QuestionI have a 2003 suzuki GSX-R600 that was involved in an accident. The frame bent in and cracked the cylinder head. I found a nice used head on the internet and proceeded to do the work myself. I have the factory issued repair manual and I followed it exactly for the alignment of the crank and cams, but the engine wont run properly. To get the engine to run it must have full throttle and even then will rev to only 3500rpm and is extremely choppy with a lot of backfiring. I believe it is a timing issue (though it may be something else, I'm not certain) and was wondering if you had any suggestions on what to do.
AnswerHi Nick,
Compliments to you for taking the initiative to do such a task.
Get the tools to do a cylinder compression test and a leak-down test. Advise me of the results: mshively1@woh.rr.com
Respectfully,
Mark Shively
Cylinder Compression & Leak-down Testing 101
You need 3 things to make a gasoline engine work, compression, fuel, and ignition. Without enough compression the engine will not function. Low compression causes you to crank the engine longer when trying to start it. As you crank the engine it slowly builds pressure inside and will eventually fire up. I cannot think of other problems it will lead to because once you have lost compression; you will not have a working engine, so nothing else can break.
What does it indicate as weak points in the engine and how is the leak down test are related? There are various possibilities for low compression and the leak down test pinpoints them fairly well. Your engine is an air pump and the leak down test measures the engine's ability to hold air. The combustion chamber has a theoretical limit of holding 100% of the air/fuel mixture that is drawn into it and compressed prior to being ignited by the spark plug.
The leak down test measures with a flow meter the actual amount of air the combustion chamber will hold. Weak points will be noted by the sound of rushing air. So, if the rings on the pistons are weak, listening at the oil filler cap while doing the leak down test will allow you to hear the sound of escaping air. A bad exhaust valve will allow air to rush out the tail pipe so you stick your ear to the tail pipe and you hear the noise of air passing by a bad exhaust valve.
The compression test uses a gauge to measure the build up of pressure inside each cylinder as you crank the engine. The cylinder leakage test uses compressed air that is forced into the combustion chamber. Both testers are screwed into the sparkplug hole.
To check the condition of the rings, you need a leak down test. This test measures the ability of the cylinders to maintain compression, thus testing the integrity of the rings and valves. The cylinders with low compression MAY have poor rings, but check the valves first. I had the valves done on mine, hoping that would cure the problem, but it was terminal; 3 of the cylinders had more than 50% leakage cold and about 70% hot - bad rings! I had a complete overhaul done. Now it runs like it just came from the factory (better, says my mechanic). If it isn't using oil, the rings are probably good for many more miles.
A compression test tells you the compression provided by the piston on the compression stroke. It is a relatively instantaneous measurement, but if you keep the meter on the port you can see how the compression decreases over time. If it decreases too quickly, something's not right; either leaky valves or rings. The leak down test quantifies the compression fall-off as a percentage over time and (usually) at cold and hot engine temperatures. It is a better indication of engine condition and is sometimes used to predict remaining engine "life." If your rings are bad, you'll usually see two things:
- low compression
- high (or higher than usual) oil consumption
You may also see white smoke from the exhaust in extreme cases. If there is no smoke and the compression tests are good, then the engine should be fine. If you still have doubts, check the valve guides and valves (if you see a puff of white smoke when you leave a stop light - after decelerating) it could be valve guides or seals. A valve job is easier and MUCH cheaper than a ring job. A skilled mechanic can usually determine if it's the valves or rings.
You should not get any more than 10% variance between cylinders. I'd start by keeping things simple. First, check all valve clearances; you can lose compression through the valves. Once this is done, check the compression again, and do it a few times to be sure it's not your compression tester that is out of calibration. There is a procedure for testing valve problems, which is also a possibility here. Pour some oil down the spark plug holes just before you take the reading. If the reading changes, then you've sealed the problem with the oil (i.e. your rings are gone), but if it doesn't then it's more likely to be your valves - which means a rebuilt head (much cheaper than the alternative).
As the cylinders wear they get a taper -bigger at the top and smaller at the bottom. You can go up to about .015'' of wear overall or .007 per side. The rings have to expand and contract every time the piston moves. So, they tend to break at high RPM but can handle 3,500 for years. Engine wear is about .001'' per 10,000 miles on average - some better and some worse. If you have a vehicle with lower compression and over 100,000 miles it's a safe bet that you have cylinder wear, the same wear that will break rings at high RPM.
You can sleeve them with no problems. If its standard bore you can go up several sizes over. This will clean out even the worst cylinder wear in most cases, but not deep gouges in the wall. Excessively deep gouges will need a new sleeve.
Re-torquing the head bolts after head re-assembly should be done after a few hours of running on a warm engine. Undo each bolt one at a time only. Start in the middle just like a regular sequence and undo a bit and then tighten up to the final amount which is usually a bit more than the initial setting. You should get another 1/8 turn out of some of them. Also use a bit of oil on the threads and washers so they screw down evenly.
