What causes a car to miss fire? - Forums at Modded Mustangs
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post #1 of 28 Old September 4th, 2008, 01:00 AM Thread Starter
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What causes a car to miss fire?

soooo my car is missing, got a more expirienced opinion (93mustank) ive known it was missing but he doesnt think its my computer or maf, what else could cause it?

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #2 of 28 Old September 4th, 2008, 01:01 AM
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Lack of spark, fuel or compression.


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post #3 of 28 Old September 4th, 2008, 01:02 AM Thread Starter
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haha i mean specifics, like plug gap? should be at .054 right? mines at .045 would that make it miss??

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #4 of 28 Old September 4th, 2008, 01:05 AM
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bad plugs, bad wires, vacuum leak, exessive back pressure, clogged fuel injectors... the list goes on and on and on... the only real way to figure it out is to monitor the pids using a scan tool... rather than spending $300 guessing at parts to throw at it, take it somewhere and spend $90 to get it propperly diagnosed
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post #5 of 28 Old September 4th, 2008, 01:06 AM
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Have you scanned the computer yet?


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post #6 of 28 Old September 4th, 2008, 01:06 AM Thread Starter
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i think my dyno tune will end up being a figure out wtfs making my car miss tune

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #7 of 28 Old September 4th, 2008, 01:07 AM Thread Starter
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Have you scanned the computer yet?

im not throwin any codes if thats what you mean, gota remember its obd1

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #8 of 28 Old September 4th, 2008, 01:10 AM
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yes and you can still scan it.... its just not as easy as OBD II. Have you tracked it down to one cylinder missing? Is it a random misfire?


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post #9 of 28 Old September 4th, 2008, 01:12 AM Thread Starter
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yes and you can still scan it.... its just not as easy as OBD II. Have you tracked it down to one cylinder missing? Is it a random misfire?

no its confusing cuz it doesnt seem to do it unless its under load, revs and sounds good in neutral

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #10 of 28 Old September 4th, 2008, 01:15 AM
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Have a spare TFI laying around?


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post #11 of 28 Old September 4th, 2008, 01:20 AM Thread Starter
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is that this?

While we find your parts, please enter your ZIP Code at PartsAmerica.com

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #12 of 28 Old September 4th, 2008, 01:27 AM
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link isn't working for me.....


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post #13 of 28 Old September 4th, 2008, 11:46 AM Thread Starter
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yeah thats what the link was to, but its called an ignition control module?

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #14 of 28 Old September 4th, 2008, 12:38 PM
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Yeah thats it. With no codes and what your describing that would be my first guess.


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post #15 of 28 Old September 4th, 2008, 01:07 PM Thread Starter
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hmmmm maybe illl just replace it this weekend, but by looking at the dizzy, how the funk does it come off? those 2 screws just holding it in??

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #16 of 28 Old September 4th, 2008, 01:10 PM
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yeah


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post #17 of 28 Old September 4th, 2008, 01:16 PM Thread Starter
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Originally Posted by C MY 4D View Post
yeah

fuck thank you! and it being bad will cause missfiring? i read it can also cause hard starting, which i have sometimes but i think its my starter not this

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #18 of 28 Old September 4th, 2008, 02:40 PM
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Quote:
Originally Posted by 93hatch View Post
fuck thank you! and it being bad will cause missfiring? i read it can also cause hard starting, which i have sometimes but i think its my starter not this
Don't thank me yet..... Its not fixed yet.


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post #19 of 28 Old September 4th, 2008, 02:55 PM Thread Starter
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Originally Posted by C MY 4D View Post
Don't thank me yet..... Its not fixed yet.

ive been reading you need a special tool to remove the tfi is this true? i cant see it really without turning my dizzy and i dont have a timing light to fix it if i throw it off

[QUOTE=93mustank;2577445]We speed tested my right arm for quickness in shifts... So he could design the cam for proper shift recovery.[/QUOTE]
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post #20 of 28 Old September 4th, 2008, 03:19 PM
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It's may be the tfi module but it is not likely. They usually work or they don't start. A scan tool is probably not going to help you much here if you're engine light isn't on as 90% of obd I codes turn it on. If it kinda bucks and jerks down the highway at medium rpm with you're foot lightly on the throttle it is likely a weak spark problem.

(This is because the combustion pressures are highest in this conditions and the fuel mixtures are very lean which makes the mixture harder to ignite so if you have a spark problem it will show up at this time. Also if the engine shakes at idle but smooths out as you rev it up suspect spark.)

If it runs very good at low rpm but doesn't miss until higher rpm (but not dependant on load) it may be weak valve springs. If it only misses under heavy load but performs better under light throttle conditions suspect fuel pressure/restriction problems or plugged exhaust.

A couple other pointers, If it doesn't have cats it's likely not a plugged exhaust, If you suspect plugged exhaust take an infared temp gun and monitor the exhaust, if there is a restriction heat will be built up until the restriction and then dissapate rapidly after (be careful as factory mufflers have engineered cooling systems built in) You can also plumb a resriction gauge into your O2 sensors bung to verify, You will also have very little exhaust flow from the tailpipe.

A couple things to do if I didn't have access to a scope is to remove the distributor cap and check the cap contacts and rotor for corrosion. ( be sure to number your plug wires as they will need to put back in the proper firing order) Also verify base engine timing. You will need to take out the shorting bar to check timing, If the engine seems to run better if the timing is retarded or advanced alot more than it should be suspect valve timing is out. (the timing chain stretched and or jumped a cog). Also pull all plugs and inspect.

You're spark plugs are the tattletale of an engine and will tell you alot. If they are white they are running lean, If they are black they are burning too much gas or oil, If the electrode is worn it's time for new ones, If one is wet that cylinder is missing. Check the resistance of all plug wires (a good rule of thumb is 3000 ohms per foot), Anything that will cause driveability problems will deviate substancially from that. I have seen twice now on chevy's were two cylinders next to each other in the cap fused together and caused crossfire (you could not tell by looking at the cap in any way). So now I ohm each cylinder to every other one to be sure. Be wary that OBDI is very primitive and will not even set a misfire code nevermind inform you of which cylinder it is.

If your diagnostic path leads you to believe fuel is the problem, buy a gauge they are relatively inexpensive and trust me it is alot easier to rule out first off than to wonder if it is a problem. Zip tie it to you're wiper and road test under misfire conditions to ensure proper fuel pressure. Also If you're plugs looked white it is possible you have a vacuum leak. A high idle and hissing noise is usually apparent with a vacuum leak. The source can be pinpointed by introducing a hydrocarbon (ie butane, propane ether) in suspected leak areas and observing engine for rpm change, If this occurs you have a vacuum leak causing it to run lean.

I have seen a few times where ect (engine coolant temperature) sensor is open causing the computer to think the engine is below freezing, The computer has a very rich mixture incorporated into it's tuning for cold start up and your engine will run like you left a choke on an old carbjob.

If you suspect you're maf is causing the problem unplug it. You're pcm/ecm will resort to failsafe fuel trims where it actually runs half decent and will not cause a miss, it may have a rolling idle but runs fine WOT. Hope this helps you. I better get back to work.

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