Head off required for a de-coke?

[andrewdmoore]

I have just had my Elan Sprint tuned and been informed that compression is down on 2 cylinders - Nos 1 & 2 are 110 (which I hope is fine), but No3 is 75 and No4 is 60. The garage reports that these cyllinders were running rich & the valves maybe coked up & not seating properly.

Can I have your thoughts please specifically: are 1 & 2 correct, and has anyone had any success de-coking with Redex or something better, before I do what every Yorkshireman hates doing and spending hard earned cash on a head off de-coke, which whilst the head is off, I may as well combine with an unleaded conversion.

Thanks, Andrew

[nebogipfel]

Andrew,

You will get much more authoritative replies from the twincam experts on the forum but FWIW my opinion is that none of your compression readings are great but particularly low on three and four. I would guess (assuming the loss of compression is not due to bore wear or broken rings) that the valves and/or seats may well have started to erode and the only way to sort it properly is cylinder head off.

As far as lead free conversion is concerned, I'm not convinced it's needed on a twincam but you will doubtless get a good consensus from the replies

[types26/36]

I have just had my Elan Sprint tuned and been informed that compression is down on 2 cylinders - Nos 1 & 2 are 110 (which I hope is fine), but No3 is 75 and No4 is 60. The garage reports that these cyllinders were running rich & the valves maybe coked up & not seating properly.

No,110 psi is no good, from memory the manual quotes 160 psi @ sea level and there should not be more then about 15 psi difference between the max to the min.
Were the valve clearence's checked although if it has been run any length of time with little clearence then damage has already been done to the valves/seats.
Were the compressions checked with a hot engine and the throttle valves full open?
Decoke???? nobody does decokes these days, the fuel/oil has improved since the model T Ford

[sprinterdave]

I would agree that 110 psi is no use. I worry about mine spewing blue smoke out of the exhaust and it runs at 175+ on all 4 cylinders.

I would suggest take the head off and examine the valves and seats carefully, the combustion chambers and inlet/outlet throats can be cleaned up easily with a rag and solvent or even 000 wire wool (dont forget to clean the port thoroughly afterwards.)

[steveww]

Sound like a head off to me. You should try a leak down test as well, this will help decide exactly where the compression is escaping.

To save some cash you can do the strip down yourself then send the head off to a machine shop for any work required. Pulling the head is not difficult and you will only need a valve spring compressor and torque wrench on top of regular tools to do the job.

If it has been a while since it was last looked at it may need new seats and guides as well. I had new seats and guides fitted by my local shop for aprox ?200. A new set of valves is a bout ?80 and worth fitting while you are at it.

Have a look at the water pump while it is all in bits, fitting a new one of those as well is a good idea.

[andrewdmoore]

Thanks alot for the feedback chaps, please keep it coming. The really puzzling thing is that the car goes like stink - or I think it does. My nervous system is obviously in for a shock if I do have the head off and 're-conditioned'.

I shouldn't mention the Porsche Boxter I overtook last month, but I really can't resist it

[steveww]

It is one thing that always amazes me about the internal combustion engine. That it will still run even when way out of tune, worn out components etc. Imaging if this were not the case trying to get an engine running after a rebuild? 10 degrees BTDC and a rough guess on mixture is usually enough to start anything.

You may think it goes like stink now, imagine how it will go with decent compression? You never notice the gradual loss in performance as components degrade and the engine cokes up until things are really bad. After a top end rebuild the 911 will have to look out

[cliveyboy]

Andrew
Are you sure they did the compression test correctly?
If they spun it over on the starter motor did they keep the throttles open?
Its just if you do the test wrong those are the sort of figures you get.
If you say the car is running well I find it hard to believe that those figures are correct.
Clive

[stuartgb100]

Andrew,

FWIW, I agree with Clive. I also think something is amiss !!

I don't see a twincam with VERY low compression figures "going like stink" and "passing a 911" ......... well, not unless it was trying to park !

Seriously though, with those kind of uneven psi figures, you should be seeing a fair bit of oil usage and/or coolant. Not to mention bad idling when hot, etc.

If't were me, I'd be checking static timing and the advance, plus valve timing and clearances. Then I'd do a correct compression test (hot engine, pedal on the floor whilst cranking, one plug out at a time).

Then I'd compare the results of my checks with what you've been told.

BTW ...... how well do you know this garage?

FWIW, my Sprint with 9k engine since professional rebuild has comp figures in the 190's...........

Regards,
Stuart.

[types26/36]

Andrew, one plug out at a time).

??????? all plugs out.

[Dave_Newcastle]

Mine are all in the range 197-202 psi. Is that right - too high? Readings were taken by a specialist engine tuner.

[types26/36]

Mine are all in the range 197-202 psi. Is that right - too high? Readings were taken by a specialist engine tuner.

It is high but if you do not have problems with detonation (pre-ign) over heating (an IC engine is essentially an heat engine) and your cooling system can handle the heat generated, also providing the pistons/rods/crank & bearings are in good condition dont worry about it.

[Dave_Newcastle]

No preignition and it has never given me any problems running hot in eleven years so I'm content. J

Just done 1600 miles to Le mans and back - mostly at around 4500rpm (and more) and 37 mpg returned over trip so nothing wrong with the performance.

I echo comments above re ensuring compression tests are done right with regards the throttles being closed. The manual sets out the procedure nicely.

[Dave_Newcastle]

.

I echo comments above re ensuring compression tests are done right with regards the throttles being closed. The manual sets out the procedure nicely.

To clarify - throttles should be fully open when doing compression test

[Dave_Newcastle]

.

I echo comments above re ensuring compression tests are done right with regards the throttles being closed. The manual sets out the procedure nicely.

To clarify - throttles should be fully open when doing compression test