87 Octane Problems vs 93 Octane

[Subzero]

This truck is a mystery to me, it used to run fine on 87 and now it runs terribly with it. Somebody borrowed my truck to use it and in replenishing the fuel they used ( almost full tank) put in regular 87 10% ethanol gas, a nice gester but kinda an issue. I didn't want them to fill it back up because of this but too late now. The truck diesels on shut down and idles badly now with the 87. I had these problems before but since switching to 93 octane they have completely vanished or decreased greatly. The shop says I have burnt valves and a carb not functioning 100%. All these problems started right after I let the truck sit for 1 month with 2 month old gas in it. The carb was rebuilt by said shop. I also always add Marvel Mystery Oil to the fuel. What is the best way to run out this 87 octane? I don't want the dieseling to ruin anything so should I try an octane booster or something?

[tnlprt]

When was the last time you checked your timing

[Subzero]

Never have since I bought the truck, I have an old Snap On light but tbh I don't know how to use it very well.

[chad67]

If your pistons and chambers have a considerable amount of carbon build up, it can increase your compression ratio to the point that it needs the extra compression. Also, the carbon acts as an insulator and can promote hot spots which can cause detonation. If the timing is correct, this would be my guess. The carbon could be interfering with your valve sealing as well. I have seen chambers on an engine burning oil get pretty closed off. Also, if your truck does burn oil, oil decreases your octane level when added to your air/fuel mixture. As far as your current predicament, add a couple bottles of octane boost and run it darn near empty before you refuel.

[chad67]

I meant to say that it can increase your compression to the point that it needs the extra OCTANE, not compression. Sorry.

[Subzero]

Now that you say that the shop did say something about carbon deposits. This one doesn't burn much oil if any, I'd say the little loss it has comes from the valve cover leaks. The Marvel Mystery Oil is meant to clean things up, hence me running it now. Last time I had this gas issue happen I drove the truck around for a long time and ran out all the gas in about 3 rides. I'll buy some of that 108 Octane Race Fuel stuff and see if that helps any. Almost at 3/4 of a tank now as of tonight. What can the dieseling do to the engine? I've heard rumors of throwing a rod...

[chad67]

The detonation can damage your pistons and is hard on the entire engine. It should be avoided if at all possible.

[Subzero]

That doesn't sound kind to the wallet. Better go get some octane booster. Any way to actually address the problem other than throwing premium fuel with additives in the tank?

[chad67]

Check your timing as mentioned before, some people swear by Berryman Chemtool fuel system cleaner to clean up carbon deposits (I take a neutral position when it comes to their effectiveness as I have not personally tested them.) I wonder if the Mystery Oil can contribute to the deposits like regular oil can? I don't have any experience with Mystery Oil, either. If the timing checks OK, it may be time for a rebuild. The start or dramatic increase in detonation with no change in ignition timing usually means a tired engine. Oh, did you have the carb rebuilt after it sat with old fuel? If so, did it help? Too lean of a mixture can cause detonation as can a vacuum leak.

[Subzero]

I will try that. Marvel Mystery Oil added to fuel claims that is cleans carbon deposits and lubricated valves, I will use some of the stuff you mentioned and see if that makes a difference. Carb rebuild helped some, the engine wouldn't idle at all before.

[1972hiboy]

I wonder if doing a compression check would reveal a carbon deposit issue? We know what a stock engine compression value should be right? If the compression readings are higher then stock because of the dispaced area taking up by the deposits then we can rule out engine tuning. We know if there are deposits it will raise the compression the same way as running higher compression pistons and needing a higher octane fuel to resist burning. but if the readings are at factory specs or lower due to normal wear and tear we should look into engine tuning. Running higher octane fuel could just be treating the symptom and not the cause.

Scenario 2. If you just have a small buildup that is glowing red during running it will cause detonation without raising your C/R. More than one now long gone service technicans have told me about dribbling water down the carbs throat while on elevated idle to break up carbon deposits. if your compression check outs fine id sip some water down its throat and see if it chabged anything. otherwise your back to engine turning for your troubles.

[Subzero]

I might do a compression check afterwards if the ATF doesn't work. I bought the Castroil ( I think) Ford trans fluid. Not that I think it matters much since it will be burned up. I forgot to stall the engine out before pulling the key at the parts store and I was treated to hearing a " clunk-clunk-clunk-clunk" filled by a hissing noise. I bought 108 Octane booster and put all of it in immediately ( treats 32 gallons). When returning home I stalled the engine out to a lower rpm with the choke then pulled the key- no issues. After this rain passes I'll try the auto trans fluid. To me I just feel better about pouring another truck related fluid other than water, but I have heard it works either way.

I will also run a vacuum gauge and see what readings I get from it.

[chad67]

Good luck, keep us posted!

[motzingg]

has wrong (too hot) plugs been considered and ruled out as a possibility?

[guhfluh]

Seafoam is also supposed to work well at removing carbon and deposits. I have also heard of using water to break up carbon deposits in the chamber, but it needs to be done when the engine is hot and I'm not sure it works well for oily carbon buildup or valve deposits. The only additives I know that work are sold at the dealerships and BG or Wynn's. They are the ones that you either suck through a vacuum port, spray directly into the carb/throttle body or tap directly into the fuel line and run solely on the cleaner. The fuel tank additives I have no experience with working, as the.cars I used it on never returned for any inspection.