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MG TD TF 1500 - Slow oil pressure build up on startup

One thing that has troubled me since I first obtained my TD (about 6 years ago), is that it's horribly slow to show oil pressure on startup. I find myself spinning the motor on the starter for quite some time (key off) to build some pressure and it takes quite a while before the needle starts to climb (we're talking 40-60 seconds!)... at that point I turn on the key and it starts right up.

Now, the car has never had poor running pressure - it never dips below 15 lbs at idle and on the road it's consistently around 50-60 lbs even when hot. Also, I use conventional 20-50 oil so it's not that it's too thin.

Essentially, it seems like the anti-drainback function in the pump or filter isn't doing it's job. However, in looking at the pump in the workshop manual (I have a later TD motor with the replaceable element filter) I can't see what, precisely, prevents drain-back.

Any thoughts as to where I should look to restore rapid oil pressure build up?
Kevin McLemore

Could it just be the capillary tube to the gauge? When you turn the engine off, does the gauge reading drop straight down or does it go down slowly? I replaced the tube on my road TC with a bigger diameter one. Since then, the gauge has responded quickly when the engine is started and also when it is turned off. Do TDs have a rubber section which might have gummed up? If that is the problem it might be improved by disconnecting the tube from the back of the gauge and flushing it out. (Perhaps by starting the engine and letting oil flow into a can.)
Bob
R L Schapel

Kevin,
Have you bleed the capillary tube to the gauge. If it is full of air it could well be soft to read the starting pressure. I just loosened the pipe at the gauge till it dripped oil and no issues after that.

Rod
Rod Jones

It drops right down when I stop the motor, and it responds rapidly as the motor speeds up and slows down, so I don't think it's a clog, but bleeding the gauge is certainly a good thought. I did have the gauge out when I restored the centre panel shortly after I obtained the car. I'll try it!
Kevin McLemore

Kevin,

I am confused by your starting procedure. Are you saying you normally don't turn the key on until you have oil pressure. If so I am wondering what that accomplishes. With my car which I have had for 47 years I always turn on the key, pull the starter and the oil pressure comes up in a second or two. The only time I would turn the engine over without the key is in the middle of winter to lube the internals which does take the amount of time you describe to indicate pressure. I would venture to say that your reticent pressure issue might be due to low RPM's in the key off scenario.

Jim
James Neel

Yes, James, I spin it with the key off because when I try to start it with the key on it still takes ages for the oil pressure to pop up. I figured that spinning it without starting is at least somewhat preferable to starting it and then having the additional hammering of the cylinders firing?
Kevin McLemore

Kevin, there is no check valve to prevent oil dain back.
See my article about how to install one :
http://ttypes.org/ttt2/pdf/TTT2-Issue37.pdf

Laurent.
LC Laurent31


Kevin, is your oil pressure gauge connected at the head or down on the block from the bottom banjo?

Further, are you using paper or felt oil filters?
Richard Cameron

I have experienced exactly the same issue as Kevin for the nearly five years my car and rebuilt engine have been back on the road, and I have dealt with it in exactly the same way, by turning the engine over with the ignition off until the oil pressure needle moves off the peg. The engine then starts start up immediately and immediately shows good oil pressure.

Laurent’s article is clearly written and very informative, but I’m in no hurry to drop the sump. He correctly states that dropping it is easy – the hard part is getting it back on without creating new leaks. I’ve been quite smug to date about how leak-free my engine is!

Inasmuch as there was no check valve originally, this must be a common issue. I haven’t yet tried bleeding the line to the gauge, but the diameter of the tube is so small that I find it hard to believe that it would take up to a minute to compress any air trapped in the line enough to register pressure.

Incidentally, I’m using a paper filter – how would a felt filter behave differently? The pressure gauge is connected to the lower banjo.

My question boils down to the same question Kevin asked: Is spinning the engine without starting at least somewhat preferable to starting it and then having the additional hammering of the cylinders firing?

Joe
Joe Olson

Fit a modern cartidge filter and the problem should go away. You will barely notice its there, except when it comes to replacing the filter, which is much easier, and its easily reversible if needs be. Dropping the sump in the car usually leads to more leaks, as you say.
Dave H
Dave Hill

Try hooking up oil pressure line to a differnt oil pressure gauge. Perhaps the gauge is faulty. Also 15 psi at idle seems low.

Cheers

Gary
79 MGB
gary hansen

To answer a few questions/

- The line to the gauge is fairly new, albeit long, and connects to the banjo down on the block.

- I use a felt filtre.

- Oil pressure at idle is normally about 20 psi. I say it never dips below 15 because if I’m in a parade and the engine is really hot sometimes it will drop to 15-17, but that’s an aberration.

I hope to get out to the (very cold) garage at some point this week and try the suggested bleeding. Given the hose is rather long, and I had the gauge out for several months, it’s possible this is the problem.


