little Help Please! p0300
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
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From: Shawnee Kansas
little Help Please! p0300
I thought some of you would have some good ideas. I Am working on a 2005 Chevy Tahoe 5.3 T engine.
Symptom were unexplained coolant loss and soft engine miss at idle 650rpm that would set P0300.
Watching the misfire counter cylinder 7 would miss only at idle at 650 rpm. Just off idle 900 rpm no misfire. I have done the usual things, the compression tested150psi across the board. Replaced the Plugs & wires, swap coils 5 & 7 , still misses cyl #7. the coolant loss was caused from cracked heads. Pulled the heads had a complete valve job, resurfaced & weld cracks. replace # 7 injector, complete head gasket set , new head bolts. Compared cam lift between cyl 7 and cyl 3, they were the same. Finished reinstalling heads. engine runs great except same p0300 @
miss at idle 550pms (miss counter 15%) no miss above 700rpm (miss counter 0%) Any help is appreciated.
Symptom were unexplained coolant loss and soft engine miss at idle 650rpm that would set P0300.
Watching the misfire counter cylinder 7 would miss only at idle at 650 rpm. Just off idle 900 rpm no misfire. I have done the usual things, the compression tested150psi across the board. Replaced the Plugs & wires, swap coils 5 & 7 , still misses cyl #7. the coolant loss was caused from cracked heads. Pulled the heads had a complete valve job, resurfaced & weld cracks. replace # 7 injector, complete head gasket set , new head bolts. Compared cam lift between cyl 7 and cyl 3, they were the same. Finished reinstalling heads. engine runs great except same p0300 @
miss at idle 550pms (miss counter 15%) no miss above 700rpm (miss counter 0%) Any help is appreciated.
Last edited by Kevin Kuse; Jan 15, 2015 at 11:05 AM. Reason: correct rpms
Uneducated autodidact
Joined: Feb 2001
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From: West of the Sabine, Texas
So, you had a cracked head allowing coolant to hit the plug causing misfire on #7, fixed and put back together and same symptom? Have you tried switching coil packs to see if that is the problem? Other than that, was the #7 injector good? I would hate to think that you did all that and still are getting coolant in the #7 cylinder....good luck. 
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
swap coils 5 & 7 , still misses cyl #7.
replace # 7 injector,
Last edited by Kevin Kuse; Jan 15, 2015 at 09:17 AM.
Melting Slicks
Joined: Jul 2012
Posts: 2,507
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Or GOOGLE is always your friend:
https://www.google.com/webhp?sourcei...+7&safe=active
Last edited by 3boystoys; Jan 15, 2015 at 09:18 AM.
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
Wonder if the block is OK on #7 or the piston?? #7 is an issue on the Vette, do a search here, you'll find many posts on people (usually running too lean) toasting the piston in #7. Not sure the 5.3 has the same issues but it wouldn't surprise me.
Or GOOGLE is always your friend:
https://www.google.com/webhp?sourcei...+7&safe=active
Or GOOGLE is always your friend:
https://www.google.com/webhp?sourcei...+7&safe=active
No Lean codes , short and long term fuel trims # all normal.
Melting Slicks
Joined: Jul 2012
Posts: 2,507
Likes: 9
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
Screw a air in into the cylinder and pressurize it to see if you have blow by out the oil fill tube or throttle body? if so then yes! no blow by.
I do appreciate all the input from you all. Thanks for all the Brain Storming!
And I am still reading the links.
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Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Melting Slicks
Joined: Jul 2012
Posts: 2,507
Likes: 9
By leak down test do you mean.
Screw a air in into the cylinder and pressurize it to see if you have blow by out the oil fill tube or throttle body? if so then yes! no blow by.
I do appreciate all the input from you all. Thanks for all the Brain Storming!
And I am still reading the links.
Screw a air in into the cylinder and pressurize it to see if you have blow by out the oil fill tube or throttle body? if so then yes! no blow by.
I do appreciate all the input from you all. Thanks for all the Brain Storming!
