Engine light on ~ can't figure out why???

wynativeOctober 18, 2006

My son and I just purchased 2 different '03 &'04 Ford's. Ever once in awhile the little 'engine' light on the dash will come on. We have both taken them to shops and no one can figure out what is causing this.

The vehicles are running great even with this mystery. If any one has had this happen - what did you do to fix it or is it just a 'glitch' that we have to put up with like I was told?



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You have to take it in where they can read the ford codes. Being that they are 3 year old fords this check engine light is going to be a regular thing. Get used to it and have your credit card handy.

    Bookmark   October 18, 2006 at 3:41PM
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my wifes car had that problem for about a yr, she took it in never showed a fault code, ran fine. turned out to be a bad seal on the gas cap, the o-ring on the cap had a thin spot in it, sometimes it leaked sometimes it didnt. they put a new cap on it, no more problems

    Bookmark   October 18, 2006 at 7:21PM
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Had same thing going on in a ford tempo for months, maybe years. Light would come on intermitantly, but always ran fine. Crawling around under it one day I found an air hose off. Reinstalled that and the light hasn't come on since. I suspect I knocked the hose off years before while replacing the starter. It was in the same vacinity.

    Bookmark   October 18, 2006 at 10:18PM
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quirkyquercus, Bill & Gary,

Thanks for your input! I will check the gas lid and hoses, hopefully that is the problem because I don't want to stress out the credit card this time of year. :)

Enjoy the rest of your week,

    Bookmark   October 19, 2006 at 9:14AM
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DD had a car that did that frequently. The problem? If the gas cap wasn't replaced exactly right, it made the light go off. If you're pretty sure your car is working okay, and if you've recently gotten gas, I'd recommend always trying tightening the gas cap before running to the repair shop. Once someone gave us that tip, it saved DD a LOT of money.

    Bookmark   October 25, 2006 at 5:16PM
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No matter what the temperary problem was the answer IS in the computer. There are three types of issues that will set a code and two that will turn on the light. Your shop will need a very good scanner, an enhanced scanner with OEM Ford software, one will set a code inone driving trip but will not turn on the light. A problem in two trips will turn on the light BUT if the problem fixes itself then the light will go out on its own. There is a history of the problem stored in the computer scan data, it is called a freeze frame. If the catalytic convertor is in danger of being damaged then the ceck engine light will come on and usually flash, when that problem goes away so will the light.

Freeze Frame scan data, get it checked out by a real shop with an OEM enhanced software scanner. It does not have to be the dealer, make some phone calls and go to the best shop, yes they may also charge the best (for them) price.

    Bookmark   November 4, 2006 at 7:06AM
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Auto Zone will read the code for free. Then go to the Ford Truck Enthsiast Forum and search the code. The code will not be specific so you'll usually have a couple options to check out.

All your shop is doing is resetting your engine light by disconnnecting the battery. If the prob isn't fixed it will just keep coming back on...

quirkyquercus - I have an 03 F150 - never had a problem with it. Only thing, I had to replace a hose at 80k miles. She's been a great truck & still going strong at 85k!

Good Luck & if you need help with the code let me know!


Here is a link that might be useful: Ford Truck Enthusiast Forum

    Bookmark   November 18, 2006 at 11:38PM
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A good shop will discover and repair the hoses - IMO, disconnecting the battery does nothing of value; for the proper repair, use the facilities where the OEM software is employed - and this is expensive - guess who pays ...
On the gas caps, hard to see how the newer ratcheting type can leak.. But I can see a O Ring with thin spots - this should be available as a "for under one dollar part" - a new gas cap could be $25 or more...
As part of the annual maintenance, this O-Ring should be lubed with a little silicone paste..

    Bookmark   November 20, 2006 at 1:11PM
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"A good shop will discover and repair the hoses"

Yikes, at what cost. Last time I tried to go that route I almost was charged over $150 for a DPFE sensor that costs $80 at the ford dealership and is takes about 1 minute to unplug the old one and replug the new one.

I always try to diagnose on my own. I do believe Autozone is using the OEM software. It never hurts to check it out when it is free, you never know how much $ you'll save.

wynative - I just realized when reading your OP that you didn't say you had a ford truck, so the forum I listed above may be of no help. Although, I am sure there are other similar forums for cars, etc.

I guess when I think of Ford all I think of is Trucks & Mustangs :-)

Cheers - Sarah

    Bookmark   November 20, 2006 at 2:53PM
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"I always try to diagnose on my own. I do believe Autozone is using the OEM software. It never hurts to check it out when it is free, you never know how much $ you'll save."

