V12 fuel sender and consumption

Hello everybody,

After my adventures on a mysterious air leak in another post (erratic idling), I discover now some others problems.

My original fuel sender was worn. I buy another one for the car, xjs, and observ that it’s “mirror made” face to the original one.
This is only model that’s fits said Limora.

I bent the stem to get a semblance of indication :slightly_smiling_face:
But now when empty, the gauge is on the top of E and when full, the gauge is 3/4 full. I don’t have other possibilities. I tried hard.

Do you know a correct model / supplier for this fuel sender ?

I drive the car (after 2 years of work :upside_down_face:)
The instant fuel consumption on the dashboard indicates
• 70km/h at 1800 rpm => 15 liters
• 80km/h at 2000 rpm => 17 liters
• 40km/h at 1000 rpm => 24 liters
Stabilized speed
These values are correct ?

As soon as I touch the gas pedal, even slightly, the indication is frightenning… 45, 50, 60 and I saw 75 for 1 or 2 sec…

Before you start buying more fuel senders you could experiment more with the one you have.
When it was on the bench did you test if it was reading full when you moved the arm all the way to the top and empty when on the bottom?
If yes, it looks like the sender needs more travel. This can accomplished by shortening the overall length of the arm.

These values are high…
Mine on a flat road and steady 80km/h speed reads 12Lt/100km (+/-1).
My trip computer is 10% optimistic though, others have reported that they are more accurate.
On our no-traffic countryside twisty and hilly roads it reads 13 - 14Lt/100km average.

First test to do would be to manually calculate consumption by filling the tank and divide with km you did. Compered to the Average on the display you will see how accurate your computer is.

Remember that the trip computer calculates consumption by the number of injections multiplied by a known Lt/Injection figure. It doesn’t know neither if an injector is leaking nor if it’s not spraying at all…

Supposing you have a healthy engine, things that have a major effect in consumption:
Timing / Vacuum advance
Bad Injectors
Bad 0² sensors
ECU always in WOT mode
Coolant Temp Sensor
Air Temp Sensor
Spark plugs / Cables / misfire

Frightening indeed, but yes… they are thirsty engines.

As I said in another topic, when I start the engine I have a very low rpm at idle 400/500 rpm.

I wait a little with engine running on P, and accelerate to increase idle. No big deal. Then I drive the car. Under 2000 rpm really no power.

I already check the AAV. The screw near AAV no effect at all.
I have a huge consumption, and it’s not a bad “instant” reading on the dashboard. It’s something else. Elswhere…

As I told before, engine don’t start at first key. Must put contact 2 - 3 times and then cranck the engine.

On the road, if we don’t think about consumption, after 2500 rpm, it’s a normal car with power and life.

I don’t recall, what is the vehicle’s history; has it ever run correctly in your possession? Is there any history of engine work (timing chains or camshaft sprockets disturbed)? Have you considered restricted exhaust? Have you verified that the TDC marks actually are accurate?
Once fully warm, what is the manifold vacuum reading?
The only sure way of checking for exhaust restrictions are to remove and inspect the catalysts, although you can get an idea by using a vacuum gauge. Hold a steady 3000 rpm until manifold vacuum stabilizes, then close throttle to idle; vacuum should momentarily go up to 21” then stabilize quickly at 17” (or whatever your idle vacuum is). If vacuum doesn’t go up to 21” on coast down or stabilize quickly, you may have a restricted exhaust. You might want to try it first on a well running engine to get an idea of what to expect.
Verifying the TDC mark would be the next step; the only accurate way involves using a positive stop on the 1A cylinder and rotating the crankshaft forward and backwards manually and noting on the damper where the stop hits the piston- TDC is exactly halfway between those marks.
Once TDC is accurately found, you can use the camshaft positioning gauges to verify camshaft position, but this involves removing the intake manifolds and cam covers.
I wouldn’t do any of this until you verify that all the basics are checked- compression, fuel pressure, base timing, and throttle synchronization. Idle vacuum will tell a lot. Unplugging the oxygen sensors and noting if there is a performance change below 2500 may give a clue to mixture issues also.
Just thought of something that I ran into on my V12- the air pump diverter valve was hooked up wrong when I got the car, which caused the air injection to remain on full time. This diluted the exhaust with fresh air, causing the oxygen sensors to drive the mixture rich. Only symptom that I noticed was horrible fuel consumption. Obviously only applies if you have the air pump and air injection system. Easy to check by removing the belt.

Hello Robert,

The car from 1987 is without catalyst.

The car was stopped for 10 years. No history known on the car, when I began to work on her 2 years ago. BUT the car was maintained and followed, you see it when you are a mechanic. Everything was in good condition, in its place, no untimely additions over the years. stock.

Of course the oils were old, the tires too. The brakes were blocked. But very little rust (a little on a rear window base). Inside everything works, no parts are broken or missing.

That being said, I check the compression 1 year 1/2 ago. I remember it was OK.
Fuel pressure was checked in another post. On B bank the fuel pressure is 28 psi.

The TDC was already check with piston 1A. Correct for me, the timing is at 18° at 3000 rpm

How can I check the manifold vacuum ??

How can I check the throttle synchronisation ?

Where are situated the oxygen sensor ? But maybe I don’t have.

Hello Jean Christophe
I’d suggest you add the exact model and year of you car in your signature, this will help the diagnostic

what have you done to stop the air leak ?
the idle rpm being so low may need one step back

to check the throttle synchronization, both butterflies should open at the same time and by the same amount
start by adjusting the gap for each bank ( very small, and easily misjudged if using a flat gauge)
I used the method suggested here, I’ll search if someone hasn’t posted yet

you can check the injectors work by using a stethoscope and check the “click” on each of them

if the car as sat for years, the rpm and pressure adjustments for ignition have probably seized,


For months, I couldn’t find any air leak.
When I removed the gearbox, a vacuum hose was running from the intake manifold on A bank, to nothing above the gearbox. I plug the hole on A bank, and no more leak.

My idle rpm was, at one time 750 rpm, but it was before I found the vaccum hose, and I obtain this value with the “air leak”.

Understand for the throttle synchronization, it’s already done.

I’m not familiar with the version of engine / gearbox you have, but iirc the depression hose is supposed to be connected to the gearbox modulator, so the gearbox can adapt the shifting to the engine use
if the engine was idling at 750rpm before you blocked this port, it needs more air to stay at 750 without the air leak

The vacuum hose to modulator is already connected.

After I start the engine, at idle and gearbox on P, if I want to accelerate I have immediatly a very slight cut / interruption of rpm and then the engine takes rpm.