VVC 160 Oil Consumption & Compression - A Puzzle!

[revilla]

I recently overhauled my VVC 160 engine. Was a full ground up overhaul, honed liners with new rings, all new bearings and seals etc. Cylinder head full overhauled including peening as per DVA recommendations followed by a skim and a 0.5mm stainless shim to restore head height and compression ratio. Valves lapped in, new valve stem seals etc.

After consultation on here, engine was run in fairly hard from the start, which may or may not have been the right thing to do.

Other than a Z&F (Kmaps) remap and some offset dowels to correct the cam timing, the engine is pretty much stock.

Performance is fantastic. Car is a heavy 585kg SV and I'm best part of 120kg and using a decent AP22 performance gauge I'm able to do quite repeatable 0-60 runs in 4.6s which is well ahead of the book figure. Compared to before the rebuild it feels quick. It's smooth and torquey, sounds beautiful, never misses a beat. Oil pressure is spot on, oil and water temperatures sit at 80-ish on the gauge whatever I do, so all sounds very happy.

Except ... it's absolutely drinking oil!

It's now done 5000 miles since rebuild so should have settled down. I've been monitoring it over the last 3000 miles and it has used an steady and consistent 1000ml / 1000 miles. It really is very consistent, if I do 250 miles it will take 250ml to bring it back to the top mark on the dipstick.

It's not leaking a drop. No signs of oil anywhere, the underside of the car and the garage floor are perfectly clean and dry. And there's no sign of anything going into the coolant which is as clean as the day I put it in.

So if it's not leaking it, it must be burning it.

First thing to say about that is there is absolutely no evidence to see that it is burning oil. It passed an MOT in the meantime with what they said were "exceptionally clean" emissions results and very low hydrocarbons (it does have a cat though). The exhaust tailpipe is a little black and sooty but no more than it has ever been, and it's not oily at all. There's absolutely no oil smoke from the exhaust, either on startup after standing or whilst running. No oily smells either, looks like it is running cleanly. I've taken the manifolds off and looks at the backs of the valves and they are clean. I've taken the plugs out and they are all a nice even matching tan colour and dry. I've also had a camera down the spark plug holes and there's a very thin flaky film of carbon on the piston crowns; where this is flaked away the piston crowns are still shiny silver metal. No real build up. The bores looked OK, you could still see a few scratches from the honing pattern but no signs of damage or vertical scuffing.

I've taken the oil filler cap off and held a sheet of paper over it; although it bounces up and down to the rhythm of the engine it is overall sucked down rather than blow off, suggesting that at idle the blowby is well within what the PCV system can handle and the crankcase is under mild depression, as I would expect.

When I took the inlet manifold off there were just a few drops of clean oil at the step between the runners and the head, which made me think it was maybe blowing a bit of oil through the breather but I'm not sure how much is normal. I know on an engine converted to TBs it would normally blow some out into the catch tank anyway which on mine will be going back into the plenum for burning so I wasn't particularly concerned. I've had the cam cover off tonight and checked that the two PCV orifices look OK, the steel wire oil separator is in place and everything looks correct.

Having subsequently seen a liner that had been honed by Dave Andrews, the conclusion I had come to was that I had honed mine rather too coarsely and that the rings hadn't seated properly. I was resigned to having to replace the liners and rings and I've bought some new liners, factory honed, but was rather surprised to see today when they arrived that the honing finish looked remarkably similar to what I had achieved. I was expecting to find it a lot more finely honed than my efforts.

While I had it apart tonight I thought I would do a compression test to see if it gave me any clues. Now the compression ratio is supposed to be around 10.5:1, it's not had any headwork other than a skim and the head has been shimmed back to the original height and the pistons are stock VVC 160 so there is no reason to expect the compression to be anything other than approximately standard. So was guessing I would see something around 10.5 bar or 150psi peak pressure.

Readings I got make no sense to me. Cylinders 1 & 2 gave me just under 18 bar and cylinders 3 & 4 gave me just over 16 bar. these readings were quite repeatable. I'm not sure how much to read into the lower pressures on 3 & 4 because the gauge was very difficult to screw into the very deep plug wells on the VVC engine, I could only screw it in by the rubber hose and the battery was getting tired as I got to the rear cylinders, so it was only cranking slowly. If I hadn't got it properly sealed into the plug hole and it was cranking slowly it may have been leaking down a bit. But I don't understand how the numbers could come out anything like that high.

The engine doesn't show any signs of detonating, there's no pinging or crackling even if you labour it a bit and when I put the camera down into the cylinders the piston edges all looked very healthy, so I don't for one minute believe I have ended up with an enormous compression ratio.

