Compression Test questions

[Jennys280Z]

Hi everyone! It's been awhile. I'm surprised I remembered my password! A few years ago, I was plagued with a cold idle issue, low manifold vacuum, a lean-mixture issue, all kinds of fun stuff. I learned so much about the EFI, tested my AFM as an issue, replaced it with a new one from Motorsport, and adjusted it to its ideal setting. Though I'm still surprised how big a difference a few teeth make going either direction. This sensitivity might be symptomatic of other deeper issues that I've discovered recently.

So I had my car in the shop last week for a brake job. Got new pads, shoes, shims, springs all the way around and when I was getting this done I also asked for a compression test. The results were not good I'm afraid.

Cyl 1: 100lb

Cyl 2: 105lb

Cyl3: 95lb

Cyl4: 100lb

Cyl5: 90lb

Cyl6: 105lb

A little history behind the car: The motor has 140k original miles. INo head work, never been rebuilt. I bought the car from a schoolteacher in 1990 when it had around 85k on it. I put the 55k on myself. The car has always been maintained, never raced or abused. I've always changed the oil every 2-3k miles.

Faulting the air regulator for the cold idle issue, the car gives about 1k RPM on startup but quickly bogs down to 600 and slowly rises as the engine temp rises. When it's fully warmed up I get a healthy idle as well as a healthy performance though I admit I haven't run it at high RPMs to know for sure. This could be a feeling relative to how bad it is when cold.

What I'm wondering now is if the issues I'm having with a cold engine are compression related. When the engine warms up, the piston rings would seal up in the cylinders and presumably give an extra 10+psi or so and I presume this make a noticable difference with the way the car runs, however I also read that motors with compression this low would be hard to start, if they even run at all. Is it even possible to have a motor running this good with compression this bad? My car starts up quickly every time.

I'm not completely confident that the low numbers aren't the fault of the mechanic doing the test so that lack of trust has made me want to test the compression again myself. Before I do, I will need a few extra tools. I already have a compression tester I bought from Actron. I am going to get an Actron remote starter and I hope that would be appropriate to the task.

For posterity, here is an excellent guide to doing a compression test on AtlanticZ:

http://atlanticz.ca/zclub/techtips/compressiontest/index.html

But as newbie proof as that might be, I still have a few questions I hope you guys can help me out with.

1. With the plugs removed, do I just leave the spark plug wires dangling? I'd like to leave them plugged into the distributor but if the coil steps up the voltage to whatever the spark plugs need and the plugs aren't there, is there any danger in this? Can this hurt the coil or the distributor? Or me?

2. What is the electric fuel pump going to be doing during this operation? Since the remote starter is going to be cranking the engine, won't there be fuel spraying into the cylinders? I'm guessing if I need about 30-40 seconds of cranking to finish the test in all cylinders, won't the cylinders be flooded with gas?

3. The guide says to hold the throttle linkage open. Does this mean to have someone else hold the linkage to full throttle? Is this necessary or does it just aid with the reliability of the test? Does this keep gasoline from injecting into the cylinders the same way a carbureted car would work?

I read another post on here where someone was getting really low compression numbers and when they corrected their stretched timing chain they got 40-50 lbs back.

I wish I could say the numbers are tight if low but with cylinder 5, I can't even say that. I'm not sure why all the numbers are divisible by 5. I guess that's as closely as the guy wanted to look at the gauge when he was doing it. 90/105 is still 85.7%, so likely not a "problem" that AtlanticZ referred to.

Let's say those low numbers are accurate. How is that possible? The cylinders/rings all wore down that much? That just doesn't seem plausible knowing the history of the motor.

I've done everything I can to tune the car up at this point (new plugs, wires, cap, rotor, filters, timing, AFM, connectors cleaned). The only part of the tuneup I haven't done yet is the valve adjustment. I need a few more tools before I do that (a torque wrench and maybe a brass hammer to tap off the cam cover if it's too sticky). My questions above would enlighten me to doing the valve adjustment myself as I presume I can use the remote starter to turn the camshaft. I'm unnerved about this too because I don't know what steps to take to make sure the engine doesn't start when I'm playing with the valvetrain

I'm sorry again for being so wordy here. I know I sometimes write disjointedly and my racing thoughts wind up splattered all over the place. Maybe thinking out loud has its merits when things need to get technical hehe. Really, I only need enlightenment on the three questions I numbered above. I hope a few of you can handhold me through my questions even if they're not that good.

