Originally Posted by 110Leaded

I posted this by mistake in the 05-12 section so I wanted to post it in the correct 99-04 section for you guys, sorry for the length but I wanted to explain my whole process... Read the red text if you just want to see what the problem was and how I fixed it. I have been driving the car for a week now since I fixed it and it runs like a dream!

I have a 2000 GT, 5 speed with 223k miles on it(yeah it's a lot but I take real good care of it) with some minor engine and exhaust upgrades and a brenspeed tune but nothing crazy. For months upon months now I was having the well known "low rpm hesitation/misfire that would smooth out above about 2000 rpm" problem. The only code it ever threw was a P0171 lean code but I knew that was due to an exhaust leak. Looking around in the forums it seems that these symptoms are relatively common with the 99-04's and no one(at least that I could find) ever really found the reason for these symptoms. Hopefully what I've found out will help some of you out there with the same issues I was having.

Having that many miles, things are bound to be worn out so I started replacing stuff because it really needed it anyways, here is the order of stuff I replaced in my quest to fix the dreaded hesitation/misfire:

Cleaned and re-oiled air filter
COP's - ran better first few mins but hesitation returned
IAC - fixed the stalling out at idle problem, but not the hesitation
Verified TPS was good - was good
Cleaned MAF - didn't do a damn thing
PCV - it just needed a new one but then I realized I had vacuum issues
Vacuum lines - replaced bad rubber lines but still had hesitation
Alternator - diode had gone out on ORIGINAL one, lol
New MAC Prochamber - Original one was rusting through
Fuel filter - time for a new one anyway
New injectors - yeah, they were original too, ran better but still no fix
Cleaned Throttle Body - was filthy but hesitation was still there

All throughout the above process I was searching all the forums to see what I could/should try next
Part of me was saying that maybe I will just have to accept this issue because there are so many miles on the engine and perhaps it was just starting to crap out from wear...

Next possible steps were to check the cam position sensor, the crank sensor, blah blah blah...

Last night I was so over the whole thing I just went back to square 1 and re-thought the problem... I'm an electrical engineer and I just cannot let seemingly simple problems like this go. The best way to describe the symptom is it's almost identical to having just washed the engine and having water in the spark plug wells causing the engine to "skip". That has happened to me in the past but it always goes away after some driving to evapoate the water. So, I took out the COP's 1 by 1 and verified that the plug wells were dry and that the plugs were not loose. Everything was dry as a bone and all plugs were tight. Reassembled everything and started the engine only to have a constant miss where before it was only an intermittent miss at idle!!!! So I made things worse

I remained calm and knew that the only things I touched were the COP's and now I had a constant miss which actually gave me a good opportunity. What I did was unplug each COP, 1 at a time to see if the idle was affected. When I got to the one where there was no change in the idle - that was the COP/cyl causing the miss. Sure enough, 3rd one I tried, cyl.7, had no change when I unplugged the COP. Now I knew exactly where to focus on. I pulled out that COP, plugged in an old plug laying around and started the engine to verify it was producing spark, and it was so WTF?!

Just for giggles I also tested one of my original COP's with the extra plug and it worked too, so I reinstalled the old COP on cyl. 7 and started the engine. It ran fine so now I was really like WTF?!

I turned my attention to the harness connector that plugs onto the COP and the tab that is supposed to keep it from backing off was not there(never has been there best I can tell) so although the connector was "connected", there was nothing keeping the connector from backing off. I pushed on the connector and got it to fully seat but as soon as I took my finger off, it would slide back a little from the dielectric grease I had on the connectors. At that moment it was like angels descended out of heaven singing the Hallelujah Chorus and I finally figured it out! Even though the connector was seemingly "connected" it would slide back just enough to break the contact to the COP pins. This explains why the car ran fine just after installing new plugs, the connector was seated enough right after install but backed off from vibration.

What I did to fix this was take a 12" zip tie(I think it was 12") and feed it between the 2 wires on the back of the connector that plugs onto the COP and loop it around the "ear" on the COP that sticks out and has the screw hole for mounting to the intake. They are not going anywhere now.
For whatever reason, only like 2 or 3 of my COP connectors had the tab intact that would keep them from backing off so I put a zip tie on each of them.

Drove it to work this morning and it was like driving a new car! After living with that annoying misfire/hesitation for so long I had forgotten just how fun this car was to drive!

Hopefully this will help some other people out there and save them some money, all you need is a few zip ties. Not to say that this is the end all be all fix for everyone's hesitation issue, but it sure was for mine and it will give you something to rule out if nothing else. Feel free to email me and I will be happy to send pics of what I did.

Thanks to all of you who post to these forums, you have been an invaluable resource for me and are a great resource for all of us!