94 GMC Suburban misfire/cough when warm

[tag346]

Found the forum yesterday and spent hours searching and reading about similar but not matching issues I've been working with a mechanic on with out suburban. It's a 1500 5.7 w/ 192k miles. Until about 4 months ago it ran strong without issue...uses a bit of oil but that's it.

Symptoms: Around town or on short trips(<20 min) on the high works fine with power and good acceleration when the pedal is applied but drive it on the highway for over 20-30 minutes start up an incline at more than about 1600RPM and the thing starts big coughing misfires. I'm not a mechanic and the shop we've taken it to (not the dealer) has checked and as needed replaced the fuel pump, o2 sensor, plugs, cables, fuel filter. I've tried seafoam and other additives to no avail.

This rig has been a great rig that we take up into the hills, etc but it's not reliable. Yesterday tried going to play in the snow and 60 minutes from home where the hills start it began coughing so bad we didn't think we'd make it back.

I've probably left out critical info...but any suggestions to this noob would be appreciated.

thanks, TJ

[2000CAYukon]

This sure sounds like a clogged cat or exhaust. The exhaust back pressure should be tested.

Something is being affected by heat. It could also be a bad coil that is failing when it gets hot.

You might also want to get a scan tool on it to see what the ECM is doing when the problem is happening.

Good luck.

//2000CAYukon

[tag346]

thank you...I'll start there...