Head gasket? Or something worse?

[kamukat]

Hey folks, I’m hoping the collective horsepower of this group can help me save my S38. Or tell me I’m SOL.

I was dropping the kid to school the other morning (Nor Cal so >50 degrees), engine braking from like 2-3k to slow down for an exit when the engine starts making a bad rattling noise. Like a clattering. Coming from inside the engine only at load at like 1.5-2k rpm.

Dropped the kid off, very gingerly drove home (<5min), car was not happy, parked her and had her towed to the shop.

Car recently had >$10k of work done to her (non engine related) and was running beautifully. I’ve owned her for 8 years, annual oil changes, premium gas etc. She’s been treated well, and I do daily drive it switching between my e39 and e28 for months at a time. Refinished wheels recently, with new tires and rotors and stainless steel brake lines, new clutch master and slave etc. She has not been neglected. 155k miles. Less than 500 miles since last oil change.

Shop dove into it, found a disconnected throttle position sensor and valves “way out of spec” (she just had a valve job not even 12 mons ago smh). Car is idling “perfectly” and making power, but still makes the rattling noise above 2k. They obv dont want to drive it.

They did a compression and leak down. All cylinders except 4 and 5 were 160+ on compression. 4/5 were sub 50. Yikes.

However, 4/5 are reading perfect on leakdown at TDC. Which is pretty strange right? They are boroscoping everything tomorrow. Shop thinks its the wrist pins making the noise (which my ignorance suggests wouldn’t impact compression?).

Naturally I called another shop I have become friendly with that works on 80s M Bimmers, they are suggesting given the proximity of the cylinders, this might be a head gasket failure at higher compression where the cylinders are fighting / cross firing causing the rattling under load. There was oil leakage from the head gasket.

Obviously an engine rebuild is a bit of an undertaking on these things, and before I go down that rabbit hole, I’m wondering if
(a) anyone has had this or something similar occur,
(b) any other obvious investigative ideas before they tear down to look at the head, or
(c) know which LS fits best under the hood (I kid, I kid).

Thanks so very much. Happy to answer any questions (as much as I can); apologies if I missed adding any pertinent info, my brain is a little frazzled at the potential cost of fixing.

Kamran

[turbodan]

Wrist pins are extremely unlikely. Rod bearings might do it, but wouldn't cause low compression. If leakdown is good, it is unlikely that this is a top end issue and I wonder if the compression test result was spurious. How do the spark plugs look? Any smoke from the exhaust?

Worst case, put a turbo M20 in it. Would be cheaper, would make more power and get better mileage. Way cooler than an LS swap.

[Preyupy]

There is the possibility that the head gasket is leaking between the cylinders. Under the relatively low pressure used in a Leak Down Test the gasket might just hold the pressure. But at above 50-60psi it might be leaking. You should be able to hear it. With all of the other spark plugs out put a plug in cylinder #1 and crank it over, listen carefully to cylinder #2. Then take the plug out of #1 and put it in Cylinder #4 and listen for a different sound coming from Cylinder #5. If you hear a difference you have a gasket failure between 4-5.

DO NOT DRIVE IT OR EVEN START IT AGAIN UNTIL YOU FIX IT. You will erode the head and make everything 3 times worse.

[austin8753]

There is the possibility that the head gasket is leaking between the cylinders. Under the relatively low pressure used in a Leak Down Test the gasket might just hold the pressure. But at above 50-60psi it might be leaking. You should be able to hear it. With all of the other spark plugs out put a plug in cylinder #1 and crank it over, listen carefully to cylinder #2. Then take the plug out of #1 and put it in Cylinder #4 and listen for a different sound coming from Cylinder #5. If you hear a difference you have a gasket failure between 4-5.

DO NOT DRIVE IT OR EVEN START IT AGAIN UNTIL YOU FIX IT. You will erode the head and make everything 3 times worse.

came to say exactly this. my suspicion is a compromised head gasket between 4-5. it would be unlikely to have piston damage on two cylinders right next to each other, but possible, i suppose.

if it just had a valve job, what was the condition of the timing chain, guides and tensioner? something else to consider, regarding the noise.

good luck, hope you can save it.

[kamukat]

Thanks so much guys. This is super helpful.

A little more info - the boroscope reveals that 4 and 5 cylinder walls are shiny clean where the others are “as expected” with soot and carbon buildup.

The shops “theory” is failed injectors pouring excess gas into those cylinders which created a hydrolock situation - although I don’t really see how that occurs - that would be a crap ton of gas. The injectors were recently rebuilt maybe it was very poorly done.

I recall perhaps that coolant leaking into the cylinders can cause shiny cylinder walls?

Which all seems to point to a failed head gasket type issue. Maybe.

Since I’m using the checkbook wrench this is all going to be painfully expensive.

Thanks for the support it’s very appreciated.

[Preyupy]

Are you losing coolant? It is possible that the gasket failed at the narrowest point between the 2 cylinders ( it’s only 6.5mm thick right there!!). If you had gotten into the water jacket you would be losing water, overheating and getting steam out of the tail pipe.

[turbodan]

Cylinder walls should be clean and shiny anywhere the rings are in contact. Cause for concern would be wear through the cross hatch or scoring.

Head gasket failure between the cylinders might not cause coolant ingress. If that is what happened you would likely only see compression loss between cylinders. This wouldn't breach any of the water passages.

[Randomg]

Had a head gasket fail during wot. Broke at the smallest spot between two cylinders. Sounded terrible, but I don't remember it being a rattle. Only drove it to get off the interstate, but it was only under load if I remember.

Failure during engine braking seems weird to me, but I guess without a TPS plugged in it would have still been injecting fuel. Maybe it was the last straw. Seems like you've gotta pull the head and hope it's just the gasket, but from my understanding it's rarely/if ever just the gasket. When it happened to me I had cracks in the head that didn't go through to coolant according to the shop, and they peened the cracks which is supposed to keep them from spreading. Got a few more years out of it before needing a new head.

[kamukat]

Thank you all so much for the help and support. This has been such an amazing community.

Made a call regretfully to sell the car to someone with more physical and emotional bandwidth to deal with it.

Not my last foray here. Thanks so much again.