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I'm pretty sure that I have built up carbon deposits in my 1986 corvette convertible with aluminum heads. I get noticeable pinging when hot and accelerating even with 93 octane gas.

My plan is to gradually mix in some E85 gas (since I already have E10) to bump it up to perhaps E35 to both increase the octane rating and to clean up my engine combustion chamber, valves, fuel injectors and pistons. This would be only maybe one tank full. The racers swear by E85 because it runs cool and keeps the engines really clean. I've been told that the E10 already would have affected any parts that might dissolve and I have been running E10 for 10 years and no rubber problems.

I understand that this may change my tuning but would it make it bad enough where it wouldn't run? Hoping to find E85 enthusiast to chime in.

Tunerpro RT - scan data of a highway run

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  • Is the engine carbureted or electronic fuel injection?
    – GdD
    Commented Jan 25, 2019 at 12:14
  • The OP used the fuel injection tag. I'm assuming this Corvette has Tuned Port Injection (TPI) like most every Corvette in 86 had.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 25, 2019 at 12:17
  • I saw the tag but I wanted to check because the OP lists it as an '86, not an '89. I'm not a 'vette expert but I didn't think FI was on the 86 models. Maybe it was added later.
    – GdD
    Commented Jan 25, 2019 at 12:21
  • Definitely tuned port fuel injection ... 2nd year corvettes had it.
    – 3dalliance
    Commented Jan 25, 2019 at 13:31
  • @GdD - Yah, saw I fat fingered that (comment corrected), thanks! As the OP stated, the 86 was the second year of the TPI system for Corvettes. They started to put them in the Camaro/Firebirds in 86 as well (I've got an 86 IROC-Z in the driveway with a TPI 305 in it). I believe they used Throttle Body Injection even before that (earlier 80's) in the Corvette, but not sure when they completely went from carb to FI on them. Well before 86, though.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 25, 2019 at 14:12

1 Answer 1

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There are several issues with what you are planning to do. If you haven't done so already, please read this answer I wrote about using E85 instead of non-E or E10 fuel.

While ethanol fuels do clean and have a higher octane rating, they also don't have the energy density of gasoline. To make it run stoich, you have to use more of it (higher ratio of fuel to air). If you dump enough ethanol fuel in your Corvette to even get it to E35, you still need more fuel than the computer can compensate for. In other words, you'll be running lean which will exacerbate your issue, causing more knocking/pinging and quite possibly damage.

I've been told that the E10 already would have affected any parts that might dissolve and I have been running E10 for 10 years and no rubber problems.

This is really poor advice. Ethanol is destructive to rubber parts. Even older vehicles can handle E10 and will not cause damage, though. Get above that and damage will occur. Just because you've been running E10 in it thus far does not mean higher concentrations of ethanol won't cause damage. The vehicle needs to handle the higher levels of ethanol or it will deteriorate.

Your best bet here is to figure out why it's knocking and fix the issue. Even if carbon buildup is the problem, there are better ways to get it cleaned out, such as fuel injector cleaner. High end products such as BG44K (no affiliation) may do the trick and will do it without damaging your fuel system. You could also do some data logging to see if the knock sensor is working correctly or if it's actually knock (or pre-ignition) which you're hearing.

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  • Good advice here, running E85 would likely require modifications or at least substantial tuning which would cost time and money, possibly making it impossible to run E10 again without spending more to change it back. Then you could find out you still have knocking, or whatever the problem is. A good fuel system cleaner and some diagnostics would be a much better choice.
    – GdD
    Commented Jan 25, 2019 at 12:26
  • I've asked this question on corvette forums and the guys that run E85 usually increase their fuel injector sizes and run a stronger fuel pump ... then retune their Calpak
    – 3dalliance
    Commented Jan 25, 2019 at 13:55
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    @3dalliance - Injectors and a tune, plus all of the other upgrades to your fuel system is what you'd have to do, but then you are stuck using E85. Really, fix the underlying cause of the issue, then you won't have to worry about bandaids or wanna-be fixes. You need to figure out if knocking/pinging is even what the issue is, then press on from there. Are you even sure you've got the timing set right? Are you sure there isn't something wrong with the distributor (like the vacuum advance is stuck wide open all the time)?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 25, 2019 at 14:07
  • @paulster2 - Thanks for the great answer. The timing is set to 6 degrees with the electronic spark advance disconnected as per FSM. Don't have a vacuum advance but have other sensors like TB temp sensor and MAF. I'm going to do a scan with tunerpro and post the results here to check if my IAC is working OK and when my knock counts go up.
    – 3dalliance
    Commented Jan 25, 2019 at 16:47

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