75' CJ5 w/ 304 Exhaust Tapping Noise and Low Compression Diagnosis Help - JeepForum.com
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post #1 of 20 Old 05-14-2020, 11:22 AM Thread Starter
PottsyCJ5
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75' CJ5 w/ 304 Exhaust Tapping Noise and Low Compression Diagnosis Help

I need some ideas/guidance on what is going on with my 304. I am going to be really thorough because I have already tried as much as I can to diagnose this... After having just recently removed my EGR valve my engine has been pretty much running better than it ever has. It's holding a pretty nice idle at 750rpm (slight fluctuations only for the most part), it's timed right at 5DBTDC, it sounds and feels strong and accelerates great with plenty of power, excellent oil pressure (70 psi @ 2k rpm's when cold, 60 PSI @ 2k rpm's when warm), has pretty good manifold vacuum (12-14 at idle, 18-20 1000-1500 RPM's), runs cool around 195 degrees, etc. BUT it has been making a tapping sound from the passenger side exhaust (I have headers and dual exhaust) which has led me to do some digging and I discovered the #4 cylinder on the passenger side has a major compression issue.



I first thought for sure it was an exhaust leak so I replaced the header gasket on that side with a new Reflex gasket (same ones I used last time I had an exhaust leak on the driver side and it fixed my issue) but it did not make any change to the tapping sound. Next I thought maybe I bent a pushrod so I popped the valve cover and took a video which I will post - it appears to be working correctly. For good measure, I pulled the pushrods and looked them over closely. I thought perhaps after years of use the pushrods were worn slightly shorter and it was perhaps causing the valves not to close completely, so I decided to try running it with two new pushrods for the #4 intake and exhaust valves, and that didn't make a difference either. I looked at the lifters with the pushrods removed and they appeared okay to me, but I wouldn't really know what a collapsed lifter would look like to be honest. You could see the cupped top of the lifter at the bottom of the hole that the push rods sits in, and there was a little bit of oil still sitting in the cup when I removed the lifter. Both pushrods were still free and clear and properly oiling the rockers as well.



I ran compression tests on the #2, #4, #6 and #8 cylinders and #4 is reading 50 PSI after repeated compression tests performed with both a hot and cold engine, while the other three cylinders are still putting up strong compression numbers around 140 PSI. When I first got this jeep about 3 1/2 years ago this same cylinder tested at 137 PSI. Back then it was the weakest of all my cylinders, which all showed around 145-150 PSI back then, but 137 wasn't terrible. So after about 6k miles and three years, the other three cylinders still look great so I think overall the engine is pretty healthy (I haven't had a chance to test the driver side yet). As far as I know this engine has never been rebuilt. Odometer had 18k on it when I bought it, but no idea of true mileage (PO was a total BS salesman). It did have an issue with a stuck intake valve on the #6 cylinder when I first bought, which threw a push rod. I was able to get the valve free and with a new pushrod it has not had an issue since.



In addition to the tapping sound which is kind of like a "tic tic tic" coming from the exhaust and low compression, I am also getting a slight almost backfire when I decelerate from a high speed quickly which it did not used to do. I am also seeing oil pressure fluctuations when idling in a fairly rhythmic pattern which seems related.



My plugs looked pretty good except for the #4. #2, 6 & 8 were a pretty nice Tan, but I might be running a little hot now after the EGR removal (I plan to adjust the air/fuel mixture on the carb to address that). The #4 plug was NOT oily like there was significant blow-by occurring but it was pretty black. I have a picture of the original #4 plug below. I tried putting a new plug in, drove it about 20 miles, and then re-checked the plug, and it was pretty ashy white. I included a pic of the new plug also. I believed based on that condition of the new plug I am still getting spark and I believe ignition in this cylinder.



I put a touch of marvel mystery oil in the crankcase and some in the fuel to see if that would make a difference but so far it has not. I also ran some seafoam through the carb to see if that would clear it would in case it was carbon on a valve, but that also hasn't made a difference. I am thinking about trying to run the seafoam through the gas tank next.



