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E85 and carbon deposits

I just had a fuel filter replaced and the fuel system cleaned out after heavy carbon deposits were found to be blocking the fuel filter on my 2004 flex-fuel Ford Sport Trac. I switched to fueling with e85 almost exclusively over a year ago, but used regular gas for about 3 years before the switch. The shop that did the fuel filter job said they’d recently seen a lot of carbon buildup on vehicles that had switched to e85 and blamed that on the e85 fuel itself. Researching the issue a little on the internet, I found some posts suggesting the e85 wasn’t the source of the carbon buildup, but was instead just loosening existing deposits left from when I used regular gas since the ethanol is a better solvent and that the loosened carbon was being caught downstream in the filter. Any ideas which explanation is more likely? Is the e85 the source of the carbon buildup or is it just loosening old deposits?

You don’t get carbon build-up in the fuel filter, regardless of the fuel you’re using. Well, maybe if you put coal in the tank.

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NYBo is correct. Either the shop that you use is dishonest or you misheard them regarding the location of the carbon deposits.

Are you sure that they said that there was carbon in the fuel system? If so, find a new mechanic before they start really overcharging you for unnecessary repair/maintenance work.

The shop said they found carbon buildup and cleaned the fuel system. I may have incorrectly put the two together to mean they found the carbon in the fuel system, but that’s what left me puzzled about that diagnosis. If the carbon buildup was elsewhere, could it be the e85 causing it (more than regular gas)? Most of the info I’ve found on ethanol indicates it burns cleaner in the engine than gas.

NYBo and VDC are correct that cars do not get carbon deposits in the fuel system. It sounds as if you got taken for the widespread scam “your car needs fuel induction service.” Take heart, most car owners have fallen for it.

How was the engine running prior to the service?

“I found some posts suggesting the e85 wasn’t the source of the carbon buildup, but was instead just loosening existing deposits left from when I used regular gas since the ethanol is a better solvent and that the loosened carbon was being caught downstream in the filter.”

Carbon deposits are found after combustion. Since the alcohol and gasoline are already burned, the solvency issue doesn’t exist - they aren’t there anymore. You might want to ask the garage if they mean carbon or some sort of tar, which might be the product of polymerization. Some hydrocarbon fluids like gasoline and alcohol might have molecules join together to form a higher molecular weight compound that sticks to a surface rather than flowing through it.

It was running fine once started, though it would have some trouble starting when cold. It would occassionally idle rough and stall, but I think that was traced to the IAC valve, which was replaced.

I just double-checked the technician’s report on the filter - he wrote “filter was clogged”. The verbal explanation was carbon, but I didn’t check the consistency of the material to see if it was tar. If polymerization is the culprit, is e85 any more likely than regular gas to produce it? Is quality of the fuel a contributor?

You need to find a new mechanic. If you took it to him because of hard starting when cold, and rough idle AND told him that you use E85, he should have been able to put the three together and tell you that these are all normal symptoms of E85 fuel and cold weather. I hope they didn’t take you for too much money. Don’t take my word for it, do a google search on “e85 cold weather starting” and see the thousands of hits it produces.

Clogged fuel filter + E85 + faulty idle air controller = hard starting and slightly rough running when cold

Notice that “Dirty intake/induction” ISN’T part of the equation. Steve hit the nail on the head- you got taken for the fuel system cleaning part of the job, unless it was just the throttle body that was cleaned.

“polymerization”? Why’d you use that term? Carbon buildup in engines, as noted above, is from incomplete combustion. Also, did you mechanic cut open the filter? If there was ‘black gunk’ on the inlet side of your system, there could be something deteriorating. E85 will do that in a non-flex fuel system. Yours is for sure flex fuel?

“polymerization”? Why’d you use that term?

I assume that is meant for me. Hydrocarbon fluids can change composition over time. When shorter polymer chains (e.g., alcohol) bond together it’s called polymerization. Does that explain what I meant clearly?

This is actually a thing that happens fyI!!!

But that carbon is not in the fuel system @Kelsey_Acheson. And years ago the cure for such carbon buildup was repeatedly accelerating at wide open throttle for half an hour. These days for hundreds of dollars walnut shells are blasted into the cylinder head to loosen the carbon then blowing it out.

I have never seen an engine that heavily coated with carbon and suspect that the engine was run for many miles despite the check engine light being on.

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