1999 Bongo 2.5TD - Valve Stem Seals / Cylinder 3 Exhaust Outlet Oil Leak

[JF117]

Hi there,

Long time no-chat! Will try and keep this as succinct as I possibly can!

June 2023
Right after Bongo Bash I had a leak in my Thermostat To Passenger Side Heater Pipe which caused loss of coolant and an overheat. At the time, I pulled over immediately and switched off the engine. We later concluded that this must have inadvertently cracked the head cylinder / led to head gasket failure.

AA man got us back on the road the same day [couldn't believe it] and I eventually had that pipe replaced. Was driving it like crazy up and down England the whole summer, so never thought anything of it.

December 2023
Had the van parked up on the drive for a few months whilst I was in Australia and when I came back, took it out for a drive and noticed the temperature gauge was acting like a rev counter, swinging back and forth and consistent with acceleration. I.E, when I accelerated, it opened [went up] and when I stopped it closed [went flat] but never beyond or into over heating territory.
After sniff test, subsequently established there was oil in the coolant and it was even sitting on top of the coolant in the header tank.

Mar-May 2024
Substantial work was carried out on the vehicle, replacing the head cylinder, all gaskets and seals, crankshaft seal, water surge tank gasket, water pump, thermostat, both temperature gauges, glow plugs, prettymuch all of the rubber hoses and a couple of metal pipes. The whole engine was out and split etc so egnerally anything that needed addressing, got addressed.
The new head cylinder from Bongo Spares came with a two way cylinder pipe not the like-for-like three way bleed pipe, which made bleeding the van awkward but still doable.
I also installed a Low Coolant Alarm and a Digi Thermometer for more accurate temperature read outs.

Summer 2024
Drove the van up and down England several times, couldn't fault it and parked it up for Winter.

Feb 2025
Had it out for a spin, no issues to report.

Mar 2025
Had it out over St Patricks day weekend, all was going well until the temperature gauge started to twitch until it eventually started acting like a rev counter, same as above. Up until this point, the low coolant alarm never sounded, the digi temp read out never recorded anything to cause concern so I thought it might be an air pocket in the system or a loose connection on the temperature gauge.
Brought it in, opened up the radiator cap, seems like the temperature gauge issue has been addressed.

However, I noticed oil around the rocker cover gasket, oil filter and front crankshaft. Rocker cover gasket seal is easily enough sorted but upon closer inspection it appears there is quite a bit of oil coming from cylinder 3's exhaust outlet, which explains the crankshaft oil, and we think it might be a bad or poorly assembled valve stem seal.

I haven't proceeded with any further diagnostic tests other than first visual, haven't got the exhaust manifold off yet or done smoke test either.

Questions:
1. Given the extent of the work above - what may have caused this?
2. What further tests might be useful in establishing the root case
3. If it is the valve stem seals, is it possible to replace the valve stem seals without splitting the engine [cause that would be sickening]?
4. If yes above, is it a difficult job and is there anything to be aware of?
5. Am I buggered?