Strange Exhaust Noise - Help with diagnosis please

[Gambit]

Hey all -
So my '88 4WD is making some interesting sounds from the exhaust - and I'm not sure what it is, might be running rich? - I've checked all the things I could think of and replaced a bunch of them - plugs, rotors, wires, coil, checked timing, looked for vacuum leaks, a bunch of sensors, auxiliary air valve etc. Not getting any codes and have kinda run out of ideas.

It otherwise is running pretty well!

Please take a listen here: https://drive.google.com/file/d/1DXNG0uWqNdAPW6GlZLFHbMaEitGSYO4j/view

Would love any ideas and thank you!

[PrecisionDriven]

I attempted to listen but have to be granted access to it on Google Drive, so I requested it. Will give it a listen once I get the permission.

[Gambit]

Thanks! I updated it to public... didn't realize nobody could access :doh:

[bikeregg]

It's not smoking, does the exhaust have a gasoline or acrid smell to it? If anything, I'd think it might be a bit lean. Idle it for 10 or so minutes, after it cools, pull the spark plugs. That will tell you the fuel/air mixture and what's going on in the combustion cylinder. You can google "read spark plugs" or something like that for reference pictures and what each means.

[JPERL]

sounds like a lean misfire to me more air in the mixture than fuel. Vacuum leaks between the AFM and intake can cause this. Also Since the exhaust and intake ports are on the same side of the engine it is possible that if the intake gasket has a leak it can be pulling in air from outside or from and ajacent exhaust port. If you have emissions testing in your state you likely would see elevated HC in the exhaust.

As a test I would be curious to see if you disconnected your 02 sensor the ECU would default to a rich fuel mixture and in doing so I would listen to the exhaust under full rich condition to see if the exhaust sound improves, less popping, then you could see for certain you have a lean condition

[PrecisionDriven]

Sorry for slow secondary response but have been very busy lately. My best guess is that the catalytic converter is wearing out internally. This article mentions some basic steps you can take to try and diagnose:
https://www.yourmechanic.com/article/how-to-identify-car-exhaust-problems-by-spencer-cates

I tried to find a collection of example sounds on YouTube but no luck.

My theory is that the baffles inside the cat are breaking down, allowing sounds from the engine operation that are normally deadened, to emit in a different sound that is more of a "raw" tone. Once you get it figured out please follow up. Good luck with it.

[Burntboot]

PD - your theory is flawed.
You are confusing mufflers with convertors, the internal construction is completely different.
While a muffler has internal baffles that can come loose, a convertor does not.

[Carbonized]

BB, the catalytic converter does not have baffles but is has a honeycomb core that can burn itself loose in the can.(Ask me how I Know) If it melt itself enough it can turn sideways in the "can" and do all kind of weird things. But you would definitely throw a code with the the post-cat OX sensor.

Cat on my Mini before melt down.

9216

The first time, it turned sideways and throw the engine on "Limp mode" at 120 mph in the middle of a pack of racing cars at Morosso racetrack : New cat. and new underwear:roflmao: Second time, I found that 6" diameter thingy stuffed in the 3inch pipe it looked like a cork half way down the neck of a bottle of wine, and the car ran fine. My exhaust guy and I where :rol:. I just don't think you can run the van exhaust hot enough for this to happen!

[Burntboot]

Am with you Carbon, I have been there and done that too.:wnk:

The point I was trying to make was that an odd sound in the exhaust, isn't typically going to be Cat related.
Cat failures usually cause drivability issues.

Limp mode at 120MPH on the track! Yikes!

[Carbonized]

Yep it get your attention :yes:.
The thing with Gambit problem is, it doesn't throw a code :cnfsd:. I don't see it on the list of things he has done but I assume he checked the air filter. It almost sound like when you leave the "choke" on for too long on a carburated engine

[Gambit]

Yep it get your attention :yes:.
The thing with Gambit problem is, it doesn't throw a code :cnfsd:. I don't see it on the list of things he has done but I assume he checked the air filter. It almost sound like when you leave the "choke" on for too long on a carburated engine

This is all awesome guys - let me add a bit more context:

NEW Recently -
Intake manifold gasket
EGR Gasket
Spark Plugs
Rotor
Wires
Used Aux Air Valve
Most sensors
Coil
Also just passed CA smog very easily

Under a year old:
Exhaust system (all new with the exception of a used flange) - Cat was a cheap California legal one
New engine head
Air Filter
Fuel Filter
Inline fuel check valve

Under 2 years old:
O2 sensors - both
Full top and bottom end engine rebuild

Man - I'm sure I'm missing some things - but that is a start.

