Having a strange misfire problem

[1quik6]

Ok, I could not find an answer anywhere in here and it is a bit of an unusual problem. I bought this car 2 months ago cheap cause it needed a clutch. Replaced the clutch, and almost immediately the SES light comes on. It had a PO300 random misfire code. The plugs and wires looked old, so I changed them. The light was out for about a day then it came back on. First, I haven't tested anything yet, I have been kinda busy.
When the car is missing it is very noticeable, it will barely make it up a slight incline. Now for the unusual part. It doesn't miss unless I have been driving on the highway for 15-20 minutes at a steady speed, then it starts missing where it will barely accelerate, but as soon as I get off the highway it is fine. In fact, if it starts missing and I push the clutch in and let the engine idle for 3-7 seconds, it clears up and acts fine for a while.
I think that that about covers my problem. I would almost think that it is the coils, but was hoping that someone could tell me if they had run in to the same problem, so that I don't have to spend an assload of money on a guess.
Thanks for any and all help.

zak

 

[1fast97gsx]

is there a 6 bolt engine in that car? If so then that's why you are getting the misfire.

 

[1quik6]

I wish that it was that easy. I noticed in searching that alot people have problems when they do the swap and all they have to do is unplug the TPS or something. But no, mine is still a 7 bolt. To give a little more info on the problem, I borrowed a scanner from work to see what it was doing on the way home, and this is what I found. The front O2 sensor sits on .000, the rear O2 reads normal .900-.980, the short term fuel trim reads 17.2(not sure what that is suppose to be). Anyhow when it starts missing the front O2 reads like .040 and so does the rear O2 and all the fuel trims go to 0.0 My boss, an excellent tech, thinks that maybe I have an ecu problem.
BTW, I also noticed that when I push the clutch in, it takes a second, but the rear O2 goes back to reading normal and the front O2 goes back to zero.
My car had a Bosch O2 in the hatch area when I bought it. Maybe they knew of the O2 problem. I don't have a box for the sensor, does anyone know which wire goes where.
Thanks again.

Zak

 

[BISHILVR]

it sounds like an ecu problem to me too, .980 is'nt actually "normal" for the o2 sensor and neither is .40, and on top of that it seems like the ecu is doing the exact opposite of what it should do under those conditions. fuel trim should be 0 in a perfectly tuned car, and if your 02 voltage is .40 the ecu should raise the trim not lower it. WTF

 

[luv2rallye]

When the car is missing it is very noticeable, it will barely make it up a slight incline. Now for the unusual part. It doesn't miss unless I have been driving on the highway for 15-20 minutes at a steady speed, then it starts missing where it will barely accelerate, but as soon as I get off the highway it is fine. In fact, if it starts missing and I push the clutch in and let the engine idle for 3-7 seconds, it clears up and acts fine for a while.

A clogged fuel filter or low fuel pressure will also exhibit these symptoms.

 

[1quik6]

Well I am going to make my first guess. I had three stored codes-random misfire, too much time before going in to closed loop, and fuel trim malfunction. In reading the troubleshooting charts for those codes the one thing that kept popping up was the front O2 sensor, so I bought one today.
I have it put in and now the front sensor is getting a reading and moving around. Also the rear O2 is now lower.
Hopefully this will fix my problem. I will let you guys know if it fixes it, or at least if I don't have any more misfiring.
Thanks
zak

 

[1quik6]

Well, it looks like the O2 sensor was not the only problem.
I haven't checked yet what code was thrown last night but it was definately missing.
Anybody else have any thoughts?