Compression test for my truck
 

Does anyone know what psi for a '93 s-10 auto 4X4 should be? I need to do a compression test...the rough idle is still there, even after the exhaust was fixed today. I'm going to do a compression test tomorrow and see if there's something off there, but I don't know what it should be at.

RE: Compression test for my truck
 

I cant think of it right now and too lazy to get the book. One thing to remember is to do it one cyl at a time and do the whole engine the same way. Books have certain right ways to do it, but if you wish to do a quick check to find a bad cylinder, just do em all the same way and if one is very low, you have found the problem. If they all are about the same it doesnt REALLY matter what else you could have done or what the right exact pressure should be, even if they are all on the low end of the scale, they would run smooth if they were all about the same.

Its the one or two with the ODD low reading that are a problem.

Does it miss at all speeds?

RE: Compression test for my truck
 

From the GM service manual:

Minimum compression recorded in any one cylinder should not be less than 70 percent of highest cylinder. No cylinder should read less than 100 psi. There example is if the highest cylinder reads 150 psi the minimum allowable at 70% would bre 105 psi.

The recommended proceedure is to remove all spark plugs, block the throttle plate wide open, disable the ignition system, zero the compression gage, crank the engine through four compression strokes. check all cylinders in the same manner. If a cylinder registers low compression, add one tablespoon of motor oil to that cylinder through the spark plug hole and re-check compression.

Hope this helps.

RE: Compression test for my truck
 

Thanks guys. I'm not sure what the issue is, I'm just trying to cover all the bases. I sent my truck in to get some exhaust work done because it has a bad shake when you gas it and a rough idle, and that has not gone away. It's not a rough as it was, but it's still there. I am FINALLY going to clean the throttle body tonight, and I figured I'd check compression while I'm at it. You never know. I had new plugs last May, but who knows?

It runs a LITTLE bit hotter right now as well. More info is on this thread

RE: Compression test for my truck
 

I just checked out your other thread, do I understand it right htat your 93 Blazer 4x4 has a 4 cylinder engine?

RE: Compression test for my truck
 

Not as lazy today, but the wife was waiting for me to turn lan off and go to bed.

Cracked exhaust manifold(s) can cause a cooler than usual exhaust and mess with readings, YES. But the reaction of the "computer" is what is (causes) the problem. A cold engine is asked to run a little richer, (in general and simplistic terms). Should you have a cold running condition for a longer length of time each time you start the vehicle, this will lead to slowly messing up your plugs for starters. Will affect your egr and its workings, quite possibly causing buildup in whatever intake arrangement you have. The intake manifold will get more and more buildup very slowly as something like a winter with exhaust leaks close to the engine. All caused by a computer reaction to feeling colder.

BTW a "colder" running engine may well not be noticed on the temp gauge. It is only running cold in one area and not neccesarily the block. The block may EVEN run a little HOTTER because additional adjustments being made by the puter can cause this. I dont say that any of this is happening on your car, but all of this is possible on all moder, computer controlled vehicles and not opften discussed. Modern cars will build problems in different areas than where the original problem started.


This can be caused by something as simply as a thermostat hanging open also. Just happened to me. Have also had exhaust leak in different vehicle altogether that caused me egr problems and a very dirty intake, also. I was told that on THAT PARTICULAR vehicle the back pressure from the exhaust was pertinent to its (egr) working properly. This does not apply to your vehicle, in my opinion. But fixing any problems will keep others from being agrevated.

In the case of 02 sensors reading funky (a leak may read more fresh air, for instance, like air injection?) cause the exhaust manifold had a problem, its possible to not get a code if the computer is able to compensate for problem. I still would have expected some sort of code. In the case of the egr valve becoming less operative than it ought to be, it too would normally set a code. A miss in the ignition system sets a code in most modern ones today. It seems strange to NOT get a code if the vehicle is noticably running BAD.

At any rate. The very first step for me would have been; plugs, roter, cap, wires. All of them. As I remove plugs, I look at them carefully to see what their condition is. I check gap at least by eyesight. Its really odd how you can set a gap and install the plug and sometimes bump it going in, actually causing a narrower gap. When chaning one plug and one wire i also check the wire all the way and each connector in the boot. One or some wires may come off quite easily and may well have been too loose, perhaps because when installed they were placed in a tension situation that caused stress on the terminal, eventually causing it to open up slightly. Yes more expensive wires have better terminals. Stress is not a good thing for any priced wire ends. Checking the rest of the wire will tell you if its been rubbing against any object, hitting a hot exhaust or a sharp edge will destroy the wires ability to funtion and perhaps ground that one cylinders spark, causing a miss.

A blacker plug with oil or soot build up may show up at the end of the wire that was cut or marred. Important to keep things in order and work in sequence so as to create the proper chain of information to yourself. A bad wire matched up to a funny burning plug would explain the plugs problem. A plug by itself looking bad and no wire problems, might indicate a deeper engine problem or simply a bad plug. In any case, it is very important to pay close attention.


In your case, you may wish to do all this when checking compression, I would NOT. Many folks lose track of what they are doing when doing all this at once and cause themselves REAL problems.

RE: Compression test for my truck
 

Yes, that's right. Trust me, it's not cheaper on gas because of that, either.

Thanks. I'll check all that stuff out tonight as well. I didn't realize it could affect all that so much.

RE: Compression test for my truck
 

Well, I think I found the issue. It's been so damn cold I haven't wanted to pop the hood, but once we finally did, I discovered a wire that was melted on the block. I am going to replace it tonight, and I'm pretty sure that I've solved the problem. It's sorta a moot point for me now, though, because we're going tonight to buy a 2002 Montana. Poor ol' Blazer is gettin' put on the back burner.

RE: Compression test for my truck
 

Ahha just what I thought in your Bad shaking post. Me so smawt.:D

RE: Compression test for my truck
 

Yes, you are so smart. Funny how actaully LOOKING under hood helps figure out stuff...something my mechanic obviously didn't do. I think he was to concerend with looking at the exhaust he didn't cover all the bases.