BAD Cylinder - what to do?? Diagnostic technique??

Hi All... I used to be a junky wayyy back in 99/00 but barely come here anymore since I've been way too distracted by trying to make a buck... I was hoping someone might have a clue where to begin with the following problem:

I have a dead cylinder (I think it's no.4) in my 92 HO 4.0 (though I should mention that I swapped it for a 98 HO back in '99!). Anyway- being in California, I had to get the thing 'smogged' to get a registration renewal. It failed the smog so I did the usual 02 sensor swap as well as replace all the plugs, wires etc and anything else I could think of. Only after failing a few times did I clue in as to what was going on...

So - SINCE I've already gone over the ignition system and replaced everything incl. verifying spark at the offending cylinder - I'm wondering what the best way to go about finding what the problem actually IS. There are no weird noises or vibes apart from the 'engine lope' of having a dead cylinder. Seems to me that it must be either the fuel system or injector (though I DOUBT that since the smog test revealed unburnt fuel coming out the other end - leads me to think that fuel's coming through okay). I have a FSM and tools at my disposal - so I can handle most minor procedures on my own probably (hoping to avoid rape by a mechanic!).

Has anyone else had this problem? Can anyone suggest a good diagnostic order of operations to try out to hunt down the issue?

Thanks
Jonathan

Do a search for testing the cylinder compression. You will want to do a wet and a dry test. That will tell you if you are fighting piston ring problems or valve problems.

old_man said:

Do a search for testing the cylinder compression. You will want to do a wet and a dry test. That will tell you if you are fighting piston ring problems or valve problems.

Click to expand...

That is the starting point.

Fuel, air, compression, spark, all at the right time in the right place = ignition.

Get a vacuum gauge. That will give you a very quick check of whether your problem is mechanical or not. Bad compression will show immediately. If the vacuum gauge is steady look to the injectors. A bad injector will probably not give you a suspicious vacuum reading. A bad injector harness should set a code, but a mechanically bad injector will not, nor will an internal engine problem, so if you're getting no codes, you should definitely test for vacuum/compression first.

Here's a quick way to test for a dead injector:

First of all figure out what kind of injector plugs you have. Some are very hard to pull and some are not. If you have the kind that use a square wire clip, it can help to remove all the clips for your testing. They'll stay plugged in for long enough but you won't kill yourself or your injectors trying to get them off in a hurry. Whatever kind of plugs you have, make sure you know how to pull them off without damage or cussing too hard even when you're standing at the side of the road with a hot running engine.

Now run the engine up to warm, and if the problem is intermittent, make sure the problem is occurring, and while idling open the hood. Pull an injector plug. If the injector has been working, the engine will bog a little, then recover. The management system is good at maintaining idle speed even with a dead cylinder or two. Replug that one, give the engine a little jab of throttle to clear it, then do the next one, and so on down the line. If you have an injector that is not firing or not squirting, the idle will not change when you pull the plug. You now know which injector is not working.

Now stop the engine briefly, and swap the plug on the failed injector with its nearest neighbor, and restart the engine. It will still run all right even with the order off. Repeat the test on the failed injector. If there is still no change when you unplug it, the problem is the injector. If it bogs and recovers this time, the problem is in the harness.

If there's a harness problem, check the voltage on the hot side of the injector plug. On this system the PCU switches the ground side, and the positive is supplied from a line coming off the distributor harness, through a number of splices. I've had a splice in that harness go bad and cause one injector to cut out when it heated up. Hard to figure out, dead easy to fix.

Before I started chasing electrical or other components, I'd do a compression test. Nothing else matters if you don't have at least 100PSI or usually 120 is needed to run right. Pull the spark plugs, jam throttle open, crank engine over 5 or 6 revolutions. If you find a low one, put a tablespoon of oil in it and try again. If it goes up, it is usually the rings. If not, a valve or head gasket.

okay - thanks for the advice so far... how easy/hard is it to just do a straight-out injector swap?? They just unplug from the fuel rail, don't they...? (forgive me if I'm talking through my ass - just going on memory here)

X2 Compression test. Guessing gets $$$$$ fast.