A couple of degrees made all the difference! I believe I've finally fixed the fuel gauge issue, it's been a hell of a job but it's done, I think.
After a week on axle stands the 7 is finally back together in time for Easter and hopefully in time for some good weather.
So if you recall from my last post I'd finally gained access to the boot and was able to photograph the orientation of the sender in the tank. I sent the photographs to Derek Howlett at Caterham Cars and after some discussion, Derek didn't feel the sender was that far out, he therefore suggested the gauge was probably at fault. Initially Derek wanted me to warranty the gauge with Oakmere, however, he later agreed to send one in the post. Before the gauge arrived I decided to do a little more digging around with my gauge.
I removed the gauge from the dash, which is really super easy, there's a plastic ring on the back of the gauge which simply unscrews and the gauge pops out. Once out I checked the single electrical connector and it all seemed good, nicely connected. I then decided to disconnect the fuel sender plug on the fuel pump housing and retest the gauge, in theory this would test the sender with an open circuit or infinite resistance. Having carefully removed the plug and turned the ignition to position two the gauge slowly but surely went round to full. In my mind, open circuit, infinite resistance is therefore a full tank, to me it suggests the wiring and gauge are both fine. The issue is in the tank.
I'd not ventured into the tank as I didn't have the correct tool to open the lockring as shown in the images. I've since found out it's secured to 61Nm, pretty tight then.
Today however I received the tool I'd ordered from Amazon which is designed to release the lock ring. I must admit it's not the best tool in the book, but it was cheap and designed for a job I'll probably never do again.
This afternoon with careful use of the tool being mindful not to slip off the plastic notches I managed to open the lockring and subsequently was able to lift the sender. The theory here is that the sender was not orientated correctly in the factory and the float is catching on the tank sides.
All I had to do was lift the sender, rotate it, sit it back down and retighten.
With no experience of this, it did turn into one of those panic situations.
Opening the fuel tank was a smelly job, with fuel vapour filling the garage even with the garage door open. After 40 minutes working around an open fuel tank you start to notice it. Rotating the sender was easy, however between the tank housing and sender is a gasket which upon removal popped away from the sender and tank rim.
The replacement part is listed below.
https://caterhamparts.co.uk/pumps-senders/1989-gasket-fuel-pump-sigma-suzuki.html
No matter what I did I couldn't get the gasket to sit back around the sender and permit the sender to sit back down into the tank. It was as if the gasket had expanded during the brief period it had been in the tank and exposed to fuel. I tried for some time to get the gasket into place, but with fuel vapour filling my nostrils I started to consider the unenviable possibility I might have a serious problem, I might be unable to re-seal the tank! Not a good situation.
I phoned Oakmere and spoke with the ever helpful Paul on service, he advised me it should be possible to refit the gasket. That at least gave me some faith it was possible. I also phoned around the local Ford dealers in a hope of locating and replacing the gasket, but none of the local dealers carried any sender gaskets. I had to solve this!
Initially, I'd been trying to fit the gasket around the sender, it seemed logical and seemed to be the way it came out. Out of dumb luck, I tried seating the gasket into the rim on the tank and then pushing the sender into it. The first time I tried this it worked, and easily too. I think Paul at Oakmere may have suggested this approach, but with the combination of mild panic and fuel vapour, I don't think I was listening very well.
After a couple of attempts positioning the sender, setting the lock ring hand tight and testing the gauge I found what I believe is the sweet spot with the sender properly registering and the gauge returning a suitable level. It's been rotated clockwise by approximately ~10 degrees, so not much! The difference can be seen in the below image by the blue marker denoting the original alignment, the displacement signifies the amount of rotation.
It was now a case of rebuilding everything up again. First up retorque the lock ring to 61Nm, not easy against the plastic notches. The boot floors were a reverse of their disassembly along with the pump control module. With respect to the rivet and cradle clip I opted for a very small bolt and nyloc combination, those I'd used previously on the carbon fibre wing guards. Use of this alternative approach allowed me to use the original hole without the risk of the rivet pulling through. I think further disassembly will be easier too.
I have to say this was a real pain in the **** and a job I could have done without. On a positive note however, I've become even more knowledgable about the components built by Caterham at the factory, bits of the car which are supplied pre-assembled. I also have a better appreciation of how the fuel gauge works. Lots of lessons learnt then, and that really has to be the mantra here. Caterham did want me to warranty the issue, but let's face it, that's not the spirit.
Tomorrow morning I intend to fill the fuel tank, yes I've not done it yet, so there is a small possibility it's still not fixed, but I'm confident. Derek has also sent me an email also agreeing that I've fixed it, so fingers crossed.
References
Original blog, https://www.caterham7diaries.com/420r/fuel-gauge-inaccuracy
2nd follow up blog, https://www.caterham7diaries.com/420r/fuel-gauge-update-and-paranoia
3rd follow up blog, https://www.caterham7diaries.com/420r/fuel-gauge-follow-up-frustration-and-cut-fingers