Flowmaster Exhaust, Cannot Get Rid Of Knocking, Help!

Discussion in 'SN95 4.6L Mustang Tech' started by Cobie, Nov 9, 2013.

  1. Stock engine, K and N replacement filter (not cold air) and cat back Flowmaster. Cannot get it to keep from knocking. Been to a number of mechanics, tried different gas (run 91 now), checked all the normal stuff (plugs, etc). Consistent advice is to get it chipped/mapped/dyno'ed, but lowest estimate is $800+.

    Suggestions? If I have to chip it, what brand/kind? Any other options besides having to chip it? I really need to keep the cost down, but it sure is irritating to not be able to drive this car with a little enthusiasm. I live in Los Angeles area, if that helps.
  2. When does it tend to knock? Part throttle? WOT any gear? WOT only 3rd and up? Every rpm, or from 2k to redline? Any codes?

    Take a close look at the ceramic portion of the plugs. Can you see metallic specs on every plug, or just a certain plug or two? The specs are evidence of detonation. If it is a particular cylinder, then it is likely due to a gummed up fuel injector. I know my GT had a bad rattle at WOT when it had 80-120k miles. I ran some e30 through it and that really cleaned up the fuel system, so no more knocks on 87 octane when I switched back. Some fuel injector cleaner might be the more common solution ;).

    If it is a part throttle issue, then suspect the EGR valve.

    Have you changed the fuel filter? Would be good to get a fuel pressure gauge on for a little bit to see if it holds pressure at WOT.
  3. Good questions. It knocks WOT, I think 3rd gear up, haven't heard it in 1st or 2nd gears. I've run lots of fuel injector cleaner through it, different brands, but don't know e30? Pretty sure we have changed the fuel filter and maybe fuel pump, but not recently (I've put 100k on the car, it now has 160k).

    I called Flowmasters and they suggested even w/out the engine check light, could be an O2 censor, or maybe the MAF sensor, but with the little mods done, computer should be able to handle that . I'd thought we'd checked all this a while back (this has been a long term problem, I just don't drive the car hard ever due to it). I don't recall any codes, but I'll take it down again and see what we get.
  4. E30 is a blend of 30% ethanol/70% gasoline. Ethanol has some pretty strong cleaning properties. My GT was able to compensate using long and short term fuel trims without throwing a code. I don't necessarily recommend it to everyone, but I was having fun playing around with the tune and testing it out. Similarly, I have been running E85 for a while now.

    Anyway, I suppose it could be a sensor like flowmaster said, or it could be a blockage in the fuel system... ie dirty fuel injector, fuel pump pick up, clogged fuel filter, ect. Checking the fuel pressure and making sure it holds steady to redline would tell you the fuel system is fine with the exception of the injectors. You could swap in some used injectors to see if that's the problem.

    There was a huge long thread on seafoam a while back. I tried it for my detonation problem, but it didn't make a difference. If the issue was carbon build up in the cylinders, then seafoam may help you.
  5. That gives me some solid things to test, thank you very much!
  6. You're running a K&N filter, presumably oiled. Have you ever cleaned the MAF sensor?

    If not: The oil from the filter can get on the sensing elements of the MAF leading to it under-reporting the amount of air entering the engine. When running oiled filters then, the MAF should be cleaned periodically. Get a can of real Mass Air Sensor cleaner, for example:


    and give the MAF a good cleaning.
  7. Sorry for the tardy follow up, been gone. I just wanted to thank you for the advice on he Mass Air Sensor. I got that cleaned, and the car worked like a champ, all the knocking just disappeared!

    Thanks again!

    trombonedemon likes this.