Home » Public Forums » GMCnet » Found engine knock
| Found engine knock [message #250125] |
Mon, 19 May 2014 15:02  |
Francois
Messages: 161
Registered: October 2012
Location: Southern California
Karma: 0
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The other day my perfect running 66,000 mi. 403 MH started running really badly with backfires and a loud knock.
Noticing that the knock seemed to be not in sync with the crankshaft but less regular, I started by taking off the valve covers. On the right side, there it was, about a half inch of space between a rocker arm and a pushrod. The rocker arm was tight, and the valve spring was intact and all the way up. So I started the process of removing the carb and the intake manifold.
I am, of course, hoping that it is a blown lifter. I figure, if it is, I'll replace them all and rebuild the carb while it's out. I am pretty sure I will have the answer by next weekend.
Does anyone have any comments on what I should look for? I would appreciate it.
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| Re: Found engine knock [message #250641 is a reply to message #250607] |
Mon, 26 May 2014 08:35  |
rjw
Messages: 697
Registered: September 2005
Karma: 4
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Larry wrote on Sun, 25 May 2014 15:59You need to add some extra ZDDP to your oil and then do the standard cam break-in. A soon as the engine starts, immediately bring the motor up to 2500RPM and hold it there for 15-20 minutes stopping the motor only if there are major oil or water leaks. This will break-in the lifters to the old cam. Failure to do this may find you with a round cam on one or more lobes. You should then change the oil at something around 500 mi or so. JWID
Odd that you should have found a "broken" lifter. Can't say I've ever seen that before. Any chance you could post some pics of that lifter.
I replaced my engine after it started to "knock", was consuming large amounts of coolant and it had over 175,000 miles. Turned out the knock was caused by a "broken" lifter.
http://www.palmbeachgmc.com/motor/motor_teardown/album/slides/DSCN9272.html
I didn't even try to figure out what was wrong before I decided to pull the motor. After I got it out and the new one in, I decided to take a look. The album of photos I took of the top tear down revealed several problems. I think that even if I had replaced the lifter, it would only have been a short matter of time before something else went.
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com
Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water heaters.
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| Re: Found engine knock [message #250655 is a reply to message #250125] |
Mon, 26 May 2014 11:25 |
Chr$
Messages: 2690
Registered: January 2004
Location: Scottsdale, AZ
Karma: 1
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Hmmm I may rethink my knock. Rockers were ok, didn't think about the lifters. I may go and take a look today.
-Chr$: Perpetual SmartAss
Scottsdale, AZ
77 Ex-Kingsley 455 SOLD!
2010 Nomad 24 Ft TT 390W PV W/MPPT, EV4010 and custom cargo door.
Photosite: Chrisc GMC:"It has Begun" TT: "The Other Woman"
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| Re: Found engine knock [message #250772 is a reply to message #250768] |
Tue, 27 May 2014 18:22 |
GMC_LES
Messages: 569
Registered: October 2009
Location: Montreal
Karma: 0
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Is the knocking still occurring after lifter replacement? if the noise is gone, then the engine may still be worth diagnosing further.
You didn`t mention if it was an intake or exhaust lifter that came apart. That info might help in further diagnosis.
I wonder if the timing chain might have jumped a tooth? Your symptoms can be caused by bad valve or ignition timing, but could also be caused by bent, sticking, or other intake valve problems.
If the bad lifter was an intake lifter, I also wonder if something caused the piston to contact that intake valve and forcefully collapse the lifter and flip the plunger? This would likely have also bent the intake valve enough to allow the backfiring through the carb. A compression test would provide more data to help diagnosis.
Les Burt
Montreal
1975 Eleganza 26ft
A work in Progress
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| Re: [GMCnet] Found engine knock [message #250776 is a reply to message #250772] |
Tue, 27 May 2014 18:56 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Sigmund,
I second Les's suggestions, compression check followed by pulling the heads and the timing chain cover.
With those two areas exposed you will be able to see what's going on in the engine before you go through all the work or R&R'ing it.
