XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

1985 XJS H.E. - Fueling Questions

 
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Old 04-20-2019, 02:34 PM
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Default 1985 XJS H.E. - Fueling Questions

Hello Friends!

Having completed the stem-to-stern restoration of electrical accessory functions (from boot lights to antennae to fog lights, wipers to windows to mirrors, etc), I now find myself ready to delve into the mechanical mysteries of this car.

Starting with the engine.

First question... Should the fuel pump run continuously with the ignition in the #2 position before starting the car? I have read that it should only run for 2 seconds or so (enough to pressurize the system) before starting.

Mine runs continuously. If this is not normal, what is the feedback loop the fuel pump is expecting to "see" in order to shut off?

Which brings me to my next question / scenario.

The car cranks and catches occasionally and will idle quite roughly until it dies (30-60 secs). During this time any attempt to apply throttle kills it immediately. I hear all injectors clicking while it runs.

My thoughts (based on the continuous run of the fuel pump prior to cranking) point to a fuel supply problem of some kind.

Trying to start from a logical point of diagnosis based on the current engine behavior. Could be ignition / electrical based solely on my observation of the engine bay wiring conditions, but I wanted to be methodical and not try to "shotgun" the fix.

Given my Jags behavior, where should I start (no pun intended).

\m/ DS
 
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Old 04-20-2019, 02:46 PM
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Old 04-20-2019, 02:58 PM
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Hi Hobe,

yes the fuel pump runs continuously and has a bypass back to the fuel pump for unused fuel in the loop.

your starting issues... I would start with the TPS (throttle position sensor) and the AAV (auxiliary air valve) plus air filters and fuel filters checked and replaced if you donít know when they were last done. There are other things but more than likely one of these may be causing it.

it could also be the CTS (cold temp sensor) that has faulted which will cause a rough cold start.

You can google these or search on this forum as there are good threads that address all of this.

you can buy a new AAV from Terrys Jaguar in the USA and a new TPS from SNG Barrett in the UK.

other guys here may chime in with repair options for the TPS and AAV if thatís something you would be into.

A new CTS is pretty easy to get hold of at most places with a good google search

Cheers
Craig
 

Last edited by Crackerbuzz; 04-20-2019 at 03:15 PM.
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Old 04-20-2019, 03:04 PM
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Old 04-20-2019, 05:01 PM
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Originally Posted by Hobejam View Post

First question... Should the fuel pump run continuously with the ignition in the #2 position before starting the car?

No.

If yours runs continuously with key on, engine off, then (most likely) a previous owners has changed to circuit. Typically this is done by permanently grounding the orange wire from the ECU to the pump relay. This is a problematic circuit in the ECU and grounding the orange wire is an easy to to 'fix' the problem.

I have read that it should only run for 2 seconds or so (enough to pressurize the system) before starting.

Correct.

Key on, engine off, the pump will run for a couple seconds.

It should also run whenever the starter is engaged.

And, naturally, it runs at all times when the engine itself is running


Mine runs continuously. If this is not normal, what is the feedback loop the fuel pump is expecting to "see" in order to shut off?
If the engine is running the pump doesn't shut off. The injectors are over-supplied. Unused fuel is returned to the tank, as mentioned..... as is typical for fuel injection systems.

Cheers
DD
 
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Old 04-20-2019, 05:28 PM
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Someone must have changed mine. what normally goes wrong with it to have to ground out?
 

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Old 04-20-2019, 10:27 PM
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Originally Posted by Crackerbuzz View Post
Someone must have changed mine. what normally goes wrong with it to have to ground out?
The 6CU ECU has a fuel pump drive circuit issue (female moment we call it),and as Doug said, the quickest/easiest "fix" is to earth the Orange wire OUT of the Black socket, thus removing the ECU from controlling the pump.

NOT the safest long term, and some Insurance companies will NOT pay if that "modification" is deemed the cause of a fuel related fire.

Perfectly OK as a diagnostic test., and a 16CU ECU is a plug and play replacement.
 
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Old 04-26-2019, 10:13 AM
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Thanks for all the replies!

Going to troubleshoot fuel supply tomorrow. Then on to engine bay electronics.

I'll post what I find.

Thanks again for being such a great resource!

\m/ DS
 
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Old 04-29-2019, 04:06 PM
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Ok, next steps taken with results below...

Disconnected fuel line at right side (supply side) regulator and ran a hose to an approved container. As my fuel pump runs continuously (yes, the PO had run the orange ECU wire to ground in the boot), I ran the tank dry using the pump. During this process I noticed the flow "surging". It was not a constant flow, but rather a rhythmic "ebb and flow" pattern from the hose into the container. The sound from the pump was constant and did not vary. Fuel was clean.

