HELP CCKW Engine knocking

[R Cubed]

I have a problem, my engine in my CCKW352 has a fairly deep knock, I will try to describe the scenario. When run on the road in normal driving it is there when ever it is put under load, accelerating, If stationary and you rev gently up to a steady 1500 or 2000 revs all is ok, if you rev hard from tick over as in a brrrrruuuummm there is a deepish knocking sound every time it is revved hard. Not on the run down from the hard rev back to tick over. Now it does not seem to sound like an exhaust blow but not positive, the knock seems to be coming from the top of the engine, also have carried out a compression test and with all plugs out it goes like this starting from front by water pump, all in PSI.

First attempt No1 135, No2 130, No3 115, No4 115, No5 120, No6 120.

Second attempt No1 130, No2 120, No3 115, No4 115, No5 130, No6 120.

 

I seem to have isolated it to No4 if you remove the plug lead to it and run the engine apart from the rougher idle from being down one cylinder and the consistent miss, the noise has gone.

 

HELP.

[Pete Ashby]

If the knock is like a sharp hammer blow then it could be a little end, if it's more like a heavy thump then it's more likely to be a big end. The fact that you have isolated to one cylinder is useful but ultimately you'll have to drop the sump and pull the head to see what the problem is. Have you noticed a sudden drop in oil pressure on idle? this may indicate a big end failure or a main bearings

however mains tend to rumble. I think your compression results make a broken ring unlikely they tend to click and scrape.

I wouldn't recommend driving the truck with the symptoms you have described.

 

Pete

[Grasshopper]

Could also be a piston crown breaking up. You've worn the poor beastie out!

 

First though- have you checked the valve clearances recently?

[Tony B]

Does the noise disapppear when you isolate the electrics? If so, check the distributor cap and leads and plug first.

[Richard Farrant]

I know you have appeared to have pinned the knock down to No.4 cyl, but before getting too deep, check the points gap and then ignition timing. If the points have a reduced clearance, it will advance the ignition, also wear on the dist. shaft bushes will not help matters. It might be that No. 4 seems the culprit but it could just be wear in the distributor. I have seen this before.

[R Cubed]

Could also be a piston crown breaking up. You've worn the poor beastie out!

 

First though- have you checked the valve clearances recently?

 

 

You might be right !!!!!!

 

No I have not checked the valve clearances yet Will put it on the list of things to check.

[R Cubed]

If the knock is like a sharp hammer blow then it could be a little end, if it's more like a heavy thump then it's more likely to be a big end. The fact that you have isolated to one cylinder is useful but ultimately you'll have to drop the sump and pull the head to see what the problem is. Have you noticed a sudden drop in oil pressure on idle? this may indicate a big end failure or a main bearings

however mains tend to rumble. I think your compression results make a broken ring unlikely they tend to click and scrape.

I wouldn't recommend driving the truck with the symptoms you have described.

 

Pete

 

Thanks for the advice I think I am going to have a rummage in a couple of weeks when I get a chance to get to it.

[R Cubed]

I know you have appeared to have pinned the knock down to No.4 cyl, but before getting too deep, check the points gap and then ignition timing. If the points have a reduced clearance, it will advance the ignition, also wear on the dist. shaft bushes will not help matters. It might be that No. 4 seems the culprit but it could just be wear in the distributor. I have seen this before.

 

Interesting point will look in to that Thanks.

[N.O.S.]

Psst! Planning on dropping the sump? Is it a deep (2 piece) sump? :banghead::cellphone: :cool2:

[Stormin]

Am I the only one thinking all those compression figures seem very low?

Not a GMC expert and I expect being military it's designed to be low compression to run on low grade fuels but anything below 120 psi must be marginal for reasonable running surely.

Have you tired a wet compression test, squirting some light oil in the bores first? This often helps to isolate whether low compression readings are a result of faulty valve sealing or piston blow by.

[N.O.S.]

