SOLVED - 1990 Suburban L05 - Crank, no start. Bad ECM?

[Road Trip]

Your plugs look textbook perfect. 7 of 8 shown, assuming the 8th matches the others?

I was taking notes, thinking that you might have a cracked reluctor magnet in the dizzy (thinking of Schurkey's pic)
...but then you mentioned trying the "known good distributor".

So thinking through the Spark, Fuel, & Compression triad, I thought well maybe too much water in the gas?

At this point, the only other thing I could think of would be bad gas, but it was running around just fine the days prior.

But since gas is ~6 1/2 lbs per gallon, and water is 8 lbs/gallon, if you happened to fill up prior to parking
the vehicle for a couple of days, it's possible that you were accidentally sold a slug of water along with the gas?

And since the water is heavier, it subsequently settled out surrounding the fuel pickup, concentrating this into
the problem you are currently experiencing?

I mention this because you were able to get the engine to run on starting fluid?

The engine will start for a second or so with a small shot of starting fluid, so you can take that as you may.

You have spark confirmed by a HEI tester.

I'm assuming no change in compression, for you didn't mention the starter sounding too fast or
with an odd cadence.

You are seeing visible fuel flow from the TBI injectors. Which wouldn't be happening unless the computer
was getting the Reference pulses from the Reluctor in the dizzy and fuel pressure from the pump.

Q1: Given the above, did you happen to refill the tank before you last parked it?

Q2: Does the fuel flow during cranking look normal? Or does it look exaggerated?
When it doesn't start, if you were to hold the gas pedal to the floor while cranking,
would it now start? (Thinking Clear Flood mode.)

Again, does the starter sound normal? Or is it spinning faster/easier than normal?

(I'm assuming normal given the compression test results you shared earlier.)

The suddeness of the stoppage plus the pretty plugs = this must be a healthy motor.

It's got that funky fuel vibe from over here.

Looking forward to what you discover.

Best of luck --

 

[dixiebandit69]

Okay boys, I've got an answer: Bad MAP sensor.

Yup, I **** you not.

After making a test rig with a bucket of fresh gas and a spare fuel pump, it still wouldn't start.

Getting desperate, I took out the injectors to inspect the screens. They looked like new.

After putting them back in, I tried the key, and it started for a second from the dribble of gas that leaked out when I pulled the injectors - clearly, this HAD to be a fueling issue.

What else controls fuel? MAP sensor (I had tested the TPS a couple of nights ago- checked out good).

I happened to have a known-good MAP sensor* lying around, I plugged it in, and it fired right up.

I shut it off, plugged the old one back in, and nothing. So I'll be running the "new" one.

Thanks for everyone's help.

*In case y'all are wondering how I have all of these "known good" TBI parts, I did a 5.3 swap on a '93 Blazer a couple of months ago, and the owner didn't want any of his old parts.

 

[Road Trip]

Okay boys, I've got an answer: Bad MAP sensor.

Yup, I **** you not.

After making a test rig with a bucket of fresh gas and a spare fuel pump, it still wouldn't start.

Getting desperate, I took out the injectors to inspect the screens. They looked like new.

After putting them back in, I tried the key, and it started for a second from the dribble of gas that leaked out when I pulled the injectors - clearly, this HAD to be a fueling issue.

What else controls fuel? MAP sensor (I had tested the TPS a couple of nights ago- checked out good).

I happened to have a known-good MAP sensor* lying around, I plugged it in, and it fired right up.

I shut it off, plugged the old one back in, and nothing. So I'll be running the "new" one.

Thanks for everyone's help.

*In case y'all are wondering how I have all of these "known good" TBI parts, I did a 5.3 swap on a '93 Blazer a couple of months ago, and the owner didn't want any of his old parts.

And if I remember correctly, your MAP sensor wasn't throwing any codes, right? That's just not fair.

Whatever you do, please tag that MAP as bad but at the same time don't throw it away. (At least not
until we can figure out how to perform a proper post mortem on the 'proven bad' part.?)

