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Discussion Starter #1
Long time reader, used to post by haven't been able to in a while because I couldn't remember which email I used but I need some info from experts now....

Got a bad miss, comes up #3 and no others with my AE.

Swapped, one-at-a-time, Coil - Plug - Injector to #1 and the miss didn't follow. Took a compression test in #3 got 60psi running - #1 was 120.

Engine's got only 80K on it. So,,, I guess I could throw oil down the cyl but rings seem unlikely at my mileage. I was wondering if there might be anything to learn by pulling the Valve Cover and looking at the valve train there (looks like the pass side would be a pain to get off). How likely am I to find a valve problem causing this? I mean, if it IS a ring/cly problem it's not like I'm going to fix it or anything, she's junk...

I've loved this car, it's AWD 1SG HUD + ACC, I'd miss her
 

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2008 STS Northstar RWD, 2000 Trans Am WS6 6 spd, 1968 LeMans convertible 6.6
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It could be possible that there is a valve spring issue. What is the compression when the car cranks but doesn't start? (Disable the fuel system somehow, maybe pull a fuse for the fuel pump or pull the relay) this usually more reliable then an engine running compression check. Usually the running compression is much less than a cranking compression, 60 psi can be normal. How about wiring problems to the coil, or injector, for #3 cylinder. Does the coil spark the plug in #3? Do you have a noid injector tester? This is a small test unit that plugs into the injector harness, simulating an injector. It will flash if the injector is getting the pulse from the PCM.
 

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Discussion Starter #3
Yea,,, that's what I was hoping for :)

A different thought process, different perspective...

So first, I really like the idea of pulling the #3 coil, and running the engine with a grounded plug in it to confirm that I actually have Spark there. I don't have a NOID tester (I look at them to be on sale at HF everytime I go, maybe this will be the motivation I need to just pony-up ;))

And next, yea, maybe 60 isn't as scary as I first felt it was if that cyl has been running with unburnt fuel in it for some time, walls are washed clean (I'm gonna try a bit of oil). I know running is often lower but, Geeze, I was taken-back by 60...

Frankly I was a bit intimidated in trying a No Start test as MY CLOCK STILL WORKS and I'm afraid I might power-down something that would screw it up (if this was my truck, I know to just pull the IDM relay). I'll figure something out and retest.

Thank You for your thoughts, I'm gonna have a new approach tomorrow!
 

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Cadillac XT5 (2017)
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Probably the most definitive test to run (or take someplace to have run) would be a leak down test. 60, in my opinion is too low (at any case).
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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60 psi, cranking or running, is bad problem low. Even 100, cranking, is low.

Service manual specification, warm cranking, is 140 psi.

Capture.PNG
 

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Discussion Starter #6
Today's Results...

I too just couldn't help feeling that any 10:1 engine having such low compression was a serious issue. I was somewhat buoyed when I went to the other bank (because I was taking these measurements from only 1 & 3, two side-by-side cylinders) and found a running compression in #2 of just under 100psi. I have Not noticed any recent loss of power until the MIL came on and could feel the miss.

BUT,,,,, here's what I found this morning;

Cyl Dry Wet Running
#1 - 80 - 80 - 120
#3 - 65 - 70 - 60
#2 - 140 - 155 - 98

All 3 plugs were out during the test except the Running (I assume the reason #1 was so much Higher running).

So I have to conclude this is a Head Gasket leak between 1 & 3 maybe?

Other thoughts?

(btw - Great Spark coming from #3's coil ;) )
 

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2008 STS Northstar RWD, 2000 Trans Am WS6 6 spd, 1968 LeMans convertible 6.6
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Cylinder 1 results seem off, like should be 120 - 80- 80. Anyway, does indeed look like a compression issue, and maybe a head gasket yes. You should take it somewhere for a leakdown test, and advise them to always have a (charged) jump pack on the battery or a battery charger connected, if they need to have the ignition on and engine off for any time over like 5-10 minutes. Tell them about the clock issue if the battery gets disconnected or drained.

Glad you confirmed spark output on #3.
 

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Discussion Starter #8
Cyl #1 readings are accurate and, to me, says that Cyl #3 is contributing 40lbs of compression when it's spark plug is installed. With the plug out, #1 is bleeding pressure to the atmosphere because of a burned gasket.

I'm perfectly capable of doing a Leak Down test myself, just not sure what it would do for me. What else is there to confirm???

Engine's Toast :(

I've got a great parts-car in CT for anyone who's interested...
 

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2008 STS Northstar RWD, 2000 Trans Am WS6 6 spd, 1968 LeMans convertible 6.6
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Time for the 5.3 junkyard swap with a small turbo lol.
 

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Discussion Starter #10
Hmmm....

No, I'm thinking either taking advantage of the CT6 situation (like maybe a '16/17) or making the move to a P90D Tesla of the same vintage.

I've unplugged #3 injector and will use it for a bit more while I look ;)
 
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