Waynerd

So trying to get the 98 through emissions. When I bought it it had no cat and throwing code for upstream 02. Two weekends ago I finally replaced the 02 and decided to throw some spark plugs in. Had been running with 02 unplugged as it ran better that way. So after the new 02 and spark plugs I cleared codes and was expecting it to run nicer. To my surprise it's still rough only different. Then a p0301 code pops up. Thought maybe was the wire so I swapped wires with cylinder 2. Clear codes. Then I get a p0301 and p0302. So I think maybe I got some dielectric grease on the plug ends so I clean them well. Now I'm getting p0300,p0301,p0302. Other things I did that day were clean up battery terminals and reattached them better to the wires because once in a while I would need to jiggle the cables for it to start. Also pulled out downstream 02 and soldered wires together because PO had them taped and wire nutted together. Also cleaned up connection at PDC.
In the next couple weeks I put two of the old plugs back into cylinders 1&2, installed cap rotor and wires from my 99. I did this because I though maybe the plugs and wires had gotten dielectric grease in the wrong places. Also removed and cleaned stator. Then I untaped injector wires to look for anything bad. Found nothing there. I sprayed around injectors and vac lines with throttle body cleaner and it didn't show any signs of leaks. The plugs I removed looked very good but I figured it wouldn't hurt to replace them. Now either I'm overlooking something or this is quite the coincidence that something would just happen at the same time as I was doing other repairs. Never a misfire code in the past. Exhaust smells a little rich and it only skips at idle and idles kinda low once warmed up. No obvious power loss or drive ability issues. If I hold it at like 2grand there is a random skip though. Going to have a garage in town check fuel pressure and do leak down and compression tests in cylinders 1&2 and have them run it on a live data scanner if I can't figure this out. Any suggestions on where to go?

Waynerd

So last night I pulled plugs from 1&2 and they were a little wet. Cleaned up new plugs and put them back in. I noticed the plugs that were in it before were the ECC plugs not LYC. Probably doesn't matter. So I also pulled the plug wires from the cap and made sure the nipples were clean for 1&2. Refreshed ground to fender. Then let it idle and only throwing p0301 now. Only other thing I did was wiggle the wires where the injector harness plugs into ecm. There is a lump there that I haven't decided wether it's worth removing the tape to find out what it is. Seems to be missing a little less when rpms are raised a little and held there. Still no drive ability issues. Just idling a little low and the misfire at idle. Can't figure out for the life of me what would cause misfire only at idle.

jatoxico

You could clean the throttle body and IAC. Just be sure to use a cleaner compatible with the throttle body.

Firestorm500

Are you experiencing any unexplained loss of coolant? Even just a little?

Waynerd

No noticeable coolant loss. Did clean the TB not removed. Think I cleaned IAC but will have to double check. Drove a mile to work then a mile back home at lunch. Reading p0300,301,& 302 again.

JK253

I had something similar recently...turns out just a sliver of the metal plug connector was sticking up past the end of one of the wire boots. It was arcing over to the block, sometimes with enough juice to fire the cylinder anyway, sometimes not, which is why it only did it at idle. Never threw any codes though, maybe it was too intermittent.

Waynerd

I've actually tried a whole cap and wires crime my 99 and also tried a couple different plugs. Was thinking of trying cap and wires from my other 98 or maybe the stator from both of them. I unwrapped wires and inspected at both #1 and 2 and at the ecm connector and nothing visibly wrong. Guess I'll need to do the compression and leak down to rule that out and run it on a live data scanner before I tear the whole injector harness apart. The ecm end had newer electrical tape around it and the look that runs from driver inner fender across the firewall is a lot cleaner than anything else underhood. Just still boggles me how it happens after a plug change and new 02. I even unplugged the 02 again and the misfire codes stayed.

Waynerd

So I just drove around for a bit and it seems to be getting better. When I got home it was idling a little higher and smoother. Since I drove it for 8 months with the 02 unplugged is there a chance that it needs to relearn parameters after installing a new 02? Just a wild guess but I have read where other circumstances required relearning.

jatoxico

I recollect there is an ECU reset procedure. Could try that or wait it out. Not sure what I would do until I did some more reading/research.

Waynerd

So things are still the same. Runs great at cold startup. Rpms around 1,000 and no skip. As soon as it begins to warm up it has the slight misfire and idles very weak. Very much a burned fuel smell from exhaust which I find odd because I assume it would be a little on the rich side at cold startup but that's when it runs best. Something must be happening when heat comes into play. I did check injectors and injector harness for resistance and it was within spec. Did it with motor cooled so I guess I need to try it again at operating temp. Also have noticed when I rev it high the rpms drop sorta slowly.

97grand4.0

How about the fuel pump and regulator test? is the distributor shaft loose?

Waynerd

So I figured something out but still have a rough idle only less rough. And still throwing p0301. Exhaust barely smells of gas now! So I was checking for resistance between injector plug and ecm plug. The picture I used to find the location of the correct pins on the ecm plug also had the color of the wire. So I noticed the injector plugs for cylinder 1&2 were swapped. It ran really well the first time I started it. Then it progressively got worse. Idle is up a little now and smoother but still a little lumpy. Can't believe it ran so well with injector wires swapped.

Waynerd

So finally got my compression and leak down done. Cylinders 2-6 were all 153 and cylinder 1, 123. Leak down they applied 40psi and cylinder 1 would only get to about 10psi. They said no air into valve cover so it's not a seal or guide issue. I don't think they listened for air in exhaust or intake for some reason. Going to make an adapter for my air hose to screw into spark plug hole and see if I can hear air in exhaust or throttle body and radiator also if I don't hear any in those locations. It doesn't seem to skip as bad or any on cold start after sitting overnight. Not sure what that would mean. Also sometimes there's a fairly loud tick from under valve cover and seems to be in time with the skip. I can shut it off and start it though and after a few times the tick will go away. I put a stock air box on and it's crazy how much more pronounced other sounds are without being able to hear the air being sucked into the intake. It always ticked before but didn't sound that bad until the stock airbox was on.

ROGR333

Had this on my 97. I did all the stuff you did and that everyone else suggested. Then I found a TSB for valve springs. Replaced those and p301 code was history...... Bad news is 6 months later it came back . Thing ran like a champ with or without the code, just misfired at idle. Motor had 205xxx when I got it and approx. 240xxx when I sold it a cpl months ago. New owner (original builder of the rig when it was lifted etc) said compression in all cylinders was 123+/-. There are numerous threads on this btw both here and several other sites. The TSB popped up on one of them and that supposedly fixed that jeepers problem. Good luck! Keep us posted

Waynerd

Thanks man. I hope that's all it is. Might not get a chance to pull the head for a bit though. Not sure if I'm going to just put my other 98 together and swap the registration to that for a while until I get some time.