Mid throttle misfire hiccup, cold weather - Third Generation F-Body Message Boards

ks4wheeler · 11-25-2008, 06:04 PM

I've got a fun one! My TBI 350, '7747 ECM, started having a slight misfire or hiccup at mid throttle a couple of years ago. At first (fall '06) it was only in cold weather (ambient below 50 deg F) at partial load and only part of the time. It got gradually worse and last winter I almost never drove it because of the misfire. This past summer ('08) it was bad enough to fault in any temperature, but still is much worse when ambient temps are below about 50 deg. It's now bad enough to consistently have the problem so I can do some trouble shooting.

Still today it won't fault below about 1800 rpm, or with not load (rev'd in neutral). It does it in 2nd & Drive, but not in 1st (prob load related), and can be almost continuous at it's worst (like shutting the key off and then back on over and over and over).

I have done some diagnosis. I checked and replaced the MAP sensor (it was very slightly out of V range). A friend and I hooked up the laptop to the ALDL and took some data. Everything was working correctly (at least for the sample rate, every 3 sec I think) except for the temp sensor was reading backward. It would drop as the engine warmed up. I don't know if this was an issue with the datataker program or with the sensor. I've done a visual inspection of the wiring to the coil, as well as the distributor.

I've got several years of experience working on all kinds of diesel and gas engines and have never seen anything like this before. It started slowly, was infrequent and gradually got worse.

Does this sound familiar to anyone? I will start diagnosing it this winter and keep the thread updated, but thought someone's else's experience might flatten the learning curve for me.

Thanks in advance for your input.