could be a sensor beginning to go out

 

Sorry, but I'd have to question the whole idea of trying to run a gauge off the stock sensors.

It's bound to be altering the signal that the computer receives back to try and control fuel delivery.

 

You might be right, but im not sure. What makes me think that's not the case is that it seems to be working fine until the random rich condition starts. Start up on a cold motor the A/F is around 12:1-11:1 (rich warm up condition) and as it gets around operating temp the ratio goes to around 15:1-14:1 (like i believe it should). It stays around 15:1-14:1 consistently until randomly it will just peg rich and run 10:1 or greater constantly. That's why i dont think it is messing with the signal going to the EEC but I may be wrong.

If anyone could chime in on this that would be great

 

The rich condition is a symptom not a cause, and most likely being commanded by the computer as it is losing sensor data to properly control the system. Rich is the default setting.

Your spliced wires may be intermittently loosing contact or otherwise interfering with the signal. Computer loses signal and defaults to rich condition. This would also set a code. A damaged O2 sensor could also be involved, but I think it less likely if you didn't have a problem before installing the A/F gauge.

It is also possible that something else entirely is at fault. As the computer is commanding the rich condition you will more than likely have codes to guide you to the problem.

Unless necessary I am not a fan of modifying or otherwise opening the factory wiring harness, especially on the wiring to critical sensors. When done it must be done correctly and with due thought because it can lead to nightmarish problems.

 

Well, you've introduced 3 possible variables.

How the sensor reads what it supposed see.

What the ecu sees.

And what the accuracy of the gauge piggybacking on the system it's suppose to monitor is actually telling you.

Just sayin'.

 

As long as the impedance of the aftermarket A/F meter is at least 10X the O2 circuit impedance there is negligible effect on normal operation.

I agree the rich condition is a symptom. With all the sensor changing from the previous epic thread, I question those "new" pieces.

Now would be a good time to look at a real time data stream device to watch signal inputs.

 

Wouldn't there be a increase in signal/noise ratio regardless ?