Back in the day when I were a nipper most people seemed to have a basic appreciation of how engines worked and how to service and maintain them. Mind you engines needed a lot more of that back then than they do nowadays I suppose. Setting points, adjusting tappets, greasing suspension nipples, tweaking carbs etc are in the main bygone pastimes which is probably a good thing. So as engine management systems have become more complex and incomprehensible to the average owner and routine maintenance less necessary most people now rarely venture under the bonnet and have little idea what's under there even if they did. When problems arise it's usually a garage job for them and that's rarely cheap. However engines themselves have really changed very little and certainly basic operating parameters are much the same as they always have been. Fault diagnosis is still a logical exercise of checking and eliminating what still works ok until you find what doesn't. It doesn't pay to assume that any problem must be a calibration issue or something wrong with a complex box of electrickery and overlook any obvious mechanical faults which manifest just as they always have done.
To run properly engines really only need the same basic things they always have done. Fuel, air, spark and compression, all at the right time and in the correct quantities. Checking these things out is not so complex. If an engine is either not running properly after modifications, or stops running properly whether standard or modified or isn't putting out the expected power while being set up on a dyno I have a set of basic rules that lets me at least find the general area causing the problem.
Rule 1 - Start with the basics and save the panicking for later
No engine will run properly if it isn't in good basic mechanical health. A few simple tests will cover most of that - a compression test, check the cam timing and check the ignition timing.
A compression test will tell you most of what you need to know about bores, rings, valves and head gasket in one fell swoop. If you run a modified engine and are serious about maintaining it properly then do a compression test as soon as you buy the car or build the engine. Then you have a baseline figure to check against later if anything goes wrong. It's not much help when the engine starts playing up if you find one cylinder is down but have no idea if it's always been like that or or if the problem is new. On race engines it pays to check after every race.
Cam timing should be correct on any standard engine that hasn't been tinkered with but if the cam belt has been changed, or has slipped or the cam itself is non standard then it pays to get this check out of the way. Performance cams are not always ground to time in properly on the standard pullies and if you have adjustable pullies fitted then you need a degree wheel and dial gauge to set things up right which I'm not going to go into here. If you can't do this yourself then find someone who can. On a standard engine it should be as simple as checking the various marks on the cam and crank pullies line up properly.
Ignition timing is easily checked with a strobe light although if the engine has been modified the standard advance curve might not be what you ideally want. If the above basic checks are ok you can move on.
Rule 2 - The last thing that got tinkered with is often the problem
Just because you recently fitted new plug leads or spark plugs or anything else doesn't mean you can ignore them ever after. One of them might have been faulty, or failed soon after installation, and you can waste a lot of time looking for other problems when it was something that got changed recently that's still not right. If any servicing or garage work has been done then make sure that while one thing was being fixed something else didn't get broken. A vacuum hose or electrical connector not being plugged back in for example. I recall a friend having his gearbox swapped by a garage who trapped part of the wiring loom between the gearbox and engine when they reinstalled it and knocked out part of the EFi system. The car never did run quite right after that even after the broken wires had supposedly been fixed.
Rule 3 - at least make sure you've got a decent spark
Hold each plug in turn against the engine block or other good earth while the engine is cranked over. It makes it easier to see the spark when it's dark or the garage door is shut. If one cylinder isn't sparking properly then swap plug leads and plugs to see if this changes anything. If not then it's distributer cap. If no cylinders have a good strong blue spark then it could be coil, rotor arm, king lead or something else common to all parts of the system.
Rule 4 - check the fuel supply
If everything looks good so far it's probably fuel. This is such a wide area I can only go into generalities. Blocked fuel filters, failing fuel pumps, a faulty injector etc. However at least you're narrowing things down.
Instead of relying on a normal garage you might want to consider taking the car to a rolling road tuning centre. Here they can check the power output of each cylinder by isolating each spark plug in turn and they'll have CO meters and oscilloscopes to check fuel and ignition parameters. They probably won't want to fix anything but they might home in the problem quicker than a main dealer who can probably only plug in a fault code reader or swap bits at random, and at your expense, until they find the fault by chance.
There are a number of common problems, or traps that the inexperienced fall into, that can prevent modified engines from making their expected power output. A few simple checks before you go to a rolling road session might save a lot of wasted time and money.
