Check engine light on 91 Explorer

[Ira]

I am having a problem that I hope someone can help me out with. Last week, my check engine light came on for the first time since I have owned it Dead Link Removed. I had been driving about 15 minutes when it happened. Have also noticed the gas mileage has dropped a couple of MPG. Since then, I bought a fuel pressure gauge and a code scanner to try to check out the problem. I did a tune-up about 2500 miles ago...plugs, wires, etc.

I first checked the codes. I was getting a code 41 (oxygen sensor signal fault),in the continuous memory (key on, engine off). When I checked the codes with the engine on, I was not getting any fault codes. I then reset the codes and checked other systems.

Next, I checked the fuel pressure. At idle, I was getting 38 PSI at the fuel rail test port. When I shut the engine off, the pressure jumped up to 45 PSI. I watched the gauge for 10-15 minutes and the pressure never did drop any. Then I disconnected the gauge and started driving.

The next morning, I left the code reader connected to see if I could catch the running code when the light came back on. It never came on.

A couple of days later, it came on again after driving 15+ minutes. Yesterday it came on again and I pulled over and connected the code reader without shutting the engine down. I received a code 42 which is rich fuel condition. I then shut the engine off and checked the stored codes again. I was getting a code 41 again.

Today, I bought and installed a new O2 sensor. I then reset my computer and reset the codes Dead Link Removed. I started it up and let it run about 10 minutes before driving it. About 5 minutes into driving it, the check engine light came on again Dead Link Removed. When I got home, I checked the codes again and had the same two codes again Dead Link Removed.

Can anyone give me some advice on what is wrong or how to figure it out before I spend a bunch of money that is not necessary? Is it possible the fuel pressure regulator decides on it's own to stick and cause the light to come on and then when I check it with the gauge, it decides to drop back to normal?

Help!!! Please!!!

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Ira

91 XLT 4x4


[This message has been edited by Ira (edited 01-11-2000).]

 

[BONeil]

I had the same problem not too long ago. It was the fuel pressure regulator. Don't know excactly what happened to to it but I had ended up changing just about every other sensor on the thing. I never did check the fuel pressure but have been told that they can be intermitient.

Hope this helps.

 

[FC2ED]

I had a check engine light problem when I bought my truck. Turned out to be a cracked hose leading to what I think is a CO2 sensor off the exhaust. I had cracked and eventually broke. Look at all your little hoses, this one happens to be on the drivers side and leads from the exhaust manifold up to a sensor.

 

[Ira]

Thanks guys!!! I will now give you an update of what I tried.

I just hooked up my fuel pressure gauge and started the engine. I sprayed carb cleaner over all the vacuum hoses and fittings and engine idle did not change. Let engine idle for about 30 mins and pressure stayed at 38 PSI. When I would rev engine, it would go to 45 and drop back down. The check engine light came on again while doing this and the pressure still stayed constant. I disconnected the vacuum line from the regulator and the pressure went up to 46. When I reconnected it, it went back to 38.

Any other suggestions? I believe the engine is possessed.

BTW, when the check engine light does come on, it stays on until the ignition is shut off. Then it will not come back on until it has run for another 15+ minutes or so.

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Ira

91 XLT 4x4

 

[slow-demon]

Try hooking up your fuel pressure gauge in such a way that you can leave it installed as you drive you truck. I usually just place the gauge in between the wiper blade and windshield so I can drive and watch the gauge the same time. When you do notice the "check engine" lite come on. Check the gauge and see if it still is within spec's. My guess is that it will be really high, like 70 psi plus... If it is then replace the fuel pressure regulator. Dead Link Removed

 

[Ira]

Well, I kinda tried that. I let the engine idle and kept an eye on the gauge and the light came on even though the pressure never changed at all. I called the local Ford dealer and they told me to bring it over and they would put it on their scanner and see what they come up with. The best part about it is that they said they would not charge me to do this Dead Link Removed. I don't feel like I can beat this price. I have to take it this afternoon. I will keep everyone informed and let you know what they find. I am beginning to wonder if I may possibly have a broken wire between the O2 sensor hookup and the computer.

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Ira

91 XLT 4x4

 

[Ira]

Another update.