Per AtlanticZ, to get my results as high as possible, I intend to:

Test on a warmed up engine

Remove all spark plugs

Keep my connection fittings tight

Keep throttle open

Test cylinder 5 twice if the results are relatively poor for me too, the 2nd time with a squirt of oil in the cylinder

Thank you all in advance! FastWoman and Zed Head and others gave me more help than I was worth about a year or two ago.

[FastWoman]

More help than you were worth?! Nah, don't say that...

Here's what you do to crank your engine without injecting fuel, creating spark, or doing anything else except turning the engine: Identify the smallest wire clipped to your starter, and pull it off. One wire of the remote starter will clip there, and the other end will clip to the positive post of your starter. Push the button, and the engine will crank. You don't have to worry about the engine starting or anything.

Specific questions:

1. Leave the plug wires wherever you like. There will be no sparks.

2. The fuel pump won't be pumping. No worries.

3. You might put a brick on the pedal. This is done to allow air to enter the cylinders freely, so that there will be compression. If your mechanic didn't do this, that might explain the low numbers.

Valve adjusting is easier than it sounds. You don't need a torque wrench. Just tighten the jam nuts roughly as tightly as they are when you loosen them. No brass hammer is needed to loosen the valve cover. Just unscrew the screws, and off it should come. Be careful to separate the gasket carefully as you're lifting the cover off. You don't want to tear it. (But if you do, your auto parts store can sell you a replacement.) Oh, and don't use the remote starter to rotate the engine (an exercise in frustration). Instead, use a big wrench on the crank pulley.

Low numbers, if they're that low, would likely be from general engine wear, especially the rings. I'm skeptical of your numbers, though. FAIW, we had a boat with a fairly well worn engine with approximately those compression numbers. The engine ran fine, although I'm sure it ran better when new.

Good luck!

[EuroDat]

I would like to add a couple of pointers to Fastwomans tips.

I generally dont pay too much attention to the pressure Im reading, but more the difference between the readings. You dont know how accurate the compression tester is. My tester was way off when I check it at my work on our calibration bench. It was reading 135psig and the test unit read 150psig.

If the vavle adjustment doesnt help much you could try a couple of squirts of engine oil down the plug hole. If rings are worn that will increase the compression. Dont use too much or it will cover the valves as well. This is just an indication. To really pin point the leakage you will need to do a leak down test.

Goodluck.

Chas

Edit: Have you downloaded the FSM XenonS30

Edited April 13, 2013 by EuroDat

[siteunseen]

I use the remote starter on hot valve adjustment, I leave the plugs in though because it does spin too fast without them. I guess I'm the only dumb butt on here that's done it but the first few times I pointed the lobes straight up without considering the engine is leaning to the passengers side. I finally made center marks on the cam towers with a Sharpie to go by. When I did my compression test, spark plugs out now, for the first time I didn't have the throttle plate open then I read it should be so I used a zippy-tie or 2 where the linkage starts on the firewall to hold it open, I guess a brick would've been too easy, it raised my numbers and I'm sorry I don't remember which but it was 10 or 20 lbs on each, probably 10, but like Chas says it's the difference between them that's important, gauges lose their accuracy after you drop them a few times :stupid:. Buy a vacuum gauge while your buying, they are a lot of fun. The remote starter goes straight to the battery and starter, it won't turn on anything but the starter.

Oh yea, you'll definately need a 17mm crows foot socket for the valve adjustment. Autozone has a metric set cheap and they also sale the remote starter cheap. I ordered my vacuum gauge from walmart.com, it will tell a lot about your motors worn-outness.

Edited April 13, 2013 by siteunseen

[LeonV]

Some good advice has been presented already but to add a few points:

- WOT during cranking should enable "flood clear" mode, not allowing the injectors to open, thus no fuel being sucked into the cylinders

- I've never used a crows foot wrench for valve adjustment. Plain open-end wrenches work just fine.

[Jennys280Z]

Hi FastWoman! I've got enough info. to proceed. Thank you! Just a matter of waiting for my starter now.

siteunseen, I already have a vacuum gauge and have run tests, primarily from the intake manifold to brake booster connection. The only time I saw 17-18mm Hg was when I jumped the TPS wires to go full rich. The car belched grey smoke and stank of gasoline but the idle vacuum looked great for the first time ever Eurodat, I'll consider the leak down test as well, thank you. I did FastWoman's infamous yogurt-cup test and couldn't detect any vacuum leaks. I saw a cool trick a guy on Youtube used to test for leaks with cigar smoke. Anyway, replacing vacuum lines was one of the very first things I did along with cleaning most of the electrical connectors. Before the yogurt test I considered the only possible place for a vacuum leak would be the bottom of the intake manifold gasket but now I have further evidence with the compression test to suggest that the low vacuum likely coincides with the low compression.

