What's the deal with this engine?

I had a compression test done in Oct.2006 @148k miles, right after a misfire code (PO303) was read and here’s what the invoice has for the compression readings.


…And here’s what I got a couple of days ago when I tested.

1:180 on the first set of cranks and then 210 on the second and third set of cranks. (I made sure the gauge was at “zero” before each set of cranks.)
2:200 on the first set of cranks and then 210 on the second set.
3:150 on the first set, 140 on the second set, then added oil to the chamber and cranked again and got 150.
4:210 on the first set and 215 on the second set.

This was my first time doing a compression test but I followed the directions in the Mazda shop manual and the directions that came with the gauge to a “T”. And I used a brand new Actron Professional gauge.

So why are the results so different? It seems like it’s getting better which seems almost impossible because I keep the oil changed which should ,at least I think, prevent carbon from building up on the rings. Or does carbon buildup have little or nothing to do with oil condition?

The only thing that I’ve been able to come up with is that the STP Oil Treatment I added increased the compression but that seems unlikely that it would work that good. And plus I stopped using it a couple of oil changes ago because there seemed to be a correlation between using it and smoke at start up.

So, should I be worried about the high compression? Is it going to keep on rising?

And also, I have a scan tool, so if there’s any info that it could provide to help get to the bottom of this , just let me know.
~98 Mazda Protege, 169k miles

My unexpert guess would be that the misfire on #3 caused the oil to be washed from the cyl causing a loss of compression. It seems OK now so be happy.

I’m curious, do you know what the sparkplug on cylinder three looked like when it was pulled to do the initial compression test? Did it show any abnormal signs when you pulled it to do your compression test? The code combined with the difference in compression on that cylinder would suggest that something is amiss. I’m wondering if the misfire was due to fouling.

When the test was done in 06, the mechanic probably didn’t do second and third tests. It was probably once and done. That means that when he got to number three, the engine had only been cranked two times, this was its third.

Now you did three tests on #1, then two on #2 so by the time you got to #3, you had cranked the engine five times. That may have pumped a little oil up to that piston by that time. If you don’t use the same procedures, you will not get the same results.

With with the difference in readings, it could be his compression gauge was out of calibration.

HOWEVER, the readings are still telling. No. 3 cylinder is showing some serious problems. In the first test, the mechanic is showing a severe compression problem with that cylinder. It is almost 50% below the other 3 cylinder average. With your test using a brand new gauge, the cylinder is still about 30% below the other 3 cylinder average.

A passing compression test would show no more than a 10% variance across all cylinders. In this case, that would be about 20 psi plus or minus. Cylinder number three is well below that. Basically it is showing a mechanical problem with that cylinder causing your mis-fire code. This could be bad rings or a burnt valve, but it is showing an internal problem. Time to make a decision.

I’d personally drive it until it croaks, and save money for a large down-payment in the meantime. The bad readings have been there for five years, so the engine hasn’t died yet. But, that cylinder could let go at any time, be that next week or another 5 years from now. I’d, bank about $200-$300 a month for a couple of years, typical for a car payment. Even with the low interest rates, the compound interest will make for a nice nest egg towards this car’s inevitable replacement do this for 3 years if this car holds out, and you could buy new with cash!

Your readings are high, but consistent. Your #3 cylinder has low compression. What to do about it, now that is the question?

When you added oil to #3 it didn’t improve.
That suggests it’s a valve problem.
Were the valves adjusted after the test in 2006?

tsm, The #3 plug looked ok to me. It’s appearance most closely matches the “normal” picture on my manual’s plug chart. It just has more grey on it than the other 3 plugs. Here’s a picture of it. It had been in the engine from 131k - 163k.

And here’s the current #3 plug that’s been in from 163k-169k

Something else I failed to mention. My scan tool pulled code P1195 which is a manufacturer specific code.

Code translation: Barometric pressure (EGR boost) sensor circuit malfunction

Detection Conditions

The ECU monitors input voltage from the EGR boost sensor when monitoring conditions are met. If input voltage at ECU terminal 3F is below 0.35 V or above 4.5 V, the ECU determines that there is an EGR boost sensor circuit malfunction.

Monitoring Conditions:
Intake air temperature is above 10 degrees C
EGR boost sensor solenoid is turned OFF (with barometric pressure applied to the EGR boost sensor).

The ECU monitors differences between intake manifold vacuum and atmospheric pressure at idle, which the EGR boost sensor detects by switching the EGR boost sensor solenoid. If the difference is below 6.43 kPa (48.2 mmHg, 1.90 inHg), the ECU determines that there is an EGR boost sensor performance problem.

Diagnostic Support Notes

This is a continuous monitor (CCM)
Check engine indicator illuminates if the ECU detects the above malfunction condition during the first drive cycle
PENDING CODE is not available
FREEZE FRAME DATA is available
DTC is stored in ECU memory

Possible Causes

EGR boost sensor malfunction
Connector or terminal malfunction
Short to ground in wiring between EGR boost sensor terminal A and ECU terminal 3F
Open circuit in wiring between EGR boost sensor terminal B and ECU terminal 3AB
Open circuit in wiring between EGR boost sensor terminal A and ECU terminal 3F
EGR boost sensor signal circuit shorts to reference voltage (Vref) supply circuit
EGR boost sensor performance problem
EGR boost sensor solenoid circuit malfunction
Loosened, damaged, misconnected, clogged or frozen moisture in vacuum hose from EGR boost sensor solenoid to EGR boost sensor
Loosened, damaged, misconnected, clogged or frozen moisture in vacuum hose from EGR boost sensor solenoid to EGR valve ECU malfunction

Is this anything to worry about?