I pulled the plugs and double checked the timing again this weekend - tried adjusting to both directions to see if it would level things out - it did not (at least not consistently)

Plugs all looked the same (and would change idle when each one was pulled while running). I will post photos soon of them shortly.

I can't find any vacuum leaks - did the propane test to see if I could get the idle to increase and didn't find anything... when I did have a leak, this was instantly recognizable with this test. (Not ruling a vacuum leak out completely, but I can't locate it - give me a quick reminder on what AFM stands for and I can try checking more closely- note that the intake manifold gasket is brand new)

Will try tonight disconnecting the o2 sensors and see if that changes anything

This is great all - I really appreciate it!

[Gambit]

Plug photos - Slight brain fart while taking the pics and didn't get the right angles. Van had been idling for about 15 minutes... that being said here they are:

9224
922592269227

[Gambit]

Looking again it appears I maybe have oil on the threads of all of the spark plugs? I didn't think too much about this since they all had it... seems weird that they all would have it? Maybe a valve cover gasket?

I am NOT going through very much oil at all, have to add like 1/3 a quart every 3-5K

[Gambit]

OK - there is definitely oil in the spark plug wells - this is almost certainly a valve cover gasket that needs repair?

9228

[Gambit]

To be clear - oil is not getting past the spark plug threads and into the spark plug "tube" but is present underneath it.

[Flecker]

There re o-rinngs that fit under the "spark plug tubes", that are more than likely leaking. This lets oil creep down into the tubes.

It won't cause the exhaust studder. I second the Catalytic malfunctioning, casing that putter.

[Gambit]

There re o-rinngs that fit under the "spark plug tubes", that are more than likely leaking. This lets oil creep down into the tubes.

It won't cause the exhaust studder. I second the Catalytic malfunctioning, casing that putter.

Thanks Flecker - I'll swap those O-Rings tomorrow and see if I can talk to an exhaust guy. I recently had the Cat replace, but my confidence level in the place that did it is low... so who knows.

[Flecker]

Thanks Flecker - I'll swap those O-Rings tomorrow and see if I can talk to an exhaust guy. I recently had the Cat replace, but my confidence level in the place that did it is low... so who knows.


Could also be worth crawling under and taking a gander for a leak. My 92 Pickup with a 22re made the same racket when I had a leak in the exhaust. I didn't see you already replaced the cat/

[bikeregg]

That first spark plug doesn't look like it is firing. It looks very clean.

Second plug looks like a slight lean condition. A touch of white ash forming on the tip.

Third plug looks like it may not be firing all the time, gas is washing carbon off the plug.

Fourth plug is what a spark plug should look like, slight brown color with a slight bit of carbon.

Now we know what they look like at idle, when you have an opportunity, take it for a short drive. Get it up to temperature and when you park it, immediately shut the engine. Don't let it idle, that will allow the gas to wash the first and third plug and skew the results.


How many miles on the engine? The diesel engines are shim adjusted, I believe there is a valve adjustment on the gasoline engines. When you pull the valve cover to replace the seals, it might be a good idea to do a valve adjustment. Might be good to do a compression check as well.


Is this a carbureted or fuel injected engine? If fuel injected, you might check out the injectors. You might have 1 or 3 that aren't quite doing the job (at least at idle). Good candidates for cleaning or replacement.

I'm not denying the possibility of an exhaust issue, but I'm trying to think through other possibilities as well.

[Carbonized]

The bugger with the bad cat theory is that it's not throwing a code! Anything that has to do with emissions is usually throwing codes left, right, under and over!

[Gambit]

That first spark plug doesn't look like it is firing. It looks very clean.