Regards,
Rob M.
Sydney, Australia
-----Original Message-----
From: Les Burt
Is the knocking still occurring after lifter replacement? if the noise is gone, then the engine may still be worth diagnosing
further.
You didn`t mention if it was an intake or exhaust lifter that came apart. That info might help in further diagnosis.
I wonder if the timing chain might have jumped a tooth? Your symptoms can be caused by bad valve or ignition timing, but could also
be caused by bent, sticking, or other intake valve problems.
If the bad lifter was an intake lifter, I also wonder if something caused the piston to contact that intake valve and forcefully
collapse the lifter and flip the plunger? This would likely have also bent the intake valve enough to allow the backfiring through
the carb. A compression test would provide more data to help diagnosis.
--
Les
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Found engine knock [message #250826 is a reply to message #250778] |
Wed, 28 May 2014 13:07 |
Gary Carlton
Messages: 59
Registered: February 2014
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A Hamilto wrote on Tue, 27 May 2014 19:06USAussie wrote on Tue, 27 May 2014 18:56Sigmund,
I second Les's suggestions, compression check followed by pulling the heads and the timing chain cover.
With those two areas exposed you will be able to see what's going on in the engine before you go through all the work or R&R'ing it.
Regards,
Rob M.
Sydney, Australia Simple stuff first. Make sure the distributor static advance is less than 10 degrees. Manual says set it to 8.
Have I missed something? 8 degrees was for the 455. He has a 403. Should that not be set to 12 degrees?
'78 Royale, "Road Grub"
Bellingham, WA
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| Re: [GMCnet] Found engine knock [message #250838 is a reply to message #250831] |
Wed, 28 May 2014 17:33 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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G'day,
Ref: MM-7725 / Section 6Y - Engine Electrical / Page 6y-16 / Paragraph - Timing Procedure
TIMING PROCEDURE
To adjust ignition' timing, proceed as follows: NOTE: Air conditioning controls in instrument panel must be "OFF" when setting
ignition timing.
1. Remove air cleaner and plug manifold vacuum fitting.
2. Disconnect distributor vacuum lines and plug vacuum source fittings.
3. Connect tachometer and adjust engine speed to correct rpm as listed on the vehicle emissions label located either on the engine
rocker cover or on the air cleaner. 1977 model vehicles with 455 cu. in. engine must be set at 1100 rpm with transmission in
"PARK;"1977 and 1978 model vehicles with 403 cu. in. engine must be set as follows: California certified engines at 2,000 rpm with
transmission in "PARK", Federal certified engines at 1,100 m with transmission in "PARK."
4. With the use of a timing light, set timing to 8° BTDC (455 cu. in. engine) or 12° BTDC (403 cu. in. engine) by loosening the
distributor clamp bolt and rotating the distributor until the specification is obtained.
NOTE: The indicator has four "V" slots, each representing 4°.
5. Tighten the distributor clamp bolt and recheck timing to make sure distributor was not moved during tightening of bolt.
6. Remove plug from vacuum source fittings and reconnect hoses to distributor and/or carburetor. Remove plug from manifold fitting,
connect vacuum hose and install air cleaner.
ALSO
FIRING ORDER - Maintenance Manual X-7525 Figure 50 on page 6Y-50 is incorrect! The # 3 and #6 posts in the distributor are reversed.
Note: all other figures in X-7525, X-7625, and X-7725 are correct.
http://www.gmcmhphotos.com/photos/showphoto.php?photo=7034
Regards,
Rob M.
Sydney, Australia
PS - The TIMING PROCEDURE above was copied and pasted from the MM that Bdub has on his website, I dont know how he did got that to
work as they are PDF! The FIRING ORDER is on Mr ERF's website: http://gmcmotorhome.info/engine.html
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of A.
Gary Carlton wrote on Wed, 28 May 2014 13:07
> Have I missed something? 8 degrees was for the 455. He has a 403. Should that not be set to 12 degrees?
Could be. The latest MM I have is for the 1975 model year.
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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