Missfire under light load when hot

[hoss109]

First let me say my 87 GN is the most trouble free car I ever owned (I bought it new). BUT it's started to missfire under light load and only after it reaches operating temp. It's fine under full throttle and before it gets hot.
It's also started upshifting to 4th directly from 1st on occasion. When this happens the converter also locks up.
I put on a new coil pack only because that and the cam sensor loosening up are the only 2 things that have ever gone wrong with it in 22 years.
Anyone have a similar problem or any ideas?? Thamks,Mike

[Keller]

Originally Posted by hoss109

It's also started upshifting to 4th directly from 1st on occasion. When this happens the converter also locks up.

Dollars to doughnuts it is your converter locking up incorrectly. The TCC switch in the trans may need replacement. Been discussed here many, many times. Especially if it feels as if it will not unlock when slowing down.

The misfire may be a module, coil pack, wires, plugs, etc. Maybe even a fuel problem. How recently was the coil replaced? Was it replaced with a GM unit?

Do you have access to a Casper's tester?

[buickpe87]

Mm,

If I understood right your explanation, your gn has problems only in closed loop, but full throttle and cold, when the car is in open loop: no problems, right?

Check also O2- sensor in addition what Scott stated. Should throw trouble code in most cases, not always. Check EGR-valve and all vacuum hoses.

MAF- failure can also cause sometimes that kind of driveability problems.

[hoss109]

I'll check the TCC switch.The coil pack is Echlin and was replaced because a terminal broke off while testing it.Plugs and wires are a couple months old.I don't have access to a Caspers tester.

No codes are showing up.The O2 sensor is 2 months old.I'll check the EGR again.All hoses are good.
If I power brake it while cold it's fine.As soon as it reaches 180 it starts to missfire,Badly!!
Thanks for all the tips,I'll check it all out and let you know.MM