93 Blazer hunting idle and misfire

[Schurkey]

it is also 0v reading to the computer

I don’t remember my scan tool showing MAP voltage

Is it showing 0v or not?

I’m thinking you have the properties of measuring the vacuum wrong. A vacuum is measured in inches of Mercury, the more vacuum the more in hg that is on the scale. Not the greater the atmosphere the greater the in hg. If there is no vacuum there is a reading of 0in hg. That’s full atmospheric pressure. And pressure is measured in psi, or bar.

https://www.reference.com/science/normal-barometric-pressure-f584ab613a620f62

Barometric Pressure of 29.80 to 30.20 inHg

If the pressure is steady or rising, expect the current conditions to stay the same. If the pressure drops slowly, there will be minimal change. If the pressure drops quickly, expect rain or snow.

Compare your local weather report (barometric pressure) against what your MAP sensor is showing (29.8)

Your engine can't develop 29.8 inches of mercury VACUUM parked at idle. Shut the engine off, turn the key back on--NO MANIFOLD VACUUM with the engine not running. See what the MAP sensor shows.

You are not reading a vacuum gauge, like the instructions in your link. You are reading a PRESSURE sensor. "0" is "0 pressure", (a deep vacuum.) 30-ish is full atmospheric pressure, (no vacuum.)

Your MAP sensor is showing 29.8 in Hg, ATMOSPHERIC PRESSURE.

 

[Massey]

Ok... after a day of messing with this pos we discovered a couple things.

first the Map Sensor. It wasn’t plugged in properly. It was plugged in far enough that the clip retained it, but when it was last plugged in by my buddy, the waterproof ring bunched up on the back side making an open connection and thus the wrong readings. Once plugged in we got a good reading, but no better running.

unplugged the vacuum hose to the brake booster and it ran great... with a huge vacuum leak slightly too high of an idle. Once I used my finger to plug the hose, it went back to surging.

a couple weeks ago when I asked him the condition of the cap and rotor he said they were fine... we looked again and they were buggered up bad. We replaced the cap, rotor, module, and wires (all under warranty) and the truck runs much better... not perfect but better. I checked the timing and right on 0 as it should be, but while I was timing it I would notice my timing light would flutter like it was cross arcing.

this is one of those times I would love to have an old engine analyzer. I think one of those would be key to figuring this out.

after the cap and rotor, and the engine dropped into closed loop mode, the LTFT turned to 128, the O2 was switching from rich to lean, and it did run better. Oh and we tried another computer from the same year and model truck, no changes at all.

 

[PlayingWithTBI]

We replaced the cap, rotor, module, and wires (all under warranty) and the truck runs much better... not perfect but better.

How's you coil? You can take a spray bottle and wet the coil with water (not the wires) while idling. If it stumbles, your coil is bad which can cause your ICM to go out too. I had a Pertronix Flame Thrower coil do that and it took out my ICM too. Both had less than 2000 miles on them. After I replaced the ICM mine ran better but still had a slight stumble - until I replaced the coil. Upon further inspection here's what I found on the coil, notice the burn marks between the body and iron frame.

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[Schurkey]

this is one of those times I would love to have an old engine analyzer. I think one of those would be key to figuring this out.

An ignition oscilloscope is a fabulous tool. They've gone out-of-fashion. About all you can get now is hand-held 'scopes with very limited functionality. God bless eBay and used equipment.

after the cap and rotor, and the engine dropped into closed loop mode, the LTFT turned to 128, the O2 was switching from rich to lean, and it did run better. Oh and we tried another computer from the same year and model truck, no changes at all.

You've made real progress.

At this point, it'd be worth doing a cylinder-balance test.
Warm-up engine, then shut engine off.

Connect a vacuum gauge to manifold vacuum.

Dull the sharp tips of eight SMALL nails on a stone or bench-grinder.

Wipe a smear of silicone dielectric grease on each of the nail points.

SLIDE the tips of the nails between the plug wire and the distributor cap boot so that the nail travels up near the metal terminal of the plug wire. DON'T PIERCE the insulation of the plug wire or the boot.

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Start engine.

Ground the lead of a test-light, or use a grounded jumper wire to touch each of the nails in turn. You'll short-out the spark to each cylinder, one at a time. Don't short the cylinder longer than 5 seconds at a time, and let the engine recover for 5 seconds minimum before shorting the next cylinder.

Watch the vacuum gauge as each cylinder is shorted. The vacuum should reduce the same amount for each cylinder. Any cylinder that DOESN'T reduce vacuum as much as others is weak. Any cylinder that doesn't reduce vacuum at all is dead. My truck had two dead cylinders at idle--#1 and #3. Turned out to be a failed intake gasket. Squeezed out of position, allowing a direct air leak into those runners.

If every-other cylinder drops less than the alternate cylinders, you've probably got an issue with one injector, or a vacuum leak into one side of a dual-plane intake manifold.

Don't forget to remove the nails after you're done testing. I forgot once, had spark shorting to the air cleaner housing when I drove the car next.

Might be worth re-checking the IAC and TPS, along with the gasket under the throttle body.

How's you coil? You can take a spray bottle and wet the coil with water (not the wires) while idling. If it stumbles, your coil is bad which can cause your ICM to go out too.

Excellent test for a cracked coil housing, or a "carbon track" in the plastic.

I also connect a spark-tester to verify coil output power. There's several kinds of spark testers, this is my favorite style. The spark tester needs to be calibrated for HEI ignitions.

https://www.amazon.com/Performance-Tool-W86553-Ignition-Tester/dp/B003WZXAWK/ref=sr_1_11?dchild=1&keywords=HEI+spark+tester&qid=1614483342&sr=8-11&th=1