Took it to the local Ford dealer. They hooked it up to their Star computer (believe that is the name). They said it was showing the code that related to "First bank oxygen sensor fault". Since the 91 only has 1 sensor, it makes it easy to figure out which one is bad. I asked the dealer if it was possible that the wiring harness from the computer to the sensor could be bad or have a broken wire and they said yes and that they have seen this problem before also. Told the dealer that I just changed the sensor. They wanted to know where I got it from and I told them Autozone. They laughed and said that's what is wrong. I don't know exactly how they meant it. The sensor that was in it said Bosch and also had a Ford logo on it. The one from Autozone is also made by Bosch and says Bosch on it. I would assume (yes, I know what assume means) Dead Link Removed, Bosch is sub-contracted by Ford to make theirs and Ford then just puts their logo on them. I have also seen other postings on here about people buying the Bosch sensors and having problems but when they make both of them, I don't see the difference.

When I left from there, I went to Autozone and told them what was going on. I then purchased another sensor and told them I would check my harness first but if I found no problem there, I know it would have to be the sensor I just put in. Told them that either way, I would be returning one of the sensors for a refund. They said that would be fine.

Tonight, I am going to make up a test harness to go between the O2 sensor connector and the connector from the computer to the O2 sensor connector. When I have this done, I can plug it into the harness and see what kind of reading I am getting thru the circuit by hooking my digital voltmeter in. Hopefully by tomorrow afternoon, I will be able to diagnose this and find out what is going on.

Has anyone else had a problem with the wires in the harness being bad? I did notice a lot of cracking and popping noise when I disconnected the 2 parts of the harness. The wires felt brittle, like they had been dried out over a period of time from the heat.

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Ira

91 XLT 4x4

 

[slow-demon]

When you took your truck to the Ford dealership did they just scan the PCM for codes. At Ford they have a scanner called a "New Generation Star" tester (NGS) which can read codes like the code scanner you bought but has more features to better help in diagnostices. One of the features is what they call "PID's" which allows you to actually watch the voltage from your O2 sensor. You can actually watch the voltage change when the sensor is detecting a rich or lean condition. Now what they should have done was road test your truck with the NGS connected and when the "check engine" light comes on see what the O2 sensor is actually doing. Whether it is indicating a rich or lean or just doesn't have any voltage at all. But, what can you expect for a free diagnostic. They probably just read the codes you have stored in the PCM.
What I would also do is actually drive with the fuel pressure gauge connected and when the "check engine" light comes on see what kind of pressures you have. Sometimes checking the pressures only at idle is not enough. I read in another post that a fuel pressure regulator is under $40 at advance auto parts. That's almost the the same price as what you paid for the fuel pressure gauge you purchased.

 

[JKristel]

Ira,

I have had the exact same expereince as you, right down to the codes, fuel pressure readings, lousy idle, poor fuel economy, check engine light on after 15 minutes, a "rich" smell from the exhaust etc, ....

Throw away your inhibitions and buy the fuel pressure regulator, it will solve your problem.

I bought mine from Wrenchead, but they shipped the wrong part, so I had to get it from Advance for 73.98

Look at it this way, if it doesn't fix the problem, you can always return the part and get your moneny back (Unlike the dealer)

 

[Ira]

Slow-demon,
I agree with you on the part about the dealer just checking the stored codes. Before I started over there, I started the engine and let it idle for about 10 minutes to help increase the chance of the light coming on. Well it did so I decided I wanted to let the engine continue running so it would not go off. Got to dealership, went in and told Service Mngr and he got one of the mechanics and told him what to do. He pulled it in the shop and...no, no, no...shut the key off Dead Link Removed.

Well, anyway, I did make the test harness but it was too late when I got home to try it out. I will tomorrow and find out what is going on. With this harness, I can also read the voltages (with my digital volt meter) being sent from the O2 sensor just like the dealer. BTW, the $40 fuel pressure regulator you are talking about is for a 93 or 94 model from what I remember. The one to fit mine is $75 aftermarket or $85-90 from dealer at discounted price. I do not regret purchasing the fuel pressure gauge because now I can use it anytime I have suspicions and I can also use it in my work. My only problem trying to read the gauge while driving is that the hose is only about 16" long and I don't believe it will stick out from under the hood so I can read it. If it will, I will try it tomorrow.

