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Old 08-27-2018, 03:49 AM   #1
SSGraybeard
 
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dropped cylinder #3/FML

So...was at the strip 2 weeks ago, yes beat the snot out of it but was all good. times are improving as expected. no problems encountered. Not to toss a sympathy card but...found out when I got home from the track my GF had passed unexpectedly. was planning on refilling nitrous bottle & running at a better local track the next day, didn't happen. car sat for 5/6 days before I drove it again while taking her son to breakfast (we had a 35+ year thing going on & no he's not my kid) but have known him all his life & his dad is a pos. was cruising down the road in granny mode & started throwing cel/service stabilitrac codes. nothing seemed out of the ordinary when it started happening/no noises etc .pulled codes/cleared 2 X indicated #3 coil was bad, swapped with a stock one as I kept them after the msd ones were installed. no change. got worse getting flashing cel.was definitely dropping a cyl. did a compression check on #3...came in at 120 psi...bad?( from what I could find on here)


Pulled valve cover trunions are all intact. haven't pulled push rods yet as I don't think would be an issue? shoved a motor camera in cyl. piston seems intact


Seeking opinions: burnt valve. rings? again had no signs of it being hurt, never made a peep.

BTW it has a very safe tune for nitrous by my local shop(trusted) on a dyno, kind of torn here. am investigating but really at this point was done spending $ on it for this season. I purposely don't keep track of mod costs, for the obvious reasons, depression/will never recoup them lol. but spent an ass load of $ already this year on the drivetrain/suspension build.

if it turns out to be lunched it will be parked in the shop until next spring (earlier than usual) & I will build the motor I was planning for earlier than planned. car was built for 1000 hp drivetrain/suspension wise earlier this year. if it needs redone plan is for 750/800 rwhp

Bad things have always seemed to happen to me in groups of 3...not optimistic on this. thoughts? sorry for being long winded, spitballing here, hoping to be able to run again this season. FML


Thanks.
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Old 08-27-2018, 05:50 AM   #2
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Firstly, engines do not fail in such a manner just cruising down the road. So I wouldnt be getting too worried in that respect just yet.

120psi is certainly not great, but quite sufficient for that cylinder to be capable of operating.

Check plugs, wires, coil ( and any wiring relating to that coil ), and of course fuel injector/wiring etc.

If wires reach you could say say 3/5 injector plug to see if that makes a difference ( to prove injector or wiring to the injector ). Or swap the coil between two cylinders ( not wiring as obviously it'd fire at the wrong time ).

Or physically swap an injector to another cylinder see if the problem moves.

test for signal to the injector , and power/signal/ground etc to the coil.
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Old 08-27-2018, 01:23 PM   #3
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Thanks for your input. I too was hoping not major failure. Gernaded lots of different motors over the years & always knew when it happened usually flogging on it. But I'm "old school" sbc/mopar/ford carb'd motors with distributors etc. this is my 1st go around with modding a modern platform (1st ls3) have switched coil packs/wires/plugs out, no change. Probably do a leak down test before messing with injectors? Had a few sbc 350's lose cam lobes in my 70 chevelle back in the day, that acted similar. of course there was no computer codes etc back then, have not heard much about cam failures in ls3's but Im not an expert on them. learning tons about them & loving them compared to the tradional sbc
Thanks for the input, hopefully have it sorted out this week.
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Old 08-27-2018, 02:37 PM   #4
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You've already done a compression test with an "ok" result...not great, but ok. But without a comparison to other cylinders, it doesnt necessarily mean a lot.

I see no point in a leakdown test. You need to establish if the problem is outside the cylinder which it most likely is, and if so where.
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Old 08-27-2018, 02:46 PM   #5
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Check continuity of the wiring from the ECM to the coil in question. That is a fairly quick and easy test to check.

The ECM side is X2-77. It is a light blue wire.
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Old 09-04-2018, 12:37 PM   #6
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Like @stevieturbo pointed out, without comparison data from the adjacent cylinders, the 120psi compression data is to vague to help pinpoint any problems. Personally, I'd think 120 is fairly low, as I'd expect to see around 180+ for a 'normal' motor. Another possibility could be a head gasket failure on that cylinder.

I just experienced that issue a couple of months ago myself. While not expensive, it is fairly labor/time intensive (I do all of my own work).

Keep us posted, and Good luck!

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Old 09-04-2018, 04:10 PM   #7
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Did you say your girl friend passed? Condolences for sure...

As far as the car, others have given you good advice to follow...

-Don
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