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Tue Sep 27, 2016 8:50 pm

'76 GD engine, Fuel Injected, Solid Lifters, stock everything

Before I begin checking my Air/Fuel mixture, I wanted to make real sure there weren't any exhaust leaks. I don't know that I have a leak, just want to know for sure one way or the other.

I got the impression from what I'd read here that blocking off the tail pipe with a wet rag should stall the engine if there were no exhaust leaks.

So I let the engine warm up, then took a thick, folded-over-twice, water-soaked rag in my gloved hand and pressed it tightly over the end of the exhaust pipe for a good, long while. The engine RPM dropped considerably, but it would not stall.

Is it true that the engine should stall if the tailpipe is blocked tightly with a wet rag? Or, is there more to it than that?

Or, would I be better off just using a smoke tester (going to build one tomorrow to check for vacuum leaks)? Has anyone here actually used one to find exhaust leaks? Any special procedure? What did you use to seal the small smoke tester hose to the tailpipe (mine is stock VW)?

Tue Sep 27, 2016 9:39 pm

I just used a regular shop towel that I held over the pipe. I've always done it cold so I could reach around the flanges. You have about 20 seconds before the manifold flange heats up to be untouchable. I had an audible leak, checked which side as described above and replaced my gasket. Good to go. I was about to build the tester, but didn't need to. If you're trying to be that precise, then do the smoke test.

Tue Sep 27, 2016 10:01 pm

ac78 wrote: I just used a regular shop towel that I held over the pipe. I've always done it cold so I could reach around the flanges. You have about 20 seconds before the manifold flange heats up to be untouchable. I had an audible leak, checked which side as described above and replaced my gasket. Good to go. I was about to build the tester, but didn't need to. If you're trying to be that precise, then do the smoke test.
Sorry, not sure I'm following what you've said here. How does this relate to my questions? Maybe I'm just not reading it right. You're saying that you held a shop towel over the tailpipe and then felt around with the other hand for exhaust leaks at the flanges?

Tue Sep 27, 2016 10:06 pm

Pressurize your exhaust with a CLEAN shop vac. Make sure the filter is clean and you've washed out the canister. You basically remove the hose from the suction side of the shop vac and put the hose in the exhaust side of the shop vac. Put the hose over or in the exhaust pipe. Use duct tape to seal it up. Turn it on and then spray soapy water on all the junctions looking for bubbles.

I had an exhaust leak that was driving me batty! Doing this above helped me find it in a couple of minutes.

Tue Sep 27, 2016 10:17 pm

wcfvw69 wrote: Pressurize your exhaust with a CLEAN shop vac. Make sure the filter is clean and you've washed out the canister. You basically remove the hose from the suction side of the shop vac and put the hose in the exhaust side of the shop vac. Put the hose over or in the exhaust pipe. Use duct tape to seal it up. Turn it on and then spray soapy water on all the junctions looking for bubbles.

I had an exhaust leak that was driving me batty! Doing this above helped me find it in a couple of minutes.
Hey, that sounds like a great idea! So long as there isn't a leak that opens only when cold, it would sure beat working around a hot exhaust. I have a fairly powerful shop-vac. Should I have any concerns about overpressurizing anything?

Tue Sep 27, 2016 10:21 pm

Credit for the idea goes to someone on youtube that I got the idea from. I was also tired of burning myself! :D

As far as over pressurizing? I didn't worry about it and had no ill effects when I did this to my bus. I literally turned the vacuum on with a hand spray bottle with soapy water in my hand and sprayed each junction. Two minutes later, I found the leak and turned the vacuum off. If you have the super duper shop vacuum, you could simply not seal up the hose to exhaust so some pressure will vent out. It really doesn't take much pressure from the vacuum for the leaks to appear with the soapy water.

From what I found when searching exhaust leaks, many folks do this exact thing with many different makes of cars/trucks.

