loquinho

This is such a simple problem I feel bad asking for help on it, but it is what it is. I'm not a mechanic, and every time something breaks on my car, I get to learn a new automotive skill. Unfortunately, this means I'm constantly a newbie when it comes to car repairs, because usually the problem is something I haven't run into before.

I'm trying to diagnose a significant problem with the vette, and in my research I discovered I should run a compression test - to rule out a host of problems.

So I look at youtube for how to run a compression test, and I go out and get the kit from Pep Boys. I manage to get the spark plugs out despite the tight clearances (It only took me breaking two to learn that 1 - they're breakable, and 2 - you have to prevent any cross torquing when removing them). When I tried to put in the tester hose, however, I discovered that the portion of the threaded end that remains outside the spark plug hole is so shallow (and completely round) that it's impossible to get enough purchase on it to screw it in tight enough to perform the test. I managed to get it screwed into 3 cylinders (and I somehow got it severely stuck in cylinder #1 - it wasn't in that tight, but I couldn't get any purchase on it to get it out), and got pressure results of 60, 0, and 0. I really don't think my engine is THAT broken, so I'm guessing I'm not getting it screwed in tight enough for the cylinder to create pressure. I threaded it as tight as I could by hand with the 1/8" I had to grip it.

The engine won't turn on currently, so I wasn't able to bring it up to operating temperature before running the test. I'm not sure if that makes a difference, I'm just trying to give you guys all the information I have.

I go online and look at compression testers to see if this threaded end of the hose is a normal size, and it looks like most testers are made the same way. The one I rented came with an adaptor for deep socket spark plugs, but I don't think that's what the chevy small block has, and I didn't want to thread that one in all the way, because I'd risk hitting the piston head on the tester.

So, am I just inexperienced and naive, or is there a better way to go about this? Also, is there any reason I shouldn't remove all the spark plugs to perform the test? I took them all out (and labeled the wires) to make the test go faster. I need to get this test done before I can move on with diagnosis, and I really don't want to pay someone to run a compression test for me.

As always, thanks for the assistance!

7T1vette

Most compression test gauges that I've seen have tapered rubber 'snouts' on their inlet. You just press it [hard] into the spark plug hole, hold the throttle plates OPEN on the carb, and crank the engine over 2-3 rounds until the cylinder pressure is maximized.

A compression gauge should have a one-way valve in it so that it will 'hold' the max pressure reading. So, it should also have a 'pressure release' button on it somewhere.

birdsmith

I use a little WD40 on the threaded end of the tester, and just screw it into the spark plug hole far enough to barely seat the O-ring.

If you're not already aware, the throttle should be held wide open and all 8 plugs should be out so the engine can crank freely as well, then crank each cylinder, watching the gage come up until it stops...usually 5-6 turns.

Good luck;hope this helps.

Duane4238

My compression gauge came with two different sized ends. That's 2 different thread sizes. If yours has 2 different ends, make sure you are using the correct end. Also make sure there is an o ring in place. You only have to have it hand tight.
Duane

hipocars1

What problems are you trying to diagnose?

loquinho

7T1vette: I actually have one of those I inherited from my stepdad, who I think got it from his father (it's old), but with the vette clearances I'm can't see how to get it firmly seated by hand into some of the cylinders. That would also involve finding someone to crank over the engine for me, and that model is so old it doesn't appear to have a one-way valve.

I did oil the end a little before installing it. Maybe I'm trying to get it too tight. If I don't need to get it very tight to work, I'm pretty worried about those two cylinders giving me a 0 reading.

hipocars1: I had a couple diagnostic threads from last summer (the car's been out of commission since then - I've had some competing priorities, and I'm just now starting to clear my schedule a little)

1st Diagnostic Thread

2nd Diagnostic Thread

There were a lot of different pieces of advice, and I still have a few more things to go through, but next on the list was the compression test.

