Author Topic: 79 deville efi sending unit question  (Read 706 times)

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
79 deville efi sending unit question
« on: July 16, 2016, 10:06:57 PM »
newbie here....
Acquired a 79 sedan deville 425 EFI from an uncle. Car is mint with 70k miles but was stored for 15 years full of gas. Dropped the tank and want to replace sending unit. Is  the efi unit different from the carb version? Where can I get one? Is there a mechanical pump on the engine and one on the sending unit? Thanks for helping.
Phil

Offline bcroe

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79 Deville efi sending unit question
« Reply #1 on: July 16, 2016, 11:30:08 PM »
Quote from: 79caddy
Acquired a 79 sedan deville 425 EFI from an uncle. Car is mint with 70k miles but was stored for 15 years full of gas. Dropped the tank and want to replace sending unit. Is  the efi unit different from the carb version? Where can I get one? Is there a mechanical pump on the engine and one on the sending unit? Thanks for helping.  Phil 

That tank unit has a low pressure fuel pump.  There is another high pressure
pump mounted in the frame, and a filter mounted on the engine where the
mechanical pump would normally go.  Not sure, but the carb tank unit may
be different.  A carb car has a mechanical pump on the engine. 

If you do replace the tank unit, I'd pay shipping to get your old one for parts. 
I work on those 75-79 EFIs.  Bruce Roe

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #2 on: July 17, 2016, 08:42:48 AM »
If you are parting out old sending units, does that mean new ones are not available?
There is s rubber hose coming down into the tank from the sending unit. It is broken off. How long was it originally?
Thanks?

Offline TJ Hopland

  • Posts: 6824
Re: 79 deville efi sending unit question
« Reply #3 on: July 17, 2016, 09:53:42 AM »
That piece hose should have a electric fuel pump hooked to it, similar to a 80's or 90's efi car. 

Are you sure this still has the EFI on it?  Maybe someone converted back?  Then the hose you saw was just shoved on the sender to bypass the in tank pump.   

If you follow the fuel line from the back to front do you see anything that looks like a fuel pump?   If the in tank one is missing I would expect to see a fancy hod rod performance looking pump somewhere down there.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
75 Eldo rusty but trusty
80 Eldo Diesel
And other assorted stuff I keep buying for some reason

Offline bcroe

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79 Deville efi sending unit question
« Reply #4 on: July 17, 2016, 11:11:55 AM »
Quote from: 79caddy
If you are parting out old sending units, does that mean new ones are not
available?  There is s rubber hose coming down into the tank from the
sending unit. It is broken off. How long was it originally? Thanks? 

There should be a 3/8" pipe coming down through a short hose to an
electric pump.  It should be as long as will fit into the space, so it can't
slip out of position from pump vibration.  There may be a longer, smaller
hose coming to the bottom.  Hoses inside the tank are a SPECIAL type
that tolerates being submerged. I have some, bought on line for about
$20 a foot. 

I don't know if you can find a replacement tank unit.  I am working on
replacing my 79 EFI tank unit pump with a high pressure type, so the
other pump in the frame is eliminated, along with its noise and priming
issues.  Some of the stuff I have has problems, so I'm collecting other
parts.  Bruce Roe

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #5 on: July 18, 2016, 11:27:04 AM »
Would love more info on how to eliminate the gas tank pump. Is it best to put a aftermarket pump in the tank or on the frame?
Here is a pic of the broken off tube in the unit. Original purpose and length ?

Offline bcroe

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79 deville efi sending unit question
« Reply #6 on: July 18, 2016, 02:43:11 PM »
For EFI you need an electric pump.  75-79 used a low pressure pump in the tank
followed by a high pressure in the frame.  If you eliminate the tank pump, the
car may sometimes have trouble sucking gas from the tank.  If you eliminate
the frame pump, you eliminate sucking problems (pump submerged), noise,
extra current, and of course one pump, but it needs to be a high pressure. 

So I'm trying to adapt a high pressure pump to a good tank unit.  Yours looks rusty. 
Your pump feeds a 3/8" pipe going to the engine.  I believe you have a 1/4" fuel
return line from the fuel pressure regulator, to that tank unit.  There should be a
piece of hose guiding returned fuel to the bottom of the tank, but it is missing. 
There is a 5/16" pipe for venting fumes at the front canister to the tank.  That is
the same convention as a carb car.  However, I think I've seen some where they
decided to use the 5/16" for fuel return and the 1/4" for vent, I would.  Bruce Roe

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #7 on: July 18, 2016, 07:41:47 PM »
I could clean the rust off with a light duty soda blaster, add new hoses and try it??? Is there a way to test the old tank pump unit before I bolt it back together and find it doesn't work?
If I eliminate the tank pump with a new standard unit (got a carb version) and add an aftermarket frame pump, how do I regulate the pressure? Anyone know what pressure the EFI motor needs?
Thanks for helping!

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #8 on: July 18, 2016, 08:07:28 PM »
Bruce,
Have you tried the Keshen 255 Ebay tank pump as a replacement for both pumps? I may try it.

