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Misfire and lower compression on even bank after heads replaced

Tiny
KEITH WARKENTIN
  • MEMBER
  • 1998 CHEVROLET SUBURBAN
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 175,000 MILES
This 5.7l Vortec had been overheated, causing the need to remove and replace the heads. Machine shop said heads were cracked, so he gave a "repaired" set in exchange. Weirdest issue with those was an oil leak, external that was unexplained (lower side of head by exhaust, where there is no oil), except after being removed for the third time, it appeared to have a crack to a bolt hole the machinist missed, leaking at the end of the head, and tracking the head gasket. Machinist finally ordered two fresh blanks, and now the leaks are gone. To the current issue with the new heads 1, 3, 5, 7 have 160-165 psi compression, while 2, 4, 6, 8 have only 130 psi, and primarily #4 shows a misfire code, but the even bank all show random misfires. I am a shop owner, and a smog certified ASE Master Tech and this is an extra vehicle of mine. So for the last year we have been searching for answers that make sense. The only thing I could think of is perhaps there are more than one set of blank heads available with different size combustion chambers. Any thoughts or ideas are welcome.
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Thursday, May 23rd, 2019 AT 1:50 PM

10 Replies

Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello,

Are you sure that they are new or are they remanufactured? If they are remanufactured I would measure the height of each head if you can. Bank 1 might have had to shaved, alot, to true it and that would reduce the combustion chamber volume and upping the compression ratio. The misfires are probably because the compression for bank 2 is either out of spec or borderline out of spec for this engine's compression ratio specifications. I might suggest a wet compression test on bank 2, but I doubt it's going to be piston rings on an entire bank of cylinders. But Bank 1 would be the bank not within the normal specs, not bank 2, unless Chevrolet made a forced induction version of this engine and lowered the compression ratio to compensate for the added volumetric efficiency. Let me do some more research on this. Please get back to us with what you are able to find out.

Thanks,
Alex
2CarPros
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Thursday, May 23rd, 2019 AT 10:05 PM
Tiny
KEITH WARKENTIN
  • MEMBER
  • 23 POSTS
The heads were new blanks ordered by the machine shop, and he transferred all the components to the new heads. My thought on that was possibly a difference in the heads; such as a smaller chamber on one side, perhaps 8.5:1 vs 9.5:1 ratios as an example, but I'm not sure if that was a possibility. The compression was dry cranking, all done at the same time. So no significant temperature difference. The compression being even, relative to each bank leads me to believe something in the heads also, but as stated, they were new. We even tried a BG fuel injection flush kit, as it only seems to misfire at idle and not a dead miss, just a little rougher than it should. 130 is still plenty to run, but I'm wondering if the difference in power, from the higher compression bank, may by causing it to lie. Since the power output would be a bit lower, it could recognize it as a misfire, without actually being one. It seems to drive completely fine, but I'm running out of ideas.
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Tuesday, May 28th, 2019 AT 8:35 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

What do the top of the pistons look like? Is there a possibility of one bank of the pistons got decarbonized and the other bank got skipped over?

Thanks,
Alex
2CarPros
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Tuesday, May 28th, 2019 AT 4:36 PM
Tiny
KEITH WARKENTIN
  • MEMBER
  • 23 POSTS
No, they were all equally clean, with no excess buildup of carbon.
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Monday, June 3rd, 2019 AT 7:55 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Okay, I'm agreeing with you. Are both of the casting numbers on the heads the same?

Thanks,
Alex
2CarPros
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Monday, June 3rd, 2019 AT 5:53 PM
Tiny
KEITH WARKENTIN
  • MEMBER
  • 23 POSTS
Sorry, was off of here for a few days. I will have to remove the valve covers to confirm, and get back to you.
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Wednesday, June 5th, 2019 AT 2:18 PM
Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello, I'm Danny.

Just a quick question. Is the your complaint just a slight misfire? If so you might want to look into the intake manifolds 'spider' injection. They are notorious for getting clogged causing a misfire, total overlooked, and very common problem on these vortecs (1996-1999) I had a 1997 K1500 and had this show up around 200,000 miles. Hope this helps and thanks for using 2CarPros.
Danny-
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Thursday, June 6th, 2019 AT 5:05 AM
Tiny
KEITH WARKENTIN
  • MEMBER
  • 23 POSTS
I have not had opportunity to pull valve covers off yet, but as far as the injectors I had looked into all of that also, but it didn't have this problem before doing the heads originally, and it runs perfectly well off idle. And killing each injector with the scanner, all are running. The only cylinder that has set a code is number 4, but the entire even bank shows the misfire counters going up as it runs at idle. On other vehicles, I have even had the internal fuel pressure regulator leak, causing misfires due to excessive fuel dripping into a cylinder. None of these have shown to be the cause, along with the lower compression on the even bank, I am still thinking the heads are not matching. I have not verified the part numbers yet though.
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Monday, June 10th, 2019 AT 9:42 AM
Tiny
KEITH WARKENTIN
  • MEMBER
  • 23 POSTS
Well oddly enough, there appear to be no casting numbers on these heads. The usual place between the valve springs, by the bolt hole, is smooth, no numbers. Is there another location that they could be? I'm not finding any other locations listed, in any searches I've done. They were supposed to be blanks, but I didn't expect that could mean no numbers.
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Wednesday, June 12th, 2019 AT 2:57 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,964 POSTS
Yep, it sounds like you are hot on the trail. The only other thing is did you check the intake valves for seal-age before installing the heads because if you have one that leaks it can shoot exhaust into the intake system effecting random or specific cylinders. Can you pull the #3 plug and upload pictures of then insulator?
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Wednesday, June 12th, 2019 AT 2:59 PM

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