Hi all, need some advice, I think I know the answer but would appreciate a second opinion. Had a noise coming from somewhere near the engine / transmission. So changed the engine, gearbox and diff oil. Ran the engine up and performed a compressions test. Result as follows;
Cyl 1 8.1 bar 125psi
Cyl 2 7.5 bar 115psi
Cyl 3 8.1 bar 125psi
Cyl 4 10.2 bar 150psi
This to me says game over but would appreciate some additional comments. The engine is. 1.8s Mk2.5 89k on the clock, W Reg.
You do not say whether it is burning excessive oil, or evidence of smoke, or whether the noise went away after changing.
Around a litre per 1,000 is deemed acceptable.
I believe they kicked off at an even 181 when new.
(According to Miata.net.)
They ought to ideally be within around 10% of each other allowing for age & wear, but that’s a bit of a tall order.
These tests often end up in heated debate ( not in this forum) as to how the tests were carried out…wet/dry, using what equipment etc.
I’d say yours look pretty uneven.
Could be valve leakage , worn oil control rings…or a number of things.
Trouble is, unless it came with detailed service history, no g’tee what previous owners failed to do maintainance wise.
I’ve seen an old 94 Roadster give better at 230,000 miles.
They do like fresh lube every 4k or so when they get on in life. Our 98k 2002 Sport runs like new with pretty even compressions.
Thanks Scot, there is sime smoke it stands for long periods so its just moisture. Goes after about 20 minutes. we’ve owned the car for about a year and a half, the car is in race spec and we have noticed a major down turn in power. When changing the oils there was no debris on the drain plug so diff and gear box no issues. We don’t go through much o, but to be honest it only gets short bursts, no run outs for hours on long roads. It’s redominantly run in 1st to 4th (ragged I think you would call it) it holds its own, but I think it’s had it’s day. Time for a recon engine I think. Just not looking forward to doing all that work ??
Hi Scott, thanks for the feedback. It doesn’t burn oil per-se. On changing the oils there was no debris on the plugs, so this is ok. She runs seeet when warmed up but with a nggling whine when starting off. We race this car so it does get a hammering, however apart from dropping coolant at Snetterton we have had no issues, apart frim what this thread is about. We have noticed a drop in power, albeit minor. Still holds its own against newer suped up models. I am at a loss as to what we should do, strip down the engine and do the seals etc… or change the engine. in changing the engine you never know what you are going to get, could be worse. oh what a decision, either way we’ve got a lot of work on our hands.
Hmm, I’d revisit the compression tests, especially if you don’t have any blow-by (eg exhaust fumes and possible mayonnaise in the cam box) so no holes in pistons and probably OK rings.
Exhaust manifold off, and a generous squirt of oil onto the valve stem of the piston being tested, enough to seal the valve to the seat. If compression goes up, then new or reground valve needed; no change then not the exhaust valve. (Put a large rag over the outlet to catch all the oil being chucked out! This applies all the way through.)
Easiest check and you might prefer to do this first, with all the plugs out squirt a teaspoon of oil onto the walls of just the piston being checked. If that makes a big difference, then there is a problem with the rings, bores, piston, etc. Expensive.
Inlet manifold off as well, and a generous squirt of oil onto the valve stem of the piston being tested, enough to seal the valve to the seat. If compression goes up, then new or reground valve needed; no change then not the inlet valve.
Something else… Head gasket? Once I found a cracked head on my old Kent engine between the oversized inlet and exhaust seats on one piston, so they effectively joined up to bypass the piston, welding fixed it.
In the old days on simple engines when it was so much easier to remove manifolds, I would always check the exhaust valves first, then the rings, then the inlet valves. This way the oil is always being blown through away from untested bits and it gives better isolation of the problem.
If it is valves, head off is cheap and simple, if it is rings then it might be cheaper to change the engine.
Good luck!
Check the bores with the oil-down-the-plugholes trick as above. If the reading(s) don’t improve markedly then it’s probably valves if it has no symptoms of HGF.
I perhaps foolishly bought a cheap Mk2 in 2011, about 90,000 miles, probably genuine. It was a poor starter. Compressions were checked, and it basically didn’t have any. Oil down the plugholes made no difference.
When the head was examined, just about every valve leaked badly. The head was fettled by http://www.corrycylinderheads.co.uk which restored the compression.
Shame that wasn’t the only thing wrong with it!
Ok update. Done the wet test, with the following results;
Cyl 1 - 9bar 130psi
Cyl 2- 7.9bar 115psi
Cyl 3- 8.2bar 120psi
Cyl 4 - 10.2bar 150psi
So if the theory holds, and it isn’t HGF, then it’s valve leakage. Start by checking the valve clearances?
I can’t remember the sequence of events with mine - some of the valve clearances had closed up completely, I think the clearances were done and that didn’t fix it so the chap who helped me took the head to Corry which is only a couple of miles from here.
I’ve corrected the link above to Corry - there was something wrong with it.