Car is 1989 Firebird 2.8 5 speed.

I've been having a problem with the car surging/missing while warm/hot under low throttle, high load situations. Once I get over ~3000 RPM the car picks up and starts running better.

It only happens when it is warm, it runs great when cold.

I did some data logging and found that my IAT was wayyy off (read 150 degrees, replaced and it read 50), so I replaced that and it helped, but did not fix the issue. I cleaned out the IAC and it helped a little with my low idle issue. Also, once it is warm it has a problem idling, the RPM will drop down it about 500 or so, then it will catch and pop back up to ~900.

Two thoughts are coil and Ignition Module due to it only doing it when it is warm. Any other thoughts on what it might be?

Thanks,
Douglas

 

Warm as in soon as it's warm or long enough for the coil to soak up some heat? And when you say surging is it a steady misfire or can you feel the motor 'cycling' up and down like it's losing and gaining power? The IAT really isn't used for much other than knowing when to turn on the ccp and some spark retard above 150* F.

 

Warm as in basically as soon as it is warm. I can normally make it 3-5 miles before it starts happening.

Surging like you described it, it's cycling up and down like it losing and gaining power.

Yeah, the IAT was just something I found through some data logging, however with the stock ECM (as you very well know) just doesn't give you much data to look at in ALDL stream.

 

What's the o2 doing? Switching quickly or does it have long lazy cycles? They don't always die like a lightbulb, often times they'll slow down as they get coated with exhaust soot.

 

At idle the O2s were switching quickly. I replaced the O2 sensor as part of a normal tune up (ok, I broke the sensor while taking off the y-pipe) ~3,000 miles ago.

I didn't datalog while driving because the laptop I normally use for tuning has a dead battery. I can datalog if need to be solve this issue.

 

Pop of the vac line to the egr (or unplug it if it's digital) and see if it goes away.

 

Found out what the problem was. I had the #1 spark plug wire touching the exhaust manifold, which was arcing so my #1 probably wasn't firing at all. My #3 plug wire had been completely toasted by an exhaust leak caused by the manifold bolts barely being in.

Spent Sunday taking both manifolds off and replacing the gaskets, plus replacing the 1 and 3 spark plug wires. The AIR system is really a pain when working with the right exhaust manifold.

It runs amazingly better now, more power and so much smoother.

Now I just need to figure out why the y-pipe on the left side isn't sealing to the exhaust manifold.

 

Does yours have those little exhaust donuts?

 

Yeah, the steel exhaust donuts