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Diagnostics on swapped-in L27

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Old 09-22-2005, 01:51 AM
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Default Diagnostics on swapped-in L27

This is the story of the swap http://www.bonnevilleclub.com/forum/...ic.php?t=39884 that was originally intended as a replacement.

I drove it again for the first time since getting it running. My wife had put about 120 miles on it, reported it ran okay but idled rough. I confirmed that when I drove it tonight, and the engine light came on once. That turned out to be low throttle position sensor voltage, adjusted that but didn't figure that had anything to do with the rough running.

Pulled the plugs. #'* 1, 2 & 5 were blacker than the others. Reverified that the spark plug wires were properly routed. Decided to do a compression check while the plugs were out.

Cyl-----Dry-----Wet
1 ------ 135 ----- 160
2 ------ 155 -----165
3 ------ 85 ------ 160
4 ------ 100 -----115
5 ----- 150 ----- 160
6 ----- 70 ------ 80

One cylinder with bad rings, at least one with bad valves, maybe one with both. Or, possibly a bad head gasket between 4 & 6.

Looks like I've got more work ahead of me. Now the decision: New cam & cam bearings in the LN3, which appeared to have good cylinders and valves before the cam went, or rebuild the L27, which should have more power potential.
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Old 09-22-2005, 07:09 AM
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Sorry to hear... just to confirm though..you took all the plugs out to do the compression test? I've seen odd readings before and I believe in your thread JR mentioned the correct way.

(I really should consider reading something besides torque spec charts)
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Old 09-22-2005, 07:52 AM
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Not to be rude or anything, but some Restore may help out the piston with bad rings. Does the engine burn any oil by chance? Friend of mine'* 91 corsica with the 3.1 had a real bad oil burning problem.. about 2qts a week, and she doesn't drive that much. Told her about restore, and now the engine not only feels more powerful, but the oil burnoff is more around 1qt every two weeks. Big improvement, IMO. We didn't do a compression check or anything, just from driver observations.
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Old 09-22-2005, 08:47 AM
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I think restore fills in small gaps and holes in the cylinder walls...

May not help enough with that low of compression.
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Old 09-22-2005, 11:07 AM
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Unless technology has changed since I learned how to do this, I removed all of the plugs, held the throttle full open, turned the engine over so that it had about 5 compression cycles on the cylinder I was testing, took the reading. I actually did all of the dry readings once, then repeated and wrote each one down. The results were similar as best as I could recall the 1st ones without having written them down. For the wet, I gave a cylinder a couple of squirts of oil, turned the engine over a couple of times to distribute it in the cylinder, ran the test on that cylinder, moved on to the next one and repeated.

I doubt Restore would do anything for the valves, but I suppose $9 isn't too big of an investment even if it doesn't produce any results. I'm sure most of what it does is desolve crud that'* causing rings to stick, which if that were the case, I've already done, but I suppose it won't hurt anything, either.
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Old 09-22-2005, 11:26 AM
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Originally Posted by jr's3800
I have seldom seem much in the way of bad rings in a 3800.... I'm shocked..
I assumed the same when I bid on the engine. If it really was the original owner, an older fellow, which is what the seller said (not himself), and it really had 78k miles on it, then that is a little over 5k miles average per year. It was shipped from the San Diego area, where I spent 9 years of my life. The condition of the coolant passages would indicate it didn't receive regular cooling system maintenance, not unusual in that part of the country where it seldom gets below freezing. The valley and head areas weren't as clean as my engine, not too surprising since I used nothing but synthetic in mine for the last 92k miles, and the L27 probably had nothing but quick lube petroleum, and probably wasn't changed any more than twice a year. The majority of its use was probably short, stop & go trips. The seller said the car needed fenders, cat, & muffler, which when put together made it more expensive to fix than it was worth.

Considering all that, I guess I shouldn't be too surprised it has a weak cylinder or two.

As for oil consumption, as best I can tell with less than 150 miles running time, it isn't using any. Haven't seen any smoke out the tailpipe or anything like that.

If I had to guess, I'd say #6 has a leaking exhaust valve, judging by the compression test & exhaust sound.

Curious the more blackened plugs were from cylinders with the best compression. When I pulled the old plugs out, they were all similarly blackened.
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Old 09-30-2005, 04:37 PM
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500 miles, same story. I haven't redone the compression test, #6 seems to be an exhaust valve leak.

I think I'll pull the heads and see what'* in there. If the cylinders appear clean, I'll either have the heads gone through, or just put the LN3 heads on if they're not. Or, maybe jsut put the LN3 heads on - no major differences in ports/valves between LN3 & L27, correct?

The wife says it seems to have better highway power than the original engine did. Perhaps it'* sealing better under speed than it is cranking/idle.

About those head bolts: Are they truely one-time-use torque to yield, or can they be reused?

Also, I assume if I put the LN3 heads on, I should use LN3 head gaskets - from what I can tell, the small coolant holes are different size than L27.
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Old 09-30-2005, 10:36 PM
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Where the torque is defined as x lb-ft then y degrees they are one time. If given as just lb-ft you can reuse them. I found one 1988 factrory 3800 manual that said if you reach 60 lb-ft, stop so that is probably a good place.
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Old 10-01-2005, 11:35 AM
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The Haynes manual says the same thing for '86-'88. The initial torque and #2 & #3 degrees are different for '89-up.

Guess it'll be gaskets & bolts, then. I think the LN3 heads are going on, fewer $'* to do that.
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