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Discussion Starter · #1 ·
Hi everyone,

1997 Ford Taurus 3.0 OHV. I recently completed a major overhaul of several systems (engine rebuild top to bottom, fuel system front to back, vacuum refit and replaced lines, full suspension, brakes...). Aside from the recommended break-in for new rings, I went on the first serious run after having replaced front bearing hubs to be rid of a serious bearing noise. On that run, I hit the dollar mark without much issue and I must say it picked up speed remarkably well. Idles well cold and hot, open loop and closed loop.

Since then I've been test driving it far more moderately the last few days. Today it did something really interesting. I was driving along at highway speeds. As soon as I exit and come to a halt to round a corner, the engine wouldn't hold in the power band for any significant amount of time at all and tends to stall out. The funny thing is that if I pop it into neutral, kill the engine and start it back up again, it goes back to normal function and drives for a few miles before doing the same thing over again. At one point it registered a code for cyl 5 misfire but I figured that the result of all the fuel issues going on because of poor combustion. I cleared the code and it hasn't come back yet with about 20+ miles driving.

Before the rebuild the ODO logged about 167k miles. The first thing I suspected was a severely clogged cat system. There's another issue that looks like a post-TB intake leak but that seems doubtful as root cause because of the whole stop and start, back to normal behavior.

Any tips for testing the cat system without a backpressure tester? For what its worth, here are some screenshots of the OBD scanner on the O2 sensor voltage readings.

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Any insight is much appreciated, - Troy
 

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It almost goes without saying that the missing vacuum hose in your other post causes this. It might be that without this hose there is not even a usable vacuum for the engine to work with. As Jeff K explained the other day this specifically causes problems when the car comes to a stop.
 

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Also, your idle RPM is rather high at an engine temperature of 216 F and that might have the same cause: the open tube sucks in air, bypassing the IAC. I guess that if you put your finger on the tube the engine initially almost stalls and then quickly recovers at a lower RPM, proving the IAC works.

That last idea is exceptionally stupid: you will surely burn your fingers touching the EGR value.
 

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The B2S2 O2 sensor voltage is WAY low. Voltage should hold around .4 V to .5 V under all conditions, just like your B1S2 sensor is showing.

If the B2 cat was failing or dead B2S2 voltage would follow the B2S1 sensor voltage and switch from .1 V to .8 V every second or 2.

What are the LTFTs on both banks at idle and driving at say 40 mph (i.e., engine under load)?
 

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The B2S2 O2 sensor voltage is WAY low. Voltage should hold around .4 V to .5 V under all conditions, just like your B1S2 sensor is showing.

If the B2 cat was failing or dead B2S2 voltage would follow the B2S1 sensor voltage and switch from .1 V to .8 V every second or 2.

What are the LTFTs on both banks at idle and driving at say 40 mph (i.e., engine under load)?
I can't make the importance of "pending" codes, which do set a light. If you do not use a reader that keeps "pending' you miss the mark.
Recent '05 with horrible misfire when warm. No codes. In "pending" misfire #3. Used coil, fixed the issue. No idea why a really bad issue will not set a code.
-chart-
 

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Discussion Starter · #6 ·
Thanks, everyone, for the replies. I did have the port plugged, but I'm going to look for that hose and try connecting it, running it again. The car seemed to run acceptably well for a while.

I'll come back with my findings.

-Troy
 

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Discussion Starter · #7 ·
I can't make the importance of "pending" codes, which do set a light. If you do not use a reader that keeps "pending' you miss the mark.
Recent '05 with horrible misfire when warm. No codes. In "pending" misfire #3. Used coil, fixed the issue. No idea why a really bad issue will not set a code.
-chart-
Thanks. Can you recommend a reader that takes note of pending codes? I use a cheap bluetooth version that relies on Torque Lite. I do not believe Torque captures pending codes. Or am I mistaken?

-Troy
 

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Discussion Starter · #8 ·
So an update...
I got the new hose and connected it between the intake manifold and the device sitting on the passenger side. The engine starts up from cold like it did before. Idle is stable at roughly the same level. The O2 sensors now read in the range of 0.6 to 0.7.

However, the car will run this way for a little while maybe 7 to 10 miles before the engine sputters. The whole off, on trick still works to regain driveability. Here's a screenshot of the sensor voltages under the new conditions.

Am I correct in saying that there is a point where the system transitions between open and closed loop and that's likely where I'm seeing the performance change?

Screenshots below. The first with the steady bank 2 readings is cruising at about 40 mph. The second is coming to a stop.

I appreciate all the input. - Troy
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The engine would typically go into Closed Loop within 1 minute after a cold start. That's when the O2 sensors are heated enough to give a correct signal. You see that happening because the RPM goes down under 1000 RPM (cold engine).

