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Check Engine light vs. Gumout

I have a Ford F150 8-cylinder with 225k miles on it. I have it serviced with scrupulous regularity. In December, I got gas at a station whose pumps were v-e-r-y slow. Within 300 miles, the ‘check engine’ light came on. I took it to the dealer, and they replaced the oxygen sensor. That solved the problem … for about a week (another 500 or 600 miles or so). This time when the light came on, I decided to add a bottle of Gumout Fuel System additive. Sure enough, within about 150 miles, the 'check engine light went out and stayed out through that tank of gas and the next one (about 600 miles). When it came back on, I added another bottle, and it went out again. I’ve now used 3 bottles in about 2000 miles.



I’m stumped. What does this mean? Should I just keep adding Gumout? Is it possible that adding the Gumout has dissolved enough accumulated crud to create a secondary problem?



Any ideas? I’d be deeply appreciative. I’ve got to keep this truck running for a l-o-n-g time to come … I have a kid who’s a sophomore in a private college!

What year is your truck? There will be a code associated with your check engine light that can tell you approximately what the problem is. Check the code, your truck is trying to tell you something. Don’t waste anymore money on Gumout.

I fear the oxygen sensor was just a guess or maybe a profit decision by the dealer.  My guess is you either did not tighten the gas cap or the gas cap is no longer properly sealing.  That can cause and error code and that error code can come and go without you or a mechanic doing anything.

 That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here. 

   Don't try fixing something until you know what to fix.  Also note that even if the code points to an oxygen sensor, that does not mean the sensor is bad, it only means the sensor is sending an error message, which could be the sensor or maybe the sensor is doing it's job and reporting a another problem.  Don't shoot the messenger.

You did the right thing the first time. You had the codes read and the truck repaired. It didn;t fix the problem, and as Joseph suggested I suspect that the dealer just went with the code rather than doing any further diagnosis, never correcting the real problem. The code only tells the tech what sensor signal(s) is(are) abnormal and gives some initial direction to head.

If you truely want to keep the truck running for a long time to come you need to again have your computer checked for stored codes, have the real problem corrected properly, and stop relying on additives. Additives don’t fix anything. While I’d normall suggest taking it back to the place that didn;t correctly fix it the first time, in this case, since it was a dealership, it’s a high mileage vehicle, and they apparently didn’t want to spend much time on it the first time around (typically dealerships don’t on high milage vehicles) I suggest taking it to an independent owner-operated shop.

Tell the shop everything including your comment about the slow gas station. It may or may not be relevant, but the more information you can give them is the better they’ll be able to do their job. It’s relevance may be to suggest a plugged EVAP system. If you have one, not only might it affect your ability to put gas in, it may also cause the CEL to light after each time you’ve used up some of the gas in the tank. As the fuel is pumped out, the gas tank breaths in air through the charcoal canister in the system. If it cannot do so, a vacuum will be created in the tank as fuel is pumped out and your pump working against the vacuum will be unable to maintain fuel pressure, causing lean operation, possible misfiring, and the CEL. This conditionb uncorrected can also lead to early fuel pump demise. Pumping against a vacuum is hard on the pump.

Oh, and that plugged EVAP system could also cause the upstream oxygen sensor(s) to send an out of the normal range signal which, by an uninterested tech, could suggest a bad oxygen sensor…

And, every time you remove the fill cap to add gumout you’ll allow the vacuum to dissipate and the CEL light will go out until some more gas is pumped out and the vacuum reappears.

You could be having a problem with an old catalytic converter. Changing the oxygen sensor could make the light stay off for a few hundred miles or so. The model year of the truck can tell us if there is a downstream O2 sensor. It makes a difference if you have to make long distance catalytic converter guesses. It looks like Joseph knows more but needs that year.

Thanks so much for taking the time to read my question. The truck is a '97 F150 Lariat. It belonged to my Dad, and I inherited it when it was 5 years old with 15K miles on it. I really appreciate this community.

Thanks so much … I think the gas cap is a VERY likely suspect, and I’m going to replace it immediately. I’ll go to the nearest Advanced Auto parts and the the codes read. I have a very good working relationship with the service manager at the dealership, too. That doesn’t mean they didn’t miss something, but I do trust them not to be rip-off artists. My sense is that this may be a combination of events. This is such a great forum, and one I really appreciate. Thanks so much … I feel smarter with every response *-)

Oh, it’s a 1997 Ford F-150 Lariat. His name is SidFord (long story). I absolutely love this truck, and not only because it belonged to my late father. I drive a LOT, and it’s all highway miles.

That makes a lot of sense. I really do have a good working relationship with the service manager at the dealership; the owner was my MYF director when I was a teenager, and the s.m. & I are both huge fans of James Burton. I know … this doesn’t mean they didn’t miss something. I know they don’t have some equipment that some larger dealerships have. I liked what you said about the plugged EVAP system, but the light doesn’t go out immediately … it takes about half the tank before it goes out, which would mean it’s going out AFTER the vacuum was formed, if that was the case. It also usually comes ON about half-way through a tankful.

