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Discussion Starter · #1 ·
The engine in question has no intake at the moment and the rad is not connected to it. For obvious reasons i cant warm it up before testing.

I started from the rear of the engine and the first test result was 120 psi.

all other cylinders were between 160 and 172

I decided to go back to the rear cylinder that i tested first then i got a result of 60psi ????????????

i checked again and i got about the same..... i repeaded it several times then i was getting results about 70psi.... now if i test it i keep getting 90 psi..

What can be causing this? how can the PSI go from 120 to 60 to 90...

PS the gauge gives steady readings on all other cylinders.

Any advice would be good

Thanks.
 

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How long has the engine been sitting?

If it was for a long time that piston may have been at BDC and the rings have seized in the piston lans.

Or one of the valves was open and it is not sealing properly.

Put about one half ounce of clean motor oil in the cylinder and crank it over a few times then redo the compression test. If the pressure goes up it is rings.

Do a leak down test. You will hear the air escaping. Since the intake is off, and I assume the exhaust also, spray WD-40 onto the closed valves chambers and look for air bubbles around the valve seats.

Wayne Monteath
Masham, Quebec.
 

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Discussion Starter · #3 ·
It hasnt been sitting long. It was my daily driver about 3 months ago. And it was last driven about a month ago. It has been turned over a few times in the last month as well (didnt start cause the battery was almost flat)

Only the intake has been taken off at the moment, The exaust is still connected.

How do i go about doing a leak down test?

Would this explain why the results on that one cylinder keep changing?
 

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I had a very similar situation with my previous motor and when I tore it down for a rebuild the cylinder that could not keep a stable reading had a broken compression ring, that is not to say that yours is the same problem but it will cause low and eratic readings.

You need a special fixture to do a leak down test but there is a way you can do a test like the one mentioned above. Rotate your engine untill the suspect piston is at tdc with both valves closed, remove the valve from your pressure tester if it is a quick release type and screw the hose in as if you are going to do a compression test but instead of putting the gage on the hose connect an air hose to it regulated down to about 30psi then you can listen for air escaping around your valves or around your piston, anywhere the air is leaking out that you can hear you have a problem.

Good luck

Miles 260/240
 

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Discussion Starter · #5 ·
to be 100 PSI down on the other cylinders there would have to be a massive leak somewhere wouldnt there.

Ill go out and put some oil in the cylinder and see what results i get.
 

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Discussion Starter · #6 ·
ok i put some oil in and now i am getting 110 in that cylinder. Stupid me didnt think to do another test right before i put the oil in . Now i dont know if it is just because it is now a fair biw warmer outside.


DOH
 

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Here's another thing to try. Get a can of top engine cleaner and put a couple good squirts into that cylinder. Then wait overnight for it to do it's work. This will soften any deposits sticking the rings in their grooves. It's not unusual for a ring to get partially jammed in the groove and that can give erratic pressure results. The next day try your pressure test again...
SoM
 

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Re: good idea

Just go to the parts store and ask for 'top engine cleaner'. It's normally sprayed in through the carbs while the engine is running to clean all the gunk out of the chambers and manifolds (but it's not carb cleaner), but for ring problems it's better to put it in through the spark plug hole and let it sit for a while. If the parts store doesn't know what it is than go to a GM parts store, they have a good product called something like 'top engine cleaner'...
SoM
 

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Re: NOTE

Hi Duff5000.
Regardless of it ' getting a little higher ' your engine is in need of a bottom-end rebuild. New crank bushings. New rod bushings. New rings. A functioning and cleaned oil punp at the sump. Do the work yourself and you'll save at least a thousand dollars or more. Goodluck. shane-
 

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Sorry...

Hi Duff5000.
Sorry but compression at 90lbs indicates a rebuild to me. If your engines compression is staying at 120lbs then it is still a weak engine and may run for 50thousand more miles but will probably die the death of a broken rod or a broken crank. Unless I misread your original post that indicated 70lbs and 90lbs as a steady state condition then the belief that it was rebuild 30thousand miles ago was presented to you falsely. Someone lied. The only way a rebuilt engine with 150 and above compression drops to 70lbs and 90lbs is to have been seriously abused. Again, my 02 is for you to attempt the bottom-end rebuild. Goodluck. shane-
 

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A leak down test is...

where you put the cylinder in question at TDC with both valves closed.

Then using an adapter with a pressure gauge connected to an air line, connected to a fitting that will screw into the sparkplug hole in the head, put at least 125 PSI of air into the cylinder / combustion chamber, and search for places where the air is going.

If the rings are bad / burned piston you will hear air escaping out the bottom end.

If the valves are bad (or poorly adjusted) you will hear air coming out of the tail pipe (exhaust valve) or the intake manifold (intake valve).

Now having said that, I bought a 79 280 ZX, from a friend. It sat in a field, under a tree, for 18 months. Nothing was done to the car, it was just dumped!

I checked the compression before I tried to start it, and got results similar to yours, # 5 was about 70 PSI, and the rest where about 145 PSI.

I sprayed about 1/2 ounce of WD-40 in each cylinder and let it sit for 24 Hrs. Then I put 1/2 ounce of motor oil in each cylinder and let it sit for another 24 Hrs.

I did another compression test afterwards and all cylinders went to 160 PSI, except #5, which went to 110 PSI.

Next I put 3 gallons of fresh fuel in the tank, new NGK spark plugs and turned the ignition key to start.

Z'ed went vrooom!

I let it get to normal operating temperature while I checked a lot of other things, as per the FSM, then I shut it off.

Next I did another compression test and all cylinders where 180 PSI, except #5, which was now 175 PSI, that car ran perfectly two weeks later and all cylinders where at 185 PSI.

Wayne Monteath
 

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Discussion Starter · #15 ·
Re: Sorry...

yeah it sounds bad...... it is in an auto so it hasnt been thrashed.

The thing is after turning it over many times i am now getting a psi of about 140...... it is like every time it turns over a bit the psi is going up....

Would this be possible if like a bug or other small thing fall into the intake port and is blocking the valves? maybe it is being squished out or something.... i just cant see why the results are getting better and better.

this could make sense, 120 PSI the first time i tested it. something could have got stuck on the first rotation. Then when i retested it next time round it was at 60 cause there was somethign stuck in there. and now it is getting better and it pushes the thing out.

seems far fetched but it could explain it.
 

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Discussion Starter · #16 ·
Re: A leak down test is...

i have a feeling if i could start it and let it run for a bit the problem would go away.

the PSI is going up and up (well slowly) every time i crank it for a bit.

it is now at 140
 

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Just a note...

You keep mentioning rotations, so this is confusing a bit. I'll add this just in case, it's worth mentioning. You should crank the engine through several rotations during each test to get the final reading on compression. Not just one rotation. Spec is to operate engine at 300 rpm with all spark plug holes open and warm.
 

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Re: Just a note...

And don't forget to short the ignition coil output to the chassis. All that juice with no place to go can damage the coil...
 

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Discussion Starter · #19 ·
Re: Just a note...

I am letting it turn over 4 or 5 times for each test and i cant have the engine at the corect temp as there is no intake or carbs on the car.
 
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