I am not seeing any blue or white smoke out my exhaust, so I don't think it's my rings. I am really thinking I have something going on with one of the valves on #4, and leaning towards the exhaust valve. When I had a stuck intake valve I had oil coming out of the exhaust and that is not happening and the cylinder appears to be firing based on the plug condition, which is why I am thinking exhaust.



I bought a leak down tester but I have never used one before. I plan to try and use that to see if I can narrow it down - if I understand how to use it correctly, I believe if it's an exhaust valve I should hear the pressurized air coming from the exhaust, intake would come from Carb, rings would come oil dipstick tube, and a blown head gasket I would see air coming out of the radiator?



In the meantime...Is there anything serviceable from the top without removing the head if there is any issue with one of my valves? Could it be a weak valve spring? Are there any valve seals or anything I can service from the top side of the head? Are there any treatments like MMO you might recommend specific to valve issues if thats truly what I have going on?



I've never pulled a head or manifold, and I just got this thing running truly right for the first time ever and I really don't want to have to start pulling the engine apart right now. I also just dumped a ton of money into this after a pretty bad wreck in November so really hoping I can resolve this one on the cheap.



Thanks for any guidance you might have on this one, as always I really appreciate it. I will let you know what I find when I get the chance to run the leak down test.

#4 plug when I first pulled it



Brand New #4 after about 30 miles



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post #2 of 20 Old 05-14-2020, 04:24 PM
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I think the spark plug threads are 14mm. I would get an adapter for your compressor for the spark plug thread
and put a little compressed air in number 4 to see where the air is going. 50 psi is really low.

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post #3 of 20 Old 05-14-2020, 04:50 PM Thread Starter
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The leak down tester comes with the correct adapter for applying compressed air.

The instructions say to remove all spark plugs - is that really correct? For a V8 do I need to remove both sides or just the side Iím testing. I got the harbor freight special, though it was still $80 bucks so hoping this thing works.

I donít have much experience with valve issues so Iím kind of grasping at straws as to what would cause such a drastic change in a cylinder compression like this. It doesnít appear to be a stuck valve, but maybe it is?

When my #6 intake valve got stuck I freed it up with MMO and a week of nightly light taps with a rubber mallet.


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post #4 of 20 Old 05-14-2020, 05:57 PM
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Hi try squirting some oil into the #4 hole and rerun the compression test if it comes up you have bad rings in the cylinder.
good luck
tim
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post #5 of 20 Old 05-14-2020, 06:12 PM Thread Starter
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Thanks Tim, I will try that.


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post #6 of 20 Old 05-14-2020, 06:35 PM
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hello

doing a leak down isnt really that hard. i do it on small aircraft engines all the time. does it have 2 gauges on the tester? if so one should be air going in. the other gauge is the pressure in the cylinder. what you have to do is bring the piston to top dead center to do it. where both valves are closed. then you screw the hose into the spark plug hole. we apply 80 psi. then we have a certain amount of air pressure in the cylinder for ours to be good. we have to have at least 60 psi left in the cyl. but thats a aircraft engine. the theory works the same for a car engine to. you are correct about where you will hear air coming from. if the valves or rings are bad. one way to see a little easier then hearing is to also put some smoke above the carb, near exhaust, near dip stick tube while doing. then you can see if the air is leaking there it will blow the smoke away quickly.
another possibility also is a lifter or cam going bad on that cyl. that can also explain the ticking you are hearing. if you take off that valve cover you can feel the rocker. if a lifter is collapsed you will feel it. also can watch the rocker while slowly turning the engine by hand and see if it looks like both valves are opening fully. can compare to the other cylinders.

just some ideas

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post #7 of 20 Old 05-15-2020, 06:14 AM Thread Starter
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Thanks for the tips old school. I did take a video of the valves operating with the valve cover off, and they appear to be working okay but I didnít take measurements or anything to compare the rocker travel.

