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Old 03-04-2011, 11:10 PM   #11
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Re: VX exhaust manifold unique

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Originally Posted by add|ct View Post
This got me thinking imz, I'll probably need to pull the shield away to access the manifold, but I remembered seeing rust around where it's exposed at the LAF on my vehicle.
I'm sure most manifolds are rusted by this age and so on my car, I hope to get a video camera, record the scan gauge's data and use it as a before and after of the GPH, steady engine RPM, etc. My theory is, that occasionally, my car won't get a good signal from the o2 sensor so it enters failsafe mode which causes the engine to rev to around 750rpm instead of 670rpm..
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Old 03-05-2011, 04:48 AM   #12
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Re: VX exhaust manifold unique

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My theory is, that occasionally, my car won't get a good signal from the o2 sensor so it enters failsafe mode which causes the engine to rev to around 750rpm instead of 670rpm..
If it's staying in closed loop then you can definitely rule that out. If it's in open loop when that happens then it's a good possibility. Do you watch open/closed loop on the SG?
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Old 03-05-2011, 07:42 PM   #13
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Re: VX exhaust manifold unique

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If it's staying in closed loop then you can definitely rule that out. If it's in open loop when that happens then it's a good possibility. Do you watch open/closed loop on the SG?
No, it's in closed loop. "Signal Noise" is certainly a normal part in any electronic circuit, but when you have a rusted out manifold, any ground the o2 sensor has to the car would be very weak. If you read the helms manual, it mentions that when the computer receives erroneous data, it goes into "failsafe mode". This isn't necessarily going to trigger a check engine light. A lot of "bad signals" from the o2 sensor could still be within range and therefore there will be no fault code. I mean think of it like this, the difference between someone playing music on the telephone vs. a compact disc.

The only reason I know that the o2 sensor uses the manifold as a ground for its signal is that when I was testing out an o2 sensor, in order to get a signal on the multimeter, one probe was on a wire on the harness and one was on the body of the o2 sensor itself. Spark plugs use the body of the vehicle to complete the circuit, so it's not a stretch for an o2 sensor to do the same.
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Old 03-05-2011, 07:59 PM   #14
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Re: VX exhaust manifold unique

Would one of the wires in the connectors help ground it?
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Old 03-05-2011, 08:19 PM   #15
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Re: VX exhaust manifold unique

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Would one of the wires in the connectors help ground it?
Just think of it like this. You have a spark plug.. Notice how you have only one wire running to the spark plug? But how is that possible if a circuit has an incoming and outgoing wire, or a point where you hook up the + and the -? That's because the part where you plug onto the spark plug is the + and the - on the spark plug is the body of the spark plug itself, which is screwed into the valve head which is on the engine which is grounded to the chassis. The O2 sensors on the manifold are heated, so they have more than one wire. I believe the o2 sensor to verify the catalyst efficiency has only a single wire. Some older vehicles also use a single wire o2 sensor for the manifold as well. Single wire o2 sensors are not heated and are narrow band. 4 wire o2 sensors are still narrow band o2 sensors but with a heater element to get them working asap. 5wire (really 7 wire I think) are wideband, heated o2 sensors. Nonetheless, they all utilize the body of the sensor itself to complete the circuit which sends the signal to the computer in regards to the Air Fuel Ratio. I believe the heater circuit is the only one that has a positive and negative wire coming out of the o2 sensor, possibly so that it doesn't interfere with the signal of the o2 sensor.

It's possible all this is an anachronism to the days of unheated o2 sensors and that they don't want to change the way they send the signal to the computer. For a more reliable signal, it would obviously make sense to just run dedicated + and - wires to the O2 sensor instead of using the body of the sensor itself to complete the circuit. They run dedicated + and - wires for the heater circuit but not for the signal part of the o2 sensor which is why a rusted manifold can certainly cause signal issues.