Ibanez540r

I did not read through your other threads, just glanced at the first one;

I think that even if you had the gauge partially in the 2 cylinders that read 0, especially hand tight, you should have got a reading. Good chance that a valve is stuck open blowing all the compression out.

birdsmith

After perusing your diagnostic threads, if that car were mine by now I would have had the engine out and up on a stand by now. There is clearly some serious age/ wear on that motor and it needs to come apart and be rebuilt.

Your previous thread was written, when, July? It's been half a year now and the motor's still not running right and it's still in the car...there's something wrong with this picture.

Too often people buy these old cars and they break and just sit broken forever or worse wind up in a scrapyard because they were simply afraid to tear into them and find out what the problem really was, or they lacked confidence in their ability to correct the problem.

I can see by your spelling and punctuation that you don't lack intelligence, and it doesn't take a rocket scientist to diagnose and fix these cars, but sometimes you've just gotta bite the proverbial bullet, quit dithering, and yank that sucker outta there so you can fix it properly...

My $1.380

loquinho

Alright Birdsmith, you guilted me into getting out there and working some more on the car quicker than I anticipated. When I say "competing priorities", I'm referring to a 3 year old kid, professional examinations, heavy church responsibilities, and two other car used as daily drivers that have a tendency to not be perfect. The corvette, while a car I've loved having for the past 10 years, is not highest on the list.

I removed the valve covers to see if I had any damaged rocker arms or obviously bent push rods. Everything looked good on both sides. I turned the engine over by hand a few times to make sure each valve was articulating, and everything appeared to be working just fine.

I figured out my compresion tester difficulties,and ran a full dry compression test. All the numbers are way low - I'm assuing this is either the adaptor I was using not getting a full seal, or a problem with the tester itself - I can't imagine what kind of problem would result in low compression occuring on both sides at once. I'll return the Pep Boys rental tool and probably purchase one myself to recheck. The results are below - though low, they're all in the ballpark of the same psi, and some of the variation could be due to how bad a seal I got on each cylinder, so it's not looking like a valve problem. I won't make a final call until I get a better tester.

1 - 64psi
3 - 46
5 - 52
7 - 52
2 - 62
4 - 53
6 - 52
8 - 50

I'm not crazy - this is really low, right? While the compression tester had an o ring between the hose and the adaptor, the adaptor that fit into the cylinder didn't have one - that's what I'm guessing the problem was.

7T1vette

Were ALL of the spark plugs removed when each test was run?

Were the throttle plates on the carb completely open when you ran each cylinder?

Does the gauge you used have a one-direction valve on it, so that it will capture [and hold] the highest reading obtained?

Did you run the starter so that the cylinder you tested 'pumped-up' at least 3 times?

If you did all those things...and the gauge is 'good' and is calibrated in PSI (pounds per square inch)...then something is not right with the valvetrain. [ie, Valve clearances wrong and valves always stay open a bit..??]

DRIVESHAFT

If your cam timing isn't correct it could cause low compression readings on all cylinders.
I read that you just replaced the timing chain, but its possible that you didnt get everything aligned properly.
You certainly wouldn't be the first person that had that happen.

7T1vette

If the cam is not oriented properly to the crank via the timing chain, that could cause major problems. Valve timing would be OK (determined by cam), but you wouldn't get full stroke for compression if cam & crank were not together.

loquinho

Alright guys, time for another update. I got the compression tester figured out - there was a missing o-ring that I had to go buy. I got it tightened up, and discovered that it didn't make much difference. Pressures were coming in just about the same. I wet tested several cylinders from both sides of the engine, and the numbers didn't change much. I got numbers over 60, but nothing over 70psi.

71 Vette: Throttle was full open, compression tester had a one way valve, and I cranked the engine over 5-6 times per test.

So, I sucked it up and dismantled the engine back down to the timing chain. Attached are a couple photos I took with my phone. I'm not crazy, right? These dots appear to be aligned just how they should be. Could it look this correct and still be off a tooth?





I'm having a lot of trouble trying to figure out what could be causing low compression on all 8 cylinders if it's not valve related. It's not like this problem has been creeping up for a long time. I was driving the car, it was acting normal, then bam, no power sputter die.

