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Discussion Starter #1
So, on my wife's 94 mercury grand marquis I cleaned the IAC, MAF, EGR Valve and EGR silnoid (?) and the car runs beter thanit has in years. However it still did the engine rock idle fluctuation, durring reving the engine in the drive way I heard this rattleing sound. I turn the car off block the tires and take a peak. The drives side tail pipe seemed to be lower than the passenger side. I grab a hold of the driver tail pipe and it rotates 90 degress. The weld is broken right at the front part of the muffler. Now my question is, durring driving the car, would unequal back pressure cause the engine to idle funny at a stop and cause the CEL to come on durring acceleration? or vise versa? I know I have to fix it, just if this car isn't going to pass smog I don't wanna dump any more money in to it.
 

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funderbird said:
So, on my wife's 94 mercury grand marquis I cleaned the IAC, MAF, EGR Valve and EGR silnoid (?) and the car runs beter thanit has in years. However it still did the engine rock idle fluctuation, durring reving the engine in the drive way I heard this rattleing sound. I turn the car off block the tires and take a peak. The drives side tail pipe seemed to be lower than the passenger side. I grab a hold of the driver tail pipe and it rotates 90 degress. The weld is broken right at the front part of the muffler. Now my question is, durring driving the car, would unequal back pressure cause the engine to idle funny at a stop and cause the CEL to come on durring acceleration? or vise versa? I know I have to fix it, just if this car isn't going to pass smog I don't wanna dump any more money in to it.
If the exhaust is stock, then the exhaust comes together at the 3rd cat and backpressure would be equalized at that point. Now I guess what could happen is that there could be some restriction in either side of the exhaust before it comes together.

It is entirely possible that an obstruction in the exhaust could cause a rough idle and CEL on acceleration. Since the CEL came on, did you get the codes read? This will tell you a lot right there. I'd find out the code before you consider throwing any money at it, besides fixing the weld at the muffler of course.
 

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No, ya'all don't understand what I mean. The Grand Marquis has true dual from motor to tail pipe with no cross over. It comes stock in the 1994. I thought the unequal pressure would cause the problems I wrote about before...
 

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I'm sorry I didn't even pay attention to the fact that you said a Grand Marquis. I was looking by your avatar. Anyway, with true dual exhaust, yes it is quite possible that an obstruction that causes increased backpressure on one side can cause problems. But the problem wouldn't only be at idle, it would be constant. And over time you would notice slightly different characteristics with the spark plugs on each side of the engine.

Now, I still say you need to find out what the code is to cause the CEL to come on. I do believe that severe enough unequal backpressure could cause a somewhat rough running condition, but I see no connection to the CEL during acceleration.
 

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Discussion Starter #7
I do have a question, which exhaust manifold is the EGR taken off? That may be the problem if it is off one side or another...
 

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You know I was thinking about the EGR and how the backpressure transducer may be setting the check engine light, but honestly on that car I don't know which side the EGR comes off of. From what I've seen, it's usually the passenger side manifold but I very well could be wrong. Any problem that I've ever seen with EGR has been from not having enough backpressure. I've actually never encountered having too much, so I don't know if it could be the cause.

However the codes you presented could very well be the problem.

118 - Engine coolant temperature sensor above maximum voltage / 0-40°C indicated

This means the circuit is open. Either the sensor is unplugged or there is a break in the wiring somewhere. This sensor is critical to how the engine runs so if the computer is not getting the right inputs, the amount of fuel delivered isn't going to be correct. And this could easily cause a rough idle and CEL during acceleration, although if this were unplugged I would think the CEL would be on constantly.

122 - Throttle Position sensor below minimum voltage

This could mean the sensor has gone bad. It's hard to say, but I'd check the wiring to the sensor first before you consider replacing it. Again, this sensor input is critical for running right and could cause a rough idle. This may be why the CEL comes on during acceleration. The computer may be expecting a higher voltage from the TPS for the measured volume of air entering the engine.

332 - Insufficient EGR flow detected

You've got the EGR code, but the coding saying it is insufficient flow doesn't ring any bells in my head in connection to too much backpressure. Perhaps someone else may have experienced a connection to this, but I haven't.
 
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