Loss Of Power

[Guest tomwp]

I have had a G reg 1.9 for a couple of years now, over that time the power seems to have dropped off. It now feels pretty sluggish.

 

So this evening I did a quick compression test:

1: 8.9 bar

2: 9.1 bar

3: 9.1 bar

4: 7.8 bar

 

All these readings seem quite low. What should a healthy engine give?

 

I put a drop of oil in cylinder 4 and got the compression up to 8.6 bar but could it be the Head Gasket as well?

 

Other clues:

+ There isn't any mayo in the oil (except in winter when it condenses in the filler).

+ It uses loads of water but I can't find the leak.

+ There is a bit of oil in spark plug 1.

 

Also I have a large crack in the exhaust downpipe and I'm currently trying to find someone to make me a stainless system locally (Oxford). I just got one quote of £450 which seems a little steep.

 

Thanks,

 

Tom.

[Anthony 1,003]

Those figures do sound distinctly on the low side, as I'd expect around 11-13 (160-190psi) bar from a healthy 1.9 8v engine.

 

However, different compression testers - particularly cheap one - have a habit of reading with varying degrees of accuracy, so I wouldn't read too much into the actual figures. What's important is the difference between cylinders, and on your engine, 3 are very close to each other and 1 is lower but not hugely so.

[stefan 4]

Did you do the test on a warm engine with the throtlle fully open?

If not, that could be the reason for the lower readings.

A very good engine (rebuilt or low milage) should show 13 bar.

This is not getting into details, because the pressure readings (compression tests) are just an aproximation.

The readings can be transformed via empiric formulae into the compression ratio, and there is an aproximation of various values here due to not knowing the exact atmospheric pressures, air temperatures of the environment where the testing is done. Furthermore if you don't know if there was any work done in the past to alter the internal dimensions of the combustion chamber (skiming of the cylinder head), change of head gasket thicknes, etc., you can't know what you're reading means, because it could give a lesser or greater pressure reading because of the chandged internal dimensions, and thus having an undefined value of the actual air (pressure) leak. The meaning of compression testing is to determine the 'air leakage' through the piston rings, headgasket or valve seats (if lather 2 are in question the reading, in most cases, will be obvious), bit this can't be done if not knowing the ratio of the swept volume and clearance volume(the volume above the piston at TDC contained in the head gasket and combustin chamber, corrected for any eventual cutouts or domes on the piston).

This is known (the compression ratio) on a stock engine because the manufacturer had already calculated it.

Edited August 10, 2010 by stefan

[Guest tomwp]

Yes the engine was hot, I had just driven home from work and the throttle was wide open.

 

As far as I know, no modifications have been made to the combustion chamber but I have never stripped it down to check.

 

I guess I'll look else where for the loss of power. Next stop the fuel filter and injectors.

 

Partly I was looking for an excuse to take the head off as I have never done it before and I love taking things apart.

[stefan 4]

Amongst other things I'd check the fuel pressure aswell, common enough I found low fuel pressure to be responsible for a car to lose power, weather it be a fuel pump or fuel pressure regulator fault.

[Guest tomwp]

Any tips on testing the fuel pressure? Not something I have done before.

[stefan 4]

Get a fuel pressure gauge, nothing too fancy just an input and output with a gauge. No need to buy a hole set with clip on or wind on connectors. Remove the fuel line from the rail and try to remove the hose from the connector end. Connect it to one side of the gauge, and use a piece of hose from the gauge to the rail. Use high pressure hose and tighten properly. Removing the connectors may prove to be a bit difficult, but can be done, heating the lines helps. The alternative is to buy connectors that can be bolted with your line. Anyway position the gauge between the filter and fuel rail.

The reading should be 2.9-3.1 bar

[large 33]

If you are using water and no leak have a look at the spark plugs are any of the tips clean?

 

Also top the header tank up to the very top and run the engine with the cap off, look for bubbles. + look for white smoke when running all signs of HG problems.

 

You do say that you put oil down cylinder 4 and the reading went up by .8 of a bar, whats the milage on the engine?

Edited August 11, 2010 by large

[Guest tomwp]

So I found the problem. I can breath straight through the vacuum advance diaphragm. Silly beggar, I should have checked that first.

 

Just to answer the other questions:

 

If you are using water and no leak have a look at the spark plugs are any of the tips clean?

- spark plugs look fine.

 

Also top the header tank up to the very top and run the engine with the cap off, look for bubbles. + look for white smoke when running all signs of HG problems.

- no bubbles and it doesn't pressurise so I guess the HG is ok.

 

You do say that you put oil down cylinder 4 and the reading went up by .8 of a bar, whats the milage on the engine?

- engine has done 130k

[Guest tomwp]

Reading around seems there arn't many places to get these anymore. Does anyone have any recommendations?

 

Thanks.

[Guest tomwp]

So i've now replaced the vacuum advance and checked its getting a vacuum. There was a small improvement but it still isn't the 205 I remember.

 

My next guess is that it isn't getting adequate fuel under high load.