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post #1 of (permalink) Old 06-26-2002, 10:21 PM
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chevy 350 compression

I have a chevy 350 that has low compression (100 psi) on cylinders 1,2&3. No noise or smoke, it happened driving down the freeway. I don't know how many miles are on the motor. The vaccum gauge indicates valve timing. I did a blow down test, the cylinders leak for sure but I can't tell from where. I am not sure what to do next,I could use some help on this one.
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post #2 of (permalink) Old 06-26-2002, 10:48 PM
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Re: chevy 350 compression

Need more info -
1, 2, 3 ?

What was the compression of the rest of the cyls?
How many pulses did it take to get to the 100 lbs? (should read 80% on the first pulse, all cyls should be pulsed the same for accurate comparison.)
How many for the rest?
Were the plugs still in or out? (All should be out for an accurate test.)
Did you block the throttle open to let air in? (blocked open for an accurate test.)

You mean the front 2 on the left bank and the front one on the right bank? That would be cyls # 1, 2, 3.

Happened on the freeway? How so? What happened? Missing?

The vaccum gauge indicates valve timing? What did it really do? High, low, pulsate, fast, slow, drifting?
What is your interpretation of a vacuum gauge saying valve timing?

"I did a blow down test, the cylinders leak for sure but I can't tell from where."
Blow down? You mean leak down? How did you do it? What was the percentage?
Where was the airpressure going? If it leaked you can hear it in the exhaust, intake, crankcase or radiator.

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post #3 of (permalink) Old 06-26-2002, 10:50 PM
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Re: chevy 350 compression

Add a few squirts, about a teaspoon of oil to the suspect cylinders and retest the compression. Add oil and retest one at a time. If compression comes up, rings/cylinder walls are indicated-a complete rebuild. If it doesn't come up, valve sealing is indicated. Possibly a burned valve(s)? The blow down test? Are you saying you pressurized the cylinders with air? Gotta make sure both valves are closed for that-top of compression stroke. Hissing in intake or exhaust indicates valve sealing problems. You may also want to pull the valve covers and turn it over to make sure the valves are actuating properly, and fully. Might have a flat cam, Chev's are known for that, but on 3 cyls at once-I doubt it. Let us know what you find.
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post #4 of (permalink) Old 06-27-2002, 10:35 PM
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Re: chevy 350 compression

The other cylinders where 155/160 psi. If I can get some time this weekend I will retest to see what they read on the first stoke, I'm not sure about the 80% on the first stroke. The carb was wide open but the plugs were still in. The symptom on the freeway was missing. Vacuum gauge on the manifold was steady, the instuctions with the vacuum gauge indicate valve timing. I don't know what % of leak down means.
Thanks for your help so far!
post #5 of (permalink) Old 06-27-2002, 11:44 PM
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Re: chevy 350 compression

OK, lets make a few assumptions first.
It would be extremely rare to lose compression on 3 cylinders all at once, especially on different heads.
It's possible the cam went that fast, but it's more likely a 747 would land in your back seat. Cams usually go flat fast, but not all of a sudden, fast may be 50 miles or so, slowly getting worse. It takes time to wear through the case hardening.

I assume the numbers you gave are the right cylinders numbers, 1, 2, & 3, left front, right front, left second back.

If you had something that caused a misfire in those cylinders, then it would naturally cause the compression reading to be low. The misfire causes the raw fuel to wash the oil off the rings, dropping your compression readings. (I've seen lots of engines get rebuilt because of that.)

Your "blowdown test", how did you do it? I asked about percentage thinking you had done a Leak Down Test with the proper equipment. The right equipment will give a percentage of leakage indicating how bad, and helps determine what's wrong. Your ears do the rest to see where the leaking air is going.

I asked what the vacuum reading was, you didn't say, just steady, that actual reading would help. I'll have to assume it's low, maybe 12"? I think that's what the el cheapo vacuum gauges call "valve timing."

If the chain had slipped, causing "valve timing" problems, all cylinders would be affected, not just 3.
If 3 were misfiring, vacuum would be low anyway.

Let's analyze what we have so far.
The 350's firing order:
1 8 4 3 6 5 7 2

_8 7_
_6 5_
_4 3x
x2 1x

Not much "in common" between them, just 2 and 1 fire back to back, and all 3 are in the front of the engine.

I'd start with ignition, like crossfiring. Check the cap for tracking. If HEI check the bottom side of the rotor for any whitish marks indicating flash through, or rust.
Check that the plug wires as they run to the front are separated, and no whitish marks on them at all. Whitish marks are actually ash or solidified smoke, indicating leakage.

How'd those plugs look? Slightly oily and dirty I assume? -- likely ignition

Or very clean and white? -- indicating severe vacuum leak - like carb base gasket. (I doubt it)

What kind of plugs did you use? Hopefully no gimmick "trick" plugs "made to sell, not to use." American of course, Japs and Krouts can't make a Chevy run. Platinums don't belong in a Chevy, no matter what the pimply faced kid at the parts house said.

When you do the compression test again, take all the plugs out, block the carb open, then watch your gauge closely. A normal hole will rise to about 80% of the final reading on the first pulse, then the next 3 or 4 pulses keep pushing it up a little more.

On the low ones, add about a teaspoon full of oil, then crank it without the gauge about 4 or 5 revolutions to spread the oil inside, then retest. I'll bet it will come up to about the rest or more. Normally if those cyls had been running OK, that would indicate leaking rings. But if those cyls were already misfiring, the normal oil sealing the rings has washed away, so don't panic - yet.

