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.

[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.

[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.

[60ratrod]

i guess that would make sense, but once i get to where i'm goin the motor does feel like its at operating temp, so i think my temp sensor might be shot, but replacing the t-stat would be a good idea too

[60ratrod]

ok so i replaced the cts, temp gauge switch and t-stat. well my gauge works a lot better now, but now its reading 2 marks above the 210 mark. is this normal? but it doesn't go any higher than that from what i can tell. i didn't feel any misfiring/stumbling, but i will find out tomorrow on my way in to work if that fixed that issue. could it be the lock out on the t/c isn't working if its reading that high?? and its a 195 stock replacement t-stat

[bwood]

I'm thinking it should read between 195 and 210. If you got Autozone parts take them back and get you money and get the OEM stuff from NAPA or Oreilly Auto, they won't be much more than what you paid for the Azone parts. I have a buddy that swears by Azone because of their LT warr, but he constantly has problems with theirs parts. One good example is their thermostats (I don't know who makes them for them), he has changed the same one on his wifes car three times last year for various reasons:
one failed closed
one kept too high of a temp
one kept too low of a temp and tripped the MIL

He finally got one from NAPA and no issues. Same goes for the ign module in his S10 and the water pump for the same truck (he is on his 4th one in 2 years first two leaked and the third one seized up on the highway, but the good news is the warr replaced them all and he is good at changing them now.

[60ratrod]

well i'll try one from csk then. would getting a 180 degree be a bad idea with my tbi 4.3??

[bwood]

Bad Idea?? Yes, the computer is looking for 195 to put the truck in closed loop. If you put in a 180 you constantly stay in open loop. Trucks that run cooler tstats will require a different/reprogrammed chip to compensate for the cooler temp. Also remember that the sender for the gauge is in the head and the one for the computer (the one that may be causing you issue) is in the intake.

[60ratrod]

yeah, found that one out the hard way. i wasn't gonna switch the cts but i was gonna change the temp switch, but come to find out that i had gotten the cts instead of the temp switch so it was back to the parts store to get the temp switch. so since i had both of those i decided to change both of them out. so now all i gotta do is re-replace the t-stat and i should be good to go. i think the old temp switch might have been ok cause the old t-stat looked like it wouldn't close up to save its life

[60ratrod]

ok so the new t-stat and cts did not fix my issue. i found it happens between 50-60 mph, when going from around 1/3 throttle to 2/3 throttle. if i press the throttle faster, it doesn't misfire/stumble. but if i go slowly it stumbles/misfires. my dad told me a couple of things. he told me that it could be the accel tune up kit that i used(cap, rotor, plugs and wires), or the egr valve. he also told me that if i disconnected the vac line going to the egr valve i should almost immediately get a check/service engine light. so i disconnected the vac line, and got no check/service light. i also thought that it might be the tbi spacer that i installed cause all of these problems that i've been getting only started after i installed the spacer and aftermarket spectre dual snorkel "muscle car" air cleaner. its one of those 2 inch straight bore spacers. could the spacer, egr valve, and/or air cleaner be causing this issue??