The problems that have plagued me from the beginning are poor cold idle, low manifold vacuum, and now the compression readings. Timing, mixture, fuel pressure and ignition are all healthy and accurate.

I do want to get a look at the camshaft and valves. Looking at repair records, the valves were last adjusted (allegedly) around 18,000 miles ago and 15 years ago. The car barely has any mileage on it since then as it was stored in a garage for over 10 of the past 15 years. If I see anything unusual about the valve train I'll post pictures of possible problems or if anything might look burned. Before I start making adjustments I might just take the cam cover off, turn the motor with the starter and take measurements and then post my cold results. If one valve is majorly out of wack I'll probably adjust it when cold to match the other valves. I'm sure I'll have more questions upon my visual inspection and will likely do a cold adjustment myself thanks to this gem of a website. I won't do a hot adjustment though....maybe on my 3rd adjustment....maybe my 2nd

The brass hammer was mostly intended for the valve adjustment screws so I can deal with things with finesse if I find them sticky

When I have the cam cover off for the compression test, I am also considering torquing the head bolts to 61 lb-ft per Tom Monroe's "How to Rebuild Your Nissan/Datsun OHC Engine". I have a 3/8" Torque Wrench good for 80 lb-ft. which should suffice. Monroe also mentions a trick to hold a dollar bill over your exhaust pipe and if the exhaust sucks the bill in at times, there's likely a burned exhaust valve.

LeonV, wasn't sure that flood clear mode applied to the fuel-injected cars.

Hope all are having a good weekend. This website rocks btw!

[mjr45]

Jenny I feel your pain, I have the same issues of poor cold idle and low vacuum, been messing with this for 6 months and still no resolution. Keep working at it, it will all come together.

[FastWoman]

Low vacuum can be caused by incorrect fuel/air ratio, the wrong ignition timing (usually too retarded), and to a small extent from bad valve timing (from a worn timing chain) or adjustment. The valve issues would be evident in the compression numbers. Most people think vacuum leaks lead to low vacuum, but that's only true to the extent that they affect fuel/air mixture. If your engine is running rich (which these Z's never seem to do on their own), then introducing a vacuum leak will actually strengthen the vacuum. Vacuum is simply a reflection of how efficiently your engine is running.

Anyway, the most common culprits seem to be mixture and timing. Of course make certain all of your cylinders are firing the same. Read the plugs. If you have any ignition failures or funky problems in just a couple of cylinders, that would drag down your vacuum numbers.

[siteunseen]

She might be experiencing coolant temp drift in her ECU? That's the Big Bang Theory for me.

[FastWoman]

Might be!

[siteunseen]

http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html

[Jennys280Z]

Hi guys, well I completed a compression test and here are the results:

Spark plugs NGK-BPR6ES after 20-25 miles of driving

Plug #1: Gassy smelling, wet

Plug #2: White and dry ground electrode

Plug #3: Looks brand new

Plug #4: unremarkable, like a barely used plug should look

Plug #5: The white insulator is mostly grey already except near the tip; some black residue along the rim.

Plug #6: Looks exactly like plug #2

Compression (psi)

Cyl1 : 136

Cyl2 : 127

Cyl3 : 147

Cyl4 : 138

Cyl5 : 136

Cyl6 : 127

The motor was cold for this test. I tested 2 and 6 twice to make sure the results were consistent and accurate.

It's interesting to me how the spark plugs from 2 and 6 looked identical and they had identical but low compression numbers. Also interesting how 3 has the highest compression and the spark plug looked brand new (tip still shiny silver, electrode pure white).

I went to the store and bought a 17 mm crows foot and a few cans of Sea Foam. Poured about 2/3 of a can in the tank with about 12 gallons of premium gasoline. The car ran good tonight. It gets up into the power band and pulls nicely. I would say it ran perfectly except I noticed when I am in gear (2nd or 3rd) and let off the gas and just coast, I hear the occasional backfire, which I think is actually a frontfire unless my ears are playing tricks on me. They were gentle tapping sounds so probably no big deal but still annoying. Well the frontfiring is a big deal...I'd rather err on the side of backfiring if I had to choose one

Valve adjustment is next, hopefully in the next 4 weeks. I'm just going to measure all the lashes first and post the results. I'm going to do this cold as well. No major surgery, just a fact-finding mission. I expect I'll adjust a few of the valves at least but not all of them. I'll take pictures of the valve train and post them if I think anything looks weird. Got a new cam cover gasket too just in case. After the valve-adjust I'm considering feeding Sea Foam via vacuum through the intake manifold to give the valves a good cleaning.