Most guys who work on cars are not very interested in the electronics part of the car and aren't able to infer repairs unless they're specifically told to do so. This is why if you ask most car guys what it means if a car battery is at 0V, they'll reply "it's dead, you need to replace it" when in fact, it's not dead, just severely discharged. The reason they think batteries with extremely low or no voltage are permanently "dead" is because battery chargers will refuse to charge batteries with a very low voltage. There are two reasons for this: 1. They don't want you charging 6v batteries with a 12v charger.. 2. If the voltage is extremely low, for all the charger knows, you don't have the battery charger hooked up at all and so you don't want the charger attempting to charge with 10amps+ of electricity when it isn't hooked up to a damned thing now do you? To get around this issue, just put a good battery with jumper cables in parallel to the bad battery. Hook up the battery charger and charge at a low amperage like 2amps. In about 12 hours, you can probably disconnect the good battery and just run the battery charger on the discharged battery itself and charge from there.
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Old 03-14-2011, 09:26 PM   #16
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Re: VX exhaust manifold unique

So I guess I shouldn't put on this new NGK 24300 oxygen sensor until I get a new manifold? That would suck because the 96 dorman cat/manifold assembly is 200 dollars. I'm spent out at this point.

I just want to fix it right the first time. I'm just not 100 percent sure that the 96 cat/manifold combo will work to the exact same specs as the 92-95 Vx cat. I know they bolt up, I just wasn't sure if they will perform the same.
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Old 03-15-2011, 12:31 AM   #17
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Re: VX exhaust manifold unique

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So I guess I shouldn't put on this new NGK 24300 oxygen sensor until I get a new manifold? That would suck because the 96 dorman cat/manifold assembly is 200 dollars. I'm spent out at this point.

I just want to fix it right the first time. I'm just not 100 percent sure that the 96 cat/manifold combo will work to the exact same specs as the 92-95 Vx cat. I know they bolt up, I just wasn't sure if they will perform the same.
why not try getting rid of the rust on your existing 92-95 manifold and see how things work. Do you have this '96 manifold or not? If you have a new manifold, just install it with the o2 sensor you have, if you don't, then just remove the rust from your existing manifold and install the new o2 sensor.

A "rusted out car" isn't going to make the catalytic converter fall apart and get into the muffler system. That would only happen if you crushed the catalytic converter with blunt force or something else.. If you're worried about rust showing up in your exhaust but don't feel like spending money to get a new manifold, then I suggest removing the existing manifold/catalytic converter, take out the part with the catalytic converter and then try removing the rust from the manifold. Let me know if you want ideas on how to remove the rust.
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Old 03-15-2011, 05:05 AM   #18
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Re: VX exhaust manifold unique

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A "rusted out car" isn't going to make the catalytic converter fall apart and get into the muffler system. That would only happen if you crushed the catalytic converter with blunt force or something else..
I agree that external rust isn't going to do it, but there are other ways that a catalytic converter gets broken up and clogs the exhaust than just blunt force trauma. Breaking up and clogging is probably the most common failure for catalytic converters.
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Old 03-15-2011, 02:38 PM   #19
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Re: VX exhaust manifold unique

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I agree that external rust isn't going to do it, but there are other ways that a catalytic converter gets broken up and clogs the exhaust than just blunt force trauma. Breaking up and clogging is probably the most common failure for catalytic converters.
While that's true, that wouldn't happen just because op had the car parked for a long period of time. Unless he has been chronically running the car rich, this wouldn't happen.
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Old 03-31-2011, 12:32 PM   #20
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Re: VX exhaust manifold unique

Well I finally got around to running a ground to the O2 sensor on the exhaust manifold. Good news is, the idle is more stable, sticking with around 670 slightly edging up to 700 if the engine gets hot but when things cool down, it goes back to around 670. This is in contrast with before where I'd notice the car idle at 670 but then randomly the idle would jump to 750. I also noticed that after driving long periods of time and being stuck at a stop light, the idle wouldn't drop to 670 but would idle at 750. The only time I could get the car to idle at 670 was if I let the car warm up gently.

However, while working on the separate ground for the o2 sensor, I figured I'd take off the heat shield and take a look around, it turns out I have a cracked manifold and the crack is right at the spot where the o2 sensor is. I don't know if I told anyone on here this but before I added the ground to the o2 sensor, I used an ohm meter and measured the ohms from the o2 sensor to the battery. I noticed on a cold engine, the ohms was lower than when the exhaust manifold was heated up and on an engine that is heating up (while running) I can slowly see the ohms drop.. This might explain why running a separate ground wire to the o2 sensor would explain a more stable idle since there is now a more steady and clean signal. However the real issue at hand is that having a crack exhaust manifold could be letting in cold air (or so I have been told) and that could make the car read lean, dumping in more fuel. I was wondering why I could smell some fumes from the engine bay but then it would dissipate.
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