Thoughts on compression and leak down test results

brizzity · 03-18-2015, 02:14 PM

I got the results of a compression and leak down test on an 88 Carrera I'm considering buying, and wanted to see what kind of feedback I might be able to get.

Compression came in about 125 across all cylinders which I was under the impression was low. However, I'm getting feedback the the leak down is the more important indicator of health and that came in at 6-8% across the board.

Relatively recent work done in the last year/5k mi:

120K service with valve adjust/tune up. New O2 sensor, fuel filter, oil pressure switch, spark plugs, distributor cap, ignition rotor and drive belt, valve cover gaskets replaced

Engine and transmission removed: rear main crankshaft seal replaced, new clutch, new flywheel, new engine oil line and left chain tensioner, new crankcase vent cover gasket, new throttle position sensor

Any thoughts with this limited amount of information?

Thanks in advance...

Derek911 · 03-18-2015, 05:35 PM

I don't see any problem as long as it is consistent across all cylinders. The main factor is price. Does the price reflect the miles and risk. On the low end of price scale you don't even do a PPI... Then on the high end of price scale everything must be checked and worth it. Price is going to be the deciding factor on how hard you nitpick. At 120,000 miles plus, sounds good depending on price.

Jcslocum · 06:59 PM

These number seem low to me. Do you know who did the test and how it was performed? There can be a bit of variability between engines. This test should be done:

DME relay Pulled
Engine hot
All spark plugs pulled
Battery fully charged and it could be on a charger to keep it strong
Throtte clamped wide open
Engine cranked until reading stops going up

Bill Douglas · 03-18-2015, 07:45 PM

I've heard... It depends on how the machines are calibrated. And it's best to ignore the actual reading and mainly focus on the amount of variation between cylinders. This one sounds good. General apperance is important too. Are bits are rusty/corroded on the engine. Or does it look reasonably fresh.

Good luck, it sounds fun.

brizzity · 03-19-2015, 11:12 AM

Thanks all for the replies.

I don't know exactly how the tests were done, as far as following all those steps being followed but from the best I can tell, the mechanic seems knowledgeable enough. The general appearance of the engine is good and looks pretty fresh so that in combination with all the other photos, service history, general condition of the car, and the consistency with these numbers has given me enough to move forward.

T77911S · 03-20-2015, 02:48 AM

how many miles.....

E Sully · E Sully's Garage

Your compression numbers are very low. They are bare minimum pressure. These engines are also known to have valve guide wear. My '86 engine needed valve guides before it reached 100,ooo miles. After a top end rebuild I had 190lbs compression.

Dave Kost · Dave Kost's Garage

Have another compression test done or do one yourself.

How strong did it run? No smoke on decel?

I would be somewhat concerned if it were to be a daily driver or track car but if just an occasionally driver on weekends , show car, garage queen not so much of a worry because you won't accumulate many miles on it to worry about a costly top end job soon.

Clean 1989's don't come up very often- I think they only made 1000 89 coupes.

If it was a strong runner buy it.

T77911S · 03-20-2015, 06:44 AM

Quote:

Originally Posted by E Sully

Your compression numbers are very low. They are bare minimum pressure. These engines are also known to have valve guide wear. My '86 engine needed valve guides before it reached 100,ooo miles. After a top end rebuild I had 190lbs compression.

yes, my 930 at 7.0 CR is 120.

i have done compression with the throttle open and closed, no difference. i just do it closed because the main thing is consistent then if they are low.

smokintr6 · 03-20-2015, 06:48 AM

My suggestion would be that there are three potential causes for your numbers:

1: The car has been sitting too much recently and might respond well to being flogged for a bit.

2: Bad valve adjustment work, causing tight clearance and leakage at the valves... You would see this in your 6-8% leakage number. If this is the case it should be remedied asap.."a tappy valve is a happy valve" to an extent of course. Ask the shop where the leak was going: intake manifold, exhaust pipe, crankcase etc.

3: Top end parts are just tired after 120k. Not unlikely at this mileage if it has never been done. You should be trying to get an idea of oil consumption.

What I would do: Ask the owner to put some miles on the car, run it hard for a tank and then have the valves readjusted and test re completed. If the numbers look better I would offer to split the cost of the work. If not, negotiate for some money off for top end rebuild. They will run for quite a while with some valve train wear. I had an SC that used a quart of oil every 450 miles, and to my knowledge it's still running like that today.

Or if it's a nice car and you just want it, make sure you get it right enough that you won't feel totally robbed if you have to do a top end while you own it, and re check everything yourself.

Chuck.H · Chuck.H's Garage

I'd be more concerned with the shop that listed 'throttle position sensor' replaced!

a) These years don't have TPS, then only have idle and WOT micro switches.
b) Do these micro switches ever fail? They seem very robust.

At 350k (no rebuilds) I did a leakdown test. 5 cylinders above 95%, one cylinder was like 92%. Still runs strong though, so no rebuild planned anytime soon.

Chuck.H
'89 TurboLookTarga, 392k miles

Jcslocum · 03-20-2015, 07:44 AM

I beg to differ on the TPS. There is a part called the Throttle Posistion Switch or sometimes called a Throttle Position Sensor:

The part in RED is the TPS.

brizzity · 03-20-2015, 08:56 AM

It's got 122k miles on it now. Someone mentioned an 89 and maybe that's where the confusion with the TPS came from. This is an 88.

E Sully · E Sully's Garage

The Carrera has a full throttle switch and an idle position switch. Very simple on or off. Idle switch shuts off fuel when decelerating from high rpm till 1500 rpm. Throttle switch activates a default program that keeps the mixture slightly rich at full throttle for safety.
This was my engine when I bought it with about 70,000 miles. Valve guids wore prematurely and a lot of carbon built up. This can keep the valves and rings from sealing well.

This was my engine after top end break in was complete.

https://youtu.be/zNoDtLu1zBI