Random miss is killing me. F/S Blazer 5.7 tbi

[Bigdog67]

For the most part I’ve always had a random misfire. Recently it got extremely worse. From a dead stop or rolling if I hit the gas she would bog down as if she had no pow. Put a new 02 sensor in and it helped pulled egr, again it helped. Ran a scan on it and I noticed she had 14 degrees of timing in. I disconnected the est wire and checked the timing and it red 6-8 degreees plugged est back in and it’s not even hitting the mark it’s so high. Trying to lose my mind over this. Checked for vacuum leak and could not find one. Injectors are questionable. As she is my daily I need to get her right. Help please!!!

 

[Schurkey]

WHAT YEAR VEHICLE?

What is the fuel pressure? How old is the fuel filter?

When was the last time you checked the plugs, plug wires, cap 'n' rotor, PCV? Did you inspect the distributor pickup including the magnet?

What readings are you getting from the coolant temp sensor, MAP, TPS, and all the rest of the sensors?

Have you verified that the EGR is working properly?

 

[DonYukon]

For the most part I’ve always had a random misfire. Recently it got extremely worse. From a dead stop or rolling if I hit the gas she would bog down as if she had no pow. Put a new 02 sensor in and it helped pulled egr, again it helped. Ran a scan on it and I noticed she had 14 degrees of timing in. I disconnected the est wire and checked the timing and it red 6-8 degreees plugged est back in and it’s not even hitting the mark it’s so high. Trying to lose my mind over this. Checked for vacuum leak and could not find one. Injectors are questionable. As she is my daily I need to get her right. Help please!!!

TBI or Vortec?

Just shotgunning some things
Is your plug wires actually Seated?
Check all plugs independently for spark?
what the fuel pressure at?
Will the truck run any better with any vacuum lines disconnected?

 

[Bigdog67]

Tbi. Checking wires last night one wasn’t seated all the way but no change. Haven’t tried the pulled vacuum line yet, but will shortly as well as spark. Did change out coil wire and did notice a change. Resistance on old wire was a little higher than my spare. Yea I still carry a spare coil wire. Old habits hard to break. FYI it’s a 93. All sensors seem to be ok. I pulled the EGR and capped it off must say that made the biggest difference. Plugs wires etc. have all been replaced about 2 months ago as well as fuel filter. Pump was changed this past week. Going to put a new thermostat in as well as check the ETS.

 

[Ron88]

Have you tried removing one wire at a time and driving the truck, or power braking it to get the miss narrowed down to a specific cylinder?

 

[Bigdog67]

Have you tried removing one wire at a time and driving the truck, or power braking it to get the miss narrowed down to a specific cylinder?

I believe I have it narrowed down to # 3 cylinder. I have to pull the plug and see what it looks like.

 

[Schurkey]

I learned from this forum that #3 is the common problem when the distributor cap fails.

Doesn't mean that has to be the problem, just that it's apparently a common result of a bad cap.

 

[BPR Speed]

IIRC the TBI SBC 305 and 350 are set to 0 degrees advanced for ignition timing with the brown wire at the firewall unplugged. TBI 454s are set at 4 degrees advanced. 6-8 degrees of base timing does make a difference after all the timing from the ECU is added in. I'd mark the dampener at 0 TDC and retime the engine at normal operating temps down to 0 degrees. After that I'd look into the plugs after a hot run around town in the truck and see which cylinder reads differently. If #3 cylinder still reads questionable, I'd move into a compression and leak down test. TBI heads beat up the valve seats over many miles. If all cylinders check out good, I'd then question electrical gremlins like grounds, ignition control module, rotor, cap and plug wires. Electrical issues seem to be the biggest trouble for many as repairs are usually filed up to the repair by replace category with little troubleshooting time for verification of good parts. That's where it gets costly and frustrating for the owner.