heliflyer

Following on from my previous thread regarding oil leaks, blue smoke and suspect high crankcase pressure after a head gasket change, I have now done a compression test and would appreciate your views on the results listed below.
I have also included pics of the plugs.
The first row of figures were taken before doing the gasket change.
The second set done today on a warm engine.
The third set immediately after with a small amount of oil added to the cylinders first.
It looks to me as though all the rings are worn to some extent (evidenced by the increase in pressures), but particularly on cyls 1-4.
Cylinder 1 appears to be particularly bad as there is almost no difference after adding the oil so I am guessing that there may be a broken ring here, and that may also be causing a higher than normal crankcase pressure???

I will pull the engine and get it stripped - but a few questions spring to mind:
1. Can I re use my nearly new head gaskets (less than 1500 miles) or should I buy another set?
2. Assuming there is no cylinder bore damage can I just buy a complete set of new rings for std size (97mm) pistons and fit them.
3. While its apart, I might as well do the main and conrod bearings also - same question as 2 - buy a standard size set of bearings to fit, or should I be planning to get the crank re ground regardless??

Thanks

Leon Speed

If I were you I would do a leak down test before you pull the engine, it will give a better picture of the condition of the internals.

Roy928tt

I'm scratching my head with this one, something doesn't make sense.

Why did the cylinder with the highest pressure prior to the head gasket swap, drop to be one of the lowest. Indeed no cylinder on that bank, significantly improved with a change of head gasket, whilst the other bank did.

Why do all of the plugs look fine, OK No. 8 is a bit darker, but I'll live with that.

What would I do? Restore the breathing system to standard, top up the oil to the regular level, put some miles on it then repeat the compression tests, warm, warm plus oil. See how the results match up.

If it is the same or No.1 is worse, the sad implication is that there is damage to No. 1 probably caused by foreign object ingress during the head gasket change.

Sadly even I wouldn't reuse the gaskets.I would probably source a replacement engine and I wouldn't put that in, without replacing the top end gaskets either.I don't think I would split a 928 crank case unless I absolutley had to, at 138 000 miles there should be very little wear down there,If it ain't broke don't mess with it!

I'm struggling with No. 1 plug and compression pressure, if it is losing so much pressure past the rings why isnt it pulling lots of oil mist back up into the cylinder, burning it and leaving deposits on the spark plug?

Good Luck

Cheers Roy

Roy928tt

Dammit ! Leakdown test ! Good Idea!

heliflyer

Hi Roy
Thanks for your comments - I agree that this is certainly confusing, however I believe the changes re the head gasket differences are down to the water passage side failing on the gaskets rather than the cylinder sealing areas. When I removed the old gaskets they were in a terrible state and the symptoms were lots of loss of coolant / white smoke but no loss in performance.
I was meticulous with cleanliness during the rebuild so I doubt FOD damage to No1 during that process, but it's always possible I guess.
I also would have expected to see a black No1 plug and that is certainly baffling.
I have also suddenly developed big oil leaks from the cam cover gaskets (they were bone dry for the first 1500 miles) so will have to pull the engine anyway to re do them. (I have tried accessing the fasteners with the engine in the car and I think it's easier to pull the engine!!)
I'll look into finding a leakdown test kit - at this stage I'm thinking just to cut my losses and source a replacement engine.
Cheers

the flyin' scotsman

Did you change both head gaskets?

Very strange how cylinders 3-8 all rise in compression with the gasket change. Any pictures of the pistons?

Plugs all look good from the pics...................another vote for a leakdown test.

If you rebuild the engine do it right.

Imo000

You run a LPG system, right? Could it be soemthing to do with that?

WallyP

I don't think that these compression readings tell you much about the rings.

- The compression isn't low.
- The lowest reading is 85% of the highest reading, which does not justify any action.
- When a wet reading (oil added to the cylinder) does not improve over a dry reading, valves are a more likely cause than rings.
- The engine runs well.

I think that you have an oil management issue, not a piston ring issue. I see nothing here (other than your leaking cam covers) that justifies an engine rebuild.

heliflyer

Malcolm - yes both head gaskets changed.
Imre - No to LPG - not sure where you got that idea from. Straight unleaded gas.

Wally - Thanks - that makes me feel slightly better. When I did the head gaskets I had the heads re faced and the machine shop also did some work on all the valves and seats and fitted new stem seals. Perhaps there is an issue with the work they have done? Would this also account for the blue smoke seen under acceleration and over run?
I will sort out a leak down test before going any further, although I will still have to pull the engine to resolve the leaking cam covers.
Thanks

GlenL

The compression test numbers don't show any standout problems and neither do the plugs.

One thing I do see is a different set of pressures bank-to-bank. I'll wager that the cam for the 1-4 bank is a half-tooth late. You can straighten that up, get even CR numbers and a bit more power by using an offset Woodruf key on the cam sprocket. Actually, you car takes a rectangular key but you can file the Woodruf into shape.

https://rennlist.com/forums/928-foru...ce-retard.html

Mrmerlin

I am with Wally and Glen on this,
the cam timing should be checked .
and the the compression numbers are all good .

Though if the heads were cut prior to reinstall then they should have seen a slight increase in pressure , however the cam timing could be masking this.

To fix the cam covers dont pull the engine just make some allen wrenches to fit the bolts.
once the covers are off fit the sealing washers to the heads of each bolt then fit the bigger capture washers, ( this increases the clamping pressure of the bolts)
then get some hondabond and inject a bead into the cam cover groove then install the gasket.

Important..... also if the front and rear cam caps were not properly fitted with locktite 574 between the head and cap then these should be correctly sealed, or they will ooze oil.

a small dab of the hondabond at each junction of cap to head should also be made prior to fitting the cam cover.

Return your oil management system to stock and inspect all of the other lines as it quite possible that you have something incorrectly connected

heliflyer

Thanks guys - will give those suggestions a try.
Merlin - sorry it's not clear in my sig but it's a 16v M28.21 engine. The cam cover bolts are straight 8*35 allen head bolts with no sealing washers.
I will take up your suggestion re the hondabond.

Jim Devine

For leaks, consider this:

Per Toyota box:

Toyota Seal Packaging 102 (fpig)- (stands for formed in place gasket)


part no. : 00295-00103


for oil pan/ camshaft housing
3 oz tube

Compare this to the Hondabond- it may be better.

before doing anything drastic, how about getting
the kit to send an oil sample out for analysis to see if there
is any signs of bearing material from running it low on oil or
coolant contamination or? Also, cut open the filter & have a look.

Mrmerlin

Ahh OK i thought we were working on a 32valver so nevermind about the cam sealant issues What i would look at is if there is somehow a small figure 8 gasket fitted around the lifter bores?? if so it might be a good idea to remove it so the outer cam gasket can properly seal use a feeler gauge to test the edges of the housing if the feeler gauge fits into the housing edge then something is levering it off and creating a leak along the edge also make sure you have a good either cork or silicone gaskets on the rear cam comers and dont overtighten them

GlenL

I never understood what those were for. I've spent a good deal of time looking and pondering, and just couldn't get it.

I believe they were required at the beginning and were left out of later 16V 928s. Possibly tied to that change from lifter sleeves to integral sleeves in the towers.