Auto Zone does not use O.E scan tools, or software. The investment to do that for every make and model sold in the US would be over $200,000, per store.

Auto Zone, Advanced, etc, attempt to pull a code and guess a part. That is NOT diagnostics, that is NOT what a real tech doing the work does. Just because you get a P0155, which is an O2 sensor heater circuit, it does not always mean the O2 sensor is at fault. Proper testing will prove if it is the sensor, or simply the fuse, a broken wire, the PCM, or some combination of the above.

Sure you can save money if you are capable of doing the work yourself. But if you guess parts instead of truly testing, you can waste much more than it would have cost you to take it to a pro.

    Bookmark   November 20, 2006 at 11:08PM
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Sorry John G, didn't mean to upset anyone. I was just trying to be helpful...

Actually, it was brakemax who had told me the DPFE Sensor needed to be replaced. They failed to mention an air intake hose that had a gapping hole in it, argh.

They told me my truck would do fine through the weekend, it barely made it home. I wonder why they didn't mention this hose since when I lifted the hood it was hissing loudly in plain sight?

When I have the code read at Autozone I DO NOT have them guess a part. I take the code to the Ford Truck forum I mentioned above and ask for help. There is actually a couple of Ford master mechanics on this forum.

"Sure you can save money if you are capable of doing the work yourself. But if you guess parts instead of truly testing, you can waste much more than it would have cost you to take it to a pro."

I've been workin on cars as long as I can remember. Helping my dad turn a wrench on his old mussle cars. It is impossible to find a good motorcycle mechanic in my city, so we also do most of this work ourselves. Perhaps it was wrong for me to assume that someone using a Auto forum might be somewhat mechanically enclined?

Perhaps it would be better to get the code from a shop with the OEM software (pay the diagnostic fee) and then look into doing the work yourself. Dealer & Brakemax wanted to charge me over $100 to replace the air intake hose. $10 and 120 minutes later problem was fixed on my own.

I do not think you need to be mechanically enclined to replace a hose or sensor.

Agsin, I didn't mean to ruffle anyones feathers. Just trying to be helpful. I think I'll stick to my motorcycle & gardening forums. (How's that for a combo!!)


    Bookmark   November 21, 2006 at 4:56AM
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You don't have to run away from here. You have a pretty good understanding of cars at this point and that's important. I have to deal with situations all of the time, where the only thing some people are thinking about is saving themselves money, and they totally forget what "fair" really means. Fair is always something that works for BOTH parties.

I can't tell you why "BrakeMax" didn't see a gaping hole in an "air intake hose". I suppose you are either describing the plenum hose between the air cleaner and MAF sensor, and the throttle body, or some other vacuum hose, or a hose that directs air for the IAC. I can envision quite a number of different systems, and I don't have specifics on exactly what vehicle you own. So I'll stick with the plenum hose, since an air leak between the MAF and the engine will severely affect performance.

A DPFE will give you a check engine light, but have little or no effect on performance. I won't rule out the possiblity that the plenum hose simply finally completely failed after the DPFE was looked at. They do have to flex constantly, and are a common failure item.

There are several master mechanics on this forum. I looked at that web site. It looks pretty good. But still taking a code, and then asking advice about what could be wrong, is still leaving the wrong impression. Follow this.

2002 Pontiac Grand Am with a 3.1l Auto. It's setting a code P0101, that is a Mass Air Flow Sensor performance code. This car is owned by a family friend, who happens to work at an Auto Zone! They pulled the code and wanted to replace his MAF. Jason chose wisely and brought the car to me to test first. By monitoring scan data and taking snapshots for playback I was able to measure the reported volumetric efficiency (VE) of the engine. VE is the measurement of the actual amount of air the engine can pump. For example, a 3.1l engine at 5000rpm should easily pump 160grams or air per second at wide open throttle. Jason's MAF sensor was reporting 90 to 105 grams per second, and struggled to get over 4500 rpm. VE measured 55%. Well that's why it had a P0101, the computer knows how much air the engine should be pumping, based on throttle opening and rpm. At this point, there are still several possible causes for the code. A bad MAF is clearly one of them. Funny you should mention it, but a torn plenum hose would do the same. Some type of an engine mechanical issue could do this, heck even a very bad air filter could actually have given the same findings. But there is one more, how about a restricted exhaust? The test for this is to remove the front O2 sensor, and install a gage and actually measure the back pressure. The spec allows for a maximum of 1.25PSI, and that's under a load. Simply speeding the engine up in neutral had this one building 14 PSI. The fix was to replace the catalytic convertor, which was still under the 8yr/80,000 mile warranty at the dealer.

