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Bad Compression Results


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#1 Maccmike8

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Posted 19 September 2020 - 12:05 PM

Hi folk.

 

Iv bought a 998 with a few faults, price was too good to say no and body is completely rot free.

 

The previous owner has had it for 6 years plus and in that time spent a fortune.

 

However, some of the things Ive found have been to a poor standard. eg wrongly helicoiled sump. Window runner put on wrong. Some tappets were too tight.

 

In the large folder of receipts includes valve stem seals 2 years ago. 

 

Its a bit smoky (blue) when first started but also when revved.

 

Engine oil and coolant are clean.

 

Compression results are as follows;

 

1 dry 100

1 bit of oil 110

 

2 105

2 130

 

3 105

3 130

 

4 100

4 130

 

Ive followed the guides on here so happy with my method.

 

88k miles so Im satisfied the engine is tired and the results in that respect pretty consistent.

 

My finger is pointing towards the head, probably valves need lapping or complete refurb.

 

What are your thoughts?

 

 



#2 Cooperman

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Posted 19 September 2020 - 12:27 PM

88k is relatively high mileage if the engine has never had a rebuild.

The comps are a bit low which, as you say, could be a valve sealing issue, but equally it could be worn bores.

Start the engine and let it warm up. Then increase the tickover by winding up the stop on the carb so that it is revving at around 1200 rpm. Then remove the oil filler cap. If it is 'puffing' smoke out of the rocker cover there is an issue with the piston rings which is normally just bore wear due to high mileage. I know this is not a definitive answer, but it is a good guide. Don't forget to slow the tickover back to normal after doing this.

A more professional way is a 'leakdown test', but this will be a good guide.



#3 Maccmike8

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Posted 19 September 2020 - 12:39 PM

Thanks Cooperman.

Its in for its mot on monday so Iv been fettling today and now it sounds like its firing on 3 cylinders. 

I can pull leads 2 or 3 off and it barely sounds any different.



#4 Maccmike8

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Posted 19 September 2020 - 12:59 PM

Definitely mis firing. Sounds aweful.

All I have done is do a compression test and I labelled the leads so they have gone back on correctly.

A temp gun on the exhaust manifold shows the cylinder 2 and 3 are half the temp of 1 and 4.

Taking little girl to the park but will recheck the plug gaps on my return.



#5 Maccmike8

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Posted 19 September 2020 - 12:59 PM

I idled it a 1200 and took the oil cap off, no smoke.



#6 Maccmike8

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Posted 19 September 2020 - 01:38 PM

Plugs were fouled from putting bit of oil in the bores. All running good now.



#7 Maccmike8

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Posted 19 September 2020 - 05:38 PM

Still seeking thoughts on the original post though.



#8 sonscar

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Posted 19 September 2020 - 06:24 PM

To my mind you have mainly two choices;run it if it passes MOT or rebuild it all.If you take some of it apart the rest of it is unlikely to be like new.Tough choice.Steve..

#9 Maccmike8

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Posted 19 September 2020 - 06:50 PM

Thanks Steve.

Could I potentially just do the rings and hone the bore (conditional the bore is scored excessively)?

Lap the valves, the stem seals, head gasket kit?



#10 Maccmike8

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Posted 19 September 2020 - 06:50 PM

Thats as a short term solution until I can afford to do it properly.



#11 Cooperman

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Posted 19 September 2020 - 10:37 PM

If you are intending on stripping the engine to hone and fit new rings, you would do better to re-bore and fit new pistons for the work involved. Honing and new rings can seem a good idea, but sometimes it will still smoke after this is done. I did this on a 998 Cooper which my son bought, but within 4000 miles we took the engine out because it was smoking again (we fitted a 970 'S' lump bored to 999 cc with all new parts. That went much better!).

At 88k miles the crank will be far from perfect as well.

Don't be afraid to do an engine build yourself. It is not that difficult and we are all here to help as you move forward.



#12 Maccmike8

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Posted 20 September 2020 - 07:22 AM

Thanks Cooperman.

Oh yes Il do the work myself, machine shop for reboring but rebuilding I will do.

Its just money at the minute.

Its food for thought.



#13 Maccmike8

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Posted 20 September 2020 - 07:23 AM

Thank you.



#14 sonscar

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Posted 20 September 2020 - 08:54 AM

As cooperman says you may extend its life,it depends on how you view removing the engine?like pulling teeth?enjoyable weekend tinkering?How many miles do you expect to drive?Personally I would just drive it,after all the gearbox is probably on borrowed time(just joking).Drive it and enjoy it.Steve..

#15 sonikk4

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Posted 20 September 2020 - 09:46 AM

If you can get it through it’s MOT and it’s not going to be a daily driver, then enjoy it for a while, save up some cash then go for a rebuild. It’s highly unlikely that a hone will sort it out so a fresh bore, pistons, crank regained etc will be the way fwd.

As for the head, a full and proper rebuild on that ie valve guides, hardened seats possibly new valves dependant on condition.

Sounds like a lot but once done you will know you have a reliable engine that will last for many many miles.




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