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Discussion Starter #1
I have a stock 1994 Ford Bronco with a MAF 302/E4OD combo. Lately after driving for 15-20 mins it feels like it begins to bog down and loses power. If you floor it it will bog, then take off like it picked up 100hp! I put the Quarterhorse on it tonight and I have some data. After the engine has been running for 15 mins and the ect is reading 200 degrees it starts to act up. The Lambse1 prior to acting up fluctuates 14.3-15.0, KAMS are .98-1.08, hego1 goes .28-.45v back and forth. When it acts up the Hego1 goes to 2.0 and stops switching, the Lambse1 falls to and fluctuates at 10.8-11.4. The truck feels like it loses all it's power and will occasionally pop in the exhaust. I pulled codes and got a 173,"hego oxygen sensor not switching, system was/is rich." Batt voltage is 14.00 the entire time, MAF voltage is in range for part throttle .25v(idle)-1.75v(part throttle cruise), and TPS voltage is in range,(.95 at idle-2.3 at part throttle.).

I conclude that the ecm is adding a ton of fuel, Lambse1 at 10.8 because the Hego1 is telling it that the engine is lean by reading .18-.20 volts. This is causing the engine to load up at part throttle. When I floor it it goes into open loop, and then uses the base OL fuel table, leans out, clears out, and gets full power back. How does my theory sound? My question is what would cause this? A bad oxygen sensor? It seems to switch fine before it hits the 15-20min mark and starts acting up. After it starts to act up it will all of a sudden clear up and the oxygen sensor will start switching again.
 

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So the HEGO acts up first, and then the engine goes rich?

Have you double checked your wiring? Make sure its not touching the exhaust, or rubbed thru the insulation? Unplug and plug back in the HEGO connector to scrape any corrosion off the pins?

A new 02 sensor is like 30 bucks? Not a spendy part to swap out and see if problem continues or goes away.

But yes, I agree your going rich, and that pop you hear could be fuel backfire. Finds a small exhaust leak, gets a little air and some heat and BOOM.
 

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Discussion Starter #3
I'm going to try an oxygen sensor from my work truck later today. I removed the vacumme from the EGR and turned it off in the tune. I removed the canister purge as well. Next I'll check oxygen sensor wiring. I've read it could be the fp reg but the oxygen sensor readings are saying its lean and not rich. It just stays in the 1.8-2.0 range and the lambse stay at 10.8.
 

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Discussion Starter #4
Quick update. Been driving it all morning without an incident or any signs of trouble in the datalogs. Looks like the EGR was the culprit. I'll know for sure as the day goes on. I'll put about 165 miles on it today for work.
 

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Quick update. Been driving it all morning without an incident or any signs of trouble in the datalogs. Looks like the EGR was the culprit. I'll know for sure as the day goes on. I'll put about 165 miles on it today for work.
I guessing your EGR system has a leak. EGR gases shouldnt have any o2 in them, but your o2 sensor is detecting an excess of o2 and then overfuels to compensate. Stomping on the throttle commands the egr to close, and your getting your power back. EGR should only work in the mid throttle range.

Also, the 15 minute warmup is probably how long the computer calculates before deciding its OK to begin using the egr system as designed.

Take a look at the tube between the EGR and the exhaust manifold. You may find a crack, or that its rotted away.
 

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I agree the EGR tube breaks on these after so many years of service. If you have not replaced it, its likely leaking. Check the outer jacket of the EGR tube up under the upper intake. If it looks black instead of off white, thats exhaust gas leaking thru the crack in the EGR tube. Mine was leaking, it looked like this.


Cracked



Also, the lead to the HEGO is also worth checking. It runs inside the jacket with the starter leads and is clamped to the block. When I did my rebuild, the point where it clamped had broken into the jacket of 3 of the wires. Upon close inspection there were several cuts in the jacket in that area. I took it all apart and sealed, heatshrinked, and taped each individual HEGO lead then shrink wrapped them all together and then ran them in a new sleeve, that way the clamp shouldnt break thru the jacket again in the future.

cuts


Repaired



my 94 had it clamped like in view F below
 

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Discussion Starter #7
115 miles today without incident. The sheath over my EGR tube is frayed and really black. If it continues to run fine for the next few hundred miles I'll be fairly certain the EGR tube is cracked. The next time I have the upper off I'll inspect it closer and repair if necessary. In the mean time I'll just leave the EGR off in the tune. If it acts up again I'll remove the oxygen sensor harness for closer inspection.
 