Sorry about you having to go through rigging up a "fresh gas" test setup. But thanks to good follow-on
troubleshooting on your end, your adventure really proves that the MAP is a critical part of the TBI's Speed Density
engine bay.

Thanks for coming back and sharing the real-world fix. Please consider adding the word (Solved) to the title,
so that others researching similar TBI No Starts know that this thread ends with a fix, and isn't just abandoned.

And congrats on being able to fix your rig with a spare MAP sensor instead of replacing the ECM & still have a
No Start on your hands.

Safe travels --

 

[Road Trip]

Okay boys, I've got an answer: Bad MAP sensor.

Circling back to the original post, you were having difficulty with the SES light and retrieving the codes.

I cannot get code 12 to come on when jumping the A and B terminals on the ALDL connector.
I verified that the ECM has B+ voltage (12+ volts), Key-on power, and good ground (less than 3 ohms). ALDL has good ground as well.

I'm now wondering if the MAP code had been triggered but it couldn't be communicated to us?

What's the post-fix behavior in this area? Do you now have a steady SES light with a KOEO? (Key On Engine Off)

And do you now get the flashing 12s when jumpering Pin A to Pin B?

Just curious. Solid communication the computer is good, especially as these machines continue to age.
Hope your setup is 100%.

 

[dixiebandit69]

Circling back to the original post, you were having difficulty with the SES light and retrieving the codes.

I'm now wondering if the MAP code had been triggered but it couldn't be communicated to us?

What's the post-fix behavior in this area? Do you now have a steady SES light with a KOEO? (Key On Engine Off)

And do you now get the flashing 12s when jumpering Pin A to Pin B?

Yes, I'm getting a continuous SES light with the key on, and code 12 with the ALDL jumped. All of the wiring in that circuit, from the fuse box, to the cluster, to the ECM is good, about 2 ohms.

My guess is that the MAP was reading far out of specs, but not badly enough to trip the SES light. I will test it later, and post results.

If I'd had a GM or similar scanner to read the data like @Schurkey had recommended, I probably would have found this much sooner.

Another fun fact: the original distributor did have a cracked magnet, and I'd known about it for awhile, and it still ran. The "new" distributor isn't cracked, and the truck seems to run a little better now.

 

[Road Trip]

Yes, I'm getting a continuous SES light with the key on, and code 12 with the ALDL jumped. All of the wiring in that circuit, from the fuse box, to the cluster, to the ECM is good, about 2 ohms.

Excellent. Whenever I read that someone can't communicate with the computer but the vehicle still runs, it
feels like the owner is on thin ice -- everything will be OK until the vehicle stops running, and then it's going to
take lots of time/effort to try to sort that out before it becomes a mad thrash due to time constraints. (!)

My guess is that the MAP was reading far out of specs, but not badly enough to trip the SES light. I will test it later, and post results.

If we can quantify the MAP's deviation from it's calibration, this will be valuable for future troubleshooting. In the perfect
world, self-test strategies & failure thresholds are established so many percent before that part is bad enough to prevent
engine starting? But in the real world sometimes the opposite occurs.

Thinking about this, I wonder if you can see a difference in the reported BARO between the 2 sensors. (Once you get ahold of
some live data capability.)

If I'd had a GM or similar scanner to read the data like @Schurkey had recommended, I probably would have found this much sooner.

Another fun fact: the original distributor did have a cracked magnet, and I'd known about it for awhile, and it still ran. The "new" distributor isn't cracked, and the truck seems to run a little better now.

All good input. The more we can learn from this, the less we will have to fear mystery MAP failures.

Again, thanks for closing the loop on this thread.

Savor the fix --

 

[Schurkey]

Y'all know how I harp on using scan tools. Don't need to go over that territory.

In this case, you could connect each MAP sensor to the wire harness, turn the key "on" (engine doesn't have to be running) and then back-probe the terminals to verify voltage using a multimeter, while varying the vacuum with a hand-held vacuum pump or even just sucking on the vacuum connection. It's way more cumbersome than a scan tool, but it'd work to get answers about how the one MAP differs from the other.