Could this HG rust expplain low compression leaking to crankcase?

Yo. So I pulled my L-block engine from my wrecked 744 to put it in my 89 gle 745 (POS timing belt broke in 240 miles).
Anyway, I did a compression test, found low compression on one cylinder, and have it on a stand with the head off. The compression readings were 120,120, 60, 120 from water pump to firewall. I added oil to the one with a low number, and it didn't help at all. Added more, still no help.

I did a leakdown test on it, and found that the bad cylinder was leaking down into the crankcase. I checked all the other cylinders and got no leakage and couldn't keep the engine from turning. My tester was jury-rigged out of fittings and a compression tester, so I don't have a %age.

The head gasket looks pretty beat up to me, and it seems to have rusted quite a bit. Pistons and rings look great from the tip.. maybe 0.010 inches play. I haven't gotten plastiguages yet, but don't think the rings are bad. There's a ridge of rusty looking deposits way up high, but the piston bores all show crosshatching and are polished with no apparent mars. They're so shiny that the crud on the piston crown reflects on the sleeves.

I think the headgasket is to blame. There is substantial rusting of the actual gasket around the coolant passages (using zerex g-05 for years, but the engine came with 147k on it). There is some of the gasket also eaten away very close to both the oil drains and the metal sealing surface where the sleeves meet the gasket.

Here are some pics (may have to edit later to add them; i'm on my phone.)

Headgasket installed on block


And separated



First rust I noticed




Here's one that looks good...




And less good...


OMG wut r u doing headgasket?




STahhhhp (this is the cylinder with compression down to 60psi)

That's the worst I've ever seen.

will740turbo said:

... The head gasket looks pretty beat up to me, and it seems to have rusted quite a bit.

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Um, yeah... headgasket is likely factory and toast.

will740turbo said:

Pistons and rings look great from the tip.. maybe 0.010 inches play. I haven't gotten plastiguages yet, but don't think the rings are bad. There's a ridge of rusty looking deposits way up high, but the piston bores all show crosshatching and are polished with no apparent mars. They're so shiny that the crud on the piston crown reflects on the sleeves...

Click to expand...

There is no way for you to check the rings without pulling the pistons (other than a leakdown test - with a gauge).

Plastigauge is not plural, FYI and is used to check bearings not rings. Your pistons are all shiny and stuff from the water you've been dousing the cylinders with.

will740turbo said:

... I think the headgasket is to blame ...

Click to expand...

So, replace it

I also suggest dumping / rinsing the radiator now and then flushing the cooling system when you get the car back together. May also be time for a waterpump and thermostat.

Just sayin'

swedefiend said:

Um, yeah... headgasket is likely factory and toast.



There is no way for you to check the rings without pulling the pistons (other than a leakdown test - with a gauge).

Plastigauge is not plural, FYI and is used to check bearings not rings. Your pistons are all shiny and stuff from the water you've been dousing the cylinders with.



So, replace it

I also suggest dumping / rinsing the radiator now and then flushing the cooling system when you get the car back together. May also be time for a waterpump and thermostat.

Just sayin'

Click to expand...

Lol yes... just wanting to know if it was extremely obvious and likely to be the cause. If it was borderline, I would look harder at the lower end. I do want to check the bearings... but would have to teach myself how.

I was not losing any coolant... maybe 8oz in 3 months. But there was some sludge in the bottom of the cooling system that surprised me. The piston tops are filthy, though the cylinders are shiny and still show crosshatching. Wouldn't coolant clean up the piston tops too?

The radiator got damaged in the accident, so itks getting tossed, and new wp and timing belt are going on too.

I thought there was a tool to check the gap between the piston and cylinder wall like a set of flexible feeler gauges made of clear plastic so they don't scratch anything.

lummert said:

That's the worst I've ever seen.

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Music to my ears!

What color is the inside of your coolant bottle? I guess brown.

ZVOLV said:

What color is the inside of your coolant bottle? I guess brown.

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I think whitish... with a little bit of rust. I could post a photo later, I guess.

Why was it rusty? Coolant leak and water was added...

Fix all coolant leaks.

Idk, it had 147k on it when I bought it. Who knows what the POs did to it.

Super duper shady shade tree test for rings when the head is off:
- Rotate the engine so the pistons are all fairly even (in between TDC and BDC, all 4 will be even, doesn't matter exactly, just roughly close).
- Pour a few inches of oil on top of all 4 pistons
- then let it sit, and compare the relative rates at which they drain down into the oil pan past the rings. Unless it's totally busted up, it should take quite a few hours
- if they're all pretty even, no problem exists in any particular cylinder, but if one or 2 of them leak the oil significantly faster, you might as well just go ahead and pull the motor and take it apart and see what's going on.

For sure change the oil cooler hoses. Very big weak point for soggy hoses. While the head is off I would do the heater hoses, a hepu water pump, and the rad hoses and thermostat too. Carefully check radiator end tanks for signs of leaky trails.

There was a leak some where at some time and somebody was adding water instead of coolant.

Post a pic of the head surface. I bet there is heavy corrosion there too that may need to be welded back up, or the head replaced.