Fuel Pump Cycle Module - Needed ???

[EDZIP]

Installing a Speed Density TPI setup in a 1994 K2500 and using the 1227730 ecm. (Have a TPI pump in the tank)

Truck has a 1619636 ECM and I'm in the middle of re-pinning to the 730.

The 94 truck has a Fuel Pump Cycle Module (FPCM -pic attached), looking at the schematic (also attached) I'm not really sure if I even need the FPCM with the TPI conversion (don't see one in my 730 schematic)?

The FPCM was used in the truck with TBI...is this something I can delete?

Thanks.

[Six_Shooter]

Not needed.

[EDZIP]

Not needed.

Thanks!

[Rocko350]

run the tpi off of the ecm that's already in the truck. The info for that is over on thirdgen.org not sure if it ever migrated over here. It used $0d mask IIRC and there was a patch to run a MAF in that code in place of the linear egr valve in the code. Fast355 on here was a big part of that over there. Oldred95 actually posted a thread over there when he performed the patch and gave some feedback.

Chris

[EDZIP]

run the tpi off of the ecm that's already in the truck. The info for that is over on thirdgen.org not sure if it ever migrated over here. It used $0d mask IIRC and there was a patch to run a MAF in that code in place of the linear egr valve in the code. Fast355 on here was a big part of that over there. Oldred95 actually posted a thread over there when he performed the patch and gave some feedback.

Chris


Thanks...but just finished the re-pin today!

[Byron454]

Does anyone here know what this component's purpose is? It is labeled as a hot fuel module in a lot of GM repair info and seems to be only installed
on over 8600 gvw trucks, is wired parallel to the fp op switch a fp relay. wondering what it does? how does it handle hot fuel.

chime in with info or opinions

Thanks Byron

[Six_Shooter]

It's a timer that's supposed to keep the fuel pump running for several seconds after the engine is shut off to avoid vapour lock.

[1project2many]

seems to be only installed on over 8600 gvw trucks

I seems to remember seeing them on LWB trucks with lower GVW and dual tanks. But my memory may not be accurate since I'm forgetting more and more of my life before I had children...

[lionelhutz]

I thought it extended the pump priming time from the ~2 seconds the ECM uses to something like 10 seconds or 20 seconds.

[1project2many]

It will do that but it will also run the pump for an extended time after key off.

[lionelhutz]

Sure, but the main purpose would be to give an extended pump prime so the engine would start quicker. Running the pump for 10 seconds after the engine shuts off is rather useless. Of course, successful operation assumes the owner will turn the key on and then wait before turning it to start.

[1project2many]

Can you imagine coming out of a convenience store on hot AZ day after the truck's been parked in the sun for 10-15 minutes? Ten seconds sitting with key on engine off would seem like an eternity when you could be running the A/C.

I only remember the module being installed in the square body trucks with dual tanks. It may have been in others though. I always thought it seemed strange that the manual said it could keep the pump running for up to twenty seconds after engine shutoff. There must have been a specific problem that engineers encountered which caused them to install this separate control module. I never checked the manual to see if HFH operated as they described but I have repeated what's in the book over the years. I don't know why they did it, but it seems important for others to know that their pump might be operating for up to 20 seconds after key off.

[lionelhutz]

Sure they would mention that for the off sequence since no-one would care about the pump running longer during the start sequence. But I can't come up with any useful reason for running the pump up to 20 seconds after you shut-off the engine.

It wouldn't be for priming because the pump is running without air and creating the proper fuel pressure already.

It wouldn't be for "vapour lock" because 20 seconds of continued pump running isn't enough time for any part of the engine to cool enough to stop radiating the same heat into the fuel system.

[1project2many]

But I can't come up with any useful reason for running the pump up to 20 seconds after you shut-off the engine.

I can't either. So either it's a misprint or we'd need to meet one of the people involved with the design to figure out what's going on.

Now that you've got me curious, maybe I'll try and dig out an old FSM.

[EDZIP]

As a FYI only..the module is in a 1994 K2500 Chevy 5.7/NV4500 cab that I installed on a 1995 Chevy 3500HD dually chassis (19.5 tires, 10 bolt wheels, I-beam axle, 15,000 GVW etc). The 3500HD trucks are unique beasts that get confused with the standard 3500 dually trucks. To help I started a small forum dedicated to 3500HD truck series at www.3500HD.com

[Byron454]

Thanks all for the input. Ive delt with these fuel systems for over 20 years have seen them on the wiring diagrams always wonder about there operating strategy,
but never remembered to pursue the information when I wasn't actually making the repair to the vehicle. sounded like a good time and place to ask the question.


Byron

[blue68deville]

Since I had the connector in my harness already, I installed the hot fuel module in the caddy last summer, just to see what it does.
It provides a 20ish second extended prime in the key on eng off position, and does not run the fp after shut down. It's back in my spare parts box, didn't see the point in keeping it installed.

[1project2many]

Interesting. FSM wording strikes again. I've got some stories about diagnostic tests that forget key steps and lead to wrong, expensive diagnosis, too.

[lionelhutz]

Since I had the connector in my harness already, I installed the hot fuel module in the caddy last summer, just to see what it does.
It provides a 20ish second extended prime in the key on eng off position, and does not run the fp after shut down. It's back in my spare parts box, didn't see the point in keeping it installed.

That would make more sense. The module just has power, ground and the wire connecting to the fuel pump. So, when it sees power first applied to the fuel pump some internal circuit connects the module power to the pump wire for 20 seconds.

It never made any sense to me that the module could start timing when the fuel pump relay turned off. It would be rather difficult to detect the relay going off using the pump wire if the module itself has connected the pump wire to power. And the module would have to be "on" when the PCM turned-off the relay or the pump would blip off for a second.

[1project2many]

It also makes more sense that it would only be needed to push vapor out of the lines at key on. A neat point is that I was never involved in diagnosing a vapor related issue on the TBI trucks, even during the 106 degree days we'd get in MT. But I have been on several road calls for Ford trucks that died due to fuel vaporizing in the lines. Ever hear someone recommend adding diesel fuel to a gas engine to cure a hot stall? It worked.