My feeling was that I got some bad gas back in December. There have been a couple of times in the last two months when I was on a freeway and got gas at stations where the pump was running very, very slowly. These were older stations. I usually stop and high-volume truck stops when possible.

I’ll get to an Advanced Auto Parts today and get those codes checked. Thanks so much … I’ll post the codes here when I get them.

You know, the catalytic converter came to mind for me, too. This is a 1997 Ford F150 Lariat. I’ll go get those codes today, if the Advance Auto Parts place in Minden (35 miles away) can get them for me. I’ll post them here as soon as I get them. Thanks so much for your response. This is such a wonderful forum, and I really appreciate it.

One other thought. I thought it was worth mentioning again that the light does not go out every time I add gas, which would seem to counterindicate the vacuum theory. It only goes out about halfway through the tankful after I add the additive. It will stay out throughout that tankful and usually all the way through the one after. Then on the 2nd tankful after, it will come on about halfway through the tankful. So it’s going approximately 700 miles (2 tankfuls) without coming on.

the light does not go out every time I add gas

That does not mean much.  It may take some time before the computer decides that it is ready to turn off the light until the next time it get the problem signal.

Thanks to all you great folks. I went to AutoZone & got their free code reader service. Who knew?! Well, obviously you did, and I appreciate the information so very much! The results were code PO H01, which they said indicated a problem with the EGR system. The diagnostic indicated that the problem would most likely be either:

  1. a failed EGR Valve (it said to be sure to inspect the DPFE/EGR Valve Pressure Feedback Sensor & hoses for proper operation and that it is common for a DPFE sensor to set a trouble code) or:

  2. the EGR Position Sensor or:

  3. the EGR Valve Pressure Sensor.

The young man who tested it said I should take it to someone who could run a real-time diagnostic, and helped me find someone in my area who could do that.

I did buy a new gas cap … I figured SidFord deserved that much. I got a locking cap ?just in case? gas prices start to head north again.

Here’s an update. I took ‘SidFord’ to the dealership this morning, told James the whole story, showed him the printout of the error code results from the test AutoZone did, and asked him to do a real time diagnostic test. He came back 2 cups of coffee later and said that when they did the diagnostic test, the conclusion was that the sensor they had replaced in December was faulty, that it was still under warranty, and they replaced it for free. It was part # EGR DPFE Sensor F77Z9J460AB.

Okay, so far.

Just to further cloud the issue, on Sunday evening I was in Shreveport, which is 65 miles away. I was basically running on fumes, and stopped at a Chevron station to fill up. I decided, “Oh, what the heck …” and filled up with premium. The ‘Check Engine’ light went out IMMEDIATELY. I’m perfectly willing to feed ‘SidFord’ premium, if that’s what it takes to keep him happy, but I’m stumped. Any ideas?

Now I’m just confused since you seemed to say that the codes were EGR system related (but check your printout b/c P0 H01 isn’t a dtc code. The last 4 characters should be all numbers. I’m going to guess P0401), and that your dealer found a bad DPFE sensor. Then you said “Okay, so far.” So what was ok? From that I got the impression that they had re-installed the DPFE, cleared codes and you were code free.

But then you said you filled up with gas and the light went out. So was the light still on when you left the service dept? It shouldn’t have been as it should have been cleared. Anyway, the premium fuel tree is the same one as the gumout - it won’t fix anything so something is still going on in there.

FYI - those DPFE sensors have been a real problem.

You’re absolutely correct about the number … I couldn’t read my own writing. It was P0401.

The sequence of events was:

  1. AutoZone on Saturday - got codes
  2. drove to Shreveport on Sun - got premium gas & CE light went out, and was still out when I
  3. went to the dealership on Monday - they replaced the DPFE sensor and, I assume, cleared the codes.

I have friends who laugh when I even mention DPFE sensor. They’ve finally just given up on even taking their cars in and now just drive with the CE light on.

With 225K, it seems reasonable (to me) that there may be some kind of build-up somewhere. It may be pure coincidence that this issue all started when I got gas from the station with the slow-running pump, but that’s basically why I used the Gumout in the first place (in case there was condensation or something going on in there). I did also replace the old gas cap for the same reason (to help prevent condensation, if it was not sealing properly).

So now that I came in late in the game, I’m assuming your problem is now fixed? Is that correct? Or has your light came back on?

It’s not on now, but I’m not gonna hold my breath!

“I have friends who laugh when I even mention DPFE sensor. They’ve finally just given up on even taking their cars in and now just drive with the CE light on.”

Many learn how a system works, but only a few learn how a system breaks. Gotta understand failure to be good at fixing things.

P0401 is simply “insufficient EGR flow”. The DPFE sensor is the “differential pressure feedback sensor”. I find myself wondering if they’ve made they’ve blamed the sensor twice erroneously when in fact the true problem was the EGR system’s inability to breath and the sensor was simply doing its job in telling the system the pressure differential was too high. I wonder if they’re in essence “blaming the messenger”.

Comments from other contributors? Does this sound viable?