Tue Sep 27, 2016 10:32 pm

wcfvw69 wrote: Credit for the idea goes to someone on youtube that I got the idea from. I was also tired of burning myself! :D

As far as over pressurizing? I didn't worry about it and had no ill effects when I did this to my bus. I literally turned the vacuum on with a hand spray bottle with soapy water in my hand and sprayed each junction. Two minutes later, I found the leak and turned the vacuum off. If you have the super duper shop vacuum, you could simply not seal up the hose to exhaust so some pressure will vent out. It really doesn't take much pressure from the vacuum for the leaks to appear with the soapy water.

From what I found when searching exhaust leaks, many folks do this exact thing with many different makes of cars/trucks.
Thanks, Bill. I'll try this out tomorrow! :D

Wed Sep 28, 2016 8:43 am

When I use my smoke tester I plug the smoke machine outlet hose into one of the FI intake plenum vacuum ports. I plug the throttle body side by wrapping the air filter with cellophane and then putting the air box back together. I plug the exhaust pipe with cellophane also. Start up the smoke machine and start blowing smoke into the engine. As you find leaks, plug them. More leaks will appear, plug those. Once all of the intake leaks are plugged, the smoke will make its way into the exhaust system through open exhaust valves. Then you'll find the exhaust leaks. So, the answer is.... make yourself a smoke machine and do all your leak checks at the same time.

Wed Sep 28, 2016 10:46 am

[/quote]
Sorry, not sure I'm following what you've said here. How does this relate to my questions? Maybe I'm just not reading it right. You're saying that you held a shop towel over the tailpipe and then felt around with the other hand for exhaust leaks at the flanges?[/quote]

Didn't have a partner so yes, cover with left hand and feel for air with the right hand. I really only needed to feel which side, it was going to be pretty obvious as you can see... I'm not sure why anyone would have used this style gasket in the first place. It is a soft foam/paper like feel with a screen in the middle.

It sounds like you are looking for a more surgical approach :wink:

Wed Sep 28, 2016 11:17 am

aeromech wrote: When I use my smoke tester I plug the smoke machine outlet hose into one of the FI intake plenum vacuum ports. I plug the throttle body side by wrapping the air filter with cellophane and then putting the air box back together. I plug the exhaust pipe with cellophane also. Start up the smoke machine and start blowing smoke into the engine. As you find leaks, plug them. More leaks will appear, plug those. Once all of the intake leaks are plugged, the smoke will make its way into the exhaust system through open exhaust valves. Then you'll find the exhaust leaks. So, the answer is.... make yourself a smoke machine and do all your leak checks at the same time.

Thanks for the smoke tester tips. That sounds like an interesting "top-down" approach, too. Didn't realize that in practice injecting smoke at an intake plenum port would reach enough volume to flood an entire exhaust system. Haven't done this before, so should be interesting!

Wed Sep 28, 2016 11:19 am

I was thinking about making my own smoke machine but decided it was just easier to buy one on eBay for around $100

Wed Sep 28, 2016 11:23 am

ac78 wrote: Quote:
Sorry, not sure I'm following what you've said here. How does this relate to my questions? Maybe I'm just not reading it right. You're saying that you held a shop towel over the tailpipe and then felt around with the other hand for exhaust leaks at the flanges?

Didn't have a partner so yes, cover with left hand and feel for air with the right hand. I really only needed to feel which side, it was going to be pretty obvious as you can see... I'm not sure why anyone would have used this style gasket in the first place. It is a soft foam/paper like feel with a screen in the middle.

It sounds like you are looking for a more surgical approach :wink:

OK, I understand now. Not sure how you could span the distance between a tailpipe on the rear right and flanges over at the front left, but it worked for you. That was one sad looking fried gasket you showed there. Probably something a PO had lying around or got some bad advice from a vendor.

Actually, not looking for a surgical approach, just wanting to check whether there are any leaks system-wide. As I mentioned in my original post, I don't know whether there is an exhaust leak or not. With the engine running, there's enough air blowing over the engine and solid lifter noise that its hard for a first time aircooled VW owner to know what's normal.

Wed Sep 28, 2016 12:03 pm

To test the Exhaust through the Intake, you may need to turn the engine over by hand in order to get one of the cylinders in overlap. Valve overlap is when both Int and Exh are open at the same time. This only happens a small percentage of the time.
Valve over lap would allow smoke to enter through the Intake valve and exit through the Exhaust valve.

