running rich and misfiring at high er speeds

[bigm-13]

running rich and misfiring at higher speeds

'98 Tahoe c1500 four door. Vehicle runs perfect around town but when accelerating on to the freeway at more than half throttle at 60mph and above, it falls on its face. Starts misfiring badly with DTC for random misfire and runs extremely rich. You can smell the cat heating up and acts to be plugged.Llet off fuel and accelerate slowly it clears up and is fine. Have replaced fuel pump, rotor, cap, wires, plugs, and installed upgraded spider injection with injectors at end of tubes, exhaust after manifolds back. nothing has helped. could use some help.

 

[lesterl]

Check the TPS sensor

 

[bigm-13]

Thanks Lesterl. I'll check I It With A Dvom. Do You Think That Would Cause This Even Tho It Wont Do It At More Than Half Throttle At Slower Than 60Ish Mph?

 

[lesterl]

It could, I would start there tho..... suprised it didnt throw a code if it is bad....

 

[bigm-13]

I Get A P0300. Occasionally I Will Get A P0420 I Keep Pushing It When It Starts Misfiring. I'll Check For Any Change In The Throw Of The Sensor. Thanks For The Input And idea.

 

[lesterl]

Ohm out the Coolant temp sensor while you have the DVOM out....

 

[lesterl]

P0420 is a Catalyst System Efficiency below Threshold (Bank 1 -Drivers side)

 

[bigm-13]

One Of The First tHings I Changed Was The Cat And All The O2 sensors BecauseYou Could Smell The Cat Burning Up. It Didnt Fix Anything. I'll Ohm Out The Coolant Temp Sensor Thru The Warm Up cycle And At Fully Warmed Up. The Things Thats Hurting this Diag Is Not HavIng A Scan Tool To Look At The Data List While Its Messing Up. Thank You Again.

 

[bigm-13]

mass air flow

Figured it out! coolant temp sensor ohm'd out good and so did tps. One of my team members had a GM mass air flow sensor laying around. Installed it and and she ran great on the freeway. No dtc's, no smell of unburnt fuel, no smelly cats, and no falling on its face. Weird that it would do this without an error or dtc for the M.A.F. with it looking clean. It worked and she ran like she was made yesterday.

 

[lesterl]

Nice, thanks for posting back what you found..... hopefully it helps someone else...