Thoughts?

Once drained, I removed the battery and checked the sump tank. To my surprise, the screen and tank were clean. No obstruction there. Reassembled. Check.

Found that the fuel filter was installed backwards. Removed, tested for obstruction. Re-installed correctly. Check.

Re-attached the fuel line to the regulator (no evidence of fuel in the vacuum line). Check.

Checked the left (return side) fuel regulator for fuel in the vacuum line. Found none. Check.

Put 5 gallons of fuel in the tank and checked sump tank for leakage. None found. Check.

Reinstalled the battery and cycled the fuel pump to checked fuel line / regulator connection. No leak. Check.

Cranked it over and got the same result. Rough, surging idle that eventually dies.

Two other points of interest. Once running, I quickly disconnected the CTS. Idle smoothed momentarily for 15 seconds or so and then reverted to the previous state before eventually dying. After reconnecting the CTS, I repeated the effort and while running, found the AAV in the open position and plugged it. Once again, the idle smoothed for 15 seconds or so and then reverted to the rough idle / die behavior.

Repeated the experiment leaving the AAV blocked and the CTS disconnected. No change in original behavior (rough idle / die). Reconnected the CTS with no change in engine behavior.

In summary, I found the fuel supply from the pump prior to the supply side regulator to be pulsing or surging as I pumped fuel from the tank. Bad pump? Also found the fuel filter reversed and fixed that. Bad filter?

Does the behavior of the motor while manipulating either the CTS or AVV suggest either is to blame? Taking for granted the AAV should not be in the open position at cold start and assume it is stuck / bad.

While further examining the engine compartment, I found numerous electrical anomalies including the A/C clutch disconnected and a 3 prong plug located on the left side of the A/C compressor bracket disconnected with a jumper between the center and one of the outside contacts.

I also found that there is no coax running from the ignition amp, but rather a wire with a spade connector that appears to be spliced into the coax about 6 inches from the amp.

It would seem that somewhere in the car's past, someone really butchered the engine compartment wiring. Short (no pun intended) of re-wiring the entire harness, I have no real faith that anything is connected the way it should be...

Plan on continuing the systematic eval of the fuel supply (checking actual rail pressures and injector function) before delving into the other electronics.

If you have any thoughts or suggestions based on this current rambling, feel free to offer it up. Otherwise stay tuned for next steps and results...

\m/ DS
 
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Old 04-30-2019, 01:07 AM
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The car should not run, should stop immediately, if the CTS on the B bank thermostat housing is disconnected. Someone has done something. More clever people than me will explain how to check the CTS wiring for continuity!
Also it is important to check that the throttle position sensor under the capstan is functioning correctly with the right voltage ramp as the throttle is opened.
 
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Old 04-30-2019, 04:21 AM
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Agreed.

Mostly, if the engine continues to run at all with the CTS unplugged, the EFI loom, down in the valley of death is shorting out.

Then comes:

TPS out of range.

ATS, and CTS plugs crossed up. Common on fiddled cars as the 2 pigtail looms are the same length, and will easily pug into the wrong sensor.

BUT

Most I have experienced with that scenario do NOT start and run at all.

The ATS, is the Air Temp Sensor, located in the trumpet of the LH air cleaner cover.

The fact someone has altered wiring for the fuel pump, also makes me suspect the ECU itself. Also the wiring under the bonnet is a suspect, as you state, as butchers have no place under a V12 bonnet.

This Pin Out I wrote years ago may assist with the systematic search.

XJ-S ECU Pinout table.doc
 

Last edited by Grant Francis; 04-30-2019 at 04:25 AM.
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Old 04-30-2019, 11:30 AM
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Thank you so much for your replies!

While not surprising, I am discouraged at the prospect of unsheathing all the wiring under the hood searching for further debauchery.

If I understand the CTS function correctly, the fact that the motor ran with it disconnected suggests that somewhere in the loom the two wires from the CTS connector are shorted together (or worse, to other bad wiring).

Based on the wiring that is visible, most, if not all the wiring under the bonnet is in dire need of refreshing. This includes a number of dodgy splices, connectors, and jumpers as well as visible cracks and missing chunks of insulation.

May be a bit late for confessions, but I purchased this Jag with the intentions of "lumping" it. When it fired and at least tried to run once I got the battery charged up, I took that as a challenge to see if I could bring the V12 back to life. Needless to say, I am verging on raising the white flag on this one. Running all the wiring harnesses in search of gremlins at this point seems too daunting a task.

Open to any additional suggestions or insight...
 
 
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