Tcash

Wed Sep 28, 2016 12:10 pm

There are a handful of threads on making a smoke tester. I haven't completed it yet, but here is my core. I will introduce the air pressure from the bottom and I have a length of small clear tubing on the top nipple to connect to the manifold. I remove the air filter and use a sandwich bag on the backside of the AFM with the big O-ring clamp. That should allow it travel everywhere it needs to go.

Someone on another smoke tester thread had a brilliant idea of using a smoke bomb instead of trying to light rags soaked in diesel fuel or other really flammable source. People are using all kinds of stuff on you tube.

Smoke bomb- No big flame and almost no heat but plenty of smoke.

Thu May 12, 2022 6:29 am

wcfvw69 wrote: Credit for the idea goes to someone on youtube that I got the idea from. I was also tired of burning myself! :D

As far as over pressurizing? I didn't worry about it and had no ill effects when I did this to my bus. I literally turned the vacuum on with a hand spray bottle with soapy water in my hand and sprayed each junction. Two minutes later, I found the leak and turned the vacuum off. If you have the super duper shop vacuum, you could simply not seal up the hose to exhaust so some pressure will vent out. It really doesn't take much pressure from the vacuum for the leaks to appear with the soapy water.

From what I found when searching exhaust leaks, many folks do this exact thing with many different makes of cars/trucks.

Revisiting this old thread, wondering if the pushrods need to be removed in order to ensure all valves are closed to maintain pressure when using the shop vac? Otherwise, you would get some leakage into the combustion chambers and out the carbs potentially.

Thu May 12, 2022 8:12 am

boxer74 wrote: wcfvw69 wrote: Credit for the idea goes to someone on youtube that I got the idea from. I was also tired of burning myself! :D

As far as over pressurizing? I didn't worry about it and had no ill effects when I did this to my bus. I literally turned the vacuum on with a hand spray bottle with soapy water in my hand and sprayed each junction. Two minutes later, I found the leak and turned the vacuum off. If you have the super duper shop vacuum, you could simply not seal up the hose to exhaust so some pressure will vent out. It really doesn't take much pressure from the vacuum for the leaks to appear with the soapy water.

From what I found when searching exhaust leaks, many folks do this exact thing with many different makes of cars/trucks.

Revisiting this old thread, wondering if the pushrods need to be removed in order to ensure all valves are closed to maintain pressure when using the shop vac? Otherwise, you would get some leakage into the combustion chambers and out the carbs potentially.
If there's enough getting by an open valve's cylinder into the case that a shop vac can't maintain any pressure in the exhaust you have much bigger issues than a little exhaust leak.
Don't bother to remove the rockers, it'll be fine.

Thu May 12, 2022 8:28 am

busdaddy wrote: boxer74 wrote: wcfvw69 wrote: Credit for the idea goes to someone on youtube that I got the idea from. I was also tired of burning myself! :D

As far as over pressurizing? I didn't worry about it and had no ill effects when I did this to my bus. I literally turned the vacuum on with a hand spray bottle with soapy water in my hand and sprayed each junction. Two minutes later, I found the leak and turned the vacuum off. If you have the super duper shop vacuum, you could simply not seal up the hose to exhaust so some pressure will vent out. It really doesn't take much pressure from the vacuum for the leaks to appear with the soapy water.

From what I found when searching exhaust leaks, many folks do this exact thing with many different makes of cars/trucks.

Revisiting this old thread, wondering if the pushrods need to be removed in order to ensure all valves are closed to maintain pressure when using the shop vac? Otherwise, you would get some leakage into the combustion chambers and out the carbs potentially.
If there's enough getting by an open valve's cylinder into the case that a shop vac can't maintain any pressure in the exhaust you have much bigger issues than a little exhaust leak.
Don't bother to remove the rockers, it'll be fine.

Thanks.

Thu May 12, 2022 11:05 am

I have a smoke tester and it's found many leaks. My buddy Ken at the busco has a BMW that's been giving him codes. He recently bought his own smoke tester and quickly found the source of his intake plenum leaks. Smoke testers are a simple way to find leaks.