Compression Test

offshorematt · Post by **offshorematt** » Tue Dec 19, 2006 1:19 pm

Finally got round to buying a compression tester. What sort of pressure should I be looking for on a standard 1.8 'K' ? I know that I'm burning oil so I suspect compression will be down anyway

Or is it more important that all the cylinders are holding about the same rather than one being mega-low (in order to rule out HGF etc?)

Stu160 · Post by **Stu160** » Tue Dec 19, 2006 1:24 pm

Hi Matt

200psi is good,I would say under 180psi is getting poor.

Remember to have the throttle open when you use it.

Stu

Baggy · Post by **Baggy** » Tue Dec 19, 2006 3:06 pm

Do your test on all four (see if they're all reading the same as you say)

squirt a wee bit of engine oil in each of the bores (not much), let it sit for a bit, test them again & if the reading improves it's the rings. If it doesn't improve it's the valves & if it's visibly burning oil I think that narrows it down to the valve stem seals.

There was a thread on seloc recently about a guy who was rebuilding his VHPD head & found the valves to be a rattling fit in the guides & the concensus seemed to be "yep they're always a bit sh*t??????" & that'll fcuk the stem seals quick enough.

On older style cast iron block engines you generally find the two middle bores a bit poorer than the outer ones "possibly cooling related but maybe with wet liners & the like that's no longer the case.

This is just how I was advised to go about it and how I've done it but I dare say there are other ways).

Could have borrowed my tester....it's not like I've used it for frikkin years.

By the way....I don't know if a compression test will give you a definitive yes/No on HGF....It may show indications but they could be missleading. H-C Sniffer or pressure test of the coolant circuit or mayo would be a more definite sign.

STU....just curious....why have the throttle open???

Cheers

Baggy

Stu160 · Post by **Stu160** » Tue Dec 19, 2006 4:11 pm

STU....just curious....why have the throttle open???

If its shut,it will create a vacuum,and not give a correct reading.
Yes,check all 4,if they are down,then use oil to see if the pressures go up,if they ,then it is a bore,ring,piston problem.

Stu

Baggy · Post by **Baggy** » Tue Dec 19, 2006 4:39 pm

Ok.....I'll go with that...

offshorematt · Post by **offshorematt** » Thu Dec 21, 2006 1:10 pm

Thanks guys. Wish I'd had time to get read this before I did the test...

Results were 9bar, 8.9bar, 8.5bar and 8.5bar (all without the throttle open) - so a damned long way from even 180psi. Can you explain the 'cause a vacuum' comment? Surely the injectors will open when you crank the engine regardles of throttle position? And would this vacuum cause my measurements to be higher or lower than actual?

I'm not to concerned about the low compression at present as the oil burning wasn't a sudden thing (there's been a nice puff of black smoke on each gear change for the last 10,000miles...

). My current poor running appears to be due to massive over fuelling but I wanted to check all the bases i.e HGF or cracked block or summat else nasty. There's no mayo and the fluid levels are stationary so I suspect that HGF isn't a problem.

Out of interest, anyone know how much a top end rebuild costs for the K?

Rich H · Post by **Rich H** » Thu Dec 21, 2006 2:33 pm

Did the oil trick help with the compression? Ebay for a new head, it's cheaper

Or in your case a new engine....

Recon head
Poss Cheap DIY rebuild <-- sorry to jump in here, but I don't think this is the right cylinder head - look at the water egress ...
Cheap direct swap if it's a high port This was exactly what I ported and fitted to mine and a 1999 should be high port
(Check casting numbers on DVA's site)

I was quoted £250+VAT for bronze valve guides, a light skim (2 thou) and 3 angle seats. Each valve insert would be £35 +VAT each

Pattern valves are cheap (Standard) mine cost £16 for a full set IIRC. Springs like wise £7.50 (All EBay specials) Lifters were expensive so I bought a scrap head off a HGF 1.4 metro and fitted them. I have a set of Cams and pulleys spare you can have for the postage if you really need them.

I would offer my spare head but it's on standby for when my DIY effort goes pop.

HTH
Rich

robin · Post by **robin** » Thu Dec 21, 2006 2:35 pm

offshorematt wrote: Results were 9bar, 8.9bar, 8.5bar and 8.5bar (all without the throttle open) - so a damned long way from even 180psi. Can you explain the 'cause a vacuum' comment? Surely the injectors will open when you crank the engine regardles of throttle position? And would this vacuum cause my measurements to be higher or lower than actual?

To compute true compression you want the maximum cylinder fill possible. With the throttle closed, less air will reach the cylinders (though at cranking speeds, you'll find they still pretty much fill because the valves are open for such a long time and the flow through the IACV opening can be enough to fill the cylinders at this low RPM). If you repeat your compression test with the throttle open you will get slightly but not dramatically higher numbers I think.