What was the engine temperature when tested each time? Were all the plugs removed and left out during the tests?

Everything points to a burned valve, most likely an exhaust valve. I’d recommend that you follow BustedKnuckles advice on this one. As for the EGR boost, considering the age of the vehicle, I’d start with the vacuum hoses, then look at the EGR valve itself. I don’t think the malfunction is anything to lose sleep over, but you should fix it so that the CEL is off. That way if something else happens, you will detect it sooner. Otherwise you will need to make a weekly code check part of your PM program.

Rod Knox,
I warmed the engine up to normal operating temp, let it cool for ten minutes, depressurized the fuel lines, removed all 4 plugs and disconnected the distributor before attaching the gauge. It probably took me less than 10 minutes to do the steps after the initial ten minute cool down so I imagine the engine was still close to normal operating temp when I performed the test. I didn’t look at the temp gauge while I was cranking so I can’t say for sure.

Thanks for the photo. Other than oil-ash deposition, not abnormal in a higher mileage engine, the plug actually looks pretty good. Which leads me to suspect that a burned valve must be causing the variance in compression readings, as others have suggested.

I’d be inclined to keep on driving it. You may have to change the #3 plug twice as often as the others to prevent misfires, but that’s certainly easy and affordable.

"Now you did three tests on #1, then two on #2 so by the time you got to #3, you had cranked the engine five times. That may have pumped a little oil up to that piston by that time. If you don’t use the same procedures, you will not get the same results. "

So would it make sense to go back and test only #3?

Can a valve become burnt from out of spec valve clearance? Because it’s never been checked but I’m going to check them soon .

Also, I’m going to replace the fuel filter (original ) and air intake duct because it’s torn and rigged with duct tape. Should I stick with OEM on these two items?

And I’m thinking about replacing the valve stem seals if the valves aren’t to wobbly. I’ve seen prices for seals all over the chart; anywhere from 15 bucks a set to $158 a set. Are the less expensive ones just as good or somewhat close in terms of quality as the more expensive ones?

There was a problem back in 2006 and not just on Cylinder 3. For the mileage the 170 and 180 could be acceptable but the 150 is going downhill and that’s a 30 PSI difference when compared to the 180. That’s out of range and abnormal. That’s all assuming the test was done properly, no gauge error, etc, etc.

Did you wedge the throttle plate wide open while doing the tests? If not, this should be done to allow air to enter the intake tract freely.

The current readings you’re getting sound too high in my opinion and should not be in the 200+ PSI range.
Whether it’s a gauge error or not I have no idea. I’m in agreement about Cylinder 3 possibly having a valve problem due to the number not rising much after the oil added, or wet, test.

Those deposits on that plug look a bit heavy for 30ish thousand miles and maybe it’s due to a worn valve guide (which can lead to a valve seating issue, and valve seal problem. This would likely point to an intake valve problem.

No, I didn’t wedge the throttle plate. The manual just says to completely depress the gas pedal or fully open the throttle valve. I chose to depress the gas pedal while cranking the engine. Is that the same thing as wedging the throttle plate wide open?

And the compression does seem too high, that’s why It seems carbon buildup on the rings and/ or cylinder wall might be a factor. The Mazda Workshop manual says standard is 195 psi, minimum 146 psi and maximum difference between cylinders 28 psi.

Would it be wise to try and lower the compression on the cylinders (except #3) by pouring something into the chambers?

circuitsmith, no, the valves have never been adjusted on this car.

“Can a valve become burnt from out of spec valve clearance?”

Yes, that’s the most common cause of a burned valve.

The shop, in Oct 2006, probably did an “electronic” compression test. These tests are based on RPM drop not actual cylinder pressure so the results can differ considerably…163K miles takes it’s toll…

When an exhaust valve becomes “a little” burned, it does not stay little very long…With flame leaking past the valve, it quickly overheats and the burning process advances rapidly, lowering compression to near zero…

Yes, depressing the accelerator pedal while performing the test is what I meant by wedging the throttle open.

There’s obviously a problem with Cylinder 3 and the others appear to be abnormally high. Let me ask this.
Has the timing belt been changed? I’m just wondering if a camshaft is off a tooth or two (maybe too advanced) and this is the cause of the abnormally high readings.

Both engine options for this car appear to be mechanical lifter motors so if Cylinder 3 is down due to a tight valve then the horses have left the barn. An exhaust valve will have burnt PDQ and an intake valve, while it won’t burn, can suffer valve face and seat damage.

You might pop the valve cover and check the valve lash. This car also appears to use the shim and bucket method so this usually involves special tools and buying some dealer only shims if an adjustment is needed.
Not that an adjustment would cure a valve face and seat problem but it could buy time if the problem is the intake valve.

Let me add something else. Don’t buy that 28 PSI difference in cylinders as being acceptable. It’s not.
That specification sounds like something out of a Chiltons manual and they’re very often wrong.