Engine Stripdown

MX5 Project, Apr-2022

Decided not to drive the car any more until I got to the bottom of the engine knock – even though it was fairly light, I didn’t want to risk further damage & expense.

So, took the engine out complete with subframe. This is a tedious job since Mazda didn’t design the car for easy strip-down. The worst part was accessing the various electrical connectors – they seem positioned for easy installation as they can be clicked into place with one hand. But removal generally needs one hand to hold the locking tab and the other hand to pull the connector free, which is sometimes impossible given the tight access in many places. The worst was the engine coolant temperature sensor which is at the back of the head and can only be accessed by removing a panel below the windscreen shuttle. Even then I found it impossible to disconnect so ended up snipping the wires. I’ll install an interim connecter during re-assembly.

Once all the wiring, coolant hoses, fuel pipe, clutch slave piping, power steering, vacuum hoses, etc, were safely tucked-away, the fluids drained, and the exhaust, prop shaft and the large differential/gearbox brace were removed, the engine & subframe were supported using a block & tackle on the lifting eye at the front of the engine and the gearbox supported by a jack. Also the brake calipers were removed & supported by wire.

The actual engine removal was then pretty straight forward; simply needed to undo the four subframe nuts and four bolts then lower the engine & subframe onto a trolley. Once this was done, a piece of timber under the chassis rails was used to lift the body clear of the engine with the block & tackle. With the engine, gearbox & subframe removed from the car, the next step was to strip away the ancilliaries and external components.

Engine/Subframe Lowered
Engine/Subframe Clear Of Body

The initial inspection showed how badly the engine was contaminated by soot caused by the exhaust gas recirculation (EGR) system. On this engine, exhaust gas is bled off from cylinder #4 into the EGR valve. The exhaust gas exits the EGR valve and enters the intake manifold adjacent to cylinder #4. I could see the amount of soot in each port of the inlet manifold was progressively thicker from cylinder #1 (best) to #4 (worst) and as reported in previous post spark plug #4 was badly contaminated by oily soot. The EGR system might work fine on newish engines but as they become worn and start burning oil, the soot from the combustion process mixes with the oil and congeals in the valves, ports and manifolds, resulting in a smelly, smoky engine.

A layer of oil covering each piston
Cylinder #4 Exhaust Choked with Soot
Cylinder #4 Exhaust Valve

Oil in the combustion chambers was almost certainly due to the piston rings. The oil scraper ring was encrusted in carbon and the oil control rings had little spring left in them and were also choked with gunk. The oil drain holes in the pistons were also completely blocked. I found that the valve stem oil seals had gone quite hard but I doubt if much oil was was getting in via the valves.

Low oil pressure and knocking was caused by this:

Big End Bearings Cyl #1
Big End Bearings Cyl #2
Big End Bearings Cyl #3
Big End Bearings Cyl #4
Bearing Remnants In The Oil Strainer

The engine most probably suffered from oil starvation evidenced by #1 & #4 big end bearing found to have “spun” in their journals. These bearings were very badly worn and getting close to disintegration. The main bearings were in decent condition but will be changed anyway. Luckily the crankshaft escaped major damage and only needed a light polish with 400 grit wet & dry to remove some scratches.

So, the engine will get new rings and main & big end bearings plus renewable items, e.g. head gasket, front & rear oil seals, new crankshaft pulley bolt, new head bolts. Also a new clutch, clutch plate and release bearing.

I’m not going to look for other problems with this engine, so the work I’m doing isn’t really a refurbishment. I’m just replacing the items that are obviously causing problems with the main aim to get it back on the road ASAP and keeping the costs down.