The compression ratio of the K is about 10:1, so given the cylinders are filling at 1 bar, I would expect a cylinder pressure of 10bar. If you factor in to that the tiny amount of space taken up by uncompressible fuel (somewhat of a variable when cranking anyway), that will raise the pressure of the rest some, but not much (a 450cc piston closes to 45cc at TDC and I would guess you have much less than 1cc of fuel in there).

Stu - any idea why you would get 180-200 psi when the compression ratio is only 10:1 - i.e. 10bar or ~150psi?

Out of interest, anyone know how much a top end rebuild costs for the K?

The gasket and seal set is about 50 quid including the head gasket, more like 75 if you want OE gaskets and seals.

You'll want the change tensioner and timing belt while you are in there - another 50-75 quid.

If you want to replace the head bolts, add another 60.

Then if you don't want to do it yourself, I would factor in 100-200 labour for someone to strip the head, reface the valves into existing valve seats. Add in another 50-100 if they have to replace the valve guides at the same time.

You would be as well to get them to do some mild fettling while they are in there; get a bit better airflow etc.

By the time you've done all that, you could have bought a factory built low mileage second hand standard head for 100-200 quid and slapped it on

BTW, your oil habit might be valve seals/guides but it might just as likely be blowing by the pistons due to wear on the pistons or piston rings. As Stu said, if you pour a little cold clean engine oil into the bores before doing the cranking compression test (and I mean a little - if you fill the thing up, it will hydraulic on you!) and the cylinder pressure rises significantly then you have piston blow by. A bottom end rebuild is almost certainly not worth the effort - less hassle and better value, in the end, to buy a low mileage used bottom end or a brand new one from PTP (1200 quid).

Cheers,
Robin

mac · Post by **mac** » Thu Dec 21, 2006 2:37 pm

Injector's only let in the fuel - you need the throttle to be open to let in air

Did you try the test again but after oiling the bores? That will let you know how much your lossing through worn cylinders.

Top end re-builld - are you taking changing the head gasket or something more?

Mac

Stu160 · Post by **Stu160** » Thu Dec 21, 2006 2:48 pm

Hi Matt

Sounds a bit low,but may be because the throttle was shut.If it is shut,it restricts the airflow into the cylinders,giving a low reading.

Stu

offshorematt · Post by **offshorematt** » Thu Dec 21, 2006 6:22 pm

Thanks

When it comes to engine internals I'm a complete amateur so would be looking for the easiest fix not necessarily the cheapest...
If I get time over Xmas I'll try the test with cold oil in the bores and throttle open but I'm still trying to get the thing running again so not too worried about the oil usage at present. Possibly getting to the point where I just need to bite the bullet and book it in somewhere - if I do, I'll get them to diagnose the oil 'habit'

Thanks again for the input...

robin · Post by **robin** » Thu Dec 21, 2006 9:05 pm

offshorematt wrote: Thanks again for the input...

Matt - is this still the won't run fuelling type problem that's been going on for a month or two? Are you around at the start of January? If so I'll come up with a box of bits and stuff and see whether we cannot get it running - how hard can it be

Do you have somewhere dry to keep it/work on it?

Cheers,
Robin

Bob vanM · Post by **Bob vanM** » Thu Dec 21, 2006 11:17 pm

robin wrote:
Stu - any idea why you would get 180-200 psi when the compression ratio is only 10:1 - i.e. 10bar or ~150psi?
Cheers,
Robin

I have tested my 90.000 miles engine ,
donno much about your psi thingies but measured
13,5 15,6 16,0 14,4 BAR (180 - 200 psi ?)
on cold engine, open throttle.
that engine did not used any oil (sold it now), used a lot on the Ring last year, for the rest about 1/4 of its life was on track.
regular oil changes

Bob vanM

Baggy · Post by **Baggy** » Fri Dec 22, 2006 11:47 am

More than welcome to use my garage (bieldside)

robin · Post by **robin** » Fri Dec 22, 2006 4:07 pm

Bob vanM wrote:
robin wrote:
Stu - any idea why you would get 180-200 psi when the compression ratio is only 10:1 - i.e. 10bar or ~150psi?
Cheers,
Robin
I have tested my 90.000 miles engine ,
donno much about your psi thingies but measured
13,5 15,6 16,0 14,4 BAR (180 - 200 psi ?)
on cold engine, open throttle.
that engine did not used any oil (sold it now), used a lot on the Ring last year, for the rest about 1/4 of its life was on track.
regular oil changes

Bob vanM

Cheers Bob - any idea why 1 bar becomes 14-16 bar when compressed 10:1?

Robin