Misfire Or Weak Spark

Discussion in 'Fox 5.0 Mustang Tech' started by Dohner90, Jul 27, 2014.

  1. Painted stock manifolds and put on, did not use high temp paint but car seems to miss and low rpm and smooth out at higher rpm, cyl 2&3 tubes on maifold are mot getting hot enough to cook paint off! All other tubes are whitish but 2&3 still black with paint!!!! Thoughts????
  2. check for vacuum leaks and double check your ignition timing and plug wires etc
  3. Make sure that all of the spark plug wire boots fully on the plug/make sure you hear a click. Also what wires are you running? I ask because i hope you are not using ford racing wires the wires look good but they don't last long..i know. Also it could be broken vacuum lines on the passenger side of the engine causing the problem. Go outside start your car..make sure it's dark ..pop the hood and see if you see any blue sparks around the spark plug wires..if so your wires are bad.
  4. They are sumitt racing 8mms blue ones. They appear to be in good shape but that dont mean anything. I will check tonight for leak outs and ill check vacuum leaks today.
  5. Start the car and use a spray bottle to mist around the bay. If your idle speed changes you have a vacuum leak. If your wires are bad, you'll see arcing, this works best when dark out
    Dohner90 likes this.
  6. Ok pretty sure i just had an issue with the IAC on my way home from work the damn car revved up to 5k and stayed there!!!!!! I had to put it in 5th and let clutch engage to pull it down from 60mph and the. Keep shutting it off when coming to stop signs and then all of a sudden it came out of it. But still missing on 2&3 and i put different wires on the. :( got some more trackin to do
  7. Try to check for codes..i have never seen the IAC to cause any mustang to rev to 5000 rpm and stay revved up.. i would check for a sticking throttle blade/bad tps.
  8. Ok found sticky throttle issue!!!! New plug wire i put i didnt put in clip and it got stuck in throttle mechanism. Also tried fixing small coolant leak and caused a major one. Put new t-stat houseing gasket on thinking it was leakin from their but guess what new gasket did not seal what so ever and just gushes coolant out now. Now i see why it was siliconed all to hell!!!!! I did check for spark on cyl 2&3 and both are firing not a blue spark but orange!!! Thoughts on tbat?
  9. Orange spark on just 2 cylinders and none of the others?
    That suggest either a plug wire(s) or a problem with the distributor cap. Check the inside and outside of the distributor cap for cracks and tracks where the spark is jumping around.

    Find a dark place and check the plug wires by using a spray bottle of water. Set the spray bottle for a fine mist and spray around the distributor cap and wires. Sparks and blue glow around the wires indicate voltage leaking out.

    Check the end to end resistance of the park plug wires. Non metal wires average from 1000 to 5000 ohms resistance per foot of length.


    View: http://www.youtube.com/watch?v=U27Gp_uQDLU
    for more help.

    Cylinder balance test: use this to find dead or weak cylinders:

    Revised 25 March 2012 to add necessity allowing the KOEO tests to finish before starting the engine and the need for a properly functioning IAB/IAC to run the cylinder balance test.

    The computer has a cylinder balance test that helps locate cylinders with low power output. You’ll need to dump the codes out of the computer and make sure that you have the A/C off, clutch depressed to the floor and the transmission in neutral. Fail to do this and you can’t do the engine running dump codes test that allows you to do the cylinder balance test.

    Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

    Be sure to turn off the A/C clutch depressed to the floor, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

    Here's how to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.



    If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

    What to expect:
    You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems

    Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

    Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

    Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

    Cylinder balance test

    If you have idle or IAC/IAB problems and the engine will not idle on its own without mechanically adjusting the base idle speed above 625-750 RPM, this test will fail with random cylinders pointed out every time it runs. The IAC/IAB must be capable of controlling the engine speed to run in the 1400-1600 RPM range. Playing with the base idle speed by adjusting it upwards will not work, the computer has to be able to control the engine speed using the IAC/IAB.

    Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Let it finish the Key On Engine Off (KOEO) code dump. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
    Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

    Do a compression test on all the cylinders.
    Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

    How to do a compression test:
    Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent/loan. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

    With the engine warmed up, remove all spark plugs and prop the throttle wide open with a plastic screwdriver handle between the throttle butterfly and the throttle housing. Crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

    I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

    A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.

    See the link to my site for details on how to build your own blow down type compression tester.
    #9 jrichker, Jul 30, 2014
    Last edited: Jul 30, 2014
  10. Holy cow man this helps alot!!!! I love the fact you can run that balance test would have never know that. Will deffinatley try that sounds like a bit of a pain but should be able to do it
  11. Ok guys jumped the port, no engine light come on. Have not checked for bad bulb yet but engine does rev up and come back down after a bit
  12. Your bulb for the engine light may be blown ..because if the engine rev and went back down to idle means that it was ready to test. But did i miss something? did you ever fix the problem you had before ?
  13. The smoking issue is pretty much gone. The high idle was spark plug wire stuck in throttle mechanism and orange spark is on all cylinders. So i want to do the balance test to see wht computer says
  14. Fix the orange spark first. It will randomly affect all cylinders if it is really weak.
    Did you do the check the resistance of the park plug wire as shown in the video?
    #14 jrichker, Aug 1, 2014
    Last edited: Aug 1, 2014
  15. I have not yet. I will tonight and do you think new coil will fix weak spark?
  16. Do the diagnostic work, then buy parts. You'll save time and money.
  17. Good news got good spark, my bro had a spare coil!!!! Bad news cyl 2 and 3 plugs i pulled back out to check and fouled out! Coated in oil :( so gonna get compression tester from work again and check compression but thinking its not good. On another good note i have a code reader tool for the car i totally forgot my uncle gave me years ago!!!! Woohoo now just tryin to read the book and figure out how to use it lol