This is the first time I've used this (Gunson High Gauge, bought new) compression tested so I guess it may over-read but that's a long way out.

Those compression pressures certainly don't suggest ring sealing problems. I tried introducing a little oil into 3 and 4 for a wet test; it made no difference on number 3 and the battery gave out before I finished number 4, but again nothing to suggest ring issues from the result from number 3.

So, I've got an engine which is somehow consuming very large quantities of oil, not leaking at all and showing very little sign of burning it! Everything else looks very healthy and performance is spot on. I don't want to just leave it because it is a nuisance to have to keep monitoring the oil this closely and topping up, if it is burning it then it will eventually cause problems and I don't like knowing that something isn't right.

I'm wondering if there's anything I could have missed before I tear it down and swap the liners and rings because at the moment the only thing that is telling me it's a liner issue that I can't think it could be anything else!

Any thoughts?

Thanks,

Andrew

[Jonathan Kay]

You're dubious about your compression readings. How about testing that tester on another engine, or another tester on your engine?

Jonathan

[SM25T]

It has to be going somewhere. Very frustrating after all that work ! Hope you get to the bottom of it soon.

[revilla]

@Jonathan - I have just done that. The identical spare engine on the trolley registered 18 bar consistently on cylinders 2 to 4 and a little higher on 1. So I guess my gauge is horribly inaccurate. They do say you should pay more attention to differences between cylinders than actual readings; that may be because gauges tend not to be that accurate but I didn't expect it to be that far out! My battery is a bit hammered now and on charge; I guess once charged I should check 3 & 4 again as they looked to be 1.5 bar down on the other two (which is probably an over estimate using this gauge, but who knows?). The liners were used items that looked OK and piston to bore clearances seemed to measure up OK but I wonder if I missed a problem with ovality or something?

[simon_h]

I guess you ran it in for 600 - 1000 miles on mineral oil and not synthetic? I have heard of a perfect engine using oil because the rings hadn't bedded in properly. A quick run with a glaze buster and new rings with proper running in cured it.

The vhpd I built years ago for a previous car was run in hard on mineral before going to synthetic at about 700 miles and that didn't use any oil to speak of. It was on scholar liners not rover so they were located at the top as well if liner flex is an issue but I don't think hard running in will be an issue for your engine myself.  

[SM25T]

Maybe you should have kept that longer dipstick ;-)

[revilla]

One other bit of info; this engine was assembled from used parts. It is not a rebuild of an engine I have previously run. So it is entirely possible that I may inadvertently bought and fitted liners with an ovality problem as I cannot say for sure whether they were burning oil in a previous incarnation.

[revilla]

@simon_h: Yes it was run on Millers CRO, straight mineral running in oil for the first few hundred miles with lots of hard acceleration and coasting before switching to regular oil.

[simon_h]

It was only a question as I cant see any reason why it should use oil then either.

[Mark Durrant]

I suggest you get a leak down test with the engine up to normal operating temperature.

[Steve_K]

I know nothing about K series engines but could oil be leaking into the cylinders down the valve stems? As long as the leak was small but continuous, it would be almost invisible.

Steve

[revilla]

It could be but I did replace all the valve stem seals at rebuild. If it was the exhaust valves I would expect to see carbon building up on the backs of the valves but they are quite clean. I think usually with valve stem leaks big enough to explain this it would also normally tend to leak when standing and then give a good puff of smoke when started, but I'm not seeing anything like that.

I'll do another compression test when the battery is recharged and the engine is fully up to temperature. If that still shows the rear two cylinders significantly down compared to the front two I would say there would be enough evidence to justify tearing it down for inspection without needing to pay for somebody to do a full leakdown test for me. I'll have a really good look at the valves and their seals while it is in pieces and put new liners and rings in throughout to be safe.

[Mark Durrant]

A Leak Down Tester is not expensive and you may be able to borrow one from a club member so all you need is access to a compressor. I would prefer to know the reason before I started to strip down an engine. I have seen a K series that had good readings on a compression test however, the leak down test revealed a problem

[21jigsaw]

Hi, Just a few thoughts :-

I tend to use the compression test gauge as a comparison between cylinders rather than a outright figure they tend to be rather inaccurate with regards.

Have you looked down the bores to see if there is any polish or pickup ?

With changing the cam timing has this created more inlet manifold depression?

Has the spark plug gaps increased? indication of detonation.

Cam timing ? difference between 1&2 and 3&4.

