end of my rope - '99 Tahoe 5.7 misfire

[dougg01]

Misfire happened first in August while on the throttle hard getting onto the interstate.  Set a 300 code for random misfire.  I'm thinking 'spider'.

 

Fast forward to September and it's doing the same thing intermittently on my commute (40-45mi one-way).  This is generally happening at low cruise rpm in OD w/o cruise when accelerating or pulling a grade.

Later in Sept it's doing it without a load on a low-rpm rev.  Get it above 2k rpm and it clears up.  It is having trouble getting over 3k.  It never fails to miss on a grade and only allows very little throttle input to climb or accelerate.

 

This condition did progress to where it is now and has not been mitigated by any of the following:

new fuel filter

new Delphi spider

new plugs (old ones were not bad)

new wires (old ones were not bad)

2 sets of caps and rotors.  Latest are AC DELCO.

Swapped another coil and ign module from a working vehicle with no symptoms.  No change.

unplugged MAF and knock sensor (makes zero difference cold or at op temp).

The cats are no longer an issue.

throttle body cleaned (while off for the spider change)

vacuum leak check around the manifold/TB detected nothing.

 

It will backfire through the TB.

 

It hesitates at tip-in and will misfire revving from idle to 1,000 - 1,500rpm.  Any constant or revving above that it clears up (no load).

 

The distributor head is firm and the rotor base is not sloppy.

 

Truck has ~140k on the clock.

 

I've inspected all the wiring and connectors and everything passes a visual.

 

Only codes are when you really push it then you get the random misfire. P0300 (I think that's the one).

 

We've even put a new battery in and have repeatedly drained the ECM battery between testing new things.

 

Pumping the brakes will cause the engine idle to slow/speed through the cycle.  Closed off the vac port to the booster with no detectible change.

 

Any ideas would be greatly appreciated.

 

I do not have a spare MAF that will work on this.  I have a bunch of a 4.3 S/T-series parts from the late 90s (OBD2 stuff).  IAT, IAC, TB, TPS, MAP, etc.  And I would have already tried them all if I'd gotten any code suggesting it.

[no HD]

Camshaft?

[Jsdirt]

Sounds like you've got a vacuum leak, even though you've checked for that. Get a plastic soda bottle and poke a hole in the cap, fill with water, and squirt all around the intake manifold WITH ENGINE COLD AFTER SITTING OVERNIGHT - if there's a leak there making the engine run this way, you'll hear it getting sucked into the leak. If that checks out, squirt ALL the vacuum lines - around the brake booster, everywhere between the MAF and throttle plate (even the big resonator box), around that big wiring connection on top & throttle body assembly. It should reveal itself.

 

Another thing to check, also in the morning with an ice cold engine, is inside the distributor cap. If you see ANY moisture at all in there, you'll need to modify the distributor housing to allow more airflow into the cap. I believe there's a TSB out for those instructing techs to knock out the screen that's covering a ventilation hole, for better airflow. Wouldn't hurt to drill a 3/8" hole somewhere through the base as well.

[dougg01]

On ‎10‎/‎15‎/‎2018 at 9:00 PM, Jsdirt said:

Sounds like you've got a vacuum leak, even though you've checked for that. Get a plastic soda bottle and poke a hole in the cap, fill with water, and squirt all around the intake manifold WITH ENGINE COLD AFTER SITTING OVERNIGHT - if there's a leak there making the engine run this way, you'll hear it getting sucked into the leak. If that checks out, squirt ALL the vacuum lines - around the brake booster, everywhere between the MAF and throttle plate (even the big resonator box), around that big wiring connection on top & throttle body assembly. It should reveal itself.

 

Another thing to check, also in the morning with an ice cold engine, is inside the distributor cap. If you see ANY moisture at all in there, you'll need to modify the distributor housing to allow more airflow into the cap. I believe there's a TSB out for those instructing techs to knock out the screen that's covering a ventilation hole, for better airflow. Wouldn't hurt to drill a 3/8" hole somewhere through the base as well.

I have been testing without the intake ductwork or the MAF so that narrows things down.

 

I am going to remove the TB and check the IAC.  I will also test with disconnecting the EGR.  I have not found a vacuum leak yet.  Doesn't mean there isn't one.

 

Someone suggested a burnt valve, but it is not consistent.  At times it will seem perfectly normal.  Burnt valves don't automagically fix themselves temporarily.

[Jsdirt]

Yeah, exactly right on that - mechanical problems 99% of the time will be consistent. If something is loose and moving around, say, like a bent pushrod, that could be intermittent, but it's not likely.

 

From your initial symptom, it points to ignition. There is a good chance you may have got defective new parts - chances are very high of that nowadays. Anything from plugs, to wires, and especially caps & rotors today - I'm finding in all my older cars that they're NOT built like they used to be.

 

Was very rare back in the day to get a bad part right out of the box. These days, the phrase shared by many techs across the globe is that NEW means, "Never Ever Worked" - so many get burned daily with bad new parts. The only way to be 100% sure that some part of the ignition system is breaking down is with a lab scope. Any tech that has one, and has used one for any length of time, will be able to tell you exactly what's wrong - it'll take more time hooking the leads up than figuring out what the problem is.

 

What I would do for the time being is make sure that the ignitor unit is getting full battery voltage, and a good ground.

 

All that said, since this engine has that damn spider assembly, that's highly suspect as well. You could have a pinhole in any one or more of the spider "legs", and that would account for the backfire through the intake (this suggests a lean condition). Might not be a bad idea to yank that upper intake manifold and see what's going on inside there.

 

I installed a new spider assembly in a '96 Tahoe about 4 or so years ago - it's an updated assembly with wires and a connector on each injector, suggesting that each injector can now be fired individually more accurately. The assemblies are pretty pricey for what they are, however. 

 

If it were me, I'd throw a pressure regulator capable of reducing it down to 5psi, and install a carburetor ... but that's just me. 

 

[Jsdirt]

Can't seem to edit my post for some reason .. but I wanted to add :

 

Any tech that is experienced using a scope can tell you if you've got a fuel problem too, just by looking at secondary waveforms.

[O_J_Simpson]

Did you change parts at random or follow an established diagnostic troubleshooting method?

[dougg01]

On ‎10‎/‎17‎/‎2018 at 6:04 PM, O_J_Simpson said:

Did you change parts at random or follow an established diagnostic troubleshooting method?

I followed my experience.  Not new to GM stuff.  Not new to Vortec stuff.  Still get curves thrown at me and cobwebs in my brain.