New here but not new to GM

92Landyacht · Sep 19, 2021

Hay folks,

I new to this forum but not new to GM power. Currently own a 2001 GMC Savanna 3500 Van with an 8.1L V8. and a 1992 Airstream Land Yacht motorhome on a P32 Chassis and a 454 TBI engine.

Just a heads up... I'm a tinkerer and a retired Electronic Engineer. The combination can get me in a bit of trouble sometimes.

Working on a project to reproduce the, no longer available, Hot Fuel Module.

I'll be posting a thread on this later.

454cid · Sep 19, 2021

92Landyacht said:
Hay folks,

I new to this forum but not new to GM power. Currently own a 2001 GMC Savanna 3500 Van with an 8.1L V8. and a 1992 Airstream Land Yacht motorhome on a P32 Chassis and a 454 TBI engine.

Just a heads up... I'm a tinkerer and a retired Electronic Engineer. The combination can get me in a bit of trouble sometimes.

Working on a project to reproduce the, no longer available, Hot Fuel Module.

I'll be posting a thread on this later.

Welcome to the forum.

What's a "Hot Fuel Module"?

92Landyacht · Sep 19, 2021

On the late 80's and early 90's big block TBI equipped vehicles there was an issue of vapor lock. GM's solution to the problem was to add a module that they inserted into the wire harness near the ECU. The module basically turned the fuel pump on for 20 seconds whenever the key was turned to ON or START. This allowed the fuel pump to purge the fuel line of any vapor. Without it, the engine would crank and the ECU would turn the fuel pump on for about 3 seconds and then turn it off until the engine started. When the engine was vapor locked, it would not start unless the key was cycled ON/OFF/ON a few times. They could have changed the timing in the ECU to keep the fuel pump on longer but this was a simpler solution without having to stock several additional part numbered ECUs.

PlayingWithTBI · Sep 19, 2021

92Landyacht said:
Hay folks,

Welcome, I see you found this forum too :waytogo:

I was going to suggest it to you but, spaced it out. Lots of great folks here plus, this forum is way more active than the GMTC one.

Eveready · Sep 19, 2021

92Landyacht said:
On the late 80's and early 90's big block TBI equipped vehicles there was an issue of vapor lock. GM's solution to the problem was to add a module that they inserted into the wire harness near the ECU. The module basically turned the fuel pump on for 20 seconds whenever the key was turned to ON or START. This allowed the fuel pump to purge the fuel line of any vapor. Without it, the engine would crank and the ECU would turn the fuel pump on for about 3 seconds and then turn it off until the engine started. When the engine was vapor locked, it would not start unless the key was cycled ON/OFF/ON a few times. They could have changed the timing in the ECU to keep the fuel pump on longer but this was a simpler solution without having to stock several additional part numbered ECUs.

Some kind of timer chip controlled by an r/c circuit and driving a relay? I'm no kind of engineer but that is how I would go after that problem. A trim pot on the r/c circuit would let you vary the timing.

kennythewelder · Sep 19, 2021

Welcome to the forum.

92Landyacht · Sep 19, 2021

Eveready said:
Some kind of timer chip controlled by an r/c circuit and driving a relay? I'm no kind of engineer but that is how I would go after that problem. A trim pot on the r/c circuit would let you vary the timing.

Actually I went the route of using an Atmel micro controller (ATTINY85) to derive the time delay and a 20A relay designed by Panasonic to make the contact. The CPU allowed me to alter the timing on the fly without any component changes and the relay was small enough to fit. I didn't want a trim POT since they do tend to change value over time.

I do a fair amount of hardware design and programming so it was only natural that I went that direction. One of the design constraints was the size of original plastic housing that the board was mounted in and the edge connector that the board plugged into. This design allowed me to meet those requirements.

92Landyacht · Sep 19, 2021

New thread started about the Hot Fuel Module in the Engine Performance + Maintenance thread.

454cid · Sep 19, 2021

92Landyacht said:
They could have changed the timing in the ECU to keep the fuel pump on longer but this was a simpler solution without having to stock several additional part numbered ECUs.

That sounds completely backwards to me, but maybe that was a limitation of that era.

92Landyacht · Sep 19, 2021

454cid said:
That sounds completely backwards to me, but maybe that was a limitation of that era.

People I've talked to seem to think that it came down to which items to stock more of under different part numbers. ECU's or harnesses. Didn't make sense to me but it is the way that GM implemented the "fix". If you look at the GM service manual diagram I posted in the other thread, it seems to indicate that the fuel module was implemented in both the HD 5.7L engines and the 7.4L engines. I guess one fix for both engines was easier to implement in changes to the harness and the addition of a common low cost module made more sense than to program and stock multiple ECU variations.

New here but not new to GM

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92Landyacht

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454cid

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