No one, reading a forum would effectively lead the poster to that diagnosis. There were more details uncovered during the testing that helped make the decision to measure the back pressure. One of them was by watching the long term and short term fuel trims on the scan tool while driving the car. If the engine was actually getting more air than was being reported, then they would have been shifted way positive, because the engine would have been too lean. But because the MAF was actually reporting how much air was truly entering the engine, they were reasonably close to neutral.

I can remember when I first started working on cars, I was 14 when I tore down my first Chevrolet 350 to put a cam in it, and had a valve job done on it. My grandfather had given my dad the car because it was running so bad. BTW, that wasn't my first engine job, I did a head gasket in a lawn tractor a few years before that.

The thing here is there are people who are born with the ability to do this work. I suspect you are one of them, just like I am. It's not easy work, and most of the time the pay actually stinks, especially when you keep dumping huge pieces of your paycheck to buy more and more tools. But if someone really likes fixing cars, there is a freedom there that few other fields can offer.

Now do you know why it's hard to find a good motorcycle mechanic? Probably because it's too hard for them to make enough money to stay in the field long enough to get really good at it. Running a repair shop, cars or motorcycles is expensive. The overhead eats some 60% of the gross profit, and then the wages and benefits start coming out before the owner finally gets his/her piece of the pie. Your couple of quotes about how much something was going to cost you if a shop had done the work, fails to show exactly how much profit the shop would have actually made if they had done it. In short, less than $125/hr of profit would have me working for free. That's how much it costs to run a shop like mine. That means, I would have to diagnose, and repair your air hose problem in about 35 minutes based on the numbers you provided, and be hard into the next job, or else I would not actually make any money at all.

I invest heavily in O.E scan tools and software. That's one of the reasons it's so expensive for my shop to operate. The facts are, if I could double my parts mark-ups, and double my labor tomorrow, (and still stay busy) and then throw all of that money into more software and O.E scan tools and I'd actually still be losing ground. In 1993 I had one scan tool, a Snap-On MT2500. I could work on anything that showed up at the door. Today that same capability would cost more than 1/4 million, and the updates, and software licensing for the tools would continue to eat tens of thousands of dollars each year to continue to support them.

You are correct, you don't have to be overly mechanically inclined to do some repairs. You do have to be very gifted for many others. Plus anymore you just about need a business degree to balance the pennies so that you have a chance to be there for the customer in the first place. The others that post here have seen some of my diatribes like this one. If you want to learn about cars stick around, just give us some room to earn a living. We should not have to apologize for trying to do that.

ASE CMAT L1 30yrs exp.

    Bookmark   November 21, 2006 at 9:10PM
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Hey John G - Thanks for the feedback... I have a '03 F-150. Right now that's my only 4 wheeled vehicle. No project cars :-( Last one was a 71 Camaro 350 SS, loved that car - never should have gotten rid of it.

I completly understand your profit making point. The DH an I own a Building Maintenance Company to help with putting him through college. I have clients who frequently expect work for free. Free?!?!

Gosh, I have had so many problems with Brakemax I wouldn't even dare to begin to list them... When my "Check Engine" light came on I was driving so I swung in and had them diagnose. This is my work truck and I don't have a lot of time for tinkering. I don't know the name for all the hoses, etc. But I am assuming it was some sort of air intake hose because air was being sucked in through the large hole. This hose is connected directly to the throttle body. I knew that was the reason my truck was dying because when I had the DH clamp his hand over the hole the truck idled normally.

Replaced the hose & cleaned the throttle body and haven't had a problem... Next on the list is cleaning the EGR valves, this looks a little complicated to get to and I don't have a weekend to give up.

Really wish I had looked under the hood before forking over $100 to brakemax to not notice the problem. Argh!

I totaly understand that mechanics with your experience will be able to find a problem I will not, but ofcourse! Sometimes it just seems better to try to figure it out on your own.

Truly, these newer vehicles scare the hell out of me. I'm very cautious when tinkering with my truck. (Can't carry ladders on the motorbike!) I'd take an old 350 any day.

I'll stick around and read posts. Perhaps I will be enlighted by more of your diatribes.

BTW, your writting/speech is impeccable. Out of pure curiosity, were you always a mechanic or is there a degree in business somewhere in there? I love writting, but I barely try to spell my words correctly here, let alone check punctuation. As any writter would know, your point will flow and be accepted much easier with proper writting.


