Fuel Pump not activating

[Jenno1981]

Hi guys,

I'm working on a 1978 280SEL KJet

Background
Has been in storage approx 5-10 years. Has 100K Kms on the clock (checks out). When I recieved the car the fuel pump was sitting in the center console. Tracked down the previous owner who stated "It was running when it went into storage then it had a fuel pump issue and I couldn't get it going again". No details of any previous testing/diagnosis, details scarce.

Actions
1. Tested old fuel pump with 12v direct, confirmed not working and replaced with operational pump (confirmed working).

2. Fuel delivery pack needed some attention (hoses, clamps, new filter etc), completed and reinstalled.

3. Old fuel (black tar...) and tank not in great shape so rigged up an Auxiliary tank and hand pumped new petrol through with the lines to Fuel Distributor (lines out). Bit yellow.

4. Dropped the oil and found it was black and 3ltrs over full (I feel this might be explained by the issue at hand... read on).

5. Ignition position 1 and 2, dash lights on, fuel pump not running.
Engine cranking (tried P, N, R), fuel pump not running.

6. Push down on air intake flap and can hear fuel pump relay activate in the Fuse box (assumed this was all that was required to confirm relay operational?).

7. Confirmed electrical plugs and fuses Seated/good condition

8. (test suggested in previous threads) Ignition off, pulled blue plug to fuel distributor with no reaction (as expected). Ignition on, pulled blue plug and fuel pump DOES activate. Unfortunately the thread I read didn't actually stipulate what the issue was. Hoping someone can finish that off for me.

I'm reading this plug has something to do with a fuel over run Safety or knock sensor (seems it has a few names) which is used for cutting off the pump when the engine is not cranking/running.

Given the CIS, this would ensure petrol is not continually pumped. Would it be correct if there was an issue with this safety, and say the previous owner in an attempt to fix it, ran the engine with the blue plug out (which has been suggested in previous posts), would it be possible 3ltrs of fuel has been pumped into the engine and made its way into the oil pan?

As always, any help in further diagnosis would be very much appreciated.

[rumb]

https://forum.w116.org/mechanicals/help-fuel-issue/msg158117/#msg158117

Test procedure in above post to diagnose.

[Randys01]

A part answer to your questions:
1. nearly for sure you have an earthing/wiring issue with the blue plug.
2. 3 litres overfill of the  oil suggests too much cranking of the engine in the past has indeed seen raw fuel end up in the pan.
: this needs to be drained/new filter asap.

[Jenno1981]

Absolute legends, both you. I'll work through that test list Rumb.

The time, heartache and parts this forum has saved me 🙏

[Jenno1981]

Hi guys,

Still struggling to get the fuel pump running on Ignition. Can I also confirm pump should run at Ignition positions 1 and 2 (1978 280SEL KJet)?

- I've independently tested the fuel pump and FP relay, they're both operational.

- I've made my way through the testing manual (supplied in link above) and get to one of the last steps "Checking Activation of Fuel Pump". The result options are "12v Fuel pump running" or "0 volts" - I get 12v but my fuel pump is not running.

- I'm not sure what else it could be other than a wiring/ground issue but I would need some suggestions on where to start. Are there common Grounding points to go too?

- Rumb, the wiring harness behind the rear seat that contained you're loose ground, was there any reason for it? Had there been work to that area or had it come loose over time?

Suggestions welcome!

[Jenno1981]

..... should have searched the manuals before posting apologies! I've got the list of Grounding points and I'll work through them.

[rumb]

I had loosened the ground removing the rear window. I also discovered my "new" pump was bad and had to replace. Sounds like your pump is dead also.