JKristel,
Thanks for your suggestion. From the readings with my gauge, I don't think the FPR is my problem. So far, all the readings have been within the specs. I may be wrong, but I want to eliminate the O2 sensor first. I am a mechanic (watercraft) by profession and I just want to diagnose the problem first. I don't like to be a parts changer unless I have to. Sometimes in the past, I have had to but don't like to. Sometimes, things do happen and just don't make sense and that is the only way of finding the problem. Someone else had a posting of a very similar problem and replaced their FPR and thought it took care of it. They fueled up after changing it, drove it for a week and found their fuel economy had only went from 11 MPG to 11.5 and now they are looking for other problems. I may fall on my face by saying this but I still think it relates back to the O2 sensor or a break in the wiring harness.

As I said before, I appreciate all suggestions and am determined to find out what is causing my problem and you can rest assured, when I do, I will post my results so it may help the next person.

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Ira

91 XLT 4x4

 

[S.Matsushige]

Ira-I understand where you are coming from. I wouldn't want to spend money on or take the time to change a part I wouldn't need. Its just too bad that as i mentioned to you earlier that these scanners don't give you an exact answer. Like I said I read a post where the scanner would read O2 sensor, but turned out to be the FPR. Go figure. Its just unfortunate that we have to be part scientist, part mechanic and part psychic to fix our Explorers. Its a good thing for this site where we can try to help each other. I don't think I would have figured half the things that went wrong if it weren't for the help of posters here. Keep plugging away Ira and keep us posted with what you find.

 

[Ira]

The verdict is in...I think Dead Link Removedand Dead Link Removed.
Here are my procedures and results. I did a wiggle test (as it is called by my code scanner manual) to check for a broken wire in the O2 wiring harness. Everything came up ok. If you remember, I just changed the O2 sensor on Tuesday so the one that is in it is still the new sensor. My next step was to plug in my homemade test harness between the O2 sensor connector and the computer connector. According to the manual, I should first test the voltage from the O2 sensor wire listed in the Haynes Manual as the signal wire. This wire on my O2 sensor is black. On the computer side of the connector, it is gray with a blue pin stripe. Manual said with the key on and engine NOT running, the voltage should read .400 to .450 volts. I found 1.050 volts...out of spec already. Step 2: Check voltage going to O2 sensor heating circuit. Spec is 10+ volts. My reading was 13.4 volts...within specs.

Now for the running tests. The manual said voltage on the signal wire should read from .100 volts (high oxygen, lean mixture) to .900 volts (low oxygen, rich mixture). It said the readings should quickly fluctuate from one extreme to another and also in the middle of this scale. My actual readings...(drum roll please) from a low reading of .946 V to a high of 1.050 V...Out of spec. This was done by constantly monitoring my meter from cold engine to hot, about 20 minutes. This told me that the O2 sensor was not sending a signal in the range it is designed for, thus, the check engine light. While doing this test, my light came on again. From my first impression, this new sensor is defective, however, since I already purchased a second one to be sure, I let the engine cool and installed it.

First test with 2nd new sensor, voltage test on sensor wire with key on, engine off. Reading was .440 V...within spec for a change. Next step, check voltage on heater circuit...again 13.4 V...within spec.

Running test. During the 20 or so minutes of engine running during the test, voltage ranged from .100 V to .900 V...just as shown in manual...within spec Dead Link Removed. The voltage did truly fluctuate all over the accepted range and changed every 1/2 second. Finally a sensor that is within spec completely. No check engine light this time. I then reset the engine codes along with the computer. Call it overkill, but I wanted to be sure.

During all of the tests, I had the fuel pressure gauge connected and kept a watch on it also. Tried to run it under hood so I could drive it first while watching it but the hood pinched the hose on the gauge enough to prevent it from reading the pressure. I did however drive it for about 10 minutes with the light on and gauge still connected to see if anything funny happened. Pressure stayed the same.

