Ira
Well-Known Member
- Joined
- October 10, 1999
- Messages
- 368
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- City, State
- Northeast Tennessee
- Year, Model & Trim Level
- 91 XLT 4x4
This information is primarily for the earlier models since that is what I am becoming familiar with. I just wanted to share this info so that you may not have the headaches like I have.
I have been plagued with the famous Check Engine Light off and on for about 3 months now. The fuel economy also seemed to suffer intermittently. I have just replaced 3 oxygen sensors in the last 2 months on mine. I know you are wondering why. The first one I replaced was bad. I know this because as soon as I put it in and drove it, 10 minutes later, the light came on again. I made a test harness and plugged it into the O2 harness and found I was not getting a changing signal like I should. I took it back and replaced it. The light came on once with the second new one and that was the first day. I later checked it and it was giving the voltages that it should. About 2 weeks ago, the light started coming on again but not everyday. I got mad because I figured I had another defective sensor so I took it back to AutoZone and they refunded my money. I told them I had better things to do than keep climbing under there and changing sensors out. I went to Ford and bought one from them. About 2 days later, the light came on again. Did I mention that I had checked the codes before and had one for a bad O2 sensor at first? After I changed it, I never got that code again. It was just saying the mixture was too rich or too lean.
A couple of weeks ago, while sitting at the drive-up at my bank, I noticed the engine would surge at idle. It would go up 200-300 RPM and then back down. It did this continuously until today. I got my fuel pressure gauge out and checked my pressure again, like I have a couple of times since the O2 deal was going on. It is supposed to read around 30-45 at idle. It was reading 38 just like it has been before. Suddenly, it jumped up to 70 and fluctuated and never dropped back down. When this happened, the idle speed surged up and down. Before doing this, I disconnected the vacuum line from the fuel pressure regulator and had NO fuel coming out of it since I suspected it was letting fuel dump back into the engine thru this line. I thought it was OK because it was dry. When I saw the pressure jump, I replaced it. Now I have more power and engine is idleing smoothly, even at 170K miles. When I restarted it after replacing the FPR, I floored it once and covered the ground with carbon that blew out of the tailpipe.
Moral of the story...just because it may test to be good, it may not be. I was just lucky to get it to finally surge while I had the meter on it. Since the early models, like mine, do not have an engine code specifically for the FPR, the O2 sensor is many times the code you will get or you will get the ones that say rich mixture or lean mixture.
Another thing to consider is that when the FPR does go bad and stick like mine did, over a short period of time, the O2 sensor can in a sense be "fouled out" by the carbon from the excess fuel being burned and cause it to read as being bad. That is what I think happened. If you pull your O2 sensor out and the end is very black, do yourself a favor and replace the FPR first before replacing the O2 sensor. If the FPR was at fault, after replacing it, the exhaust may possibly get hot enough to burn the carbon off the O2 sensor and it will start working.
Been there...done that.
BTW, my check engine light would not come on until after driving for 10-15 minutes at least. I think what happened at AutoZone was that I did in fact have a defective O2 sensor at first and the second one went out because of the FPR.
From what I understand about the way the FPR works, fuel is supplied to one fitting, vacuum is supplied to another fitting, and it has a third fitting that goes into the fuel rail (fuel at regulated pressure). As fuel is supplied from the fuel pump to the inlet fuel fitting, it is under high pressure. The purpose of the regulator is to keep a constant lower pressure (30-45 PSI) supplied to the engine, or fuel rail. As the engine RPM increases, the vacuum will compensate for the extra fuel needed or used by the engine and keep it regulated.
From what I have heard and have figured out on my own, either of 2 things can happen causing it to fail. The first is that the FPR will leak fuel internally from the high pressure side thru the vacuum and this fuel will cause the engine to run RICH. The second thing is that the regulator will stick (which is what happened to mine) and dump high pressure fuel into the fuel rail. It is possible that when it sticks like mine did, it can be intermittent. It can stick for a few seconds, minutes, or even days and then decide to loosen up and work correctly again. This is why my fuel economy seemed to be up on one tank and down on another tank when the same conditions were present on each tank (highway/city combined mileages were similar on each trip/tank).
I just wanted to post this to try and help out other people with the same problem. I know there are plenty of other posts on the subject, but none of them seemed to directly describe what was happening with mine.
Good luck and hopefully this will prevent you from getting the headaches like I had.
