sensor all swapped today

Cluster is installed and working as expected

GEMS: 260ohm at 90c (engine fully warmed up after a drive)
Thor: 280ohm sitting in a cup of water out the kettle (will be close to 90c but perhaps a little less)

Seems close enough for me! Same reading at 5c and same reading at 90c.

Now i just need to find time to fit it!

bad fuse box perhaps?

Seems like a lot of unrelated circuits losing power at the same time?

on a "traditional" vacuum referenced system the pressure will absolutely fluctuate, thats the whole point of the vacuum reference. On boosted cars the swings are even larger due to the boost reference. If there is 1bar of boost in your manifold, the fuel pressure rises by 1 bar to match it, to ensure injector flow rate remains the same.

My A4 runs a base pressure of 3 bar. however at idle its pulling 20 inhg of vacuum (about 0.6bar) and the rail pressure drops to around 2.5. At max boost it runs 1.5bar and the rail pressure rises to 4.5bar.

Early returnless systems, ran a fixed pressure, and the ECU compensates for the flow variations using a MAP sensor and a calibrated model of how the injector responds to the varying manifold conditions. Newer returnless systems have full PWM control over the pump and can adjust pressure at will as defined within their engine mapping.

Stick a pressure guage on your GEMS engine and you'll see it for yourself. Fuel pump running and engine off (no vacuum) will see about 2.5bar of fuel pressure. Once the engine starts and the vacuum acts on the regulator, it'll lower the pressure to around 2bar. Open the throttle and the pressure will rise back towards 2.5bar. Pull the vacuum line off the regulator and it will rise back towards 2.5bar. At wide open you'll see 2.5bar, and part load will vary between 2 and 2.5 depending on the exact levels of manifold vacuum.

I suspect my engine is a bit knackered 🤣

Were up to three rusty coolant stained spark plugs now, and one oil-caked plug

Head gaskets getting done as soon as my Audi gets an MOT and see where it goes from there?

Gilbertd wrote:

You'll need to add an extra ground too. The early GEMS had a single wire to the gauge sender with the ground through the body (later GEMS have a different sender with two pins so has a ground wire to it), the Thor one has a 4 pin connection, two signal and two ground wires.

Yeah I noticed the extra wire, I guess I'll fashion up a wire to the manifold to provide that. Still haven't managed to fully test the Thor sensor yet. Maybe tonight!

Pete12345 wrote:

Surely the easier method is to make sure the correct engine type is selected in the BECM. The BECM converts the analog reading from temp sender & fuel sensor to a value from 0-255, which then drives the gauges. It's more likely the A-D conversion changes than the gauge settings ?

But then there are many different instrument pack part numbers

My suspicion is the becm simply measures the voltage and sends the signal across to the cluster as a 0-255 digital value. It doesn't do anything to the reading other than measure and pass it on.

The cluster takes that digital value and applies a lookup table for the guage deflection.

The reason I suspect this is the becm diagnostic doesnt actually show the temperature, it only displays the digital value.

I tried resetting the gems engine type and writing settings again just incase doing that would somehow pass it on to the cluster, but it didn't make any difference.

is the fuel pressure regulator manifold referenced? If so its normal that it will fluctuate around.

The aim is to maintain say 3bar across the injector. If the manifold is under 0.5bar of vacuum, the pressure in the rail will drop to 2.5bar. If there is 1 bar of boost in the manifold, the pressure at the rail will increase to 4 bar.

Returnless systems are not referenced like that, but the ECM can alter the fuel pump speed via PWM, so you may also get similar variations in pressure as the pump adjusts its speed to meet demand.

I'd be looking at the three odd cylinders and figuring out whats going on. Might be worth getting the injectors properly ultrasonically cleaned and tested? Its usually fairly cheap and can be a good thing on an older engine anyway. A free test would be to move the injector to another cylinder and see if the issue follows.

19 is pretty good.

I drove mine home 40miles the other night on petrol as the LPG had run out. Reset the trip computer when i left mums as i wanted to know what it was going to have cost me on petrol and did most of the 40miles on cruise at 70, MPG was showing 15.5 when i got home. 😭

From googling online the Thor dual sensor and the GEMS engine sensor both use the same thread. So it should screw right in there.

