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Random Misfire

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striker2

Official CEG Vendor
Joined
Mar 15, 2004
Messages
7,903
Location
Arvada, CO
97 zetec MTX w/253Kmiles,
random misfire but no CEL. it happens under high load most often in the lower RPMs but has recently started to happen up till about 4K RPM. plugs and fuel filter are new, coil pack and plug wires are about 6 months old. I'm leaning towards a dieng fuel pump that just cant keep up anymore or a plugged cat (although it should happen all the time and idle extremly poor if it were the cat). sometimes i can get it to miss without any load on it at all.
anybody got any other ideas or tests?
 
striker2 said:
97 zetec MTX w/253Kmiles,
random misfire but no CEL. ....... sometimes i can get it to miss without any load on it at all.
anybody got any other ideas or tests?
Click to expand...
Are you sure it is misfiring? Usually a misfire will cause a CEL to come on.

If it were my car, with those conditions, this is what I would check:

1) Vacuum leaks at intake manifold (and associated piping)
2) Ignition (Ignition coil, spark plugs, wires, CKP, CMP and associated wiring)
3) Fuel injectors or pump (change fuel filter first).
 
im almost positive its a misfire.

1) no vacuum leaks, if there were it would idle a little rough and the vacuum gauge would not be sitting at 18" at idle
2) plugs-less than 5k Miles, coil and wires-about six months old, CKP and CMP both have been tested and both were good
3) fuel filter just changed with the plugs, injectors-checked them when i replaced the coil and wires, pump-tested with the injectors and is only flowing about 8oz in 30 seconds (reason why im leaning towards pump).

one thought i just had is try bumping my fuel pressure up and see if that helps (i have an adjustable FPR)
 
the wiring tubing is starting to get kinda brittle in some places but the wires inside all look good.
 
striker2 said:
the wiring tubing is starting to get kinda brittle in some places...
Click to expand...

Smoking gun!

You may want to consider using "Liquid Rubber" sold at Home Depot or Lowes. That's what I have now for my positive battery cable.
 
well im not convinced that its the wiring harness as its only the plastic tubing thats a little brittle and only in some places, not the injectors or coilpack BTW. plus it doesnt do it all the time, sometimes i can punch it and it will run just fine no miss, others i punch it and it misses horribly but still no CEL (yes the bulb does work).
 
well the wires move and sometimes maybe they have a better connection then at other times ... I would look really closely at the wires, I didn't think my fuel injector harness had cracks in it but it does ... now $500 for a harness or car payments ...
 
what years was the recall for? i know im way out of the limits but mine was also one of the last 97s to be produced. hell it was made after some E0 SVTs
 
some more info.
It only does it sometimes when im going from part throttle to WOT while below about 3.5 -4K. this is leading me to believe its fuel related. if it were ignition related it would do it everytime i did anything to make the AFR go lean. If it was the wiring harness for the injectors i would think it would be more random than only when i go PT->WOT. this leaves me with either clogged injectors (again should happen more often and feel a loss of power particularly in the higher RPMs) or the fuel pump. the AFR from my dyno run several months ago leads me to believe it was having problems even then with the fuel.
 
i replaced the pump and it definitely improved. the misfiring doesn't occur near as often now. I'm gonna check the plugs sometime tonight and see if possibly changing the gap will help any as well as run some fuel injector cleaner through it. any other ideas?
 
Driving on my home tonight the check engine light came on. P0303, Misfire cylinder 3. im gonna check the plugs tomorrow night and see if i cant get a hold of a compression tester and cylinder leakdown tester. any other suggestions?
 
As the ignition coil uses a waste fire system, make sure you check both sp wiring 3 and 4, plug gaps 3 and 4, and test ignition coil 3-4. The other thing is, of course injectors, 3 and 4 (one way would be having freeze frames or live data).
 
Tony2005 said:
As the ignition coil uses a waste fire system, make sure you check both sp wiring 3 and 4, plug gaps 3 and 4, and test ignition coil 3-4. The other thing is, of course injectors, 3 and 4 (one way would be having freeze frames or live data).
Click to expand...

It will be cylinders 2 and 3 since its a zetec but i know what you mean.

BK4293 said:
Are you still running those 9mm wires?
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Yeah. im gonna check those as well. How many miles were on them before i got them?

Edit: pulled the number 3 plug and discover that the porceline is cracked. i went and got 4 new plugs and installed them. i now have 3 used spare plugs and no misfire :D
 
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