98 Tahoe 5.7 - Compression Test


jsetche
08-28-2009, 09:31 PM
I want to conduct a compression test on my truck. Does anyone know what the cylinder compression should be, or the range, for a '98 350? I can't find the spec anywhere. Thanks in advance.

RL2000
08-29-2009, 09:45 AM
While I can't tell you what the specs are, I'll tell you how I usually do it... I just test each cylinder, writing down each one, and if there's a large variance between the cylinders, it's an indication that there's a problem. For example, if all the cylinders test out at 150psi, and one is at 70psi, I would suspect trouble. Be sure to double-check the low cylinder before deeming it bad.

It's normal to have a little difference between cylinders, but I think anything more than 25 or 30psi off from the average is a reason to be concerned.

Of course, I'm just a backyard mechanic... so you might want to get a 2nd opinion... :)

rovertec1958
08-31-2009, 08:36 PM
The compression ratio is 9.4 to 1 compression pressure on a good engine
should be around 175 psi.

Scrapper
08-31-2009, 08:41 PM
i'm prety sure it 150 to 175 psi. but take plugs out and see if there burning white on the tips of plugs.

good luck......

Blue Bowtie
08-31-2009, 10:00 PM
The compression ratio is 9.4 to 1 compression pressure on a good engine should be around 175 psi.

Mt 10.8:1 engine cranked at about 145 PSI on a fresh rebuild. Cranking compression pressure depends heavily on the cam profile and valve timing. Some very high performance engines will have relatively low cranking compression pressure, but very significant dynamic compression pressure. Engines somewhere between high performance and street/stock will vary between those points. The more reasonable method is to compare the cranking compression pressures and suspect any low readings, as stated above.

The better method is to use a cylinder leakage tester and forget about compression testing altogether. Any more than 4-5% leakage on a new engine is a flag, and more than 35% leakage on a well worn engine can indicate a problem.

LastOne
09-07-2009, 05:54 PM
My 98 tests out at 180-185 on all 8 cyl, 150K on the motor.

MT-2500
09-07-2009, 06:33 PM
I want to conduct a compression test on my truck. Does anyone know what the cylinder compression should be, or the range, for a '98 350? I can't find the spec anywhere. Thanks in advance.

The main thing is that all cylinders be even.
Not over 10% difference in any cylinder.

Qver 15-20-25% difference can give a miss or rough idle.
And no compression or very low on one or more you will have a bad engine miss and bad running engine for sure.

Always do a compression test with spark and fuel disabled.
With all plugs out and TBI butterfly open some Just jump the starter for test.

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