------------------
Ira
91 XLT 4x4
Once I thought I was wrong, but I was mistaken.
[This message has been edited by Ira (edited 03-23-2000).]
I have been plagued with the famous Check Engine Light off and on for about 3 months now. The fuel economy also seemed to suffer intermittently. I have just replaced 3 oxygen sensors in the last 2 months on mine. I know you are wondering why. The first one I replaced was bad. I know this because as soon as I put it in and drove it, 10 minutes later, the light came on again. I made a test harness and plugged it into the O2 harness and found I was not getting a changing signal like I should. I took it back and replaced it. The light came on once with the second new one and that was the first day. I later checked it and it was giving the voltages that it should. About 2 weeks ago, the light started coming on again but not everyday. I got mad because I figured I had another defective sensor so I took it back to AutoZone and they refunded my money. I told them I had better things to do than keep climbing under there and changing sensors out. I went to Ford and bought one from them. About 2 days later, the light came on again. Did I mention that I had checked the codes before and had one for a bad O2 sensor at first? After I changed it, I never got that code again. It was just saying the mixture was too rich or too lean.
A couple of weeks ago, while sitting at the drive-up at my bank, I noticed the engine would surge at idle. It would go up 200-300 RPM and then back down. It did this continuously until today. I got my fuel pressure gauge out and checked my pressure again, like I have a couple of times since the O2 deal was going on. It is supposed to read around 30-45 at idle. It was reading 38 just like it has been before. Suddenly, it jumped up to 70 and fluctuated and never dropped back down. When this happened, the idle speed surged up and down. Before doing this, I disconnected the vacuum line from the fuel pressure regulator and had NO fuel coming out of it since I suspected it was letting fuel dump back into the engine thru this line. I thought it was OK because it was dry. When I saw the pressure jump, I replaced it. Now I have more power and engine is idleing smoothly, even at 170K miles. When I restarted it after replacing the FPR, I floored it once and covered the ground with carbon that blew out of the tailpipe.
Moral of the story...just because it may test to be good, it may not be. I was just lucky to get it to finally surge while I had the meter on it. Since the early models, like mine, do not have an engine code specifically for the FPR, the O2 sensor is many times the code you will get or you will get the ones that say rich mixture or lean mixture.
Another thing to consider is that when the FPR does go bad and stick like mine did, over a short period of time, the O2 sensor can in a sense be "fouled out" by the carbon from the excess fuel being burned and cause it to read as being bad. That is what I think happened. If you pull your O2 sensor out and the end is very black, do yourself a favor and replace the FPR first before replacing the O2 sensor. If the FPR was at fault, after replacing it, the exhaust may possibly get hot enough to burn the carbon off the O2 sensor and it will start working.
Been there...done that.
BTW, my check engine light would not come on until after driving for 10-15 minutes at least. I think what happened at AutoZone was that I did in fact have a defective O2 sensor at first and the second one went out because of the FPR.
From what I understand about the way the FPR works, fuel is supplied to one fitting, vacuum is supplied to another fitting, and it has a third fitting that goes into the fuel rail (fuel at regulated pressure). As fuel is supplied from the fuel pump to the inlet fuel fitting, it is under high pressure. The purpose of the regulator is to keep a constant lower pressure (30-45 PSI) supplied to the engine, or fuel rail. As the engine RPM increases, the vacuum will compensate for the extra fuel needed or used by the engine and keep it regulated.
From what I have heard and have figured out on my own, either of 2 things can happen causing it to fail. The first is that the FPR will leak fuel internally from the high pressure side thru the vacuum and this fuel will cause the engine to run RICH. The second thing is that the regulator will stick (which is what happened to mine) and dump high pressure fuel into the fuel rail. It is possible that when it sticks like mine did, it can be intermittent. It can stick for a few seconds, minutes, or even days and then decide to loosen up and work correctly again. This is why my fuel economy seemed to be up on one tank and down on another tank when the same conditions were present on each tank (highway/city combined mileages were similar on each trip/tank).
I just wanted to post this to try and help out other people with the same problem. I know there are plenty of other posts on the subject, but none of them seemed to directly describe what was happening with mine.
Good luck and hopefully this will prevent you from getting the headaches like I had.
------------------
Ira
91 XLT 4x4
Once I thought I was wrong, but I was mistaken.
[This message has been edited by Ira (edited 03-23-2000).]