1) Are you really getting full throttle? If I had a quid etc etc for every time I've seen a non standard carb or throttle body system where the butterflies aren't opening fully because the linkage or throttle pedal isn't set up properly. The butterflies should open fully to about the horizontal position inside the bores. If the linkage isn't right they might either not open all the way or even go over centre and start closing on the other side. There should be a positive stop fitted to prevent that happening. Whip the air filter off, wedge the throttle pedal fully open and look down the intakes. If the butterflies aren't edge on to your line of sight it needs fixing.
2) Dodgy air filter setups. No matter what type of filter you use - paper, oiled cotton, foam etc, you need a certain minimum amount of filtration area per bhp the engine is expected to make. Usually about 25 square inches per 100 bhp will get the job done with maybe a bit more needed for foam filters which generally flow less efficiently than paper or cotton elements. The absolute worst thing you can do is fit those filter sock things that go tight up against the end of the rampipes. Your filtration area is now only the rampipe ends which in total is only about 12 square inches on an average 4 cylinder engine and that can easily knock 20% or more off your engine's potential power output. For a 200 bhp four cylinder engine you'd be wanting that much filtration area for EACH rampipe rather than for all four together. That means that any type of sock filter must be at least 2 or 3 inches long to have the required surface area per rampipe and you can run similar calculations if the engine breathes through a single panel or cone type filter. You can't go wrong by having too much filtration area rather than too little so seek the manufacturer's advice and when in doubt err on the large side.
The other thing to watch for is rampipe ends too close to the walls of air boxes or filter elements. Ideally you want the rampipe end to be no closer than half its diameter away from any obstruction.
3) Insufficient fuel supply. If the engine has been really heavily modified the standard fuel pump or even the fuel lines themselves might not have enough flow capacity. To be safe you need about 1 pint per minute for every 100 bhp.
4) If your engine runs fine up to about 2/3 of its normal rev range and then suddenly hits a brick wall at a specific rpm and refuses to rev higher it can indicate broken, faulty or incorrectly shimmed up valve springs which are letting the valve train float at high rpm. If the engine has hydraulic lifters they'll do exactly what they're designed to do and try and take up any clearance in the valve train once the valves start floating. This means that they might then continue to hold the valves open for a while even when the revs are back down and the engine will cough and splutter and run like a dog for a bit until the lifters have drained back down. With solid lifters the problem will just appear at a very specific rpm and disappear again immediately below this. Clueless rolling road operators can bugger about for hours experimenting with tiny, and completely pointless, changes to fueling and ignition trying to cure this when the problem was obviously never calibration in the first place but a basic mechanical fault. Fueling or ignition timing, even if they're not quite ideal, that let the engine work fine at X rpm can't magically make it stop running at all at X plus 100 rpm! It's not rocket surgery people - it's just common sense. Get the mechanical problems sorted out first before wasting hundreds of quid on setup that will all have to be done again later anyway.
A similar effect can be obtained on engines with hydraulic lifters if the oil pressure is so high that the lifters overcome the valve spring pressure and hold the valves open although this will manifest at any rpm where the oil pressure is high enough. So called "high performance" oil pumps or standard ones that have had their relief valve springs tinkered with to raise the pressure or just the use of oil that's too thick can be the cause. I've yet to see an engine that needs more than about 60 psi at high rpm but apparently as with all things engine there is no shortage of people who think more of something must be better in every case. A well meaning owner of one of my Ford CVH race engines once decided without asking me first that despite nearly everyone in the series using Castrol GTX without problems for year after year he'd try some super special thick racing oil to get higher oil pressure during hot races in the summer. Unfortunately what he didn't take into account when trying to run the engine in on a bitterly cold day in February was that having treacle in the sump when snow was falling was putting the oil pressure off the gauge and pumping the lifters and hence the valves open. The car would get half way round the track, splutter to a halt, start up again several minutes later once the lifters had drained down and then repeat ad infinitum. Luckily they had a smart enough mechanic there to twig the problem and with normal oil back in the sump it ran fine again I'm told. You tinker with things at your own peril!
5) Turbocharged engines not running properly at full throttle, high rpm or high boost levels can be losing the spark at the plug because the gap is too big. It takes more voltage to fire a spark through a given sized plug gap when there are more air and fuel molecules in the way so forced induction engines need to run smaller plug gaps than normally aspirated ones or the spark can get "blown out" as the boost level rises. Try reducing the normal 0.8mm to 1mm gap for N/A engines down to about 0.6mm and see if this helps. A higher voltage ignition system may also be a good idea.
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First written 4th April 2010.