Compression Test Numbers

[Conehead396]

Did a compression check today, motor has been surging badly at idle until warmed up. Already checked vacuum leaks, timing, etc. Motor is a 350 TBI 1994 C1500.

Results

#1 170 #2 180
#3 100 #4 182
#5 92 #6 175
#7 180 #8 182

Added small amount of oil to #3 and #5 and the recheck showed about 120 psi. No oil in coolant and not loosing coolant. Ideas?

 

[PlayingWithTBI]

Does it run hot? I'd guess blown head gasket but, that may not be the reason your engine surges. Although it can be a contributing factor.

 

[Conehead396]

No, temp is good. Holds 190 to 200 with 195 stat. I am thinking head gasket also, but no oil in coolent or coolent loss.

 

[Hipster]

With it being 3/5 I would suspect a head gasket. You don't always have water consumption but the flame travel between cylinders can cut a groove in the deck or head.

 

[thinger2]

Except 3 and 5, those numbers are really good. It either has really low miles on it or I suspect its been rebuilt.
Loss of compression between two adjacent cylinders with no noticeable coolant loss is exactly the situation which will burn a groove into the block and head and trash them both.
The gasket is still holding around the water jackets but has failed at the narrowest part between cylinders.
Its possible that 3 and 5 both have stuck or bad valves but I kinda doubt it A leakdown test might help.
I suspect its been rebuilt and the gasket failed or wasnt torqued right or they didnt chase the crud out of the bolt holes or a bunch of other reasons.
Good thing is they are the easiest head gasket to replace

 

[Pinger]

With it being 3/5 I would suspect a head gasket. You don't always have water consumption but the flame travel between cylinders can cut a groove in the deck or head.

If the firing order is 1-8-4-3-6-5-7-2 then cylinders 3 and 5 are only 180 apart. It should be possible then to arrange for the valves on both cylinders to be closed (one one at mid compression stroke, other at mid combustion stroke). Then, blow through one spark plug hole and listen at the other?
(Better still - blow smoke).

 

[Erik the Awful]

The fact that your compression went up when you added oil leads me to believe it's not a head gasket, but rather rings. It's feasible that if you added enough oil it could be squishing into a gap in the head gasket, but that's wishful thinking.

 

[Conehead396]

Thanks for all the responses. Not rebuilt as I bought it new in 94. Guess I will pull the head this winter. Just painted it and plan on passing it down to my son. If block is grooved, it's time for a short block I guess.

 

[Schurkey]

I burned gouges into both the block and the heads on my K1500 5.7L. Failed head gasket between #3 and #5.

Engine knocked like mad. Sounded like a rod knock. In fact, it seems to have been the hot combustion gasses from #3 igniting the mixture in #5 prematurely.

If the block is grooved...you'd cut the block anyway to get the pistons to zero deck. Might not be a problem. Mine is pretty bad, it may or may not clean-up with decking. If the heads are grooved, they're scrap. Swirl-port heads aren't worth putting money into when Summit sells aluminum replacements for eleven hundred dollars, less with promo or sale pricing. You might also need hardened pushrods, if you use the guideplates instead of self-aligning rockers.
https://www.summitracing.com/parts/sum-162108

Do a leakdown test before you tear anything apart. For all you know, you've got two weak or broken valve springs.

Would be worthwhile to verify the compression tester gauge is accurate. 180-ish seems really high.

 

[Erik the Awful]

Also, phone-based borescopes are dirt cheap now. I think I paid $30 for mine.