(G) Cylinder Head Skimming and Engine Performance

Hi Everyone,

I managed to fix my Fiat finally . My cylinder head was skimmed, new gaskets, thermostat and some other stuff. Water is not vanishing anymore at last but ..... another problem evolved. My engine is not working as before. It seems to have a lot of power but it is not moving as fast as it used to do. Fuel seems to be going down faster too ........... what might be the problem ?

Its probably a raise in compression you're feeling due to the head being skimmed....makes the engine feel a bit more punchy. Problem is with higher compression comes higher stress to internals and you can also get detonation if you use lower octane fuels.

I take it it's a turbo

If so i;d advise getting it on a Rolling Road where they know what they are doing and get the feulling checked over just to be on the safe side

It isnt a Turbo mate .... it is a Fiat Uno Fire Mk1 1989 model. How can I solve the problem ?

Does the performance drop off the more you use it or like the warmer the engine gets?

I think that the performance drops off a bit while the engine gets hotter. You got me to think about this issue.

It could well be that the compression has been raised quite a lot. How much was taken off the head? Also with the age of the car and the nature of these engines it's probably been skimmed before now anyway.

I don't know how much was taking off the head to be honest. The cylinder head wasn't skimmed before. This is the first skimming. After work I am taking my car to the mechanic to try to solve this problem but I would like to gather as much as possible information as to help my mechanic regarding this problem.

How do you know it's not been skimmed before have you owned it from new or been told this?
I know it's not the most desireable thing but try running it on super unleaded (Tesco do a 99 octane one) and see if it makes any difference. If it does then its likely the compression has been rasied too much for it to cope on normal unleaded. Only other thing it could possibly be is the timing...seen as though the heads been off etc etc it could possibly have been put back on and timed up slightly wrong. If its running too much advance the higher octane should help it run better from that point of view too.

Btw if you already have a load of unleaded in the tank throwing some octane booster in could be worth a try...but I don't know how well it'll mix and how effective it'll be just throwing some in. Basically though I'd have thought like said before it'd be one of two things...either timing slightly out or raised compression. Not much else that could go wrong when putting the head back on that could cause this I don't think. Might be worth compression testing the cylinders against another motor.

Excessive fuel consumption and 'odd' power characteristics - sounds like the camshaft timing is incorrect and/ or the ignition timing. The cambelt and most probably the distributor would have been removed during the head removal. You need to get the cambelt timing double checked - use the marks on the crank sprocket and camshaft sprocket and line up with the timing marks on the cylinder head and crankcase. You should also have the ignition timing checked/ reset dynamically with a timing light. I think the dynamic ignition setting for a 999cc FIRE is 2 degrees BTDC (with vacuum advance disconnected and plugged).

I doubt if the skimming would have caused any problems. You'd be getting 'pinking' (pre-ignition) if the compression ratio was too high as a result of excessive skimming. If you hear a metallic 'tinkling' sound when you put the engine under load, i.e. high gear at low speed and you push the accelerator hard, then you have pinking. If you don't then chances are you haven't.

Double check the obvious first!

...check that the camshaft pulley mark lines up with the notch JUST ABOVE the join between cylinder head and cam cover (13 minutes to the hour, so to speak) - *not* the horizontal join itself at 15 minutes (quarter-two) - when the flywheel is at the TDC position (forward-most pointer on the gearbox bellhousing). Setting the camwheel with the mark horizontal is a common mistake.

LOL @ jamie assuming that it's a Turbo when they're probably 1% of the models on the road But good advice all the same for a fuel-injected car to be checked on a rolling road

-Alex

Someone more technical will explain why, but if you have a faulty vacuum advance, then the idle speed, and subsequently advance will be wrong. I had a 60 FIRE that was tuned to idle and run with a faulty vacuum advance and was getting 10km per litre. With a new advance and reset idle and advance now getting 16-17km per litre and better engine response. A new vacuum advance is the answer to any problem.

An interesting thought, and it may not really be an issue. But since you have skimmed the head this will put the cam pully closer to the crank pully (I know its not much). Many people have told me I need to check my cam shaft timing using the old fashioned method (ie ignore the timing marks and use a protractor and an adjustable cam pully) because I have taken 1mm off the cam shaft box.

I thought that...

Pinking (Pre ingestion) is when the spark is too early, igniting the mix before it should.

Detonation can happen when the spark is too late, or when the mix is over compressed so much that it self ignites.

Skimming the head will increase the compression ratio, so in theory, it’s possible that (as above) if you take too much off, by nature, lower octane fuel will detonate before higher, hence the knock sensor on more complexes incision sensors. Don’t know it it’s possible to take too much off the fire’s head.

None are good, but detonation is so much worse than pinking as it blows holes in pistons. But a good place to start is with the timing and Vac advance diaphragm.