...Have you tired a wet compression test, squirting some light oil in the bores first? This often helps to isolate whether low compression readings are a result of faulty valve sealing or piston blow by.

 

That would be a useful test, but I suspect the sump might have to come off for the knocking What the heck, something to do on Saturday at Hardwick

[Richard Farrant]

Am I the only one thinking all those compression figures seem very low?

 

 

The average of these figures is not too bad for this type of engine, but your post has made me go back and look at the figures, which I did not do before. I noticed on both attempts, that figures for No.3 and 4 are both equal, and lower than other cylinders. This could signify a blown head gasket between those cylinders.

[Pete Ashby]

The average of these figures is not too bad for this type of engine, but your post has made me go back and look at the figures, which I did not do before. I noticed on both attempts, that figures for No.3 and 4 are both equal, and lower than other cylinders. This could signify a blown head gasket between those cylinders.

 

Good point Richard I'd not paid too much attention to the numbers either, but I would have thought that this would produce a very

noticeable off beat at the exhaust?

 

I think my moneys still on big ends, if I'm wrong I'll stand you a pint at W&P David and I are coming up for the day on the Friday

 

Pete

[Richard Farrant]

Good point Richard I'd not paid too much attention to the numbers either, but I would have thought that this would produce a very

noticeable off beat at the exhaust?

 

I think my moneys still on big ends, if I'm wrong I'll stand you a pint at W&P David and I are coming up for the day on the Friday

 

 

Hi Pete,

 

Diagnosis from afar is difficult when you only have limited information. We will have to see what RCubed finds.

 

Look forward to catching up with you and David at W&P.

 

regards, Richard

[deadline]

While this is dated, the info is correct and should be used as a guide:

 

http://www.tm9-801.com/searchTM9/tm9_pages/209.pdf

 

Most likely cause is valve timing. Set the valves to .012 intake and .016 exhaust. Must be done with the engine hot and running. Yes, its fun.... and the special tool to do it makes it 1000% easier.

 

Most people are not used to mechanical lifters and the noise of an improperly set tappet can seem like a loose pebble in the cylinder.

 

If you have a vacuum gauge improper valve timing shows itself as pulses from the average. Say you should get a steady 19in reading... on the 'bad' valves it may drop down to 15in or so.

 

This page explains the vacuum readings for the 270:

 

http://www.tm9-801.com/searchTM9/tm9_pages/228.pdf

 

These engines are low compression... like 7:1 due to the poor quality gas in the 40's. There is no spec in the TM9, but 'over 100psi' is specified, and variance cylinder to cylinder should be no more than 10psi:

 

http://www.tm9-801.com/searchTM9/tm9_pages/229.pdf

 

I'm sure there are books that list a compression... but it number itself is not critical... you want uniform numbers on all 6 tubes.

[R Cubed]

Psst! Planning on dropping the sump? Is it a deep (2 piece) sump? :banghead::cellphone: :cool2:

 

 

Errrr yes its a deep 2 piece sump...... Do you know something that I am about to find out !!!!!:red:

[N.O.S.]

Only that if you try to remove the main sump pan (having removed the lower section and then the oil pick-up unit) with the truck just sat there, you could end up getting very frustrated like I did last week.

 

Before you try wrangling with the pan, simply put a jack under the middle of the front bumper and raise the truck away from the front axle by about 4". In addition you may well need to loosen the front engine mounts and lift the engine up in the frame by an inch or so too. I had taken the rad out so we did it anyway - anyone got away without doing that?

 

Also take the little front tin cover off the bellhousing as the rear of the pan catches that too - only by 1/8", but it saves flying into yet another tantrum and throwing ones tools about :blush:

 

One benefit of the job is that by the time it's all boxed up again your hair will have been nicely conditioned by all the engine oil dripping onto your head :-)

Edited June 26, 2011 by N.O.S.