Can a lifter be pulled out and replaced through the pushrod hole without removing the head? I am wondering if I can pull those out and drop new ones in there. That would allow me to possibly see the cam lobe too. I guess a worn lobe could definitely be it...a worn cam lobe would ultimately reduce the stroke of the pushrod and in turn not fully close a valve.

Iím going to try and get the leak down test performed today. Thanks for the smoke tip, thatís really helpful. I have a length of rubber hose I can use to listen.

I should note I also used a mechanics stethoscope and listened all over the engine and cannot find where the sound is coming from. Thatís why I thought for sure it was just an exhaust leak, until I got the bad compression test.

Would an exhaust valve not fully closing make a tap tap sound in the exhaust?


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post #8 of 20 Old 05-15-2020, 11:05 AM
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hello

hopefully you will get a direction once you do the leak down check. if rings are sticking. you can try and put some marvel in the sparkplug hole. sometimes it will loosen the crap that is sticking the rings.
the beauty of a v8 is you can remove the lifters without taking a head off. but you do have to remove the intake manifold to do so. that isnt really all that bad. once you remove the intake and valley tray(gasket) and you will see in the valley the tops of the lifters. a easy way to remove them is with a small 90 degree pick and hook the retaining clip inside the lifter.

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post #9 of 20 Old 05-17-2020, 12:03 PM Thread Starter
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I was able to perform the leak down test day and it looks like my initial diagnosis was actually on the right track. At TDC there is close to 100% loss of pressure through the exhaust, as shown by air flowing out of the passenger side exhaust pipe. To be sure the intake valve was operating correctly, I kept the pressure on the cylinder and hand cranked the engine through all four strokes. You could hear when the intake valve opened by coming out of the carb, and then it stopped when the intake valve closed. No matter what I did I could not get the air to stop coming out of the exhaust.

I never heard any sound come through the oil dipstick or saw any motion or bubbles in the coolant in the radiator so that is good news.

So it looks like I have a significant issue with the exhaust valve on #4 and that is the cause of the tapping sound I am hearing. Would a stuck open exhaust valve also cause oil pressure fluctuations and backfires?

The pushrod is not bent so it is not that. I recorded the valves operating with the valve clover off and the coil disconnected to see if the rockets were going up and down as they should and they were. It would be really hard to tell though if one is off by a millimeter versus the others though.

Without knowing if itís a seal, valve stem or something else, are there any tricks you all know I can try to possibly get this valve working again before I have to remove the head? I am hoping to avoid needing to take the head off for a valve job right now.

I am going to start sourcing a backup 304 and/or new heads - either rebuild an entire motor, just the heads, or get a crate motor. Will compare costs. Being the second valve issue I have had on this same head I donít think the valves are in great shape all around.

Would love to know if anyone has a tip to try and get this valve working just as a short term until I can get this repaired or replaced correctly.

Thanks.


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post #10 of 20 Old 05-17-2020, 12:41 PM
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hello

how many miles do you have on the engine? with those other compression numbers if it was me i would pull just the heads and fix the exhaust valve. then change oil seals, and check all the other valves. in the old days we did that all the time. there is no sense in rebuilding a whole motor for just one cylinder that can be fixed. on aircraft piston engines we do a cylinder at a time all the time. no one want to spend 12k to 40k to rebuild the aircraft piston engine when its just one cylinder leaking. especially for a valve. just something to think about.

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post #11 of 20 Old 05-17-2020, 12:59 PM Thread Starter
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Hey old school - I definitely might do that and I appreciate your advice. I guess what I mean is depending on what the costs end up looking like, those are the other options I am considering. I do think it seems like the engine is in good shape with the other compression numbers and maybe it just needs a good valve job. Iím assuming that was probably routine maintenance on these things back in the day.

