High compression question

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ajgreen

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Hi, New here but not new to car problems. Here's the latest. 1999 Yukon 5.7L. I'm chasing a P0305 missfire. Changed plugs, wires, cap and rotor. SES light came back in a day. Yesterday I pulled all the plugs and did a compression test. All cylinders were between 190 and 200 PSI! I checked a couple with another gauge I had and it actually read a couple PSI higher. My first guess is carbon buidup. Anybody else want to guess?
 

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vacuum leak/intake leak?, double check the work you did, if it ran fine for a day then maybe something failed already that you replaced., do you have a scanner that can read live data?
 
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ajgreen

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vacuum leak/intake leak?, double check the work you did, if it ran fine for a day then maybe something failed already that you replaced., do you have a scanner that can read live data?
Double checked everything, all is good. I assumed a vacuum leak would give me a lean condition error which it does not. I have an old scanner that will give me live data. What should I be looking for in the data stream?
 

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Double checked everything, all is good. I assumed a vacuum leak would give me a lean condition error which it does not. I have an old scanner that will give me live data. What should I be looking for in the data stream?
I would start with monitoring the misfires and see if that points you anywhere specific, like if there consistent on certain cylinders or just all over the place random, I haven't had a obs in a long time but don't those have a coil? or is it all in the distributor? I think it's called HEI? maybe the distributor has issue's? like I said it's been a while, maybe @swathdiver or @07Burb may have some idea's
 
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ajgreen

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I would start with monitoring the misfires and see if that points you anywhere specific, like if there consistent on certain cylinders or just all over the place random, I haven't had a obs in a long time but don't those have a coil? or is it all in the distributor? I think it's called HEI? maybe the distributor has issue's? like I said it's been a while, maybe @swathdiver or @07Burb may have some idea's
It does have a coil. But, I did cheap out on the tune up. it has an after market cap and rotor. Think I'll get an OEM part and see if that helps. I also read somewhere that the distributors are all made of plastic! I wonder if it's worn to junk?
 

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It does have a coil. But, I did cheap out on the tune up. it has an after market cap and rotor. Think I'll get an OEM part and see if that helps. I also read somewhere that the distributors are all made of plastic! I wonder if it's worn to junk?
Well cheap parts would explain a lot, lol I would at least get a decent cap & rotor, like borg-warner or ac delco, but borg warner probably makes them for delco anyway.
 

bigfootchiro

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Didn't Nathan go through this before his big block swap? @bigfootchiro

Oh yes indeedy! So my answer to solve all issues is big block swap lol.

Alright, so you have a Cylinder 5 misfire. Good job on compression testing. How many miles does the motor have? How long has the misfire been happening? Did you do anything just prior to this happening? The code only sets once the misfire counts go above a certain number.

You can have a vacuum leak and not have a lean condition code set. That code only sets when the fuel trims go way into the positive range (above 25 I believe, someone correct me if I’m wrong).

Definitely setup a scanner with live data and look at misfire counts per cylinder (history and current), CMP Retard (should be between -2 and +2), short term and long term fuel trims for Banks 1 and 2, RPM, aaaand I think that may be it for now... try that and report back with some numbers at Idle, 1500, 2500, then while driving, as well. If you don’t want to keep an eye on the scanner while you’re driving, record your screen if you have an iPhone.
 
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ajgreen

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Oh yes indeedy! So my answer to solve all issues is big block swap lol.

Alright, so you have a Cylinder 5 misfire. Good job on compression testing. How many miles does the motor have? How long has the misfire been happening? Did you do anything just prior to this happening? The code only sets once the misfire counts go above a certain number.

You can have a vacuum leak and not have a lean condition code set. That code only sets when the fuel trims go way into the positive range (above 25 I believe, someone correct me if I’m wrong).

Definitely setup a scanner with live data and look at misfire counts per cylinder (history and current), CMP Retard (should be between -2 and +2), short term and long term fuel trims for Banks 1 and 2, RPM, aaaand I think that may be it for now... try that and report back with some numbers at Idle, 1500, 2500, then while driving, as well. If you don’t want to keep an eye on the scanner while you’re driving, record your screen if you have an iPhone.
Don't have such an advanced scanner so may have to take it in to someone who does. Truck has 178K miles. Had the missfire code several weeks ago before I did plugs etc. Came right back.
Nobody has addressed the high compression. With 190 to 200 PSI wouldn't it ping like crazy on regular gas? Compression ratio would be 13:1!
 

bigfootchiro

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Don't have such an advanced scanner so may have to take it in to someone who does. Truck has 178K miles. Had the missfire code several weeks ago before I did plugs etc. Came right back.
Nobody has addressed the high compression. With 190 to 200 PSI wouldn't it ping like crazy on regular gas? Compression ratio would be 13:1!

Don’t need an advanced scanner. You can order a Fosseal WiFi OBD2 on Amazon and use DashCommand app. Buy the $10 add on pack for your year inside the app, and you’ll be able to read everything you need.

I would double check compression with another tool. If they are ALL between 190-200, that’s some pretty equally distributed carbon deposits lol.
 
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