1997 5.7 Vortec misfire/stumble

[DaHoe]

It doesn't happen necessarily when the converter locks up but it's present while the converter is locked up. In other words I don't feel a thing through all the gears and when it gets to 4th and converter is locked and all is good. I can modulate the throttle and make it do it almost on command but the converter must remain locked to keep a load on it. Parts I have changed so far with zero difference:

- new distributor with offset set dead on 0
- new ignition module
- new coil
-complete tune up
-verified fuel pressure while driving
-replaced EGR with a known good one
-verified TPS, MAF, O2 cross counts, RPM signals with snap on scanner while driving
-cleaned and verified grounds on TStat housing and back of passenger cyl head
-verified TCC slip RPM..I can't see anything jumping around on the data graph.

I have shaken and yanked on the engine harness all over to try and see if I can get a stumble while it's running. The truck is plenty driveable but it's just annoying because I know it's not right in my mind. I'll keep at it when I have the time and thank you for your input we seem to be on the same path. I'll definitely report back once I figure it out or try other suggestions. My next step is getting a known good ECU just to try. I have 2 spare 99s but mines a 97 go figure

 

[docstoy]

Did spark plugs all look the same ( build up of any kind ) or pull your O2 and inspect make sure it doesn't have any buildup .

 

[DaHoe]

No build up at all looked like they had been changed within 30k miles or so. Truck only has 120k original miles and doesn't smoke or burn any oil. I pulled out delco plugs and put delcos back in.

 

[mjskins]

It doesn't happen necessarily when the converter locks up but it's present while the converter is locked up. In other words I don't feel a thing through all the gears and when it gets to 4th and converter is locked and all is good. I can modulate the throttle and make it do it almost on command but the converter must remain locked to keep a load on it. Parts I have changed so far with zero difference:

- new distributor with offset set dead on 0
- new ignition module
- new coil
-complete tune up
-verified fuel pressure while driving
-replaced EGR with a known good one
-verified TPS, MAF, O2 cross counts, RPM signals with snap on scanner while driving
-cleaned and verified grounds on TStat housing and back of passenger cyl head
-verified TCC slip RPM..I can't see anything jumping around on the data graph.

I have shaken and yanked on the engine harness all over to try and see if I can get a stumble while it's running. The truck is plenty driveable but it's just annoying because I know it's not right in my mind. I'll keep at it when I have the time and thank you for your input we seem to be on the same path. I'll definitely report back once I figure it out or try other suggestions. My next step is getting a known good ECU just to try. I have 2 spare 99s but mines a 97 go figure

I have a 99 4wd 5.7 Tahoe that kinda does the same thing. But mine misses when it's in overdrive on a pull ,like going up a incline to the point where it's like it's flooding it.

 

[docstoy]

Have you serviced trans. With fluid and filter ? If 4 ck at front of drives haft for to much play could be bushing /bearing bind saw that on another thread, ck MAF sensor plug in to make sure wires are seated . Let us know,good luck

 

[Schurkey]

99 4wd 5.7 Tahoe

mine misses when it's in overdrive on a pull ,like going up a incline to the point where it's like it's flooding it.

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Assure the fuel filter isn't plugged.

2. Verify all "tune-up" items--parts and procedures. Distributor cap, rotor, plug wires, spark plugs. O2 sensor. Check PCV, EGR, EVAP systems, and the heated air intake (if used.) Check initial timing, and see that the electronic advance works. Verify cranking compression pressure.

3. Connect a scan tool, look for "codes", but more important, verify EVERY sensor and computer output. Look at the misfire COUNTS and fuel trims when the engine is acting up.

 

[docstoy]

mjskins , I was responding to DaHoe and you were entering your post. I have slow internet service in remote location and usually have to put whatever device I'm using to get enough service to function and it is slow . Now DaHoe need you to read my last post and respond, been out mowing grass.