Kevin McLemore

Pardon the dirty motor... spring cleaning is yet to come.

Kevin McLemore

The banjo...

Kevin McLemore

Kevin, check the oil pressure relief valve ball (item 14 in the Moss catalog. I've seen these balls worn or even egg shaped from wear against the seat in the pump. They have to be perfectly round to close properly. I believe you can check it while the pump is still in place on the car.

Also, I've noticed on my cars that the paper filter seems to flow the oil through them more freely. I'd try a paper filter to see if it makes a difference in your situation.

I believe Dave Hill's Input to use a cartridge filter conversion is because most modern canister filters have a built in anti drain back valve incorporated.
Richard Cameron

Technically speaking, there is no need to bleed the oil pressure gauge pipe. Pressure is pressure, and you will never detect the difference in pressure build-up between air and oil. 60 PSI of oil will compress the minuscule amount of air in the pipe in a fraction of a second. A larger pipe could actually build slower, because it will take a larger quantity of mass to enter the pipe before maximum pressure is achieved. If a larger pipe makes a difference then I would suspect the smaller pipe was clogged, pinched or had some other restriction.
Steve Simmons

Kevin, is it possible you have an early engine with a by-pass relief valve incorporated in the engine block and a later pump body which also has a by-pass valve. Refer Workshop Manual - The Engine Fig. A.7. and A.8.
I suffered irregular oil pressure before discovering this problem when I first bought my TD.
Rob Woodfull

Drop your oil viscosity. You may be pumping the bulk of it out the relief, actually starving the engine.

With high viscosity lube, oil pressure at the oil pump can be much higher than the gauge reads and your oil is racing right back into the sump.

If it has broken in (after 6 years) and doesn't burn or drip a lot of oil, you might treat it to a nice synthetic.

I'd rather have a lot of slippery, thin oil circulating as opposed to a trickle of diesel oil.

Oil filter doesn't really enter the equation because the filter bypass valve will reroute oil directly to the engine if backpressure is too much.

I have restricting banjo bolts at the top and bottom of different engines. Which of your banjo bolts has the restricting orifice? If it is at the bottom, then there will be a lag to fill the rocker shaft at startup.
JIM N

Jim could have answer. Many banjo bolts were restricted by solder/drilling to make up for worn rocker bushes. If your lower bolt is restricted that could be the cause. Easy to remove and check. Not sure I go with thinner oil and synthetic however. George
George Butz III

For Jim, Are you sure about the bypass dumping back to the sump? I thought it just bypassed the filter.

Regards, Tom
tm peterson

We have two "relief" valves built into the engine, the pressure relief valve and the filter bypass relief valve.

For some engines, the pressure relief valve dumps back into the sump. In the XPEG/XPAGs, the relief valve short circuits the oil from the pump output right back into the pump inlet. In both cases, pressure is limited by REDUCING THE FLOW INTO THE ENGINE.

When the oil viscosity is high, due to cold temperature or thick oil grade, the oil flow is reduced when you need it most, at startup.

The filter bypass relief lets the oil bypass the filter if filter backpressure is excessive, due to cold oil, high viscosity oil grade or the filter is clogged. The engine still gets 100% of what's coming out of the pump, it just doesn't all go through the filer.

John Twist gives a good description of pump and relief valves... https://www.youtube.com/watch?v=ShHAaDKL3gM

Workshop manual only had straight weights to choose from: 30W with temps down to freezing; 20W down to 0F; 10W when temp could be below 0F.

20W50 probably won't hurt and likely isn't responsible for the slow pressure rise, but face it, it probably isn't flowing as much volume as 10W40, cold anyway. Someone could do some testing by blocking the relief valve and reading pressures with various weight oils. I'm not volunteering. I do know of a TF running 100psi.

With our supercharged engine, in worst case scenarios, it would cruise at 230F (normally cruises sunder 200F), so I've run 15W30 and kept a close eye on the oil pressure gauge.

I've seen numerous Honda engines that call for 5W30, but have thick oil or it gets low- the oil pressure light is satisfied but bearings spin wasting the engine.

I recently had to overhaul my son's brand new 465 hp ZZ454 crate motor since the so-called mechanics were complete idiots. They ADDED a filter bypass valve on the filter output with nowhere for it to go- all the oil had to waste the first 10psi opening the valve and then it all had to flow through that 1/4" hole. The engine calls for 5W30 synthetic, yes, "five-W-thirty", and of course they filled it with heavy duty, high viscosity oil. They stuck a cast iron distributor gear in it when GM warns it requires a hardened steel gear. That $10,000 engine lasted 400 miles. The $10,000 engine before it lasted 700 miles.

Jim
Ann Arbor Hydraulics







JIM N

This thread was discussed between 24/01/2019 and 31/01/2019

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