I have a 92 k1500 5.7 TBI. I have been trying to diagnose this thing and fix a misfire under very light load. At Idle it runs smooth and mild throttle to WOT it runs smooth, but the transition from idle to slightly giving throttle results in a misfire like a lope as well as the tachometer jumping up and down sometimes by 500 rpm but have been as high as 1000 rpms and then falls back down to "true" rpm.
This had me thinking that the PCM was receiving false high rpm signals causing excessive fueling. I checked the sensors on Tunerpro and everything looks good. Took a datalog while it was misfiring and looking back nothing really looked out of the ordinary except the tach would spike up to 2500 rpm while holding at 8 mph.
What I have done is to replace the coil and ICM with no change. I then replaced the distributor with another on hand and no change. Other that having two distributors with bad pickup coils I am out of Ideas. Not sure if anyone here has had similar issues.
Hello Mr. Churchill,
If you didn't have the jumpy tach coincident with the misfire, I'd have you take a closer look at the following:
1) TPS sensor. Is there a worn spot where the throttle spends a high percentage of time there? (Think of
a volume knob on an old stereo setup where there's a scratchy spot on the pot?) With TunerPro during
KOEO, is it possible to monitor the voltage from the TPS sensor versus position? Or, just watch the TPS
position signal change smoothly, correctly tracking the way you are moving the throttle plates?
2) MAP sensor. Can you monitor the MAP sensor output while applying a varying vacuum via a Mityvac or similar
instrumented hand vacuum pump? Again we're looking for smooth voltage changes to track with input changes.
3) A popular issue in GMT400 land is that at some unknown point in the past the PO had replaced the EGR valve.
Unfortunately for a lot of mechanics out there, if it will bolt up it must be the correct one, right? Take a careful look
at the varying orifice diameters on the vacuum port to these EGR valves. These fine orifices cause the EGR application
to ramp up over time instead of just a shock of EGR with no orifice when the engine is most sensitive to this type of
disruption. (Lightly loaded, not much A/F charge for this inert gas to be mixed into.)
EGR orifice differences to help with driveability.
****
But as Schurkey pointed out, the erratic tach when this is occurring must be accounted for. The 3 scenarios are:
1) Tach is accurate. It is reporting erratic behavior in the ignition system, whether at the beginning (Pickup coil in the distributor)
the middle (ICM & PCM) or at the end of the chain where the High Voltage pulses are generated. (Coil > Distributor Cap, Rotor, Spark Plug Wires, etc)
2) Tach is flaky, is
not reporting the ignition system activity accurately, but at the same time is also
not causing the malfunction. Ignition system
is actually steady, but for other yet to be determined reason(s) we have a rough-running engine just off-idle, and this roughness is stimulating a
cold solder joint or other intermittent condition in the tach/wiring driving the tach.
3) Tach
is the faulty part in the circuit, and is actually the root cause of the failure due to the fact that it's connected to the coil. This would be a rare
failure, but if you were to temporatily disconnect the tach and the problem disappears, then as improbable as it seems this would be the
highest probability answer.
Since you have changed the Coil, ICM, & distributor, this covers a lot of the real estate involved in the ignition system. But as
Schurkey was suggesting, you should closely monitor the TunerPro RPM being reported while your tach is wack. IF they are
both showing instability, then this ignition system issue must be sorted out. On the other hand, IF the TunerPro reading is
steady, but the tach is flaky, then the tach is simply a seperate issue that can be dealt with after the engine is repaired and
running smooth.
****
Unfortunately I'm not familiar with what TunerPro can and can not show when connected to a OBD1 system. Since there is
no CMP & CKP sensor in the TBI system, I'm sure that we don't have misfire counts by cylinder. Or total misfire counts for
that matter. But at the same time we should be able to look at the 'fuel trims' and O2 crossover counts? (TBI speak is
Integrator and Block Learn, whereas OBD2 terminology was standardized as Short Term and Long Term fuel trims.)
In English, if we have a persistent lean or rich condition and the computer is trying to tune around it, then this would
show up in your TunerPro data, and this might give us a hint as to what direction to go.
****
This reply might seem to be all over the place, but in reality the TPS & MAP sensors have a lot of authority in a speed density
system like the TBI, and the wrong EGR valve can be a real speed bump just off-idle in driveability land.
And we also have to answer what's going on with the tach. Is it giving us valuable visual hints, or is it just showing it's
age and muddying up the troubleshooting waters? At this point all I can offer is more questions than answers. Hopefully
in the above there's something that proves helpful.
Let us know what you discover.
Best of luck --