Second plug looks like a slight lean condition. A touch of white ash forming on the tip.

Third plug looks like it may not be firing all the time, gas is washing carbon off the plug.

Fourth plug is what a spark plug should look like, slight brown color with a slight bit of carbon.

Now we know what they look like at idle, when you have an opportunity, take it for a short drive. Get it up to temperature and when you park it, immediately shut the engine. Don't let it idle, that will allow the gas to wash the first and third plug and skew the results.


How many miles on the engine?
The diesel engines are shim adjusted, I believe there is a valve adjustment on the gasoline engines. When you pull the valve cover to replace the seals, it might be a good idea to do a valve adjustment. Might be good to do a compression check as well.


Is this a carbureted or fuel injected engine? If fuel injected, you might check out the injectors. You might have 1 or 3 that aren't quite doing the job (at least at idle). Good candidates for cleaning or replacement.

I'm not denying the possibility of an exhaust issue, but I'm trying to think through other possibilities as well.



Thanks this is all super helpful.

On the plugs - wouldn't pulling each one and having the idle change mean that they were all firing? I'm happy to pull them again and take a look without letting them idle.

These were pulled with engine up to temp (180). Engine has 310k miles :) But had top and bottom end done recently.

Injectors were just cleaned while intake manifold was off for the Auxillary air valve replacement... so I'm hoping they are OK, but not ruling them out completely yet. I had removed the intake manifold and done the air valve - which didn't fix another issue I was having (cold start & high idle along with this exhaust sound), so I took it to a shop who thought it was all the same issues I had, they figured I screwed something up in there. So they also removed the intake manifold and while they were in there cleaned the injectors (all of this did nothing to remedy the current issues)

[Gambit]

Am I off base with the valve cover gasket and thinking it is leaking?

[bikeregg]

Am I off base with the valve cover gasket and thinking it is leaking?
Yes, the O ring around the spark plug tunnel is leaking, but doesn't normally cause an issue until the tunnel fills with oil, over the top of the spark plug boot and prevents a good connection.

Ideally, all the spark plugs should be in the same condition. If a plug looks different, there is a different condition occurring in that cylinder than the others. All of your spark plugs look different. I'll say it again, when you pull the valve cover, check the valve clearance. Nowadays, with hydraulic valves and computer controlled variable valve timing, it would be very easy to overlook the valve clearance as a culprit. And, just because the injectors were cleaned does not mean they are working properly. You need special equipment to check the injectors for proper operation. RockAuto has a couple of vendors that offer injector rebuild services or you could look for someone in your local area.

[Gambit]

Yes, the O ring around the spark plug tunnel is leaking, but doesn't normally cause an issue until the tunnel fills with oil, over the top of the spark plug boot and prevents a good connection.

Ideally, all the spark plugs should be in the same condition. If a plug looks different, there is a different condition occurring in that cylinder than the others. All of your spark plugs look different. I'll say it again, when you pull the valve cover, check the valve clearance. Nowadays, with hydraulic valves and computer controlled variable valve timing, it would be very easy to overlook the valve clearance as a culprit. And, just because the injectors were cleaned does not mean they are working properly. You need special equipment to check the injectors for proper operation. RockAuto has a couple of vendors that offer injector rebuild services or you could look for someone in your local area.

Thanks - really appreciate your patience, I am still learning about reading plugs and well, lots of other things. I wouldn't be shocked if it is the injectors. I'm having my exhaust guy take a look tomorrow AM. If that yields no luck, I guess I am pulling the intake manifold again...

[Flecker]

The bugger with the bad cat theory is that it's not throwing a code! Anything that has to do with emissions is usually throwing codes left, right, under and over!

I don't have a cat, and mine throws no codes. O2 Sensor was upstream of it anyways on my 87

[Gambit]

I don't have a cat, and mine throws no codes. O2 Sensor was upstream of it anyways on my 87

Unfortunately they started using 2 at some point - my '88 has one pre and post Cat

[Gambit]

Exhaust checked out as all tight with no leaks and they said the cat was good. Next steps I suppose is to get back into the injectors, it will be a little time until I am able to do so. Thanks again to everyone for the help - I'll report back.