High compression results

[aoeu]

I did a compression test on my 1985 3AC engine and the results don't sound right. Each cylinder was around 200psi.

Results (including pump up)
cylinder 1: 87, 115, 145, 165, 175, ..., 205 (13 readings for max)
cylinder 2: 90, 125, 148, 165, 172, ..., 205 (14 readings for max)
cylinder 3: 92, 130, 150, 165, 175, ..., 200 (12 readings for max)
cylinder 4: 92, 131, 152, 166, 178, ..., 202 (17 readings for max)

The manual says each cylinder should be 178 psi, minimum 128 psi, and all within 14 psi of each other. The max compression was oscillating around 200 (198-205) for each cylinder. This car has 150,000 miles on it so I doubt it has that high of compression. What do you think? Bad gauge or something else?

[aoeu]

I am running the cheap NGK plugs, not titanium or anything like that. Also regular gas, not high octane.

[Jarf]

Assuming the head has not been shaved or other mods performed.
I would suspect major carbon build-up in the cylinders.
Run a can of Seafoam through it, flush out as much as you c an and recheck.
Carbon build-up on that scale is not a good thing.
Just as plaque in the bloodstream can break off and cause catastrophic damage in the body, same can happen in the engine.
Usually resulting in scored cylinder walls.
Some people prefer to do a steam clean with water misted into the carb but that method can have disastrous side effects too.
Care must be taken. Seafoam is nearly idiot proof.

Once all the symptoms are dealt with, make sure you what caused of the build-up in the first place.
Possibilities are endless, from infrequent oil changes to blown valve seals but if not corrected, it will return with a vengeance.

[Jarf]

I just re-read that, what do you mean by "including pump up"?

A proper compression test is done with the engine warm, throttle WIDE open and ALL the plugs out, crank 6-8 revolutions. That's it, that's all.
Most importantly, is to crank the same number of times for each cylinder.
You do not keep cranking until the gauge stops climbing or you will get inaccurate readings.
Easiest way to crank the engine over is to make up a jumper wire with a female 1/4" spade connector on one end and bare the other, disconnect the solenoid wire, install jumper and touch the bared end to positive post of the battery, much easier than running back and forth to the drivers seat plus you can crank it with the ignition off so no possibility of distributor damage.
Hope that helps

[aoeu]

Hi Jarf, thanks for the input. I have already ran 1/2 a can of Seafoam in the oil but I haven't done the vacuum lines yet. I'll run it through there next to see if that helps. A smaller cylinder volume due to carbon buildup would cause a higher pressure so that might be the culprit.

The "pump up" is just the gauge reading as the gauge increases, or pumps up. I recorded the value for each revolution and how many revolutions it took to reach max psi to determine if all cylinders were increasing pressure at the same rate (similar readings for each revolution). The 5th revolution reached the psi listed in the FSM.

[4wdchico]

My motor had very similar numbers at just under 150k miles. I did a seafoam soak, then the regular seafoam treatment followed by a combustion chamber steam cleaning with distilled water. Then I changed the oil and filter. It brought the numbers down to the mid-180's.

I do not recommend the steam cleaning unless you have really studied up on the issue first.

[Petros]

They are all close, so that means good and even compression all the way across. The high number is likely carbon build up, but if the car is not ping or knocking in the warm air with crap federal gasoline, the high compression actually gives better fuel economy and slightly more power output.

I would just leave it.

[aoeu]

It has an intermittent knock which is why I did the compression test. Strange thing is that it runs smooth and quite after checking compression but then after a while begins to idle rough and the occasional knocking comes back. I'm doing a Seafoam treatment tomorrow so hopefully that helps.

[Petros]

If it was run low on oil, and damaged the bearings, it will still knock and it will not show up on the compression test.

You might put on an oil pressure gauge (the sender is behind alternator on the side of the block), and check your oil pressure. If it drops off at idle to less than 10 psi most likely the bearings are shot. YOu have to drop the oil pan and replace the connecting rod bearings. Usually the mains are fine even if the connecting rod bearings are shot.

Good luck.

[aoeu]

Update:
The problem was in fact carbon buildup. I pulled the head and cleaned the cylinders, head, valves, and replaced the valve seals. Total was under $100 and well worth it. The car is idling smooth, knock is gone, dieseling is gone, and compression is lower.

Compression results
Cylinder 1: 185
Cylinder 2: 177
Cylinder 3: 190
Cylinder 4: 187

I'm attaching the before and after pics. Also, I did 2 Seafoam treatments before I pulled the head and you can see how much carbon buildup is still there.

[dlb]

WOW, super clean! what did you clean it with?

[aoeu]

I used carb cleaner and a steel brush for the big stuff, then used a brass brush when I got close to the metal, then finally a nylon brush at the end.

[dlb]

unreal. how long did it take you? it looks awesome.

[aoeu]

It took 2 afternoons. I probably spent about 3-4 hours taking the valves out and cleaning everything, then a few hours actually getting the head off and on. I got the gasket set for $65 from autozone and then I had to get some cleaner and a few other things, so the total was probably $80.

[deejay1272]

Total was under $100 and well worth it.

In addition to the head gasket - what other gaskets did you get in order to do this job?


Thanks,
DJ