My next step, if timing is aligned, is to buy a compression tester I know is new and run the tests again to make sure the gauge wasn't faulty. The tester seemed to be in really good condition (except that o ring), so I doubt that's it, but... who knows.

Any more ideas guys?

birdsmith

At this point if you rule out 8 burned exhaust valves or 8 pistons with holes in them the only other explanation would be over-tightened valves. Have you readjusted the valves lately, and are you absolutely positive that they are correctly adjusted?

If that's not the problem, and you performed the compression test as instructed (plugs out/throttle open) the only other possible diagnosis is major internal wear and/or damage.

While I don't have a lot of good tools, I do have a Sears compression tester; and I have noticed that the China cheapies from Autozone are difficult to use and flimsy...I saw the hose pull right off of one when it was screwed into the cylinder.

Generally when there is some kind of internal engine failure it's only going to affect one cylinder(i.e.,dropped/burned valve) or it will result in comprehensive destruction(high-rpm blowup resulting from a broken crank or connecting rod). You're not seeing either of those so I'm a little inclined to think that there's something wrong with the testing procedure.

Do go and get yourself a known good-quality tester, double-check to make sure you have a good strong battery in there that will crank quickly enough to build compression, and re-run the test. If the valves are properly adjusted you should see a significant increase in compression. Once you get there, if you're NOT seeing that, be ready to pull it out and tear it down....

loquinho

I started reading threads outside this forum on low compression in all engine cylinders, and having a good battery lept out at me. The battery has been dead for a while, and I've been turning the engine over using heavy duty jumper cables attached to the battery of my running pickup. It sounded a little lazy turning over, but i didn't think it mattered since the cylinder has to compress just as tightly no matter the speed. the more I do wrong, the more I learn. I think i need to get myself a new battery before trying again.

The valves are however they were when I got the car back in 2002. I haven't touched them.

birdsmith

Ok...it's starting to sound like you're not getting accurate compression readings because of weak battery-induced slow cranking. In reality though if correcting that doesn't get you numbers of well over 100 psi on every cylinder that's cause for concern, and as indicated awhile back disassembly is the only thing that's gonna reveal the true nature of the problem.

Probably not what you want to hear, but not really the end of the world either. Read Fatcat's engine rebuild thread...he's on a budget and managed to walk through an engine rebuild in a couple months' time...pretty impressive really. An old Corvette motor is greasy and nasty to be sure, but most of all they are SIMPLE.

loquinho

I'll try to pick up a new battery this weekend, and hopefully report back with some better news. I'm actually looking forward to doing a rebuild someday... I'd just prefer to drive it a little more beforehand. I'll check out Fatcat's thread.

loquinho

So, good news to report. I got a new battery and tested again. 165-170 on both sides. So, turns out a good battery vs. a set of jumper cables makes all the difference in the world. Unfortunately, that means I haven't found the problem yet, so I'm going to ressurect my old "Diagnostic Help II" thread and see if I can get some more help. I think I've isolated it down to the ignition system.

REELAV8R

Just came across this thread today. If it was as you said, bam sputter die then compression was not ever an issue. Even though those previous compressions were low they would not prevent the engine from running or cause that kind of failure.
Diagnostically speaking always start with the basics.

1. do you have spark to the plugs
2. do you have fuel to the carburetor

If one of these is missing it will not run, so always start here for an engine mysteriously failing to start or run.
Pull a plug and ground the threads to the block or intake mainifold, have someone crank it over your should see the spark in the plug gap. If not work your way back to the ignition and all it's components for the failure to get the spark.
When cranking can you smell gas if you pump the accelerator. If not you have a fuel delivery problem and that is the direction you will need to go.
Once you have determined the presence of both of these items and it still won't run then it's time to dig into things like timing and compression etc.
Looks like your starting in the right direction now though. Good luck.

gcusmano74

That's one of the problems with a compression test; you test things that are not related to compression. In your case the compression test revealed a weak battery.
To really know the condition of an engine, you need to perform a leakdown test. You need a source of compressed air for that.