The status of the Injection System is OBD-II Live Parameter PID 03 that should switch from 0 (Open Loop) to 1 (meaning Closed Loop).

In that state the upstream sensors (1x1 and 2x1) should really switch up and down every 2 seconds like Jeff K said. The second picture in your first post is exactly what you want.
 

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Discussion Starter · #10 ·
So I've checked PID 03. Thankfully Torque reports out that PID and you are correct, I'm definitely in closed loop well before any of this trouble starts. Here are some screenshots showing fuel trim and O2 bank voltages as the car starts to limp along. The interesting feature of all this is that all of the O2 voltages go dead as the car sputters. Could a massive exhaust leak cause this? Extremely high backpressure due to broken down cat converter? Extremely low back pressure due to broken down muffler?

I feel as though the cat system is working as evidenced by the fact that I'm clearly in closed loop mode and the O2 bank 1's are cycling while the O2 bank 2's are holding relatively steady (after connecting that vacuum hose).

This tends to lead me away from the emissions control system and toward something else. What exactly I'm not sure because I don't get intuitively what system failure could cause the car to run acceptably well for the first so many miles then suddenly lose power like that.

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Discussion Starter · #11 ·
Solved it.

It seemed like the vacuum issue was two-fold. The cap I had on the intake post-TB wasn't perfect so it leaked. Also, the device that connected to intake vacuum was rendered ineffective and its an emissions control device - in other words influences A/F ratio.

Today after running it in idle in the garage for a little bit, I took it out for a run and the stalling condition completely went away. Now, the engine will peter out at a certain load. How I take this is that the engine load is proportional to the extent of combustion products flowing through the engine, or in other words a certain extent of catalytic converter loading. Something interesting to note is that the O2 sensor voltages upstream and downstream all bottom out when the engine peters out at a certain load percentage. I take this to mean that the EFI system delivers fuel relative to the MAF sensor. When that combustion product reaches the O2 sensor, the soaked cats can't covert a large portion of the remaining fuel and the downstream sensor then reads super rich which leads the ECU to think that it must drastically cut A/F ratio, hence the overall bottoming out of O2 sensor voltages.

Previously under the vacuum leak, the ECU falsely determined there to be a lean condition due to the excess air and flooded the engine with fuel. This impacted the spark plugs and the catalytic converter I'm fairly certain. I am somewhat floored that the O2 sensors still read although their accuracy at this point is a toss-up.

When the cats flooded with fuel, it took a while for them to clear up. Bear in mind, I ran under the artificially rich condition for about 150 miles, burning roughly 11.4 gallons of fuel which is extreme.

My bet is that I will regain a certain percentage of catalytic converter function over the course of time. My initial run recently yielded highway MPG of 30.4 in one direction and 27.8 in the reverse direction for an average of 29.1 mpg overall. This is definitely a good result, but before the overhaul and the oil burning issues that started all of this, I used to get anywhere in the range of 33 to 35 mpg highway, so I know there are some losses due to fouling and loading on the cats causing inaccurate downstream O2 sensor readings.

So I have an appointment tomorrow with the muffler shop to replace the muffler because it sounds terrible and has several holes in it. I also acquired the cat assembly while I was at it. My question for you guys at this point is, should I hold off on the cat replacement since the O2 sensors are questionable or is it likely that the O2 sensors are unimpacted by the artificially lean, rich running condition?

I appreciate all the input so far. This is a great forum.

-Troy
 

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Now, the engine will peter out at a certain load.
What load exactly? Does it also do that above a certain RPM? You revved up to 3400 before. So this is no longer after 7 or 10 miles?
Something interesting to note is that the O2 sensor voltages upstream and downstream all bottom out when the engine peters out at a certain load percentage.
I'd say the injectors are switched off for some reason. Then the engine loses power and the O2 sensors report "lean".
Bear in mind, I ran under the artificially rich condition for about 150 miles, burning roughly 11.4 gallons of fuel which is extreme.
That is an MPG of 13.2. The hose that I erroneously called a vacuum hose indeed is a EVAP hose and this means that the gas vapors disappeared into thin air instead of being fed back to the engine. That is bad for mileage, but this is indeed extreme.
I used to get anywhere in the range of 33 to 35 mpg highway, so I know there are some losses due to fouling and loading on the cats causing inaccurate downstream O2 sensor readings.
Since your mileage used to be pretty good, I'd say the injectors are not causing this problem. The cats might perhaps cause inaccurate downstream O2 sensor readings, but these readings have no effect on the running of the engine, so in first instance they can be ignored.
 
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