1970s 1976 Eldorado fuel injection help

[johnpaul]

Hello - I have a 1976 Eldorado EFI that is behaving strangely. At first start up it is at its best, fairly smooth but with a bit of hesitation and some mild engine vibration when cruising that is intermittent, with periods of perfect smooth running.  After it is fully warm it transitions to horrible hesitation, even noticeable when cruising - power surges alternating with missing, causing the car to coarsely vibrate. Then all of a sudden it starts running pretty smoothly again. I'm thinking that only a computer-related problem could cause something like this.

[V63]

Some suggestions

If alcohol blended fuel is used...the EFI fuel pumps do not like it, ultimately causes them to thermal out.  Try maintaining  marvel mystery oil to fuel to lubricate fuel pumps. 2 oz per 5 gallon.

The alcohol adds a 'resistance' to the fuel pumps adding stress to its electrical system components. Of coarse the engineers could not anticipate alcohol blend  use in 1975 . RR models that used the Bosch system in  later models after the alcohol blended fuel availability has in their owners manual "NO ALCOHOL BLENDED FUELS". I found (FINALLY)  with numerous fuel delivery issues with early EFI the problem was the alcohol blended fuel!!! I figured out a FIX, add lubricant to the fuel and the fuel pump(s) are much happier. I had a car with known fuel system delivery issues and I started adding lubricant to fuel and never had a symptom again. I never changed the pumps either.

Verify no engine vacuum leaks.

Verify (OHM) the 2 brass EFI manifold sensors.

[TJ Hopland]

We have a couple guys here that know, repair, and even make parts for these systems and I'm sure one or both of them will be along soon with better advice I can give.

From reading I do know that the power for the fuel pumps runs through a relay on the ECU board and its a common problem to have issues with the traces and connectors for that power.  Once the damage is repaired the fix is to go to an external relay to save stress on the ECU internals. 

Map sensor is on the ECU board so its a long vacuum line to get there.  If its leaking maybe you are not getting a good signal to the sensor so once it comes off the 'choke warm up' mode its just lost.

[bcroe]

Maintaining fuel pressure is critical, it may
vary from 39psi coasting down a hill, to high
20s under power.  I finally installed a modern
solid state fuel pressure gauge to keep track
of my 79, and other cars.

Could well be your ECU is running lean, trouble
starts as soon as cold enrichment ends.  This
may be caused by a bad MAP or related circuitry. 
I have been testing and repairing the 70s ECUs
for a while, email me bcroe@juno.com
Bruce Roe  CLC #14630

[79 Eldorado]

JP,
You should have the Cadillac troubleshooting manual. It's often on eBay in paper form and a member on the CLC posted a pdf. See post nr 6 of this thread:
https://forums.cadillaclasalleclub.org/index.php?topic=167249.msg511579#msg511579

Our cars don't have closed loop so the computer reacts to the inputs and adjusts the conditions regardless of the result. Bruce mentioned after cold enrichment ends but if a sensor fails with lower than normal resistance the ECU will think the car is always cold. If the sensor fails with too high resistance the ECU will think the car is always above the temperature where there's no longer adjustment. One of the easiest things to check is the temperature sensor resistance. Based on what Bruce has mentioned previously the coolant temp sensor has the most influence.

Bruce can check the ECU but you could check the vacuum line going to the ECU and the sensors easily. The ECU is using 5 inputs: 2 coolant sensors, the speed sensor in the distributor, the MAP signal at the ECU, and the TPS on the TB. The system also adjusts fuel pressure based on a vacuum signal to the pressure regulator (opposite corner from the fuel supply on the fuel rail). It would be a good idea to look for vacuum line leaks in general.

Less vacuum at the pressure regulator increases fuel pressure. If it more-or-less is consistently bad hot you could shut it off once it runs poorly and check the plugs. You could also remove the vacuum line from the fuel pressure regulator and see if increased pressure helps or hurts (plug the line when you remove it to prevent a general vacuum leak). Regarding vacuum leaks one place they can be hard to find is in the brake booster. If it's consistent and you can check things out in Park you could plug the vacuum source to the vacuum booster to see if there's any change as well.

Scott

[Chopper1942]

This going to be a little long winded. It defiantly sound like a fuel delivery issue with your car.

I also had a fuel delivery issue with my 79 El Dorado 350 EFI. This is basically the same fuel system as in your car.