And I am still reading the links.
http://ls1tech.com/forums/generation...lp-please.html
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
read this, especially starting at #6 reply:
http://ls1tech.com/forums/generation...lp-please.html
http://ls1tech.com/forums/generation...lp-please.html
When the heads were off & the pistons cleaned & inspected there were no visible cracks.
to Correct my original post #7 miss at idle 550pms (miss counter 15%) no miss above 700rpm (miss counter 0%)
where that dam idle adjusting screw when you need it! LOL
Last edited by Kevin Kuse; Jan 15, 2015 at 11:06 AM.
Melting Slicks
Joined: Jul 2012
Posts: 2,507
Likes: 9
Good info, I will do the amp clamp cranking test!
When the heads were off & the pistons cleaned & inspected there were no visible cracks.
to Correct my original post #7 miss at idle 550pms (miss counter 15%) no miss above 700rpm (miss counter 0%)
where that dam idle adjusting screw when you need it! LOL
When the heads were off & the pistons cleaned & inspected there were no visible cracks.
to Correct my original post #7 miss at idle 550pms (miss counter 15%) no miss above 700rpm (miss counter 0%)
where that dam idle adjusting screw when you need it! LOL
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
No, We just had the heads off, we were only able to see the piston top. Once the carbon was cleaned off no obvious problem. And the Cylinder wall when the piston was at the bottom of the stroke. again to sign of trouble. never expected a lower end problem with all cylinders at 150 psi with Closed Throttle.
Right now I am searching for a High Amp Probe, mine is a low amp probe.
So I can check amp draw during cranking to compare Cylinder balance. I have Modis to show the graph, but wrong probe.
Right now I am searching for a High Amp Probe, mine is a low amp probe.
So I can check amp draw during cranking to compare Cylinder balance. I have Modis to show the graph, but wrong probe.
Melting Slicks
Joined: Jul 2012
Posts: 2,507
Likes: 9
No, We just had the heads off, we were only able to see the piston top. Once the carbon was cleaned off no obvious problem. And the Cylinder wall when the piston was at the bottom of the stroke. again to sign of trouble. never expected a lower end problem with all cylinders at 150psi.
Right now I am searching for a High Amp Probe, mine is a low amp probe.
So I can check amp draw during cranking to compare Cylinder balance. I have Modis to show the graph, but wrong probe.
Right now I am searching for a High Amp Probe, mine is a low amp probe.
So I can check amp draw during cranking to compare Cylinder balance. I have Modis to show the graph, but wrong probe.
This is an extreme case:
http://ls1tech.com/forums/generation...on-busted.html
Lots of pics:
https://www.google.com/search?q=ls1+...IGQDg#imgdii=_
Some pics of the 5.3 doing this:
https://www.google.com/search?q=gm+5...w&ved=0CB0QsAQ
I assume you used a new intake manifold gasket when you put it back together, a leak around #7 would allow too much air in and you'll get a lean misfire at low speed since that's when vacuum is the highest. Spray some carb cleaner around #7 intake runner and see if the idle changes.
Last edited by 3boystoys; Jan 15, 2015 at 04:15 PM.
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
Many posts on #7 piston cracking a ring land on that piston and the cylinder bore holding it together with no scoring of the cylinder wall.
This is an extreme case:
http://ls1tech.com/forums/generation...on-busted.html
Lots of pics:
https://www.google.com/search?q=ls1+...IGQDg#imgdii=_
Some pics of the 5.3 doing this:
https://www.google.com/search?q=gm+5...w&ved=0CB0QsAQ
I assume you used a new intake manifold gasket when you put it back together, a leak around #7 would allow too much air in and you'll get a lean misfire at low speed since that's when vacuum is the highest. Spray some carb cleaner around #7 intake runner and see if the idle changes.