Offline bcroe

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79 deville efi sending unit question
« Reply #9 on: July 18, 2016, 08:39:20 PM »
Quote from: 79caddy
    Bruce,
Have you tried the Keshen 255 Ebay tank pump as a replacement for both pumps?

I have a Walbro high pressure pump, about a 195 (liter per hour).  Everybody
gets those 255 LPH pumps, which will supply enough gasoline for a huge souped
up performance engine.  Someone here knows how many horsepower that is. 
Seems to me my mild mannered 403 has no need for that big a pump, which
will draw more current and strain my fuel pump circuit. 

Now to actually get the job done....     Bruce Roe

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #10 on: July 20, 2016, 02:50:12 PM »
Ordered a PRECISE low pressure IN TANK PUMP from Rockauto.com. Will try to retro fit this onto the existing sending unit. Wish me luck.

Offline bcroe

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79 deville efi sending unit question
« Reply #11 on: July 20, 2016, 04:00:01 PM »
I could clean the rust off with a light duty soda blaster, add new hoses and try it??? Is there a way to test the old tank pump unit before I bolt it back together and find it doesn't work?
If I eliminate the tank pump with a new standard unit (got a carb version) and add an aftermarket frame pump, how do I regulate the pressure? Anyone know what pressure the EFI motor needs?   

The EFI operates between 29 and 39 PSI, depending on manifold vacuum.  The
pressure reg is on the injector fuel rail.  Don't forget you need some hose rated
for submerged use. 

I think some injector test fluid could be used to test a pump if gas is too
dangerous.  I wouldn't run it dry.     Bruce Roe

Offline TJ Hopland

  • Posts: 6824
Re: 79 deville efi sending unit question
« Reply #12 on: July 20, 2016, 04:00:55 PM »
What are the specs on the pump?   Low pressure today would likely be the GM TBI stuff which was 13psi and that was achieved with a regulator in the throttle body, deadhead they would do 30-40 psi.  Low pressure in this case is more likely like a carb and in the 3-5 range.   This would be like what they call a lift pump on an older diesel which were usually pulse pumps.   On the GM diesel stuff these were not in tank.   Not sure if extra pressure gong into the high pressure pump would be a good or bad thing. 

You also have to be careful about the electrical load.   

Ultimately a single in tank high pressure pump would be the way to go if one could be found and easily fitted. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
75 Eldo rusty but trusty
80 Eldo Diesel
And other assorted stuff I keep buying for some reason

Offline bcroe

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79 deville efi sending unit question
« Reply #13 on: July 20, 2016, 09:37:55 PM »
Quote from: TJ Hopland
You also have to be careful about the electrical load.   

The 75-79 EFI Cads and the Cosworth Vega with the same pump system,
both had frequent failures in the pump power supply.  For a while I have
encouraged owners to install a relay to handle the pumps.  Maybe a relay
could go, but that is way better than burning out the injection computer. 
Details are on my PHOTOBUCKET, first album.  Bruce Roe

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #14 on: July 23, 2016, 02:08:08 PM »
Rockauto.com had parts to recondition old unit with a little help from a soda blaster. We will see how it works.

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #15 on: July 23, 2016, 02:13:13 PM »
About 25 bucks by the way  :).

Offline bcroe

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79 deville efi sending unit question
« Reply #16 on: July 23, 2016, 03:54:39 PM »
Quote from: 79caddy
   About 25 bucks by the way

Rock sold you the big O ring gasket, the special small hose, and the pump
with the large hose, all for $25?  Sounds like a deal.  However my version
will be with a single high pressure tank. 

Since its out, you might as well check that the float circuit reads somewhere
near zero ohms empty, 100 ohms full.  You will note that short float arm
when elevated only goes about 2/3 of the way up to the top of the tank. 
Because of that it will just read full for the first 1/3 of a tank, then suddenly
drop very fast.  I always lengthen and adjust that arm so the float can just
touch the top and the bottom of the tank, which makes the gauge WAY
more accurate.      Bruce Roe

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #17 on: July 24, 2016, 11:56:44 PM »
Large o-ring and special short hose came with pump for 16.01 including various clamps and other o-rings in the kit. Strainer was 4.55....  check it out at the rock!
Had long fi hose stock laying around my garage.
Thanks for tip on float!

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #18 on: July 31, 2016, 08:19:33 PM »
Tank pump working now  :) and have pressure to fuel rail now (7.0L) bUT injectors not firing. Also pulled frame pump and  bench tested it. Seems to function well. Car has not run in 13 years but will fires up briefly and sounds great if I pour gas in top.I will check fuel pressure tomorrow. What pressure should I have at release valve?  Are there other obvious reasons that injectors are not pulsing? I have not replaced fuel filter yet either. Thanks for your thoughts!

Offline 79caddy

  • Posts: 16
  • Name: Phil Landes
Re: 79 deville efi sending unit question
« Reply #19 on: July 31, 2016, 09:01:05 PM »
By the way HORN ON THIS DEVILLE IS AMAZING.

 

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