96 Suburban misses on downhill

[W6XRL4]

My 96 Suburban has a 5.7 Vortec. I got it at about 70k miles and it now has about 105k. It has had the problem since I have owned it. Basically, it runs great in town up to 45 mph. When it gets on the highway (which is infrequent), after about 20-50 miles, the check engine light comes on. When the accelerator is lifted, a small miss is felt. When you lift going downhill, the miss is really felt and the check engine light flashes. After the accelerator is applied, the light ceases flashing and goes back to steady. The fuel pump and inline filter have been replaced, and also the plugs. It was scanned and only read "random intermittent miss". It has two cats with four O2 sensors. Any ideas would be appreciated.

[j cAT]

Originally Posted by W6XRL4

My 96 Suburban has a 5.7 Vortec. I got it at about 70k miles and it now has about 105k. It has had the problem since I have owned it. Basically, it runs great in town up to 45 mph. When it gets on the highway (which is infrequent), after about 20-50 miles, the check engine light comes on. When the accelerator is lifted, a small miss is felt. When you lift going downhill, the miss is really felt and the check engine light flashes. After the accelerator is applied, the light ceases flashing and goes back to steady. The fuel pump and inline filter have been replaced, and also the plugs. It was scanned and only read "random intermittent miss". It has two cats with four O2 sensors. Any ideas would be appreciated.

carefully check the vacuum lines...for cracks etc..check fuel pressure...with guage...

[2000CAYukon]

When it was scanned, it should have shown the misfire count per cylinder and that would help especially if there was a clear pattern to the misfires.

If you pull the dist cap, is there a lot of play turning the rotor? Since you feel the misfire when you let off the gas, I wonder if the dist drive gear is worn.

Another idea is that when you let off the gas, the engine vacuum is very high. Like j_cAT said, check for vacuum leaks. The fuel pressure regulator could also be leaking when the vacuum is high causing a misfire.

//2000CAYukon