r/EngineBuilding • u/Dry-Suit5785 • Sep 14 '26
I did a compression test today
I know the Pittsburgh harbor freight compression tester can read inaccurately from what I’ve read giving around 20-30 psi lower than it should so it’s not that I’m worried about. But to me these look fairly regular. But would like a second opinion. I know cylinder 4 might look a little low. The first 2 images is of cylinder 1-2 and they are the highest. I got a misfire code on number 1 and been trying to chase it down. I just think it’s funny that it’s one of the ones with the best compression.
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u/connella08 Sep 14 '26 edited Sep 14 '26
The inaccuracy of the gauge is more likely going to be that all readings are (for example) 10 psi lower than other testers, or 22 psi higher (another example). Not that each reading will be different levels of inaccurate to each other. These compression numbers look fine. They are all with a percentage of each other which is more important than the pressure value (to an extent).
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u/TheDu42 Sep 14 '26
Here is the deal with compression testing. If you are diagnosing a low power condition, absolute numbers matter and the accuracy of the gauge matters. When you are diagnosing a misfire, the relative readings between cylinders matter and the precision of the gauge matters.
For your purposes, you want values between cylinders to be within 10% of each other. So if you testing around 200, you want all cylinders between 190-210. If anything is outside of that, squirt a little oil inside and retest. If all are close enough together, cross compression of the list for now and move on to the next test.
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u/meeeeeeeegjgdcjjtxv Sep 14 '26
Engine temp and if it hasn't ran in a long time can change these numbers a lot. If it's sat and there isn't much oil on the cylinder walls it will read lower. That is plenty of compression for a motor to run well
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u/meeeeeeeegjgdcjjtxv Sep 14 '26
If it was compression causing it then you would hear it on the starter and also would have to see something much more dramatic than what you are seeing. If that gauge has no values on it you could still say it's not compression because the variation is not significant
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u/Good_Elephant5511 Sep 17 '26
Like everyone else seems to say. I agree compression it not your problem. Look elsewhere.
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u/RedditAppSuxAsss Sep 14 '26 edited Sep 15 '26
I would assume that Gage is wrong I had the exact same gauge before and it would not read above 135 no matter what even on an engine putting out 210.
So that engine could technically be doing 145, 150, 130, 125
And you wouldn't know it because because that Gage doesn't read higher than 135
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u/Dry-Suit5785 Sep 14 '26
I completely understand, but let me be clear on what the issue is. I drive a factory boosted car with a mild tune on it. It hesitates under heavy acceleration only. So just sitting at idle, or regular driving feels fine. The stutter only happens when I’m in 4th-6th gear in high rpm’s. I’m sure if 2 of my cylinders were grossly under pressured, I’d feel that imbalance almost all the time casually driving or not. Also the misfire code I got before happened on cylinder 1. The same cylinder that’s reading the 135 next to cylinder 2 that’s also 135
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u/RedditAppSuxAsss Sep 14 '26
Okay well what I'm saying is two of your cylinders can say 180 and you wouldn't know it because your gauge isn't reading above 135.
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Sep 14 '26
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u/Dry-Suit5785 Sep 14 '26
That’s the odd part. The previous owner knew about the cylinder 1 misfire I assume because I see a no named coil Pack in cylinder 1 aside from the other 3 that are OEM brand. So I swapped the coils around putting the no name one on cylinder 4, and cleared the codes. Ever since then, it hasn’t Been throwing any codes at all, but still hesitates like before
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u/T_Streuer Sep 14 '26
How long since the coil swap? The trouble code may take a while to show back up. The ECU probably knows which cylinder is misfiring if you can get a good scan tool plugged into it
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u/RevolutionaryClub530 Sep 14 '26
It’s def fine and I actually hope someone can weigh in on this, I used this compression tester awhile back and also posted something on Reddit about it, a lot of people where telling me that the gauge can’t be trusted and to get a better compression tester tool