I Experienced Vapor Lock on a TBI

[studigggs]

1990 5.7. Was hot in CA last week (110F) and I sat idling with A/C blasting for 30 minutes which I don’t often do. Short trip home and I had to go out again 20 min later. Crank no start. Sprayed some carb cleaner down the intake and she fired. Had to be a fuel issue.

The pump would prime upon KO but it sounded more muted than normal. That’s about all the diagnosing I did as I Had to skeedattle and grabbed another vehicle for my errand. When I came back she fired no problem. Been running normally since.

Did some research on if vapor lock is even possible on a TBI and saw mixed opinions. Digging into the troubleshooting manual, there is actually a hot fuel module that is specifically intended to prevent vapor lock on TBi 454 and R body 5.7s by energizing the fuel pump for 20s on KO instead of the standard 2s. This leads me to believe that vapor lock is possible for a TBI.

Now, I live in the desert, so driving in heat is the norm. It took a long time of what I would consider extreme environmental operation to cause this symptom so I’m not too worried about it. Will keep it in the back of my mind for future reference. Perhaps I’ll make up a jumper so I can manually engage the fuel pump from the ALDL if I don’t have time to wait for a cool down.

 

[PlayingWithTBI]

there is actually a hot fuel module that is specifically intended to prevent vapor lock on TBi 454 and R body 5.7s by energizing the fuel pump for 20s on KO instead of the standard 2s.

yeah, there was a guy on here a while back that designed and built his own circuit board for that since, replacements are pretty much NLA.

Now, I live in the desert, so driving in heat is the norm. It took a long time of what I would consider extreme environmental operation to cause this symptom so I’m not too worried about it. Will keep it in the back of my mind for future reference. Perhaps I’ll make up a jumper so I can manually engage the fuel pump from the ALDL if I don’t have time to wait for a cool down.

What desert do you live in, the Mohave? I'm lucky I guess, I haven't had any issues with hot starts here. I did have a '60 Impala with a weak fuel pump back in the 70s when I lived in Milwaukee, I had to wait almost 3 hours for it to fire up. I couldn't afford a new pimp so I lived with it for a summer

 

[GoToGuy]

How long did you crank? The module on the big blocks installed for the much greater amount of heat generated, specifically upon stopping . On shutdown , without cooling flowing through. Under hood temp heat soak would rise dramatically. Therefore with higher pressure fuel flow at a longer rate the vapor boiled fuel would be pushed out.
You can crank for 30 seconds, 1 minute off, 30 seconds , 1 minute off, 30 seconds then 5 minutes off.
That's the advantage pressurized fuel , pump prime or oil pressure signal operation will overcome a vapor lock.

 

[studigggs]

How long did you crank? The module on the big blocks installed for the much greater amount of heat generated, specifically upon stopping . On shutdown , without cooling flowing through. Under hood temp heat soak would rise dramatically. Therefore with higher pressure fuel flow at a longer rate the vapor boiled fuel would be pushed out.
You can crank for 30 seconds, 1 minute off, 30 seconds , 1 minute off, 30 seconds then 5 minutes off.
That's the advantage pressurized fuel , pump prime or oil pressure signal operation will overcome a vapor lock.

Good to know. So the abnormal sound of the fuel pump was because the high pressure vapor empties the supply tube back to the tank?

Big blocks makes sense, but what interested me was that the hot fuel module was also used in r bodies with the 5.7 according to the factory manual. I’m guessing R bodies in ‘90 would have been 3/4 and 1 ton square body service trucks and such?

Longest crank was probably 30s. I cranked it long enough to see oil pressure + 5-10s to see if I may have had a fuel pump relay or control problem. Because the fuel pump didn’t sound right, my next step was to check fuel pressure, but it fixed itself.

 

[studigggs]

What desert do you live in, the Mohave?

That’s the one.

 

[L31MaxExpress]

The G-vans had that hot fuel module. Mine tried to catch on fire. They burn up and let the smoke out.

 

[Road Trip]

yeah, there was a guy on here a while back that designed and built his own circuit board for that since, replacements are pretty much NLA.