First, a little history about my '79. Here in Iowa, 10% ethanol blended fuels have been available since the mid-70's. My '79 has 218k miles on the odometer. It's a driver! It has never had the heads or pan off. The oil and filter changed every 3k. When I ran out of oil with zinc phosphate additive, I began adding zinc additive for addition engine protection. I use Mobil 1 oils in all my vehicles ever since it became available. Other than trans filters and fluid changes at 24k, I have only had to replace the trans filler and shift shaft seals.

Now let's talk about my and maybe your fuel delivery issue. Periodically when I haven't driven it for a while, I back the car out of the garage and let it run until it reaches normal operating temperature. It was outside running and when I went out to drive it back in the garage, it was not running. It would not start. What was the issue. Diag. time.
1.  Gas gauge 1/4 tank
2.  Checked spark with spark tester. Good.
3.  Cycled the key several times. Can hear the high pressure pump running. Good.
4.  Removed air cleaner to access fuel pressure test port on fuel rail
5.  Connect fuel pressure gauge to pressure port.  Cycle key several times. 0 psi. Not good.
6.  Jacked up car, put on stands, removed fuel hose from fuel filter, put in a container, and cycled key. No Fuel.
    Removed fuel filter and blow through filter. OK.
All this just confirmed my suspicion that the low pressure fuel pump in the tank has died. I dropped the tank and removed the fuel pump/fuel sender unit. What I found was the rubber coupling between the fuel pump and fuel line of the sender had come off. I had purchased a new fuel pump, so I installed the new pump, fuel pump sock, and coupling. The old pump tested OK, so I kept it as a spare.

Now, let's talked about how to diag. the EFI fuel system.  The fuel pressure delivered to the injectors is controlled by the fuel pressure regulator. The fuel pressure regulator is connected to manifold vacuum and maintains fuel pressure between 39-65 psi depending on manifold vacuum. 0 in.Hg.= 65 psi. 18 in.Hg. = 39 psi.  The Bosch high pressure pump has the capability to produce < 100 psi. if you deadhead the return line from the fuel pressure regulator.

The in-tank low pressure supply pump for the external high pressure pump should develop around 5 psi.

Explanation:
1. 0 psi fuel pressure. If high pressure pump is bad, should have about 5 psi pressure. O psi either filter is
   plugged or the low pressure pump is bad.

Let's diag. your system.
1.  Connect an accurate fuel pressure gauge to the pressure port.  Cycle the key on and off several times. Should
    have 65 psi. If not, remove the vacuum hose from the fuel pressure regulator and see if there is fuel in the
    hose. If so, needs fuel pressure regulator.
2.  Unplug the high pressure pump and see if you have 5 psi. If not, jack up the car and remove the fuel filter and
    blow through it. Should be easy. If no, replace the filter. Remove the supply hose from the high pressure pump
    and and have someone cycle the key. Should have a solid stream of fuel for about 2 seconds. If not, the problem
    is either voltage supply to then in-tank pump, pump sock, bad pump, or rubber connector. Before you drop the
    tank, check the voltage at the connector at the tank and the tank ground. Often the ground wire for the fuel pump
    gets corroded where it attaches to the sender.
3.  If you have good fuel supply from the tank, verify the voltage and ground for the high pressure pump. If OK, the
    high pressure pump is going away.

Oh, by the way, for all the ethanol naysayers, my 79 has its OE high pressure pump and the low pressure pump tested OK after 218k of ethanol fuels.

 

[79 Eldorado]

Larry,
Those soft line couplings in the tank must be replaced with fuel line rated as submersible. Normal fuel hose is not and some people miss that when they change it. I wanted to add that point because, like you mentioned, the hose does need to be changed occasionally as they can deteriorate; crack/leak.

Scott

[Chopper1942]

Thanks 79 Eldorado. I failed to mention that I did not and do not use the rubber coupling.  I use high pressure fuel injection hose for in-tank high pressure pumps, even though it is kind of overkill, but that's what I keep on hand.

[79 Eldorado]

Larry,
But even EFI fuel hose, for use outside the tank, is still not rated for submersion. I never realized that there was a different standard until I came across it on the Gates website. I was looking at their Barricade hose at the time. The SAE specification for submersible hose I found is SAE 30R10.

I've made (significantly modified) a couple of sending units for cars where I wanted an in-tank pump. Here's a link to the one I made for my 1968 Impala:
https://www.chevytalk.org/fusionbb/showtopic.php?tid/369576/

Here's a capture of the Gates package for 5/16". Somewhere I have the PN for 3/8" as well.

Scott

[Chopper1942]

That's the stuff I use. It is high pressure FI hose rated to 100 psi.

[bcroe]

Meanwhile the original poster disappeared with
no solution.  Bruce Roe