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Low compression, thoughts please!

2K views 16 replies 6 participants last post by  Dr Dave 
#1 ·
MkII 214SLi (16v mpi)

I've been having trouble with my car for months now, rough starting/low economy/occasional cutting out when slowing. It's not electrical, I've checked everything, and I've just carried out a compression test which confirms low compression (as reported by the garage that had it for nearly two weeks!).

Haynes say minimum compression should be 10.3bar, and I was getting between 5 to 5-1/2bar (dry) and up to 8bar (wet). I only did the wet test on 1+4 as they were lowest.

The invoice from the garage stated "suspect inlet valves leaking", and on this diagnosis they tried flushing the oil and using thin (5w/40) oil to try and help the tappets (not unusual on a K-series apparently). I've tried treating it for sticky valves, and it seems to have helped a little, but as you can see the compression is still about 50% on all 4!! I'm tempted to lift the head, and hope to fix it from the top, but the wet test suggest it's the bottom end thats f:cus:d, but I don't want to believe this!!

Any views on what I should do?? I'd love to fix it, as other wise it's a great car, and I'm itching to do some mods, I just don't want to spend too much money on a not particularly valuable car! car!! Bit of a dilemma really!
 
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#2 ·
The increase in the compressions with the wet test would point to the rings, I'm afraid.

What mileage does it have?

Whatever the problem is, the head will have to come off anyway. So there is nothing to be lost by yanking the head off and having a look around.
 
#3 ·
It's only done 65k. I'm not entirely convinced it is the rings as I would expect more serious symptoms if it was! I am trying to convince myself it could be that the thin oil is not helping, and reverting to the thicker stuff might help reduce (not necessarily cure) the problem. Other compression loss might then be helped by attention to the head!

If it is the rings, then I think I'll just run it till it gives up completely as I suspect it will be an expensive job, so not worth the cost. I think I'm gonna get the head off, and see what happens anyway, for experience sake if nothing else! You never know it might cure it!

I wan't to get some alloys, and lower it aswell, but it seems silly to spend money modding a car thats not running right! I might do it anyway, as it won't be wasted money like a repair could be. I'll just swap the mods onto my coupe when the insurance people let me have one!
 
#6 ·
Try adding Wynns Hydraulic valve lifter treatment to the oil. Oil flush helping points to valve problems, the flush will do for a start, but if it's bad several treatments could be needed. The oil you quote is too thin, so I would add the Wynns, run it for a couple of weeks or 500 miles, add another flush (STP) to the oil and run at a fast idle for 30 minutes. Drain the oil when hot, replace filter and add a good semi synthetic oil of the right grade. Add another Hydraulic Valve lifter treatment, and put a fuel system cleaner in the tank.

If this doesn't work (it might take a little while), then it's probably head off, and new rings or similar. You can do all the above yourself for about £40, which might be a good investment.
 
#10 ·
Thanks for the ideas guys.

The garage I took it to is owned/run by a friend of my dad's and they've been looking after our cars for years (including an engine rebuild on our old lotus). So I trust them and wouldn't doubt they're diagnosis. In two weeks, with all the mechanics scratching their heads as to what's going on I'd be suprised if they hadn't traced it through a leakdown test. This time though, my own tests have suggested it's the rings not the valves that are the problem as they suggest.

I have done about 500 miles (inc. 3rd gear motorway runs) since adding the Wynns tappet treatment, and have tried several system cleaners: Nitrox, Redex, and Wynns Extreme at the moment. What chance with a thicker oil and a bit more cleaning things might improve? It's two weeks before I'll have time to do anything major (head off) so I'll hope this works in the mean time.

If it does need new rings though what sort of job (DIY) am I looking at? Can the pistons be removed past the crank, or should I do it when the heads off?
 
#11 ·
Pistons come out when you lift the head but be sure not to upset the liners ... or they'll need to come out to, to re-seat them.

You need to drop the sump off for access to the big-end bolts but remember NOT to rotate the crank when the head is off. You can make your own liner locking tools with some copper pipe and suitable sized washers if you need.

Personally I find it easier with the crank at the TDC position on the bottom pulley because then the big ends are all in line (although not at the same height).

You'll need to re-seal your oil rail when it goes back on, and if your liners need attention you can hone them. I would suggest using a 'flex-hone' honing tool. If you're dad's mate owns the garage maybe you can buy one off him becuase they're not especially cheap.

Andy
 
#12 ·
Cheers Andy. Looks like this could turn into a big job! Problem is I won't know exactly whats wrong until I lift the head, at which point I have no means of collecting parts!!:irked:

The car runs well enough most of the time, so I dont want to spend loads of time and money on the off chance that I bugger things up!:rolleyes:

What to do??
Decisions, decisions!:confused:
 
#15 ·
Just checked my invoice, it says under the description of work done " carry out cylinder loss test" and concludes "suspect inlet valves leaking". Is this the same as a leakdown test? I could always phone Ray and ask him!

Gonna try and change the oil this week, so any suggestions on what oil (and flush) to use appreciated!
 
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