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MG Midget and Sprite Technical - Bouncing oil gauge

I was wondering if anyone can help. I treated myself and took the frogeye out for a rare drive today (948 cc standard car). It ran nicely, and was fun.
But when idling at traffic lights etc., the oil gauge bounced continually like a pendulum between 40 & 50 lbs. As soon as the revs increased, the gauge went up and it stopped "bouncing".

Any ideas please? Many thanks

Graham V

I wonder if the PRV is "cycling" at tickover due to reduced flow from the pump at low speed.

Hi Greybeard
Thanks, I guess it could be the pressure relief valve, but I haven't noticed it do that in the past.
I was thinking maybe the gauge needs bleeding but then I cant see how the air would have got in.

I am ashamed to admit that I haven't changed the oil for several years but I do a very low mileage (less than 1,000 since the oil change. I dont know if that would effect it
Graham V

Graham, my guess is that at tickover speed the pump is delivering so little flow that it takes time to build pressure to the point that it lifts the PRV. Then as soon as the valve lifts the pressure drops to the point that the valve closes and the cycle repeats. You notice that as your OP gauge is so direct. I wouldn't see it in my car as my gauge is electric and the instrument hysteresis would mask it.

I stress it's a guess though. Another possibility is wear in the pump. A tiny excess of clearance between the rotor and body would reduce the delivery volume at tickover speed.

First thing I would try is an oil and filter change. Even though your mileage is so low I would do it annually anyway. There is no greater kindness you can do for an engine than fresh oil. It's such an easy simple and cheap thing to try that it would be my first step in your place. I'm doing mine this weekend with 1199 miles on the oil.

If the symptom goes away it means that your old oil and filter were past their best and ready for renewal. It isn't just mileage that deteriorates the oil.


The first thing I would do is stop worrying, mate. 40 psi at tickover is perfectly respectable I think. I wish I had it, but I never will because it's a Tr++++h engine!

BTW what pressure do you have at "driving" revs when the engine is hot?


Thanks Greybeard. That all sounds logical. I will do the oil and filter change over the next few weeks as soon as I get some time to play!
It gets up to 60+ psi once the revs go up. It didn't really get "hot" today. Ambient temperature was about 9degrees, and I only drove about 3 miles! Got a bit nervous when I saw the gauge.
Graham V

Ooops Graham, I meant to add that I don't think your gauge needs to be bled. The fact that it is recording pressure changes so immediately suggests to me that it is working really well. And 60+ psi is good, as long as it is stable. Hope this helps.

Graham, change your oil before you damage your engine no matter what the gauge is doing. The worst thing you can do is leave old oil in. With short runs and age the oil becomes very acidic and any part of the engine under physical pressure ie rings, cam follower to cam lobe and yes oil pressure relief valve to seat can and does get damaged by the acid. It tends to show up as little black pits. Change the oil once a year before you put it away for winter, just get oil pressure up on starter motor, squirt some oil in the bores, spin it over again and leave it unless you use the engine properly.

Peter Burgess Tuning

"-- haven't changed the oil for several years -- " " -- less than 1,000 since the oil change. -- "

And on top of what Peter just said, if you leave oil standing that long, before agitating it, it begins to sepearate into some of it's constituent parts. I had a container of GTX do exactly that when left on a shelf for about 18 months.

I don't observe the oil change intervals that closely either, but I do use the car a lot.

Even if you aren't going to go for a drive, I would run the engine until hot, and leave it running for 10 mins or so at fast tickover, every 3 months or so, just to swirl up the oil, and burn off the inevitable water vapour(condensation) that's in there.
Lawrence Slater

Thanks guys. I didn't realise. I can't do anything for a week, but it will be at top of the list.
Out of interest, does oil have a shelf life. I have never noticed a "use by" date on oil in the shops I.e. Do I need to be sure it's not been standing in the store for ages?
Thanks again, appreciate the advice
Graham V

I'm not aware of limited shelf life of engine oil, properly stored. It's not something I've ever noticed. Lawrence's comment about his tin of GTX going off was a surprise to me.
Not that I would doubt Lawrence for an instant, but I haven't come across the issue before. Maybe it's something lacking in my education.

Interesting Q.

Thanks Lawrence.

The flickering of the pressure gauge when the relief valve starts to cut in, a feature of B series engines as well, provides a very informative guide to engine health - the lower the speed it happens, the less oil is escaping through worn bearings or pressure not maintained by a worn pump.

If the engine is properly hot and it is hapening at idle, the engine should be in fine fettle.

Though if the oil is thick enough pressure can be held even with a run bearing.
Paul Walbran

The fluctuation at idle is normal for an A Series engine. Idle oil pressure usually is a bit lower at idle, and the needle swings around 40 PSI.
Glenn Mallory

Reporting back!
The good news is I now have fresh oil onboard. But the bad news is its spurting out of the filter!
I believe I have a Tecalemit filter as it has a domed washer welded on the bottom. But it is leaking from the seal at the top.
It was a real pain to fit, I had to hold it in with grease and thought that in the end I did s decent job, clearly not!
So my questions please:----
1. Could I have over-tightened it? I couldn't see a reference to a torque setting, and was a bit nervous that if it was only hand tight, it could work loose.
2. Or is there a trick in fitting it? Its not easy to get to from underneath because of the chassis frame, and I could have done with a third hand!
3. I was thinking of fitting a spin-on adaptor. Once on, it should make my future changes easier, but I guess just as difficult this time around?

Thanks very much, Graham
Graham V

Did you renew the thin rubber washer in the top of the filter? They can be a real pain to seat with the result oil runs down the side of the housing. I converted my engine to a spin on using a filter head from a later mini/midget/sprite
Bob Beaumont

Easiest access to the oil filter for fitting or removal is via the triangular apertures from the back of the wheel arch. Jack the car up, secure on axle stands and either turn to full left lock, or remove the wheel.

Usual reason for leakage is over enthusiastic tightening which distorts the rubber seal.
Guy W

Yes Bob, I replaced the rubber seal. The only way I could get the new one to stay in and defy gravity, was to smother it in grease.
I think Guy may have the answer. Its likely I over-tightened it but how tight should it be? Cant really risk it loosening off in mid-flight!
And thinking about it, if it is covered in grease, the seal might slip out more easily when I was tightening it up?

Graham V

That is good news Graham. But what about the oil pressure?
As to the filter I can't help. I have a 1500 with a spin-on cartridge and if it's not too onerous I suggest it's a good solution.

5 *****'s

for a spin on oil filter assembly

Prop and the Blackhole Midget

Thanks for all the help. Its now back in with a new seal and i think sealing ok.

I say "I think" as have only tested it on the starter motor, as for some reason the car wont now start! Very odd - it started just before to squirt the oil over the garage floor, but not now. Fuel seems to be getting through ok. Plugs have a spark, but it looks quite a weak spark to me (yellow colour).
Graham V

Reporting back. Cleaned the plugs and its running fine.

Just wanted to say thanks to all the feedback. It's very helpful and thought provoking.

Interestingly, oil pressure is now at 60 on idle and about 65 on about 3000 rpm. Not sure why, but the engine was still cold (as its pouring down here so just ticked it over for a few minutes in garage) and new thicker oil I guess.

Happy days!
Graham V

Thanks for YOUR feedback Graham. It's good when people post resolutions. It's good to know what worked and it doesn't happen enough - I'm often left wondering what happened.

This thread was discussed between 22/10/2015 and 06/11/2015

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