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MG Midget and Sprite Technical - 1500 engine knock

Hi guys,

I couldn't get back to my old post t continue it so I'll start again. A couple of months ago I was having trouble with a knocking coming from the engine and low oil pressure. I tried all sorts of fixes and in the end have installed a fuel pressure regulator at 2psi which has fixed the oil pressure problem. That being said the knocking is still there. Last night I ran it again and the knocking got much worse. I turned the car off and the knocking continued! Could it be a cooling problem? I'm genuinely stumped!

As ever, your assistance is hugely appreciated!

Jamie Watt

I'm confused. Why would a fuel pressure regulator have an effect on oil pressure?
Chris Hasluck

I think the tale was that the oil had become diluted with petrol, eventually tracked to a too- high fuel pressure overcoming the float chamber inlet valves and allowing fuel to overspill into the bores.

But I am more confused by a knocking that continues after the engine is switched off?! Jamie, do you mean that when you switch off, the engine continues to run, roughly and with knocks and lots of vibration. Its called "dieseling" when an engine runs on like this.

Have you checked the ignition timing?
Guy W

....and how does it continue to knock after it has been turned off! Seismic activity or maybe there is a slight misunderstanding here!
Lindsay Sampford

Jamie, I think I remember your previous thread now. Are you sure it is diluted oil? Make sure that the oil in the sump is still fluid. I had similar symptoms a while ago, sudden loss of oil pressure with the associated big end rattle. What had happened was the coolant (50% oat antifreeze) had got into the sump via a poor quality head-gasket and turned the oil to a big lump of jelly, a bit of a struggle for the oil pump to pick up! I initially thought the oil pump had stopped turning for some reason, so went to drain the sump prior to removing it, only to find that with the drain plug removed, nothing came out! I replaced the head gasket with a Payen one and also replaced the big-end shells as they were a bit scuffed and it all seems to be OK now. If you haven't already checked your oil for 'jelling', try draining a little out to see what it is like. If it is petrol dilution, you ought to be able to smell it.
Lindsay Sampford

Just been out to have another bash and I think I've cracked it - essentially I think there was a substantial air lock somewhere in the system and the knocking was actually boiling water, hence why the noise continued briefly even after the engine had stopped. Kicking myself for not checking it sooner and hoping no permanent damage has been done.

Re the fueling issue, the facet pump was running at about 5psi which was flooding the cylinders and allowing a good amount of petrol past the rings and into my oil. It took 3 oil changes and a change back to SUs from a webber but I'm happy it's all sorted.
Jamie Watt

If you took it out of gear, turned off the engine and coasted and the knocking continued I would suspect a Universal Joint about to expire. If you were not coasting then I'm confused also.

Rob aka MG Moneypit

I remember this

If it is dieseling... the 5 most common cause are,

Carbs to lean,
wrong temp spark plugs,
vacume leak around the carbs/ intake manifold,
Bad fuel
timing advanced to far

I cant remember in the last post if thrust washers/bearings where with in spec, but Ill throw that out there just in case as a redundancy to cover the that is common issue on the 1500s

Does the oil still smell fuel... im assuming you changed it

Prop and the Blackhole Midget

All is well folks! Oil pressure is about 50psi on start up coming down to 25psi once warm. No more knocking and nicely running. A huge thanks to all of you for being so patient with me!

As an aside, is there a way to dig up old posts which are no longer on the front page?
Jamie Watt

Yup look in the archives for the thread, and simply post a comment and submit it. It will live again.

If you look at your previous thread, title, "Engine Knock (1500)", you'll find that overpressure from the facit is precisely what I, and Nick, said was the cause of the diluted fuel.

3 lost sumps full of oil huh?

I seem to recall I also suggested finding the fault prior to replacing the oil. ;).
Lawrence Slater

Perfect, thanks, I'll look it up.

3 lost sumps of oil cost 60 and my fuel regulator also cost 60 so I was damned if I did and damned if I didn't! Still good to know that there isn't anything more sinister going on.
Jamie Watt

I beg to differ Jamie. :). After the 1st oil change, you'd have been better off exploring all the possibilities before adding more oil.

Swapping out old bits for new bits, is one way to find a fault, and it usually works, but it's also an expensive way to go about it. If you have deep pockets it doesn't matter at all, but to a tight sod like me, it hurts. lol.

Pssst. Don't tell Nigel you spent 60 on 3 oil changes, he might not believe you. ;).
Lawrence Slater

60 for three oil changes (with or without filter(s)) sounds about right

particularly for a 1500 following the refill instructions in the Driver's Handbook should help to avoid airlocks and hotspots

are you sure it was an airlock and not a blockage of some kind that has come loose - if you get the car running unusually hot in the future I'd check the whole cooling system in case the blockage has moved back or to elsewhere (and I'm not saying it was a blockage and not an airlock)

coolant changes are every 2 years with ordinary coolant so a thorough drain, clean and flush/back-flush might be useful at next change

I'm glad for you it's sorted
Nigel Atkins

Jamie what grade oil are you using?, 25PSI still sounds a tad low.
R W Bowers

This thread was discussed between 26/11/2014 and 28/11/2014

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