::standfirst Healthy oil pressure is the best single indicator of a T-Series engine's condition. A rough rule is at least ~10 psi per 1,000 rpm, with strong pressure once warm; low or wildly fluctuating pressure points to wear — usually the bearings, oil pump or relief valve — and shouldn't be ignored, as it precedes expensive damage.
What's normal
The traditional rule of thumb for the XPAG and XPEG is a minimum of about 10 psi per 1,000 rpm on the road, once the engine is warm. A healthy engine will comfortably beat that — pressure climbs strongly as soon as revs rise from idle, sits reassuringly high through the mid-range, and holds steady at cruise.
At hot idle, pressure naturally falls. A modest drop as the oil thins with heat and the pump slows is normal; a healthy engine may sit around 15–25 psi at hot idle and still be fine, provided pressure comes up firmly the moment you blip the throttle. Verify the exact figures for your engine and gauge against the workshop manual — they are guides, not gospel.
Watch how pressure behaves, not just what it reads. A firm rise off idle matters more than the peak number.
What low or fluctuating pressure means
Persistent low pressure, a slow decline over the miles, or a needle that swings around in a way it did not use to — none of these should be ignored. Work through the likely causes in a sensible order rather than jumping to the worst case.
1. Gauge accuracy. Before condemning the engine, confirm the gauge. Fit a known-good mechanical gauge in parallel — a dashboard gauge or its capillary line can lie, and it is cheap to rule out.
2. Oil-pressure relief (bypass) valve. The relief valve on the side of the block bleeds excess pressure back to the sump. A worn ball or damaged seat lets pressure leak away permanently. Remove the plug, spring and ball, inspect for pitting on the ball and its seat, and reseat lightly if needed. Renew the spring if it has gone soft.
3. Oil pump. A tired pump — worn gears or excessive end-float — cannot generate the pressure the engine needs, especially at idle. On the XPAG the pump body cap is surface-ground and fitted with no gasket; anyone who reassembles it with a paper gasket or without cleaning the mating faces has just given the pump extra internal clearance, and pressure will suffer accordingly.
4. Top-end bleed. A worn rocker shaft and bushes let oil escape from the top-end feed faster than the pump can supply it. This shows up as low pressure at idle in particular, often with a noisy top end. Rebushing the rocker assembly restores both.
5. Bottom-end wear. Worn main and big-end bearing shells are the classic cause of low hot pressure with a heavy rumble under load. If pressure is poor after the relief valve, pump and top end have been ruled out, the shells are the next stop — and the sooner they are dealt with, the less likely the crank journals themselves are damaged.
The right oil
The XPAG and XPEG are flat-tappet engines. Their camshaft and followers rely on the zinc/phosphorus (ZDDP) additive package in the oil to survive; modern low-SAPS oils formulated for catalyst-equipped engines have that package deliberately reduced and are not appropriate here.
Use a classic-engine oil with a proper ZDDP package — most of the major oil companies and specialist suppliers offer one explicitly aimed at pre-emissions engines. A 20W-50 viscosity suits typical UK/European use; hotter climates or a tired engine may take a thicker grade, colder climates a thinner one. Verify against the workshop manual and your own conditions.
Change the oil and the filter together, at least annually or at the mileage interval — whichever comes first. Old oil that has done a season's worth of short trips is more damaging than fresh oil worked hard.
Protecting the bottom end
Oil pressure protects the crank; you protect oil pressure with a handful of quiet habits.
- Warm the engine gently. Do not lean on the throttle until the oil is up to temperature — cold, thick oil does not reach the top end quickly, and the bearings are the last thing you want to load before pressure is fully established.
- Keep to the service intervals. Annual oil and filter changes are the cheapest engine work you will ever do.
- Watch the gauge. Learn what your car normally reads warm at idle, at 30 mph in top, and at a fast cruise. Any persistent shift from that baseline is worth investigating.
- Treat a persistent drop as a warning. A gauge that has quietly lost 10 psi over a season is telling you something. Diagnose it now, at the cost of a relief-valve strip or an oil-pump inspection, rather than at the cost of a crank regrind later.
Frequently asked questions
Why does my tuned XPAG misfire under load?
Misfire under load is almost always ignition on an XPAG, not carburation. Check the plug gaps and colour first — sooty plugs mean the mixture is rich or the ignition is weak; white or blistered electrodes mean it is lean. Then confirm the points gap and dwell, the rotor arm and condenser, and that the coil is producing a strong blue spark. On a Stage 1 or Stage 2 engine, also check the static timing has not crept too far advanced; a pinking engine often feels like a misfire and will eventually burn a hole in a piston. Finally, balance the SUs again — an unbalanced pair can leave one cylinder running lean enough to misfire at wide throttle.