I was so happy Dead Link Removed. The light did not come back on. Decided it was time to load up the family and take the first (defective) O2 sensor back to Autozone for a refund. Pulled up, left engine running, returned part, pulled out, and shortly after, that f*&$in (don't worry, it's French) check engine light came on again Dead Link Removed. Pulled in at Walmart and rechecked my codes again. Still the same...rich condition and sensor not switching. Went in, done some shopping and started to drive around before coming home. During this 45 minutes of driving around (give or take), the light never came back on.

My theory, I will drive it for a couple of days and see if the light comes back on. If it does not, fine, I will keep this sensor. If light does come on, sensor comes out, goes back for a refund, and I am going to Ford to buy one.

My summation of the whole thing: I think the problem was the O2 sensor and I also think that both of the ones from Autozone were defective. I am not trying to bad mouth Autozone or anything like that. I know that it is possible to have a bad run of anything produced. At least they can usually get the right part for my application on my first trip in instead of having to go back 2 or 3 times, like I have had to do with some of their competition. I will probably end up taking this one back and getting one from Ford and hope for the best.

I just feel sorry for all the people who have changed sensors or anything else like this and assumed that the new one is good and not be able to go thru all the testing I have, only to find the same problem again. I know it makes you think that it was not the problem to start with and you start looking at other possibilities. I mean, you think to yourself, what is the chance of 2 of the same new parts being defective right out of the box.

I hope that I have not made anyone mad with the amount and extent of this posting. I just like to get to the bottom of a problem like this and since I am not the only one who has had this problem (from what I have recently read on this board), I wanted to share my findings so nobody else would have to go thru the headaches I have. I have learned a lot from your postings and just wanted to contribute what I could to help out.

Thanks again to all who have replied. I will keep you informed of what happens with this problem in the next couple of days.

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Ira

91 XLT 4x4


[This message has been edited by Ira (edited 01-16-2000).]

 

[JJW]

S.Matsushige, I aree with you on how helpful this board has been in trouble shooting and modifying our explorers. It has been a BIG help for me. I'm the guy that changed the FPR without checking it first Dead Link Removed I was hopeing it was a standard fix for a common problem with high mileage first generation explorers. (anyone need a used FPR? Dead Link Removed).I am curious about the Bosch o2 sensor and if there is a difference in dealer and aftermarket.Even though the FPR was not at fault,I hope the one I put in will last as many miles as the OEM did before premature removal. Dead Link Removed 88,300. Ira, I wish all mechanics were as dedicated to diagnostics as you. Unfortunately I have had the "parts changers" that make me feel I can do the same and save the labor. Good mechanics are hard to find. I hope the harness does the trick.Keep us posted. JJW

 

[slow-demon]

Ira, congatualtions on finding the root cause of all your problems.

 

[Ira]

Slow-demon,
Thanks for the kudos. So far it has been 4 days since I replaced the new O2 sensor and the light has not come back on Dead Link Removed. I hope it stays that way.

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Ira

91 XLT 4x4

 

[kb5zcs]

hello name here is michael,and i can tell you right now repalce you (fuel pressure regulator)i also have a 1991 with 140.000 miles on it and i had to replace it 6 months a go what happens is the regulator is not mataning the right pressure at idel or driving speed and it lets the computer burn a richer fuel mix and then what happens is the rich fuel mix causes the o2 sensor to start going out every time you keep changing the o2 sensor its going to keep messing it up also.sence you are running a rich fuel mix you will smell gas,you will get bad gas millage,and it will run like crap,and pretty sone it want run at all and i mean at all so change the (regulator)befor it leaves you sitting on the freeway some far from home.i sounds like it not to ot bad yet but beleve me it will get worse. michael of brownsvill,texas.............

 

[Ira]

Michael,
Thanks for your reply but it was not the fuel pressure regulator. As you can see from my previous posts, I did check everything out that I could. I ended up finding out the 1st new O2 sensor was bad. I replaced it and so far, (12 days now), no sign of a check engine light. I was afraid it was the regulator but with the pressure readings on my gauge, I was hesitant to believe it was faulty. After further diagnosis, it did come up to be the O2 sensor.

Thanks for your help.

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Ira

91 XLT 4x4