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Discussion Starter #8
I spoke too soon. Just started acting up. I notice the idle drop then looked at the laptop. Oxygen sensor dropped to .2 volts. Lambse froze at 14.7 for 15 seconds then fell to 10.8. I'm going to look at swapping oxygen sensors and the wiring harness for the sensor as well. Dang! Never easy.
 

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Its possible that the bad EGR tube has caused the engine to run too rich too often, and has fouled the o2 sensor. One of those "one part going bad, breaks a different part" scenarios that mechanics love to troubleshoot.
 

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Discussion Starter #10
I've got 70 miles with the O2 out of my 93 Lightning and no issues. Going to put about 150 miles on her tomorrow and hopefully they will be issue free.
 

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Discussion Starter #11
Ok new oxygen sensor an still having issues. As soon as I pull up to a stop the Lambse1 begins to drop from 15.0 to 10.8. The wb goes from switching properly to .15-.20 volts and won't raise. This is pulling down the Lambse because it thinks it's lean. Any thoughts on this? Wb is only reading the pass bank.
 

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Doing some reading, see if this makes sense to you. It could be a bad ground, or poor wiring to the heated side of your sensor.

4 Wire Hego's

It is imperative that the exhaust be completely sealed (no exhaust leaks) and that the HEGO's get a good clean and solid ground. The older 3 wire sensors do not have a dedicated
ground, they receive their ground through the exhaust pipe=not ideal. HEGO's were redesigned to include a 4th wire which is a dedicated ground. Most 92+ fords utilize a 4 wire
HEGO (excluding the foxbody).

One issue that can have you 'chasing your ass' is faulty heater power +12v. A HEGO with the heating element burnt out or not receiving heater power at all will typically result in the
hego reporting lean at higher loads where the excess exhaust flow has cooled the sensor. This is something to be mindful of, at WOT all HEGOs should be reporting rich 0.8v+, if they
do not and the HEGOs are reporting accurate then the engine is lean = not good under high loads.
ECU Hego Ground Pin

Henry thought it was wise to ground the hego through the ecu, some eec 4s (ALL 91- and older ecu's) have a hego ground pin that runs through the engine harness and grounds at the
back of the head or the firewall in that vicinity. I don't believe it was ever documented as to the specific drug(s) Henry's engineers were ingesting, but one can only infer they were
influenced by mind altering substances.
 

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I would do as I said earlier and pull the O2 Sensor lead and inspect, repair, or replace it. If theres a cut in the jacket where the wiring loom is clamped, it could intermittently ground to the engine.
 

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Discussion Starter #14
Going down the hwy it works flawless. Each and every time I roll to a stop it drops the hego to .15-.20 and the Lambse go to 10.8. It idles rough.
 

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Going down the hwy it works flawless. Each and every time I roll to a stop it drops the hego to .15-.20 and the Lambse go to 10.8. It idles rough.
Going back to your first post, your o2 swing was .28 to .45. Thats too narrow of a swing, and always on the lean side. You should be swinging something like .25 to .75. Ideal mix is .45, and you should swing above it and below it so that the average works out to .45

Your also dealing with literally one volt (ok, .1 to .9) for a signal. At that low of a signal, your connections and wiring are critical.

The o2 sensor makes it own voltage signal. If its not getting hot enough to operate correctly, it wont send the right signal. Thats where checking the heater wires is critical also.

You've swapped o2 sensors, so you can pretty much rule out the sensor itself, that leaves the wiring back to the computer. But lets not rule out why you had such a low swing to begin with. You may have an intake or vacuum leak somewhere.

Hook up your computer, and spray around the intake with carb cleaner and see if the o2 sensor reacts and goes heavy on the rich side. That would mean the engine is sucking in the carb cleaner somewhere.

Also, look at your injector commands. See what your pulse width and duty cycle are. Wouldnt hurt to get a shot of your short term fuel trims if your software will show you. Those trim numbers are what get changed by readings from the o2 sensor in closed loop mode.
 

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Discussion Starter #16
I found a leak at an exhaust weld 1" from the O2 bung. Tomorrow morning I'm going to weld a bung for the O2 on the driver side down pipe. If it works fine then I know it's something on the pass side. Actually I'm going to weld up the pinhole near the sensor first, then if that does nothing I'll move the sensor to the other side. If neither fix it then the wiring is coming out.
 

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Discussion Starter #17
I fixed the exhaust leak this morning and the issue still exists. It runs perfect then all of a sudden after idling for 15-20 secs the O2 bottoms out saying its lean and the Lambse fall to 10.8. I'm moving the O2 to the driver side down pipe today. Maybe it is something like a lower intake leak on the pass side only. If it still acts up on the DS too then vac leak at the upper, bad wiring, or bad ECM .
 
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