Bad O2's = Detonation?

misl302

Member
Apr 29, 2003
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WI
Hey All,
Started to notice some pinging after installing LT headers. Happens when loading the engine at low rpm. My O2 sensors are likely original with 68K miles. They've been through many on-road/off-road x/h-pipe swaps (darn emissions tests) and now also have extensions due to LT's. Timing is 14° BTDC, fuel press has been anywhere from 36-42 psi, but no difference in pinging. Close to stock compression ratio.

Any ideas?
 
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Long tube headers move the O2 sensors back from the optimum position to a location further back on the pipe. That means they tend to run cooler and are less efficient.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
“Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage.

When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.” End Quote

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.


A non functioning EGR will also cause a low RPM/cruse range ping. My stang has been doing it for a while and I haven’t gotten around to fixing it yet.
 
Good advice above. It probably is simply the new location of the O2 sensors in the exhaust gas stream (further downstream) causing the problem, but if not, and the sensors check good, start looking at the air entering the engine.

One other consideration is that if you have the room and skill to do so, consider moving the O2 sensor bungs to a position in the tubes similar to their "old" position (closer to the engine), and cap off the original holes in the LTs. This may alleviate your problem as the O2 sensors will then heat properly and relay "more correct" relative information to the computer for adjustment of the profile.

In line with the EGR-related issue noted, the possibility of a vacuum problem, or incorrectly functioning PCV system may also be causing the computer to feel the engine is running rich, or retarded, and cause the pinging, as well. I replaced the O2 sensors on the wife's '98 Expy (4 total) because the computer was interpreting the O2 sensors' inputs as rich, causing leaning, and subsequent pinging. It turned out that the PCV hose rubber on the back of the IBV was rotted, had pinched, and was allowing too little air to enter the system via the PVC port, causing the lean condition and pinging. 200 bucks for O2 sensors and 2 very bloody knuckles, when all I needed was a 50 cent hose. Ah, computers....
 
Thanks for all the feedback everyone, I really appreciate it. I also suspect it is the new, further downstream location of the O2's. Without the collector geometry fresh in my memory, I would think BBK would have put the O2 bung as far upstream as possible to avoid this problem.

Hissin50, I'm not sure I've experimented with full WOT since I get a little nervous when the detonation occurs. I'll give it a try next time out (could give me an excuse for that new stroker shortblock!). So, correct me if I'm wrong. At WOT the O2's have no influence. Therefore, if the detonation still occurs at WOT then it is NOT due to bad O2's.
 
misl302 said:
So, correct me if I'm wrong. At WOT the O2's have no influence. Therefore, if the detonation still occurs at WOT then it is NOT due to bad O2's.
That was kind of where I was going. Now I would not go out and stomp on it like it is - I was just asking in case you had taken it to WOT.

Good luck.
 
All I have to add is that the last time My O2's went bad I hooked up the diagnostics computer and seen the 02's were in a closed loop making the car run rich as snot... so rich it ran like a disiel tractor or like it had the worlds largest cam in it... Anyways, I have always thought that when 02's fail they just send the engine signals to run really rich as a sort of insurance policy... I could be wrong there but I have seen them do it with my own eyes...
 
Did some WOT experimenting yesterday. At WOT the pinging goes away. Hmm...what does that mean? Does it mean the O2's are reading too rich and telling the ecu to go leaner?
 
misl302 said:
Did some WOT experimenting yesterday. At WOT the pinging goes away. Hmm...what does that mean? Does it mean the O2's are reading too rich and telling the ecu to go leaner?
More likely that you have the same problem I do - a poorly functioning EGR that isn't opening enough at cruse. The EGR gases burn slower than normal, so the computer adds more timing. No EGR gases and you get a part throttle ping at cruse and light to medium acceleration.
 
Thanks jrichker. I'll have to check out the EGR then. Don't know too much about it. I assume it's a matter of replacing the EGR valve. Any idea how much $? Although I still wonder about the LT O2 sensor location since the pinging was noticed after LT install. If that is the problem then I guess the only remedy is to move the O2's upstream. Not too crazy about that.
 
Some basic theory to clarify how things work is in order…

The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine to ping. It can also reduce HC emissions by reducing fuel consumption.

The EGR system has a vacuum source (line from the intake manifold) that goes to the EVR, computer operated electronic vacuum regulator. The EVR is located on the back of the passenger side shock strut tower. The EGR valve and the passages in the heads and intake manifold route exhaust gas to the EGR spacer (throttle body spacer). The computer uses RPM, Load. and some other factors to tell the EVR to pass vacuum to open the EGR valve. The EGR sensor tells the computer how far the EGR valve is open. Then computer adjusts the signal sent to the EVR to hold, increase or decrease the vacuum. The computer adds spark advance to compensate for the recirculated gases and the slower rate they burn at.

Diagram courtesy of Tmoss & Stang&2birds.
mustangFoxFordVacuumDiagram.jpg


The EGR sensor is basically a variable resistor, like the volume control on a radio. One end is 5 volt VREF power from the computer (red/orange wire). One end is computer signal ground (black/white), and the middle wire (brown/lt green) is the signal output from the EGR sensor. It is designed to always have some small voltage output from it anytime the ignition switch is the Run position. That way the computer knows the sensor & the wiring is OK. No voltage on computer pin 27 (brown/lt green wire) and the computer thinks the sensor is bad or the wire is broken and sets code 31. The voltage output can range from approximately .6-.85 volt.

EGR test procedure courtesy of cjones

to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 5in vacuum?

if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500 RPM.
should read about 5in vacuum
 
I Like JR's EGR idea (I had a light ping at very low load in high gear on the '94 myself. Unfortunately the EGR was relatively clean and flowing well. :bang: ).

If all else fails, I would make sure the O2's are switching alright. They might not be out of spec but can cause a lean condition from a lack of calibration. And as mentioned, their input is disregarded at WOT...........

Good luck.