Any colour difference between the 1&2 and 3&4 plugs

Dave

[revilla]

Dave,

Thanks for your inputs.

I take your point on the compression tester.

I'm not sure what you mean by polish or pickup. Could you give me a bit more info and tell me what I should be looking for?

I have only adjusted the cam timing by about five degrees as recommended by DVA using an offset dowel on each front cam pulley, so one to advance the inlets on cylinders 1 & 2 and one to advance the exhaust and therefore indirectly the inlets on cylinders 3 & 4. Being a VVC it's not easy tweak the timing as you would with verniers but I do have gauges so I could compare the timing on the front and rear inlets to see if there is much difference to explain the different readings. I guess after going through three roll pins the rear inlet timing is prone to being a bit out. I'm really not sure if this will have affected the manifold depression significantly; I guess younare suggesting that increased manifold vacuum my be causing it to draw off more oil via the PCV hoses?

The spark plug gaps didn't look yo have increased although I hadn't thought to measure them yet; I have no indication of detonation at all though, I only mentioned it because if those compression readings were absolutely correct it would imply a silly compression ratio and I would expect it to detonate horribly, which it clearly doesn't. I thinkbthe absolute pressure reading was just a red herring as you point out.

No difference in plug colour between any of the cylinders, an even healthy looking light tan colour on all of them.

Would a leak down test reliably indicate all possible cylinder issues that could lead to high oil consumption? I can see how it would reveal blowby that could be carrying oil to the plenum but would it show anything abnormal if for example the oil control rings weren't sealing and oil was entering the combustion chambers on the cylinder walls? Or is this unlikely to be the problem. I'm reaching the limits of my experience here.

 

[21jigsaw]

I would look for scuffing or polishing on the thrust faces and compare to the non thrust areas, have you put oil down the bores to isolate the piston / rings determine any effect on the compressions.

There are no visible differences between the cylinders with regards to colour, compression, plugs, which would tend to rule out an issue with individual cylinders.

Increase in manifold compression can increase oil past both pistons / bores and valve / guides.

On the VVC inlet manifold depression is measured on the t map sensor have you the facility to measure the depression and compare to a standard vvc cam timing that would rule out that possibility.( through the full rev range)

Dave

[revilla]

Dave (and anyone else ...)

I did put a little oil into number 3 and it made little difference. The battery was getting pretty clobbered before I had chance to repeat the test on number 4. I will try to get a decent borescope down there to look at the cylinder walls and make the comparisons you suggest.

I've taken some measurements of MAP at steady state cruise and coasting down at various RPM, all in 3rd gear. I've also measured MAP at idle. At WOT the MAP went to atmospheric 100kPa at all RPM.

Without taking all the timing pulleys off and removing the dowels I don't have standard VVC figures for comparison but to me they look normal enough. What do you think? 34kPa (so 66kPa of depression) certainly sounds normal for a stock engine at idle. It rises with increasing load to 48kPa (52kPa depression) at 6000rpm steady cruise in 3rd gear. On the overrun it's pulling a fair vacuum but it's only around another 20kPa of depression over the idle value.

I will also try to measure up the timing on the two sets of inlets; given the complexities of the VVC cam drive train and the fact that the two front compressions matched and the two rears matched, I suspect you may be right and the difference between the fronts and rears could well be just cam timing, but I will check.

This all seems even more puzzling to me now though; it's looking less and less like cylinder problems (unless they are all coincidentally affected to the same extent) but I can't think what else would affect the engine as a whole. I did check that the oil separator wire mesh was in place in the baffles in the cam cover, the PCV orifices looked OK (i.e. not blocked or molested by a previous owner).

Do you think we would learn much from disconnecting the PCV pipes, blocking them off at the plenum end and running the cam cover breather out to a temporary catch tank? My thinking is that if it is excessive manifold vacuum that would result in a reduced oil consumption, if it is breathing heavily in blow-by it will probably throw quite a lot out but if it is getting past the rings it will probably continue to burn oil and not blow much into the tank. I'm guessing!

[john milner]

For reference the oil consumption of my 160 VVC is negligible and is rarely topped up between changes (circa 6-7,000 miles). I would worry if it started going through a pint every 250 miles.

If it is not leaking the oil is still in the engine as a liquid or is burnt off. I doubt it is burnt off as you would probably notice so if it is staying as a liquid where is there that could hold a few pints? Bellhouse? Blockage somewhere so the oil is pumped to the top and can't get back down quickly? Oil getting into the coolant system?

Comparing dipstick readings with the engine on and after being off for half an hour or so might give some clues.

