Compression Test questions

[Jennys280Z]

The noise is intermittent in the video too. FWIW, I was revving the motor a lot higher than that off-video and I didn't hear any weird noises most of the time.

I'm worried now because I did start hearing noises whooshing noises under the hood right after using Sea Foam in the gas. :paranoid:

I'll take the cam cover off again and investigate if there's no other easier ideas that might save me from doing so. I'd like to start with #2 if it got the lowest compression and then I put a couple good squirts of oil into the cylinder about an hour before starting it.

Edited May 4, 2013 by Jennys280Z

[Zed Head]

Yes, it would be from an exhaust valve not opening completely. Pressure pops back through the intake valve in to the manifold. It might not be that at all, hard to tell from the video. You'd probably hear more noise all the time, so maybe not. On a carbureted V8 you'll actually see fumes coming out of the carburetor when it happens.

The other possibility is knocking (detonation) from timing too advanced or from low octane gas. Again, just possibilities that sound somewhat like the noise on the video. I don't know what SeaFoam does to the octane rating of gasoline. Timing too advanced could be from a stuck advance plate in the distributor. That might be more sporadic.

These are all just guesses. You might just check timing first, rev the engine while checking and make sure it doesn't stay advanced when the throttle is back at idle.

[EuroDat]

The 10psi increase is normal with oil. I had a car with a broken top ring which increased 30 psi with a couple of squirts of oil.

I cant watch the video where I am now, so Im just going by your description. If the noise started after you started using SeaFoam it might be causing your problem or increasing it to the point that its noticable.

I like Zed's reasoning about the timing. Its also easy to check. You can map the advance curve by increasing the revs every 500rpm and checking the advance (without the vacuum connected). Check it with vacuum connected to make sure its working as well. I cant remember if the FSM has a curve?

The cover is to protect the electronics in the AFM, but Im not sure if the shaft is sealed or the cover does the sealing? If the cover does seal it and the shaft is worn it could make a differance, but it would be marginal if any. This is an MSA rebuild so it should not be worn.

Goodluck

Chas

[Jennys280Z]

Okay, I bet if I took a poll, the consensus on this noise would probably be "Oh you just adjusted valves? It's the valves." I would think that too if I was reading someone else's post. I'm really confident in my work though and I'd/I'll be shocked if one of the valves came loose.

After meditating under the waterfall all morning my gut tells me that this is detonation. I will elaborate...

Maybe a few of you guys remember back in the day when I was starting to deal with the car, I removed those hot 5-heat-rating plugs out for some 7s and advanced the timing from 10 to 12 degrees thinking that if I was running lean, these wouldn't be bad ideas in the interim. The first time out driving I came back and said I heard "crackling" under the hood. The noise in the video above is reminiscent of that sound. After adding Sea Foam, I reported that I was hearing whooshing or crackling under the hood. I highly suspect this is the same noise. My first impulse was to think this was carbon deposits burning off and causing its own detonation.

If I had that big a result going from 10 to 12, my instincts are telling me to retard the timing from 10 to 8 as long as this Sea Foam is in the tank. When I burn through this gas and fill up again on pure Chevron 93 I can return to 10 perhaps. FSM recommends 8 anyway if I remember right. I suspect my car is sensitive to octane right now and has a tendency to curve advance, for whatever reason. The theory that Sea Foam lowers octane is something I would have never thought of....very interesting! I remember another day on my adventure when I hadn't run the car in months and started it up for the first time and heard popping. I drained the old gas, put new 93 in the tank and the sounds were gone. It's moments like these that made diagnosing my car so difficult. It was like a different car every time I got in and started it up. A box of chocolates...I never knew which one I was gonna get lolz

FWIW before driving the car last night and hearing this noise, I poured the rest of the can of Sea Foam into the half-full tank. At the same time I also adjusted the AFM one tooth richer before driving, and it seems intuitive that having a bit more gasoline in the mix would make a tendency to detonate even more likely if the gasoline is the culprit.

EuroDat, I don't see anything in the FSM. I don't want to map the curve right now while I'm not certain what's wrong with my car. If someone else has a great-running 280Z it'd be great to get some benchmarks and I'd be happy to trace my results to their curves and see how close it looks. I've been suspect of this distributor from the very beginning. The vacuum advance seems to be non-existent (clogged) for one thing. There's got to be good/rebuilt distributors for these cars floating around out there somewhere. Why can't I find any online? I know Autozone does a swap but no you'll have to pry my distributor from my cold dead hands. I don't want just anyone rebuilding/restoring it so they mess up the curve and I install it and my car runs like poopie, and my distributor is gone forever because I gave it up as a core....ummm not gonna happen

Thanks you guys are the best! :)

[Jennys280Z]

For the record, the formerly white plugs in #2 and #6 turned beige from one good drive after the valve adjustment!

I retarded the timing to what should be ~8 deg-btdc. I hope this stops the detonation and gets vacuum to a steady 18mm. I'm going to do a forum search now to find what timings people are typically running on their cars!

[siteunseen]

Jenny's enthusiasm and Leon's knowledge got me to check the valves on my 240 I bought a few months ago. I use the "Fits or Don't Fit" method and all 12 were "Don't Fit" so I loosened them up and WOW, what a difference. Cranks on the first bump of the key, previously it took 3 and I just thought that was the way the choke works. Brought the idle down and it now has a slight lope. It's pulling between 17 and 18 off the brake booster vacuum bung. I'm very pleased to say the least. Thanks!

[Jennys280Z]

Nice to hear that about your pretty car siteunseen! Now that you've shaved your crow's foot you've created a much better tool than mine. I ran into a problem with #5's exhaust valve where the position the wrench needed was so far to the side the wrench head was making contact with the head bolt. That was the only one I had to break loose with a regular 17mm. So I want to shave mine now too. I will still use it for this job but hopefully not to break anything loose anymore but to torque it to spec.

Hmm...I wonder how fast these valves tighten over time on average? I am also wondering about proper tuning. Is it better to improve cylinder #2's compression via fine-tuning its lash, or is it the better convention to keep the valves as close to specs as possible no matter what?

Edited May 6, 2013 by Jennys280Z

[siteunseen]

They're like my blue jeans I guess, they get tighter over time, so I'll have to keep them loosened up no matter what.

[LeonV]

After break-in, the valves take a while to get out of spec. Nissan's recommended interval (from 240Z FSM) is 10k miles for the first 50k, and every 25-30k miles thereafter.

Keep lash the same, otherwise your cylinders will be ingesting different amounts of air/fuel mixture and will be unbalanced. Slightly lower compression has a much lesser effect on power production than choking off a cylinder.

If you truly want to know why a cylinder is down on compression, perform a leakdown test and try to listen for pressure escaping from the intake/exhaust/block (breather tube).

[Jennys280Z]

After break-in, the valves take a while to get out of spec. Nissan's recommended interval (from 240Z FSM) is 10k miles for the first 50k, and every 25-30k miles thereafter.

Keep lash the same, otherwise your cylinders will be ingesting different amounts of air/fuel mixture and will be unbalanced. Slightly lower compression has a much lesser effect on power production than choking off a cylinder.

If you truly want to know why a cylinder is down on compression, perform a leakdown test and try to listen for pressure escaping from the intake/exhaust/block (breather tube).

Thanks so much Leon!

[LeonV]

Thanks so much Leon!

Sure thing.

[EuroDat]

My compression tester has a quick conector that fits my ompressor hose. Ideal for a leakdown test..