[R Cubed]

Here are two videos, the first was to show the knocking sound but has not turned out very well !!

 

 

 

 

The second shows the vacuum readings mostly at idle and then revved sharply followed by the fall back to idle.

 

 

 

Hope this helps.

Edited June 26, 2011 by R Cubed

[deadline]

I'll have these available for sale shortly... printer is still proofing the layout One on each side of the upper oil pan for

for two piece units.

 

[deadline]

Just watched both vids and the vacuum guage should not be bouncing around that much... a *slight* 'vibration' like movement but you were losing 1-3+ inches. On a 6 or 8 cylinder engine the needle should not move much at idle. A 4 cylinder will bounce a bit... but a 6 or 8 not that much.

 

Give it a good tune up. Set the gap (points and plug) retime the engine and set the valves.

 

A big problem with jeep engines is timing chain slop/stretch.... you could have worn timing gears or a stretched chain... but I would not start unbolting things till you get to a good starting point (aka tune it up). Check the manifolds for tightness and that the exhaust flange is tight and the donut is in place.

 

After that it gets more difficult.

 

Here's a good vacuum guide:

 

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

[R Cubed]

This was before any adjustments. Now I have gapped the plugs, they were opened up a very little, now right at 0.025 thou. Checked points gap, this had closed up a bit cleaned the bump off and set them to 0.020 thou also timed it to a sweet idle, the vac gauge is now dead still at 21 Inch Hg at idle.

 

The timing is way off the 5 deg BTDC that it should be, but suspect this is due to the higher octane fuel, I would say that it is idling nicely at about 10 deg BTDC as the ball marker in the flywheel ( which indicates 5 deg BTDC ) is to the left as you look through the window of the bell housing at the flywheel.

 

Now is this too retarded should it be running nearer to TDC so between 5 deg and 0 deg ? What are others timed at.

[Richard Farrant]

This was before any adjustments. Now I have gapped the plugs, they were opened up a very little, now right at 0.025 thou. Checked points gap, this had closed up a bit cleaned the bump off and set them to 0.020 thou also timed it to a sweet idle, the vac gauge is now dead still at 21 Inch Hg at idle.

 

The timing is way off the 5 deg BTDC that it should be, but suspect this is due to the higher octane fuel, I would say that it is idling nicely at about 10 deg BTDC as the ball marker in the flywheel ( which indicates 5 deg BTDC ) is to the left as you look through the window of the bell housing at the flywheel.

 

Now is this too retarded should it be running nearer to TDC so between 5 deg and 0 deg ? What are others timed at.

 

Richard,

 

Reading what you have posted, it seems you are too far advanced, if you reckon it to be about 10 degs BTDC, and it would have been even more advanced if you had to remove a bump off the points. This is the point I was making in an earlier post. So, is the knock still there?

[deadline]

Set the engine as it should be.. don't try and compensate for octane difference... the engine will run fine on the gas we have now set up for the gas they had then (gas in wwii was 70 octane... now everything is 85-87+.. much better fuel).

 

If you want to tweak the timing... hook up the vacuum gauge and shoot for whatever setting gets you the highest vacuum reading... then back off 1-2inches (on the gauge). That should set you at optimum timing... but may need to be retarded as you drive it a little.

Edited June 26, 2011 by deadline

[Adrian Barrell]

I would advance a little, higher octane fuels burn slower and need more time to completely combust. Running at book figure will be fine but you will get more power and better economy with it advanced a little but probably only 2 degrees over book.

[R Cubed]

I would advance a little, higher octane fuels burn slower and need more time to completely combust. Running at book figure will be fine but you will get more power and better economy with it advanced a little but probably only 2 degrees over book.

 

Book says 5 deg btdc, the truck itself seems to idle better at about 10 deg btdc !! so you are saying then it would be better to advance it, when you say advance do you mean move closer to tdc or further away from tdc ?

 

 

I will play more this week will carry out some road tests too.