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Discussion Starter #1
My 2002 Focus ZX5was running well, but recently I found coolant leaking from a hose near the thermostat so I decided to replace a couple of hoses while at it. Upon further inspection, I found a vacuum leak in a hose that runs between the engine intake manifold and a fuel vapor purge valve. EGR valve was all rusted and PCV valve was not replaced since forever. Valve cover gasket was leaking oil as well. Long story short, I replaced the following parts:

1. Fuel vapor purge valve with: YS4Z-9C915-AB
2. Fixed a vacuum leak in the hose that runs between the engine intake manifold and a fuel vapor purge valve
3. EGR valve with: XS6E-9D475-DA
4. PCV valve with: YS4Z-6A666-BA
5. Valve cover gasket with: YS4Z-6584-BA
6. Spark plug wires with: XS2Z-12259-AA
7. Thermostat housing, ring, thermostat

Now, this is where the fun started. I started the car after all the repairs above and noticed that it is idling very rough! Putting it in reverse or drive almost stalls it, one time it stalled completely and I had to start it again. Check engine light came on with code P0301 – Cylinder 1 Misfire. I went ahead and re-checked all the newly installed spark plug wires on all the cylinders and coil and even cleaned them inside using some rubbing alcohol and cotton swabs. No change, still idling really rough. Re-installing old spark plug wires did not help either. The next step to troubleshoot that I took was to remove all spark plugs and check them for damage. Cylinder number 1 and number 3 spark plugs had what appeared to be a larger gap compared to the rest of the two spark plugs. I also noticed that cylinder’s 1 piston head appeared to be ‘wet’ while the rest of the heads on the remaining three cylinders appeared to be ‘dry’ upon a flashlight inspection. Swapping out all four spark plugs with brand new premium autolite spark plugs helped a little bit but it idles still very rough. I checked PCV hose that runs behind the engine, PCV valve boot hose, all the connections to the fuel vapor valve and even took off and re-installed EGR valve to make sure everything is secure – no change. Upon even further inspection, I noticed that one of the coil pack connector wires was a little bit damaged and had a few copper strands sticking out. This happened because I disconnected the old aftermarket remote start system that I had installed over nine years ago, but it was not functioning since forever and had one of its sensing wires tapped into the ignition wire so I decided to remove it along with the siren and an open hood sensor that were no longer needed. To fix the ignition coil connector wire, I cut the wire in half and re-soldered it hoping it was somehow causing coil to not produce a sufficient spark, but – no change. That being said, I ordered a new ignition coil and a new connector for it as well as a new PCV house and PCV boot hose. It should arrive sometime this week but I was wondering if anyone can point me to anything else that I may have done wrong. After thinking about all the steps:

1. I don’t think that rough idling has anything to do with me removing a wiretap on the ignition coil wire from an old aftermarket remote start system that I had which was not used in years. From what I understand this wire is used in the aftermarket remote starts to sense the engine start therefor it is not needed for the OEM function and was not there before the install. Could I be wrong?

2. I don’t think it’s a new PCV valve or EGR valve

3. Maybe it is caused by a new fuel vapor purge valve? I still have the old one laying around, so I may try swapping it out if a new coil pack does not remedy the issue
4. What about the cylinder head on cylinder 1 that appeared to be ‘wet’? Is that an indication of no spark and maybe fuel on top? I’m hoping that’s the issue and a new coil pack would fix it

5. Could a damaged coil pack connector wire somehow damaged the coil pack by not supplying it enough voltage and damaging it?

6. Would air in cooling system cause a misfire? That sounds super unlikely.

Any advice is appreciated and thanks in advance.
 

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2004 Focus Wagon, Zetec DOHC, Auto
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It seems that much of the work you did would not cause the P0301 code.

One thing that I see as I read through this is the comment "checked all the newly installed spark plug wires on all the cylinders and coil and even cleaned them inside using some rubbing alcohol and cotton swabs."

The plug wires need the silicone grease. It not only prevents the boots from sticking on the plugs, but it also helps prevent the spark from jumping down the outside of the plug. That may not be the cause of your misfire, but just saying.....
 

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Hi. No need to keep repeating "rough idle" when you have a P0301 and a fuel wet spark plug. Your misfire is most likely being caused by; a bad coil pack, defect of wiring to the coil pack, or the wire repair you did.

A coil pack will develop hairline cracks and causes lots of problems, sometimes without setting a misfire DTC. The wiring leading up to the coil pack also has a history of becoming damaged, -helped by the twist the wire harness makes just before it connects on the coil pack.
 

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Discussion Starter #4
Thank you for all your comments. I will report back once I repair coil pack connector, install a new OEM coil pack and PCV hoses. If that does not work...i was reading that it may be Idle Air Control Valve or Throttle Position Sensor even though I am doubtful. Waiting for parts at this point :) Thanks again for all your help guys
 

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If car ran fine before the work, easy things.............pull the EGR valve hose and the same for the vapor valve, assuming you are talking about the one on pass side firewall. Plug those two hoses and then recheck the idle. If either the vapor system or EGR the problem should be gone unless by some odd circumstance the new EGR is stuck slightly open. Then go to your other things.

Coil connector will not short term damage coil, maybe over longer time.....air in cooling system does nothing too. Any messing around with coil wires (valve cover change) commonly faults an old wiring connector, they crack the wires too easy and sometimes up inside connector where you can't see it. The coils tend to go bad as well.

If higher mileage then a new PCV can often crack a hardened rubber 'vee' connector there to make a vacuum leak. Assuming a zetec motor here.
 

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Try switching plug wires. If you still get a Cyl 1 misfire, then it's coil or harness. If the misfire switches to another cylinder, you have a bad plug wire or boot.

Betting dollars to doughnuts that it's the harness. I tried rewiring mine to no avail...the wire was frayed right at the rubber plug, and the time required to split it and redo all the wires wasn't worth it.

Install the harness first and test before putting in a coil you maybe don't need, since they're usually non-returnable once installed.
 

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Discussion Starter #7
After removing the coil and inspecting it, I realized that it was completely FUBAR with cracks everywhere! It's been on there since 2002. Bought new OEM DG536 from Amazon for $66 (dealer wanted $112 + tax, f that) and car's running smoothly! Pigtail power connector to the coil has also been replaced. Thanks to everyone who decided to chime in :)
 
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