    Bookmark   November 22, 2006 at 8:10AM
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I've been a mechanic all of my life.When I graduated high school what I really wanted to do was become an aviator, and be landing jets on an aircraft carrier. To that end the Marine Corps gave me a shot at the Naval Academy, but I had one thing that took away that dream. I actually can't write, not with my hands anyway. That's the actual reason for my nickname the cardoc. I've always been told my handwriting is so bad I should have been a doctor. :)

Being able to write here is partially a result of what has occured over the ten years I've on the net in automotive chat rooms and forums. In the initial write up screen I won't see the run-on sentences and the typo's. But when the preview message screen comes up and I re-read with the different looking page I see them right away. I have to laugh a little, if some of my high school english teachers saw just how much I do write these days, well lets just say you better have a chair and some oxygen ready for them. :)

Back to the hose sucking air. You have now better described a PCV hose, or something similar. That would allow air to enter the engine, without it being measured by the MAF sensor. That would drive the system lean overall, but could also drive a few cylinders leaner than some of the rest and that will cause a pretty poor idle, even though the computer would try and compensate for it. The plenum hose will give you more trouble under a higher engine load because of how much air is moving, as compared to an idle speed.

What I try to teach service writers, and technicians is when the customer comes in, they have to get as much information as they can. If they only thing that they got you to tell them was the "check engine light" is on, and for whatever reason the truck simply does not display any idle quality issues when the tech drives it. Then he/she is going to pull the code, check the circuit idicated and advise on the repair based on exactly what is found.

A bad DPFE.

Now if the tech is told that the engine is running rough, stalling, or almost stalling etc. plus the MIL just came on. Now the tech would approach the vehicle with possibly a longer road test, and look at some other things a little closer. A DPFE failure can be diagnosed, without the engine running while the tech is under the hood. Plus if the hose I'm thinking of is the one that failed. It turns pretty gummy then it finally tears, and occasionally it will collapse in on itself and actually seal the leak every once in a while. That could make it undetectable on occasion even if the tech was looking for something that can cause the engine to run rough. Now take this situation, and picture the thousands of different engine and control system designs that a tech has to work with that can all have their individual quirks and there are way too many pattern failures to even try and remember. The tech has only his/her skills, and what the vehicle shows him/her for the few minutes that they have it, and the customers description of the problem(s) to work with.

A Plausible senario.

OK, NOW put yourself under the hood of your truck as a tech. The customer report is a MIL on and nothing else is told to you. You drive the truck into the shop, and do not notice any idling issues, (remember the hose can collapse in on itself one time, and may not the next the engine is started when it has failed) and you find a DPFE code. You perform the steps required to diagnose and confrim a DPFE, many of which are done without the engine operating. The result is you don't find anything else wrong. What should you tell the customer at that point?

Taking this one awkward step further. I have had customers where I have found something like one of these hoses bad, that wasn't causing a big driveability issue at that point turn around and accuse me of damaging it because they weren't having a problem with that before. Put yourself back under the hood again as a tech, and now how would you handle that? You've just done an exemplary job, and been accused of being dishonest for doing it.

"I completly understand your profit making point. The DH an I own a Building Maintenance Company to help with putting him through college. I have clients who frequently expect work for free. Free?!?!"

Now imagine spending an extra $9000 for a tool that the other maintenance contractors around you refuse to buy, and have to use it when you do some of that work for free. Now imagine spending another $40,000 on top of that for similar but different tools. Now you have the situation that I do, when people talk about having AZ pull codes for free and call it diagnostics.

This huge dollar number represent the O.E. scan tools that I have bought, that allow me to do everything the dealerships can for the manufacturers those tools support. The aftermarket scan tools (not to be confused with a code puller) have gaps in their vehicle support. The contention of the the aftermarket tool manufacturers is that if they developed the software for their tools that could do everything the O.E. tools do, we would not be able to afford their tool. So they give us the 70% to 80% that they feel we are most likely to need. That strategy forces me to still have their tool so that I at least can do something with most cars, but I also have to own the O.E. tool for the cars that I work on the most. That's why I could spend a couple hundred thousand more today on just those kinds of tools, and be losing ground in just how many different jobs I can take in the door.

A full aftermarket scan tool, like a New Snap On graphing MT2500 will run you about $4000, a fully loaded Snap-on MODIS $9000. Many shops attempt to run with just these tools, and they are indeed excellent, for 70% to 80% of the work that is likely to show up at a shop. I keep my MT2500, non graphing because I have had it for some fifteen years fully up to date. It costs me $1100 a year just for the bi-annual updates. When you see a code puller at a parts store, it does about 2% of what the MT2500 does.

    Bookmark   November 23, 2006 at 10:21AM
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