1987 454 TBI stalls at idle after a short time

[beady]

Hello again, I have this new to me 1987 454 TBI suburban that I’ve been chasing problems down in. Today’s issue is stalling after a few mins of idling.

The previous problem was power being way down, to the point where it would max out at 47 mph and you could only use literally the first 1/4” of throttle travel before it would rev seemingly ok, but make even less power. This turned out to be a loose/worn distributor/cap. I replaced distributor and more, and it ran pretty well for a week. I put maybe 50 miles on it, but last week I started it and walked away and it stalled a minute or so later. Now seems to be the truck’s MO. What I’ve replaced or checked:

Brand new:
DUI distributor, coil, wires
Delco plugs
MAP sensor
Delco injectors
Delco FPR rebuild kit

Fuel pressure was 13 psig after filter
Timing set at 4-5 dtbc with ignition wire disconnected

Here’s a video of it stalling I took earlier today:

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The last thing I did was the distributor, coil, plugs, wires all at once. The idle sounds great, nice and smooth, no bounce or bauble. I’d love any thoughts or ideas on what to do/check next.

Thanks,

Mike

 

[Vbb199]

Have you checked for any vacuum leaks ? How old is the map sensor.

 

[beady]

Have you checked for any vacuum leaks ? How old is the map sensor.

MAP sensor is a couple months old, a couple hundred miles old, tops. I haven’t tried spraying starting fluid around intake to check for leaks yet. But it doesn’t have a high idle, and doesn’t hunt either.

 

[Vbb199]

If you had a means to scan the obd1 interface, you could check manifold pressure.... but since those all seem to be gone from the internet and me and handful of people are the only people to own them now, you can alternatively back probe your MAP's output wire for its 0-5 volt signal, if youre DMM savy... getting a manifold pressure reading is pretty helpful in diagnosis of the old beloved tbi systems....


These systems are simple.

They need a map reading for fueling, a coolant temp sensor reading for fuel flow offset, a TPS sensor for the obvious, and a o2 sensor to adjust fuel finally.

If you care to check the map pressure reading, i can give you some instructions.

If you want to check your TPS voltage, i can give you some instructions, but they both require a DMM.

The map might be good, but it could be doing it job, and reading manifold pressure problems... we wont know without a scan tool , or checking it with a DMM, or at least using a vacuum gauge on the tbi.


Whats the story with the TPS sensor? New? Old?
Coolant temp sensor ?

O2 sensor?

 

[fast 99]

FYI, TBI primarily uses MAP for fuel trim. O2 sensor only effects trim after reaching operating temp and usually will not stay hot enough after idling a minute or so.

As stated above, data would help immensely with diagnostics.

Another item check for excessive AC voltage.

Would be helpful to know manifold vacuum reading. If low would cause MAP to dump fuel causing stall. Without more information it's all a guess, from a bad ground to leaking EGR.

 

[beady]

If you had a means to scan the obd1 interface, you could check manifold pressure.... but since those all seem to be gone from the internet and me and handful of people are the only people to own them now, you can alternatively back probe your MAP's output wire for its 0-5 volt signal, if youre DMM savy... getting a manifold pressure reading is pretty helpful in diagnosis of the old beloved tbi systems....


These systems are simple.

They need a map reading for fueling, a coolant temp sensor reading for fuel flow offset, a TPS sensor for the obvious, and a o2 sensor to adjust fuel finally.

If you care to check the map pressure reading, i can give you some instructions.

If you want to check your TPS voltage, i can give you some instructions, but they both require a DMM.

The map might be good, but it could be doing it job, and reading manifold pressure problems... we wont know without a scan tool , or checking it with a DMM, or at least using a vacuum gauge on the tbi.


Whats the story with the TPS sensor? New? Old?
Coolant temp sensor ?

O2 sensor?

Thanks for the detailed reply. I had a scanner that would give ALDL live data until about a week ago. My son borrowed it for his friend to use, and the kid’s car got broken into and the scanner stolen from his trunk : (

I’m handy with a DMM (love my fluke 179 that has a thermocouple for temp readings), and have repaired a couple audio amps in the past, so would welcome what reading I should be looking for at the various sensors.

I’ve only had the truck a few months, so don’t know the age of anything I didn’t mention I’ve replaced.

I’m at my gf’s farm and not sure if my vacuum gauge is here or at my house 80 miles away. I haven’t checked vacuum yet. I do have a mightyvac type hand pump here, might be able to rig something up to use that…..

Thanks,

Mike

 

[Vbb199]

Thanks for the detailed reply. I had a scanner that would give ALDL live data until about a week ago. My son borrowed it for his friend to use, and the kid’s car got broken into and the scanner stolen from his trunk : (

I’m handy with a DMM (love my fluke 179 that has a thermocouple for temp readings), and have repaired a couple audio amps in the past, so would welcome what reading I should be looking for at the various sensors.

I’ve only had the truck a few months, so don’t know the age of anything I didn’t mention I’ve replaced.

I’m at my gf’s farm and not sure if my vacuum gauge is here or at my house 80 miles away. I haven’t checked vacuum yet. I do have a mightyvac type hand pump here, might be able to rig something up to use that…..

Thanks,

Mike

Ok, so on the map sensor, you have 3 terminals at the plug.

5 volt reference (grey/term c), MAP signal (green/term b), and ground (black/term A)

First, with key on engine off, back probe the 5 volt reference, just make sure its showing 5v. It should be. The truck probably wouldnt run without it.

Check the signal wire, and it should also be 5v or so, maybe a little less. Which is 100-105kPa. Thats normal key-on, engine off reading.

With the probes still in the reference and ground, turn the truck on, and see what the reading is. It will be between 0-5v. if the trucks running right it will presumably be about 1.5-2.0v at idle.

Let us know what you find there.




On the TPS sensor, you also have 3 terminals.
Sensor ground (black/term A), TPS signal (blue/term B), and 5 volt reference (grey/term C )

The same thing goes here... with key on, engine off, you ought to see 5 volts on the reference.

And over on the signal wire, you should be seeing like, .6-1.0 volts with the engine on, key off.
It all depends on sensor orientation.. and mfg, but the important part is that we see somewhere in that range with the throttle blades closed.


For chits and giggles, slowly open the throttle , and see if the TPS voltage rises in correlation with your movement...

Report back on what you find with those sensors to start.