This is an extreme case:
http://ls1tech.com/forums/generation...on-busted.html
Lots of pics:
https://www.google.com/search?q=ls1+...IGQDg#imgdii=_
Some pics of the 5.3 doing this:
https://www.google.com/search?q=gm+5...w&ved=0CB0QsAQ
I assume you used a new intake manifold gasket when you put it back together, a leak around #7 would allow too much air in and you'll get a lean misfire at low speed since that's when vacuum is the highest. Spray some carb cleaner around #7 intake runner and see if the idle changes.
That would suck! I wounder if #7 piston will come out in the truck?
I know the oil pan and head will. I can't think of any way to check for a Broken Ring or lower Land in the block can anyone? Would that show on the High Amp Cranking Test? it would have too, right?
Melting Slicks
Joined: Jul 2012
Posts: 2,507
Likes: 9
Try checking for the vacuum leak 1st. If you pull the piston, you can take head off on that side and if you can get the oil pan off, you can get the piston out the top. How did your other test go?
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
Screw a air in into the cylinder #7 and pressurize it to see if you have any blow by out the oil fill tube or throttle body? if so then yes! no blow by.
I just did a running compression test (pull one plug install gauge start, let idle 550rpm) on each cylinder. All Cylinder were about 85psi except #8 is was 77psi. add oil to #8 and read 85psi. #8 is one before #7 in the firing order. 1-8-7-2-6-5-4-3
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
By leak down test do you mean.
Screw a air in into the cylinder #7 and pressurize it to see if you have any blow by out the oil fill tube or throttle body? if so then yes! no blow by.
I just did a running compression test (pull one plug install gauge start, let idle 550rpm) on each cylinder. All Cylinder were about 85psi except #8 is was 77psi. add oil to #8 and read 85psi. #8 is one before #7 in the firing order. 1-8-7-2-6-5-4-3
Screw a air in into the cylinder #7 and pressurize it to see if you have any blow by out the oil fill tube or throttle body? if so then yes! no blow by.
I just did a running compression test (pull one plug install gauge start, let idle 550rpm) on each cylinder. All Cylinder were about 85psi except #8 is was 77psi. add oil to #8 and read 85psi. #8 is one before #7 in the firing order. 1-8-7-2-6-5-4-3
There was a lot of slug in the oil areas. So I am praying that I may have a stuck ring in cylinder #8. I pour a complete can of Sea Foam into that Cylinder and let sit over night. This morning cranked it over a few times, then re installed the spark plug. Started the engine and man did she smoke. I took the truck for a long test drive after the smoke cleared. I cleared all the codes & let it idle for 30 minutes. So far so good No misfire codes.
Melting Slicks
Joined: Jul 2012
Posts: 2,507
Likes: 9
Update: With the coolant leaking into the engine (from the cracked heads)
There was a lot of slug in the oil areas. So I am praying that I may have a stuck ring in cylinder #8. I pour a complete can of Sea Foam into that Cylinder and let sit over night. This morning cranked it over a few times, then re installed the spark plug. Started the engine and man did she smoke. I took the truck for a long test drive after the smoke cleared. I cleared all the codes & let it idle for 30 minutes. So far so good No misfire codes.
There was a lot of slug in the oil areas. So I am praying that I may have a stuck ring in cylinder #8. I pour a complete can of Sea Foam into that Cylinder and let sit over night. This morning cranked it over a few times, then re installed the spark plug. Started the engine and man did she smoke. I took the truck for a long test drive after the smoke cleared. I cleared all the codes & let it idle for 30 minutes. So far so good No misfire codes.
Thread Starter
Racer
Joined: Jul 2010
Posts: 262
Likes: 2
From: Shawnee Kansas
Spoke to soon, the miss creeper back in. Yes I have a Snap-0n Modis and it shows the misfire counter. And yes the counter is showing #7 as missing at Idle (550rpm only) approx 15%. But with everything checking good in that cyl. New Elec, New injector, & compression 155psi 5 sec. crank, 90psi running & 125 on snap throttle.(similar # on all Cyl except 8) So I looked at the cyl before 7 in the firing order, maybe it was lagging. That would be cyl #8 and it was lower on the running compression so I ran the SF in that Cyl. and it did bring that Running psi up. And the miss was gone for a few hours.