I remember reading that thread, talk about not taking NLA for an answer: Airstream owner recreates Hot Fuel Module

Did some research on if vapor lock is even possible on a TBI and saw mixed opinions. Digging into the troubleshooting manual, there is actually a hot fuel module that is specifically intended to prevent vapor lock on TBi 454 and R body 5.7s by energizing the fuel pump for 20s on KO instead of the standard 2s. This leads me to believe that vapor lock is possible for a TBI.

Now, I live in the desert, so driving in heat is the norm. It took a long time of what I would consider extreme environmental operation to cause this symptom so I’m not too worried about it. Will keep it in the back of my mind for future reference. Perhaps I’ll make up a jumper so I can manually engage the fuel pump from the ALDL if I don’t have time to wait for a cool down.

Thanks for taking the time to share what happened to you. I'll bet you a dollar that the Hot Fuel module was developed
way back when this issue must have cropped up during prototype hot weather testing at GM's desert proving grounds in Mesa, AZ.

NOTE: For anyone researching the FSMs, I discovered that when searching for the common term ALDL (Assembly Line Diagnostic Link)
that this comes up empty when searching, at least in the FSM wiring diagram .pdf files. However, if you search for DLC (Data Link Connector)
everything shows up. Just like there is OBD in the old manuals, but not OBD1. (This term didn't exist prior to OBD2, at least not
in the FSMs.)

Anyway, your comment about engaging the fuel pump from the ALDL reminded me of a diagram I saw. I pulled it up,
decluttered the page of unrelated stuff, and ended up with this: (Look for Pin G)

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With the fuel pump relay off, the internal contact of the switch is connected to relay pin C,
which in turn is connected to Pin G of the DLC (ALDL) connector with the circuit #490 Orange
wire. So if you apply +12v to pin G you will end up driving circuit #120, which is the Gray wire
connected to the fuel pump.

FWIW, I've attached a drawing of the ALDL (DLC) connector that identifies the pinout.

****

I share all this because it would be a simple matter to replace the original Hot Fuel Module
(made of unobtanium) with a simple 'momentary on' switch, wired on one side to the
correct fuse in the fuse block, and the other side to the Orange wire connected to Pin G of the
underdash connector, and you are in business. (Maybe install the switch along the bottom of
the dash. Out of sight but not out of mind.)

Maybe not a priority here in upstate NY, but if you actually live in a desert clime I would add
this momentary on switch and verify it's operation sooner rather than later. (Think of a scenario
where you need to go to the hospital right now, you can't wait for everything to cool off, and the
backup vehicle is apart for maintenance.)

And in order to pace yourself while operating the switch, singing 'Happy Birthday' takes ~22 seconds,
give or take a couple. :0)

So there you have it, a full manual implementation of the Hot Fuel module that was originally used on
the >8500 GVW TBI GMT400s. And affordable to implement -- less than $10 worth of switch, wire,
heat shrink, solder, etc.

 

[GoToGuy]

It is discussed in the CK-2 manual chapter 3. The ALDC use and circuits.
Terminology in identifying certain items or objects, in engineering has standards. Meaning adhering agreed upon rules, or definitions. So we all speak the same language, and are understood.
Why would it surprise you that there was only OBD. That's all there was. It is what it is. Until the revised and more restrictive OBD I I , standards were implemented beginning in 1996. Second phase of On Board Diagnostic.
If look in the 1996 CK-1 manual. General Information, chapter 0A-17, SAE J1930 Standards Conversion. Look down to lines seven and eight. It not for nothing, there are reasons why things are done certain ways.

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[Sean Buick 76]

1. Isolate the throttle body from the intake using a 1/2” thick plywood spacer. This will reduce the temperature of the throttle body and the fuel in it.

2. Shield the fuel lines from heat

 

[GoToGuy]

Plywood, now think about this for minute. It's flammable. It soak up liquids. It can swell from moisture. It dry's out , shrink from heat. It does not have a true flat surfaces. It can be porus, containing air channels. Need more?
This is not a problem with regular trucks styles. That's why pressurized fuel injection beats carburetor. The electric pump pushing fuel negates any boiled fuel in the lines.
Is this a joke. Are you trying to fix a problem that's not there.
There isn't much fuel in the throttle body to begin with. If you don't understand the physics. Haven't you ever wondered why no one has mentioned vapor lock in there fuel injection auto's?