Hey guys, I have a 1995 gt 5.0, this is the first mustang I have owned and I'm having a spark knock issue.....I always use premium fuel (91+ octane) I have checked the timing, went through the fuel system and it still seems to be having a spark knock issue, here is the catch though, if I use turbo blue racing fuel it goes away, I'm stumped, I'm a full time mechanic and this is driving me nuts but I'm not going to give up on it, I know I'm overlooking something and hoping maybe someone on here can help me out with this, any input is appreciated. I will check this post periodically throughout the day, but I do live in the country, so I may not be able to look at it much throughout the day. Thank you for any advice, Jerome

 

My 1993 HO 5.0 pinged too. Turned out it was in need of a new MAF sensor. Buy one on Rockauto.com and put it in. That should do the trick.
Maybe also check out the idle air sensor and any other part where air is flowing through a tube...

 

Just to make sure,you did remove the spout plug attached to the tfi module wiring before/when you checked/set
timing,correct?? Timing advances under ecu control when that spout plug is in,so it has to be removed when setting base timing.If you just got the car recently,its possible the previous owner could have dropped the distributor in incorrectly,which could have it in an advanced position already before base timing was even set.I would remove the #1 spark plug and rotate the crank until compression is felt at the plug hole,then continue rotating the crank a tiny bit more until the 0° mark on the balancer is aligned with the timing pointer then remove the cap and see if the rotor is pointed at the #1 position on the cap.If it isnt,the timing could be advanced just from the distributor being indexed incorrectly. Once youve got the 0° mark and pointer aligned,start the car,remove the spout and advance timing to the 10° mark (12-14° if more low end power is needed) then reinstall the spout plug.If youve got it set above 10° and its pinging,back it off 1-2° then recheck it with a test drive.Some other things that could cause pinging are excessive carbon buildup on the pistons,an egr valve thats not opening or stuck open when it shouldn't be (egr gases lower combustion chamber temps + the ecu leans the fuel mixture and advances timing when the egr is open.When its stuck open,it causes a vacuum leak which leans the fuel mixture.It should only be open during part throttle/cruise conditions) a canister purge valve thats stuck open will allow unmetered fuel to enter the system,which may cause the o2 sensors to read rich which will in turn trigger the ecu to lean the fuel mixture to compensate for the unmetered fuel condition.Check the condition and routing of the egr valve/
evr solenoid/purge valve vacuum lines.A leaking vacuum line will trigger a lean condition and if the wrong line is ran to the evr solenoid or egr valve,the egr wont open correctly or it will be open all the time.The lines for the egr and purge valve start at the evr solenoid and canister behind the passenger fenderwell.The passenger front rim and splash shield must be removed to access them.The purge valve hoses are notorious for dry rotting.Check all vacuum lines under the hood,fittings,gaskets,etc,etc for vacuum leaks.

 

You can try cleaning the maf sensor also as dirty elements will cause erratic readings.CRC Maf cleaner.Dont use carb cleaner.

 

I will try cleaning the MAF, if that doesn't work I will try replacing it with a known good one, I take it this is a pretty common issue with these cars?

 

And yes I did remove the spout plug when I checked the timing, the engine has never been touched in this car so I doubt the distributor has even been removed, but I will check it.

 

Make sure the balancer is good too.When the outer portion begins
slipping,the marks will be thrown off.