stumbling/misfiring at 2500-3000 rpms

[60ratrod]

ok so lately, as in the past few days, my truck(94 gmc stepside 4.3l auto) has been stumbling/misfiring at around 2500-3000 rpms(i can't tell exactly since i don't have a tach) while going down the highway. it only happens when i'm cruising, trying to maintain a steady speed. otherwise during any kind of acceleration, like gentle part throttle/hard wot acceleration she doesn't stumble, just when i'm cruising trying to maintain speed from 45mph-55mph. any ideas?? i checked the cap and rotor, i don't have any visible fuel leaks, i have spark on all 6 cylinders, only pulled one plug cause the motor was still too warm to attempt to pull off any of the other wires and it looked alright. the only mods that i have done are an intake, tbi spacer, msd coil, accel tune up kit(plugs wires cap rotor). could it be a ign module in the dizzy?? oh and the motor has about 145xxx so far as i can tell. oil was changed at 143,143(exactly), along with tranny fluid, filter, trans pan gasket, trans tail housing seal and bushing

[68gmsee]

Sounds like a good problem....

Seems to me that if it runs well at any time other than a steady speed, you shouldn't have ignition problems.

The question is, what is happening when you're at a steady speed? You're not moving the throttle much but at acceleration or wot you are. Could it be throttle position sensor? EGR valve? Vacuum leak? Just throwing out suggestions.

[60ratrod]

throttle movement isn't much, maybe an 1/8th movement, usually in the 1/4 throttle to 1/2 throttle range. i don't think its a vac leak otherwise i think i would be seeing other symptoms like increased idle. could possibly be an egr valve, its the one on the back side of the intake behind the tbi right?? i don't know where the tps is. one thing that i just remembered was it's been throwing codes at me at least once a week, but once i shut the truck off it goes away for about a week or so, but monday-wednesday it thew codes all 3 days. i haven't had time to go have the codes read since being put on nights. could it be an O2 sensor??

[faribran]

if we are guessing.....
i have seen a small block produce these kinda symtoms due to a worn out timing chain and gears...

[60ratrod]

well i did notice when i put my timing light on the motor i saw the timing mark jump around when i was trying to see where my timing was sitting. could this be a timing jump issue?? i'm gonna try to get my ecu read this weekend.

[bwood]

Originally Posted by faribran

if we are guessing.....
i have seen a small block produce these kinda symtoms due to a worn out timing chain and gears...

Originally Posted by Green Machine

Could it be the torque convertor locking/unlocking? Try resting you foot on the brake enough to turn on the brake lights next time it happens.

These are what I was thinking. The torque convertor locking/unlocking can also be cause from a brake light switch that is failing (makes the brake light flicker) or a bad Coolant temp sensor (makes the ECM think that the truck may not be up to operating temp.

Originally Posted by 60ratrod

well i did notice when i put my timing light on the motor i saw the timing mark jump around when i was trying to see where my timing was sitting. could this be a timing jump issue?? i'm gonna try to get my ecu read this weekend.

Timing wll jump around if you have not unhookrd EST wire.

[60ratrod]

well i have been thinking that my temp sensor is kinda faulty, cause the temp gauge doesn't get up very high once warm. i'll try the sensor first. then the brake switch. which one is the est wire??

[bwood]

The sensor in question actually doesn't operate the gauge and is mounted in the intake near the thermostat. The gauge sensor is actually mounted in the driver side cylinder head.

Here is a thread that has info on testing the CTS, check post #9
http://www.67-72chevytrucks.com/vboa...d.php?t=355350

Edit: Just reread your last post. You say that your temp gauge isn't going up as high, you may have a thermostat that is not modulating at 195. It may be maintaining a lower temp (staying open too long). This would cause the CTS to see a lower temp all the time or cause it to go in and out of closed loop which would cause the TCC lockup to turn off/on.

[Green Machine]

Could it be the torque convertor locking/unlocking ? Try resting you foot on the brake enough to turn on the brake lights next time it happens.