Just need to see if the Engine part of the Thor sensor uses the same resistance range as the GEMS one. I only have one datapoint for the GEMS engine sensor currently: ~4.5k at 5c

However the Thor sensor was around 2.8k at 20c, and some googlefu suggests that sensors reading ~2.8k at 20c will often read around 4.5k at 5c

I've put it in the fridge and will measure it tonight to get some more points to confirm. But we may be in luck and i can just fit the Thor sensor in place of the ECM one, rewire and job done.

Yeah I could have gone that route, but if it went wrong I'd be staring at two broken clusters 🤣

I figured swapping them would be easier.

Just need to figure out either a matching sensor that will fit the 1/8" NPT hole, or a way of attaching the Thor sensor to the gems manifold.

Okay additional data points

Fuel guage behaves the same way when I plug the original cluster back in. I guess the becm will dial itself in once I'm driving.

Temp guage I suspect there is a calibration difference between the gems and Thor engines.

Measured the sensor on the engine at 150ohms at around 50c and according to some online resources measures about 600ohms at room temp.

Measured my Thor sensor and it's at 150ohms around 80c and ~1200ohms at room temp.

Some months ago while in the local U-Pullit i happened across a late model P38 with a updated cluster.

The guage faces have a green background and the legends etc are all a bit more modern, so i bought it.

Finally went to fit it today, and after sorting some niggles (airbag warning light/message due to a missing bulb) and syncing the odometer, it all seemed to be working, except the two small guages.

The fuel guage seemed to display 1/4 tank. The petrol tank is nearly empty and with the old cluster the light was on and the guage was just off the red. Though at various points it did correctly display "fumes" as it should, it also at various points returned to showing 1/4. I took it out for a drive for a mile or so and it slooooowly dropped to the line before the red, however the light didnt come on. I dont know if it would have eventually sorted itself out. When i got back, i cycled the ignition a few times and eventually it did show the low fuel light, but after checking some things with nanocom, it went back to showing 1/4 tank again....

The test drive also highlighted a second issue. Once it warmed up the temp gauge pinned its self round against the right hand side and the red warning light came on. Checked with nanocom, and the ECM was reporting 93c as normal. Does the temp sensor perhaps differ between Gems and Thor? Any way to alter the calibration?

The shiny areas are heavily grooved and there are other deep gouges all over the shafts. I would guess there is 0.5mm of wear on those shiny areas. There is also ridges from where the pedestals sat.

From reading online it seems bits of debris from camshaft/lifter wear etc gets embedded in the soft aluminium of the rocker and eats thru the shaft. So reusing those rockers on new shafts is also a no-go.

Problem is it seems aftermarket rockers are also complete junk 🙁

I think these might be knackered 😂

I've got a pair of rocker shafts from my spare engine which I'd like to disassemble and clean to go back on the heads before install them to the engine in the car.

Then I wondered, do these parts need to marry up with the original components? I don't have the pushrods from the spare engine, and the rocker shafts have been mixed up so aren't going back on the original heads either...

Assuming I can still use them, is there anything specific I need to check or measure to check for wear?

Yeah I'll need to dig the bits out. Headgaskets will be happening soon so ideal opportunity to do some comparison.

The crank pulley is the same as is the crankshaft itself? So the lower pulley is in the same place on both engines?

I cant see how the alternator can therefore operate in a different plane?

The tensioner arrangement is different, but the tensioner itself seems to be the same part, the Thor just gains an additional idler (presumably for more belt wrap?)

So a thor alternator+bracket+tensioner should work on a GEMS unless it clashes with something?

Does the Thor use a different crank pulley or something?

I actually have a full Thor engine spare but the alternator was snagged before they took it out the car, so I have the brackets but not the alternator itself.

I've always felt my alternator lacking. Voltage will sag right down when using heated screen for instance.

Had a look just now and its a Denso 076334-1353 unit, which seems to be 100A from what i can find online.

The Thor units seem more powerful, 150A versions are fairly readily available on ebay... are there any other options for Gems?

Do you know if a Thor bracket will fit a GEMS engine (and still work with the other anciliaries)?