Recently, I replaced the heater core and after putting it all back together, I now have a slight misfire at idle. I can put the car in gear, hold the brake, accelerate and it will still miss, but barely. On the road, it accelerates and drives fine. Obviously, I went back and double checked everything since I unhooked some things. To make it easier to get at the hoses on the core, I unhooked the MAF sensor, it's vaccum line, the heater control valve vaccum line, all right side plug wires from the cap, and the harness attached to the firewall that runs into the A/C heat harness. On the inside, I unhooked the ECM, speed buffer module and put them both in the house.

My main question is before I go futher diagnosing, does the ECM need time to "relearn" after being unhooked or does it even matter? I had it and the battery unhooked for about 3 days while working on the core a couple hours a night. I've gone back and triple checked all vacuum lines, plug wire order, etc and cannot account for a misfire cause. It was not doing it before the heater core install, so I did something, just haven't figured it out yet, but I wanted to rule out the ECM being unhooked for so long. 89 Firebird 305 TBI

 

probably a plug wire went bad when they were disturbed-could ohm them or sub out with known good ones.

 

Well, strangely enough, it turned out to be the EGR valve...AGAIN!! I just replaced that item in October because it was stuck closed. This new one is stuck open. It's amazing how it did this at the same time the heater core went out making me think I caused it. After diagnosing all day, I discovered the EGR was stuck, so I took it off and put on my block off plate temporarily and presto, no more misfire. Called the parts store and they ordered me another one. We'll see how long the next one lasts. This thing is 54.00 and at least they are swapping me out. The thing still looks brand new and there is nothing stuck in it or the intake...it just malfunctioned. The plunger won't move at all. I'm not even surprised anymore. All this stuff is made in China and it just sucks.

 

I would stick with the block off plate if at all possible. There's no getting away from the china junk anymore.

 

Yea, I was thinking about that, but this '89 is completely factory with all the original smog equipment functioning, all harnesses intact and nothing and I mean nothing is bypassed, cut, or screwed with. I really found a rare one and I'd really like to keep it that way, but yea the China parts are really getting under my skin.

At least it's a simple and quick item to replace. I'll give it one more go and if the next one malfunctions in less than a year...then I guess it will get the block-off put on permanently. I'm just a little concerned of how that would affect my engine by just disabling that one item and capping the vac line.

 

Ok, so the new EGR didn't fix the problem, even though the one I just took off was stuck as in would not move with a screwdriver and a hammer on the pin, so I thought a new EGR would fix the misfire since it was stuck open. The new EGR works great, but the same misfire is there. It's a weird one since it does not affect acclerating or driving around. Just does it at idle in drive/reverse. Looks like I'm going to have to do a compression test now. I have a bad feeling the exhaust leak I posted about in the exhaust section might have screwed me here. Funny how this only started after completing the heater core install. Car was running flawlessly before all that. I swear I've done something, but I sure as heck can't find it.

 

Forgot to add, went back to the block off plate again and still misfires. For some reason, a couple days ago when I put the block off plate on it didn't misfire, but I didn't run it that long either, just long enough to see if it were missing. Must have been a fluke. Now it misses with EGR or with block-off. Crazy car.

 

Do you perhaps have a pro comp distributor???

 

I finally found it. I knew it had to be something I did when I changed the heater core. I went back and re-checked all vacuum lines again, finally pulled off T at rear of manifold again, stuck my finger over it and car smoothed out for me this time. Started putting one section at a time back on the T at manifold and once I got to the A/C heat line..BAM hissing sound and miss. Somehow, I managed to break both plastic lines inside the taped wireloom going into the interior. They were both cracked way down in the harness. I could wiggle the main one some and it would seal which might explain why it didn't miss when I put the block-off plate on as I must have moved it just right that time. Figures, but it's fixed. That was not an easy splice to do. I would have replaced the whole line with rubber, but it's no easy task to fish all that stupid plastic line out and re-run new line, so I just spliced it with small rubber vacuum way down low and it still looks factory after it's all taped up and works perfectly. I'll never understand all the hard plastic lines, it's nothing but a headache once it gets some age on it.