Exhaust manifold question

[2000flair]

A while ago we had a big exhaust leak on my 2000 Flair, Workhorse 7.4L. We took it to the shop and they replaced both manifolds.
2 days after we got it back, we went on the road and after 20 minutes the problem came back on the drive side only. We took it back to the shop and they fixed it under warranty, they said the manifold they installed was defective.
We drove it a couple of times, about 500km each time without a problem.
Last Friday we went out again and at some point the engine started stuttering, then vibrating hard enough that the steering wheel was shaking some. I slowed down and the vibration went away but the RV lost a lot of power and it started smelling like gas inside. I pulled over turned the engine off and looked around but didn't see anything abnormal (I am definitely not a mechanic but I tried all the same).
I started it again and went back on the road. The driving was better but the tapping noise from the manifold started a couple of minutes after that. Not very loud at first but by the time we reach our destination it was like the first time we had the problem.
I imagine the heads are not the problem since the shop replaced the manifold twice and never mentioned that to be a potential issue.
Is it possible that it's coming loose? Does anyone have an idea of what may be causing this to occur?
Thanks.

[2000flair]

Thanks for the advice, I’ll ask for a replacement kit in those lines.

[udidwht]

Upgrade the manifold bolts with ARP bolts. Much better quality than OEM.

[3_tons]

When you install a new exhaust manifold you have to use new OEM bolts…

3 tons

[2000flair]

udidwht wrote:
Sounds as if 1 or more spark plug wires may have burned. Check all your wires carefully. It helps to wrap them with high temp sheaths (8) from the end of the wire.

Also check each of your exhaust donuts where the exhaust manifold meets the pipes. The donuts can burn up and you'll get excessive noise combined with power loss.

Thanks, I’ll check the wires once more but they were replaced at the same time the manifolds were replaced the first time and they also added the temp sheaths.

I think there may be a problem with the combustion that makes the engine shake and loosens the manifold. The shop that installed them will take in the rig in 2 weeks to fix under warranty but seeing that it’s the 2nd time it happens i’d like to find the root cause so there is nit a third.
I will consider all the tips and advice everyone has given me and hopefully it’ll be corrected for good.

[udidwht]

Sounds as if 1 or more spark plug wires may have burned. Check all your wires carefully. It helps to wrap them with high temp sheaths (8) from the end of the wire.

Also check each of your exhaust donuts where the exhaust manifold meets the pipes. The donuts can burn up and you'll get excessive noise combined with power loss.

[theoldwizard1]

Likely no OEM manifolds available.

Aftermarket are "questionable" quality.

[2000flair]

RLS7201 wrote:

2000flair wrote:

RLS7201 wrote:
I suspect you may have a catalytic converter that is not flowing freely.
That will cause the exhaust manifold problems and power loss.

Richard

good point, is there a way for me to check that or is it something they must do a the shop?

A pressure gauge in the O2 sensor port would tell a lot.

Richard

Will take a loot at that for sure. Thanks

[2000flair]

fyrflie wrote:
I recently had my manifolds resurfaced. I installed them exactly to factory torque specs. New studs, nuts, etc.
After several hundred miles, I found two nuts that were loose. Retorqued to factory spec and so far no more issues.
Did the shop check the new manifolds to make sure the surfaces were flat ? Just because they’re new doesn’t guarantee anything.
Good luck and let us know how it all turns out.

Thank you, I'll ask them to check for that.

[RLS7201]

2000flair wrote:

RLS7201 wrote:
I suspect you may have a catalytic converter that is not flowing freely.
That will cause the exhaust manifold problems and power loss.

Richard

good point, is there a way for me to check that or is it something they must do a the shop?

A pressure gauge in the O2 sensor port would tell a lot.

Richard

[fyrflie]

I recently had my manifolds resurfaced. I installed them exactly to factory torque specs. New studs, nuts, etc.
After several hundred miles, I found two nuts that were loose. Retorqued to factory spec and so far no more issues.
Did the shop check the new manifolds to make sure the surfaces were flat ? Just because they’re new doesn’t guarantee anything.
Good luck and let us know how it all turns out.

[2000flair]

RLS7201 wrote:
I suspect you may have a catalytic converter that is not flowing freely.
That will cause the exhaust manifold problems and power loss.

Richard

good point, is there a way for me to check that or is it something they must do a the shop?

[2000flair]

jimh406 wrote:
Definitely possible for the manifold to be loose.

I have a feeling that may be it but I'm only learning all of this so I appreciate the input of more experienced people. https://www.rv.net/SharedContent/cfb/images/smile.gif

[2000flair]

IB853347201 wrote:
Could be an ignition issue. When was the last time you had your spark plugs and wires replaced?

They were replaced the first time the change the manifolds in July.

[RLS7201]

I suspect you may have a catalytic converter that is not flowing freely.
That will cause the exhaust manifold problems and power loss.

Richard