Why does my XPAG hesitate when I open the throttle?
Hesitation on pick-up is the classic SU weak-mixture signal. The piston in the carburettor is lifting too fast for the fuel to follow, usually because the jet is set too lean, the needle is worn, or the dashpot oil is too thin. On a Stage 1 car with standard 1.25 in SUs, start by checking the damper oil level and weight — SAE 20 is correct — and that the piston rises smoothly without sticking. Then richen each jet by one flat, test again, and confirm with the lifting-pin method. On a Stage 2 car with 1.5 in SUs the same rule applies, but the needle profile is more critical; a worn or mismatched needle will hesitate whatever the jet setting.
Why is my tuned XPAG hard to start when hot?
Hot-starting problems on a tuned XPAG usually come down to fuel evaporation or flooding. A free-flow exhaust and a hot engine bay can boil the fuel in the float chamber and carburettor body, making the mixture over-rich. Check the float level first — it should be at the workshop-manual figure, not simply "looks about right". If the car starts cold but floods hot, try easing the throttle fully open while cranking to clear the cylinders, then start with no throttle. On Stage 2 engines with raised compression the starter also has to turn a higher-compression engine against a hot block; a good battery and clean earth straps matter more than on a standard car.
My Stage 1 XPAG pings on acceleration — what do I adjust?
Pinking means the combustion pressure peak is arriving too early. On a Stage 1 tune running on modern 95 RON petrol, retard the static timing by one or two degrees first and re-test on a warm day under load. If it still pings, check that someone has not already advanced the distributor beyond the Stage 1 figure, verify the compression ratio has not been raised by a previous skim, and make sure you are not running the engine below about 1,500 rpm in a high gear. Do not simply richen the mixture to mask pinking — it helps a little, but the real fix is timing or compression.
After fitting 1.5 in SUs my Stage 2 idle hunts — what is wrong?
An uneven idle after a Stage 2 carburettor swap is usually one of three things: the throttle spindles are worn and drawing air, the inlet manifold is not matched to the head ports and is causing reversion pulses, or the jets and needles are not centred and matched. Re-check the manifold face and gasket match, recentre both jets with the piston-drop test, and confirm both carburettors are drawing the same airflow at idle. Stage 2 engines with a fast-road cam also have less manifold vacuum at idle, so the idle speed may need to be set a little higher than standard — typically 800–900 rpm — to keep the SUs stable.
How do I know if my tune is running lean?
Lean running shows up as a flat spot under load, popping in the exhaust on the overrun, white or grey spark-plug insulators, and elevated exhaust-manifold colour after a run. On a rolling road the air/fuel ratio will read above 14.5:1 at cruise or above 13.0:1 at wide throttle. Fix it by richening the mixture a flat at a time on the SUs, checking for air leaks around the manifold and balance pipe, and confirming the fuel pump is delivering at least 1.5 psi at the carbs. Lean mixture is the fastest way to burn exhaust valves on a tuned XPAG, so do not ignore it.
Related reading
- Cooling the XPAG and XPEG — heat and oil pressure are two sides of the same story; hot oil is thin oil.
- Ignition and Charging — under-advanced ignition dumps heat into the engine and thins the oil in the process.
FAQ
What oil pressure should an MG XPAG have? A rough guide is at least 10 psi per 1,000 rpm, with strong pressure once warm and a modest drop at hot idle. Verify the exact figure for your engine and gauge against the workshop manual.
Why is my T-Type's oil pressure low? Diagnose in order: confirm the gauge, then the pressure-relief valve, then the oil pump (including whether its cap has been reassembled correctly, with no gasket), then top-end wear at the rocker shaft, and finally worn main and big-end bearing shells.
What oil should I use in an XPAG? A classic-engine oil with a proper ZDDP additive package — 20W-50 suits typical use. Modern low-SAPS oils formulated for emissions-controlled engines lack the zinc/phosphorus these flat-tappet engines need and risk cam and follower wear.
Is a drop in oil pressure at idle normal? Yes, within reason. Pressure naturally falls as the oil warms and thins and the pump slows at idle. What matters is that it comes up firmly the moment revs rise and holds steady at cruise; a slow, persistent decline over months is not normal.