Looking here for guidance. I have posted multiple times about 92 K1500 Regular cab shortbed that I purchased in November. I have been through a diagnosis jungle since then. I have a good friend who is remote from me who has helped me try to troubleshoot, (he is smarter than me) by the way. It is is newly rebuilt motor, almost no miles, done around 2019 and I think at least 2 people ahead of me in trying to get straight. 190 compression all cylinders. Holds steady 19 to 20 in. vacuum. Idles great. Had an exhaust valve stuck open when I got it and thought for sure I had it fixed. Have been through the learning curve on sensors, injection, fuel pump, etc. etc. etc. It is a pretty much rust free Oregon truck and I intend to make it my daily driver with lots of work and even paint at the end.
Greetings Anchor,
With 190 psi compression test #s and 19-20" of vacuum at idle/smooth idle, there's a *lot* that's right with
this motor. And I understand that if you can't ever get the desired results that pulling the motor in order
to figure out the root cause is occasionally the answer of last resort. And knowing that the TBI is sensitive
to cam changes (from stock) ...makes me very curious about the cam you have.
But at the same time you are getting 19-20" of vacuum from what's in there, so it must be a lot closer to stock
than the 'low-vacuum at idle Mother Thumper' cams that can tie the TBI system into knots?
****
But before we pull the motor, let's first prove that there is nothing we can do to make this run better. Since
you are driving a '92, I'm assuming that you are still running the TBI setup. This is a good thing, for you
can change the Initial timing (the foundation that all the computer tables run from) simply with a twist of
the distributor housing.
I would ask you to check the current base timing at a hot idle (by following the procedure in the FSM)
and write it down. And in order to find out what the engine
wants, let's add a couple of degrees of
advance to the Initial timing and see if that makes a difference. (ie: IF the original base timing setting
was at 0°, set it to 2° BTDC. {Before Top Dead Center})
And if this seems to make an improvement, then add another 2° on top of that. And for whatever
compression / cam specs / cam to crank timing combo you have, you may end up adding a total of
5/6/7/8+ degrees before you hit the diminishing returns -or- you start hearing light pinging at
certain throttle angles or rpm bands. (Or the computer starts to pile up excessive KS counts.)
At that point, you then back off the timing 'til the noise just stops, and that is the best timing
you can get for whatever octane fuel is in the tank.
Of course, just how much you can advance your Initial (base) timing is based on several
variables including compression ratio, short vs long duration cam, whether you live in temperate
Oregon vs 110° Phoenix, and of course the octane of the fuel you are feeding it.
But to pull the engine before trying to figure out what it wants more of/less of would be
counterproductive, because we need to probe/understand the limits of the 'goodness' that
this engine will deliver before we decide to pull it out.
NOTE: If you are getting 190 psi of compression at cranking speeds, then you should
only be able to add 4-5° of Initial Timing before the engine starts to protest at certain
speeds/loads on 87-octane gas. But
light pinging is not going to hurt anything, and in
the tuning process it can be trusted even more than timing marks that have slipped a
bit, or the cam/crank phasing is 'dot to dot' but for whatever reason is still not ideal.
Engine starts, runs ok but in accelerating and cruising speed it feels like a plug wire or 2 are off and fuel mileage is pitiful.
As for the roughness at highway cruise, that could be an EGR not metering properly,
tired O2 sensors, or the two injectors are not metering the same. Or something
as simple as counterfeit/sub-OEM quality plugs, spark plug wires, distributor cap, coil,
or even ICM.
How is it possible to have really good looking plugs yet still have an ignition-related
driveability issue? The answer lies in how many
Kilo
Volts does the spark voltage have
to rise to in order for the failure to occur? Let's say that a
healthy HEI system on a
TBI truck can generate up to 35
KV reliably. Let's go one step further and say that
the fresh engine with fresh plugs in it only needs anywhere from 5-25KV in order to
cover ALL aspects of your daily driving. Guess what? With that 10 KV of ignition system
headroom, the driver *never* experiences anything but smooth power no matter what the
driving occasion calls for.
On the other side of this continuum, imagine an older twin of the above motor. Due to the
condition of the cap (or coil, or wires, or even ICM) the max KV this ignition system can
develop is only 25 KV before the spark finds an easier path than the plug gap to ground.
Meanwhile, because of the forgotten/excessively worn plugs installed, when all the variables
line up exactly wrong the plugs need 30+ KV in order to jump the gap and fire the
A/F mixture. But at idle it only needs 6-7KV in order to deliver the smoothness?
*This* is when you can have intermittent driveability issues despite having a mechanically
healthy motor.
****
Thinking outside of the (pure stock) box, we may have a situation where either the Knock Sensor
is too sensitive (they tend to do this if overtightened during installation or sometimes due to sheer
age) ...or the aftermarket valvetrain makes more than stock amounts of chatter which gets picked
up by the Knock Sensor (which is basically just a piezo microphone) and your computer interprets
this unexpected sound as a need to back off the timing?
Don't know if the MT2500 will show you knock counts, but if it does then that would be interesting
to find out. Also if we can find out what your total timing is, then if an issue in the knock sensor
feedback loop is causing the computer to pull out handfuls of timing then this might help to explain
what you are experiencing during cruise?
These are all just informed guesses, but this is what comes to mind while reading your observations.
I for one didn't realize until after the fact just how much a bad actor Knock Sensor could cripple the
behavior of an otherwise good motor by overreacting to normal engine sounds and pulling large
amounts of timing out of the mix in response. (I had a similar issue in a DD that had me close
to pulling the engine before the bad sensor finally crossed the magic threshold and kicked a CEL
indicting the bad KS.)
****
Could it be too-tight valve guides? Absolutely, but normally they have a very specific failure footprint.
(Misfires only occur during sustained heavy demands, but clear up once back to steady, light cruise.)
Somewhere in all of the above lies the answer to your question. But before you pull the motor in
order to check to see if the cam is the root cause, let's try all the simpler stuff first and eliminate
all that as possible causes. Because you want to be sure that if you pull the engine that when it
goes back in it's 100% that it will meet your expectations.
Check out the above, experiment with the initial timing settings a bit, and report back with
your findings.
Best of luck --