As for mileage I donít really know. The PO tried to tell me it was 18k miles all original (and promoter read 18k when I bought it) but the rest of the Jeep wasnít all stock so I really donít know if he was telling the truth. Itís the original 304 but thatís all I know for sure.

If itís easy enough to pull a head off and fix a valve I am definitely up for tackling it like I have everything else on this Jeep. I was under the impression head jobs required machining which I wouldnít be able to do and donít know if a shop around here that does that kind of work.

As a backup I have a lead on two new heads for a 304 but I need to find out what they cost.

What would be the most common cause for this kind of single valve failure and whatís involved to repair it? When I had a stuck intake valve I put MMO on it and tapped it with a mallet. I might try that just to see if it shakes anything up.


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post #12 of 20 Old 05-17-2020, 01:50 PM
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hello

yes doing valve work does include some machining. having a lead on a set of heads might be a good option. if your heads are original 75 heads the valve seats are probably not hardened exhaust seats. in 75 the gas was still leaded so it lubricated the valves and help keep the seats cool. with unleaded gas it doesnt have the lead anymore. it wasnt till about the early 80's they learned they had to harden the exhaust valve seat to keep from burning up. it was pretty common back in the 80's to do alot of valve jobs because of the unleaded gas on the old heads. we used to have hardened seats installed to take care of the problems of unleaded gasoline.

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post #13 of 20 Old 05-18-2020, 11:56 AM
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My thought is maybe you have a choke heat riser issue in the head. There is a passage that goes from the intake to the exhaust port in cylinder #4. These get filled with carbon and could be the reason for your exhaust leak.

Pull the intake and look for this. See attached.

I have a 1972 Jeep with 57,000 miles. I change the oil every year (average about 200 miles a year) and I was starting to see issue with this.
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1972 CJ5 304 V8, 3 speed manual trans
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post #14 of 20 Old 05-19-2020, 09:45 AM Thread Starter
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Interesting NorCal. That is a LOT of carbon buildup, crazy. If that passage gets clogged like that would it cause the low compression I am seeing? Did you have tapping sounds in your exhaust too from that buildup?

I've never pulled my intake so I am gonna start looking into what I need to do. From what I understand I would need to pull the intake to remove the heads anyway.

In your case did you simply clean out that passage and put everything back together and all was good?

I also change the oil every year, sometimes twice, but I have bene putting closer to 3k miles a year on the motor since I got it.

In the meantime I am trying to find out what this guy wants for the new heads and also see if I can find a place near me that would do a valve job on my motor for a reasonable price if I source the parts.

Quote:
Originally Posted by Nor Cal CJ5 View Post
My thought is maybe you have a choke heat riser issue in the head. There is a passage that goes from the intake to the exhaust port in cylinder #4. These get filled with carbon and could be the reason for your exhaust leak.

Pull the intake and look for this. See attached.

I have a 1972 Jeep with 57,000 miles. I change the oil every year (average about 200 miles a year) and I was starting to see issue with this.
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post #15 of 20 Old 05-19-2020, 07:19 PM
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I had the heads rebuilt. This port that is clogged goes directly into the #4 cylinder. There is a similar port on the other head. There was some marks on the top of the piston from carbon getting into the cylinder on my engine. I caught the issue before something more serious happened. I didn't have a specific issue yet - but I did have smoke out the exhaust from engine braking down grades and knew that the heads needed some care. I have a great head guy in Carson City, NV. He has built many a turbo head for me in the past. So good to work with someone you know and trust.

If your port is clogged like mine, you may have carbon into the cylinder and effect a ring as mentioned previously in this thread.

If you don't find an answer with your leak down tests, my recommendation would be to pull the intake and see what you find. You can always pull the choke heat tube from the top of the intake and see how clogged it is before you remove the intake, but I'd bet heavily that the heat tube location is full of carbon...

1972 CJ5 304 V8, 3 speed manual trans
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