Let us know what you find.
My guess - crossfiring, or wrong heat range plugs or "mickey mouse" plugs. No biggie!

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post #6 of (permalink) Old 06-28-2002, 09:06 PM
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Re: chevy 350 compression


You need to learn to be more politically correct. "Krouts" is spelled "Krauts". I wouldn't want you to accidently offend anyone.

Chris in Texas
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post #7 of (permalink) Old 06-30-2002, 01:32 PM
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Re: chevy 350 compression

OK, I did some more tests this morning and here's what I have.
1) Vaccum gauge is steady at 12.

2) The under side of the rotor is rusty but no sign of whitish marks, it is very dirty inside the distributer.

3) Also dirty and ugly but no signs off whitish marks. They are seperated and look to be good wires but who knows?

4) Plugs are autolite with a number 16 on them. Unfortunatly when I did the first compression test a long time ago I didn't put the same plugs back in the same hole so I can't tell what that cylinder is doing by the looks of the plugs. Some look ok some don't,none look great. All where DRY. Might be old??

5) Compression-To try to keep it short I will get to the point. First pulse % is closer to 60% on just about all of them. Putting oil in 1,2&3 did bring them up. Here is what I got:
Before oil (final) #1 = 80 After oil (final) 140
#2 = 125 140
#3 = 130 140

All others no oil, just rechecked.
#4 = 175
#5 = 175
#6 = 175
#7 = 160
#8 = 160

I did verify the cylinder #s and I am correct on the Numbers. I have been running the engine every week or two for about 6 months and these numbers are better then the first ones ???
It doesn't sound all that bad but something is wrong for sure. My neighbor have a tool to cut open a oil filter, I think I will use it and see if there is metal in it. Back to the cam idea - is there a way to check lift or something without removing the cam? Troubleshoot tips??
post #8 of (permalink) Old 06-30-2002, 03:46 PM
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Re: chevy 350 compression

Well those readings do indicate worn rings, but -

Low compression doesn't "just happen" that fast.
Vacuum is low, 12v like I guessed, but wouldn't be steady if it were bad rings, or a cam going south, or even bad valves. It would be "ticking" up and down every time those cylinders fired, or tried to.

I still think it's ignition problems. But first try something.
Since it's all in the front cylinders I wonder if it's carb base gasket or something. There's very little those cylinders have in common with each other.
Is there a vacuum fitting in front of the carb? If so, pinch off the hoses to it to see if the RPM changes at slow idle. Something that's connected there may be leaking.
Also spray some carb cleaner around that fitting, the carb base, and the manifold to head gaskets. Listen for an RPM change. Use a carb cleaner that says it's flammable, like Gumout, or use starting fluid. Keep it off the distributor obviously.

If no good there, you said the distrubutor is rusty inside. Rust is very conductive. Use rubbing alcohol to clean up the inside of the cap, best to just replace the rotor.

Clean the plug wires with laquer thinner, dirty wires are very conductive.

The obvious, I shouldn't ask, but I will anyway. The firing order is still correct - a wire didn't get inadvertantly switched?

Is there anything "modified" on the engine or is it bone stock? What year and what's it in?
How many miles on it?

You said it was missfiring - just at idle, at low RPM under load? At high RPM no load? At high RPM under load?
All clues.

As you can see I'm trying to find something little, smaller and cheaper than a new engine. (Wouldn't it be frustrating to put in a new engine and have the same problem? Seen it done lots of times.)
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post #9 of (permalink) Old 07-02-2002, 10:45 PM
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Re: chevy 350 compression

Today I changed the oil, oil filter, Distributor cap, rotor & plugs with quality napa parts. I cleaned the wires and found a couple that have heat marks and some cuts through the insulation, I will try to get some new ones tomarrow. They are on correct.
The oil was very very thin like water. The carb. does have a vaccum fitting in front,I sprayed starting fluid on hoses and intake gasket area,carb base gasket. No change in idle.
I opened the oil filter and found some (not much) small pepper size metal.
The engine has no modifacation and did past the Ca. smog check a year or two ago. It is a 1979. I don't know how many miles are on it, I might be able to find out through a people search?? It is in a 1979 XJ6L (converstion). I drove the car around today out on the road after oil change and tune-up for the first time (non-opp. status) and it's not that bad BUT I can tell it's not quite 100%.
What about lifts or valve adjustment? I to am looking for something small and I would really like to Troubleshoot this thing and not just rebuild something that doesn't need it or rebuild it and never know what was wrong. I have the time and I am learning something!
Thanks for the help so far!
post #10 of (permalink) Old 07-03-2002, 12:06 AM
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Re: chevy 350 compression

Wait till you get a set of wires on, that could be the problem. Too bad there's no easy way to tell if they are shot without a good scope. Ohmmeter readings often are very misleading. I've seen them read fine with an Ohmmeter, but not be able to carry the current to the plug.

If you really want to check the valve train, take off the rocker covers, one at a time so you don't make too big a mess.
Run the engine and check to make sure all the lifters are moving the same amount. Put your finger loosely on each one at a time to tell, or you can use a scale to measure them. You don't need to be really accurate, since if one's worn enough to make a difference you can feel, it'll be really obvious.

If they are all the same you know the cam's not gone flat.
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