Qustion about a broken part I found.
10-05-2010, 03:41 PM
#1
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Join Date: Apr 2005
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Posts: 602
Qustion about a broken part I found.
What is the black plastic part that sits on the drivers side fuel rail, it has a vacuum line and an electrical connection going to it. The reason I ask I noticed today the vacuum line was broke off and sucking some air thru the line. I don't know how long it's been like this, the car seemed to run fine. Did I hurt anything from this being off.
10-05-2010, 04:38 PM
#2
2nd Gear Member
Join Date: Jul 2008
Location: Billings, MT
Posts: 462
yes, that is the fuel rail pressure sensor, and if the vacuum line is broken off, the PCM is reading actual pressure, not delta pressure, and it will not be providing enough fuel, which will make the fuel trims work overtime to compensate to keep the AF ratio right. You will probably throw a too lean code if you get on the throttle hard when the engine is cold with it this way.
Definately want to get this part replaced. I think they cost around $100-140..
*edit - I see you have a supercharger, so it is even more important because you will be going lean when not in boost and rich when in boost, and the o2 sensors will be compensating by throwing the long term fuel trims back and forth.. this can cause all kinds of issues. The injectors are rated for the flow at 39-40 psi of fuel, so if the PCM doesn't "see" manifold pressure (from the vacuum line), it will assume you have no vacuum or boost and deliver 40 psi.. so under vacuum, the "delta" pressure, or the effective pressure pulling the fuel out of the injectors will be 48 psi, which will mean that the injectors feed too much fuel. Under boost, the opposite happens, and not enough fuel pressure will exist, so you could lean out. The PCM will adjust as it can, but if it tries to adjust more than 25% for more than a few seconds, it will throw a code. At 20 in-hg of vacuum or 8 psi of boost, you are right at the edge of that 25%, so you may very well throw a code, particularly when the engine is cold and needs more fuel anyway..
Definately want to get this part replaced. I think they cost around $100-140..
*edit - I see you have a supercharger, so it is even more important because you will be going lean when not in boost and rich when in boost, and the o2 sensors will be compensating by throwing the long term fuel trims back and forth.. this can cause all kinds of issues. The injectors are rated for the flow at 39-40 psi of fuel, so if the PCM doesn't "see" manifold pressure (from the vacuum line), it will assume you have no vacuum or boost and deliver 40 psi.. so under vacuum, the "delta" pressure, or the effective pressure pulling the fuel out of the injectors will be 48 psi, which will mean that the injectors feed too much fuel. Under boost, the opposite happens, and not enough fuel pressure will exist, so you could lean out. The PCM will adjust as it can, but if it tries to adjust more than 25% for more than a few seconds, it will throw a code. At 20 in-hg of vacuum or 8 psi of boost, you are right at the edge of that 25%, so you may very well throw a code, particularly when the engine is cold and needs more fuel anyway..
Last edited by dkersten; 10-05-2010 at 04:51 PM.
10-05-2010, 07:42 PM
#3
3rd Gear Member
Thread Starter
Join Date: Apr 2005
Location:
Posts: 602
yes, that is the fuel rail pressure sensor, and if the vacuum line is broken off, the PCM is reading actual pressure, not delta pressure, and it will not be providing enough fuel, which will make the fuel trims work overtime to compensate to keep the AF ratio right. You will probably throw a too lean code if you get on the throttle hard when the engine is cold with it this way.
Definately want to get this part replaced. I think they cost around $100-140..
*edit - I see you have a supercharger, so it is even more important because you will be going lean when not in boost and rich when in boost, and the o2 sensors will be compensating by throwing the long term fuel trims back and forth.. this can cause all kinds of issues. The injectors are rated for the flow at 39-40 psi of fuel, so if the PCM doesn't "see" manifold pressure (from the vacuum line), it will assume you have no vacuum or boost and deliver 40 psi.. so under vacuum, the "delta" pressure, or the effective pressure pulling the fuel out of the injectors will be 48 psi, which will mean that the injectors feed too much fuel. Under boost, the opposite happens, and not enough fuel pressure will exist, so you could lean out. The PCM will adjust as it can, but if it tries to adjust more than 25% for more than a few seconds, it will throw a code. At 20 in-hg of vacuum or 8 psi of boost, you are right at the edge of that 25%, so you may very well throw a code, particularly when the engine is cold and needs more fuel anyway..
Definately want to get this part replaced. I think they cost around $100-140..
*edit - I see you have a supercharger, so it is even more important because you will be going lean when not in boost and rich when in boost, and the o2 sensors will be compensating by throwing the long term fuel trims back and forth.. this can cause all kinds of issues. The injectors are rated for the flow at 39-40 psi of fuel, so if the PCM doesn't "see" manifold pressure (from the vacuum line), it will assume you have no vacuum or boost and deliver 40 psi.. so under vacuum, the "delta" pressure, or the effective pressure pulling the fuel out of the injectors will be 48 psi, which will mean that the injectors feed too much fuel. Under boost, the opposite happens, and not enough fuel pressure will exist, so you could lean out. The PCM will adjust as it can, but if it tries to adjust more than 25% for more than a few seconds, it will throw a code. At 20 in-hg of vacuum or 8 psi of boost, you are right at the edge of that 25%, so you may very well throw a code, particularly when the engine is cold and needs more fuel anyway..
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