[Ivaan]

Were the oil rings spaced correctly on rebuild?

 

[revilla]

@john milner - Yes my other VVC engines have been like that, absolutely negligible oil consumption, which is precisely why I am so concerned about this one.

The oil is not being retained in the engine anywhere, I'm quite sure of that. There's a good five litres of the stuff disappeared now and it is disappearing at a very steady rate relative to miles travelled. The bellhousing isn't even vaguely sealed; a little oil may collect in the bottom but with five litres it would be coming out all over the place and the clutch would be shot. And there isn't the capacity to retain it anywhere else in the engine; yes I know the VVC can retain a little oil in the head, especially when canted over at an angle, but not on that scale. It's not going into the coolant, the whole cooling system is literally spotless. The coolant is as perfectly clear and pink as the day I put it in. Last time I drained it I ran the entire lot through a filter paper to remove and dirt before reusing it and I think I got one tiny spec, smaller than a grain of salt, out of the whole lot. And I know it's not leaking externally either - the engine and underside of the car are clean enough to eat your dinner off, so it has to be burning. It just seems to be burning it very cleanly!

@Ivaan, yes all of the rings were clocked exactly as specified in the K Series Overhaul Manual on assembly, but I'm now worried / confused about the piston ring specification ...

The rings I used were Goetze 08-522700-00. These I believe are OEM spec and equivalent to Rover LFP101320. However now looking on Rimmer for the MG TF, LFP101320 is specified for "Except VVC" and (per piston). LFP000110 is specified as "1.8 VVC" and (engine set). However for the MG ZR, both LFP101320 and LFP000110 are listed as "MPi" and nothing is specified specifically for the VVC. So I think that the difference in part numbers are just the difference between (per piston) and (engine set) and the rest of the MPi/VVC bit is just rather random labeling on behalf of Rimmer. This seems to be the opinion on other forums on the internet to.

If this is correct then I have used the correct rings. I know the bore is the same at 80.0mm and I did measure the ring clearances in the piston grooves and ring end gaps when installing them and everything was within spec. The oil rings were 3-piece with two scrapers and an expander.

Can anyone confirm the above for sure please?

[SM25T]

Mine doesn't use any oil ... At 88k miles.

[7 wonders of the world]

Spacing the ring gaps is fine but they move around in operation anyway.

Did you check the ring gaps in the bore

Did you check the rings in the piston grooves if they are a very loose fit you get ring pumping where oil is pumped around the back of the rings and allowed into the chamber.

They need to be a snug fit in the grooves.

Were the compression rings put in the correct way up too?

Were the stem seals tapped onto the guides otherwise they ride up the valve stem and do sweet FA.

Is there any noticeable crankcase pressure build up if you remove the oil filler cap whilst running

[revilla]

No Ian, it really shouldn't. Something somewhere is horribly wrong. Question is ... What?

[revilla]

@7 wonders of the world:

Yes I measured every ring gap in the bore following the procedure given in the overhaul manual and every single one was within spec. I also measured the piston to bore clearances following the specified procedure exactly and all were fine. I also measured the fit of the rings in the pistons and the clearances were very small and within spec. I did all of these checks for every single ring and I'm pretty sure they were all installed the right way up too as I took time and great care over what I was doing. I was also careful to make sure that the ends of the oil control expanders butted up to each other rather than being interlocked.

The stem seals were tapped gently onto the guides, but on the K the stem seals are integral with the spring seats; in other words they are held down in position by the valve spring pressure so I don't think they could ride up the stems like they could on some engines.

No noticeable crankcase pressure, in fact quite the opposite; a gentle crankcase depression / suction. If you remove the oil filler cap and hold a flat sheet of paper over the hole, although it does flap up and down a bit in response to pressure pulses generated by the engine, overall it is gently sucked down onto the opening rather than being blown off, so the manifold vacuum is drawing gas out of the crankcase through the PCV system faster than it is blowing by the pistons.

[revilla]

I've been keeping records of mileage and oil added over the last 2500 or so miles. Each time I've topped it up back to the top mark on the dipstick with the engine hot and running. the maroon line represents 1000ml/1000miles. The blue line is my oil usage. Each point is an oil topup.

One interesting feature is the fact that the last segment suggests it is getting worse; however in this segment I spent 160 miles chasing two 420Rs and an Audi R8 V10 Plus around Warwickshire and basically had to drive the backside off it to keep up. I was certainly revving it and pushing it harder than I normally would. So it does appear to use a bit more when driven hard, if that gives any clues.