Part of the front suspension (lower ball joints) has to be taken apart to remove & replace the engine, so there is your dummy at work.

Code 420 almost always means the cat is worn out (or has been hollowed out to be a straight pipe) and is not reducing pollution like it should, but that situation alone should not cause any major driveability problems.

 

A stuffed cat will cause stalling.

http://www.aa1car.com/library/exhaust_backpressure.htm - read this, get a backpressure gauge, report with results.

 

Since OP says the engine can rev high when standing still with clutch in or out, I would guess the cat is not clogged bad enough to cause engine to operate anomaly. But there is a slight possibility the mechanic who had performed engine swap may have dropped in the pipe that is floating inside and causing partial blockage as the car running. This FO may explain that the car is behaving in the way it is, intermittently.

Having visual inspection is the best way to find out but light taps with hammer on the exhaust system may reveal something floating inside the pipes.

Good luck with your car.

 

Either way a back pressure check will find if the issue is in the exhaust.

 

I saw the link. I don't know about other people, but, to me, it doesn't look like a cost or time effective way to check the exhaust system.

I doubt if the check will be effective or meaningful for the subject car on this thread.

 

And I doubt that you have EVER even seen this check performed. It is VERY Effective and about the only real way to diagnose the exhaust system quickly. Unscrew the front O2 sensor, screw in gauge, check readings. Unscrew rear O2 sensor, screw in gauge, check readings. Compare. If there's a big pressure drop across the cat on a warm car at 3K RPM than the cat is bad. If there's back pressure over 3-4psi at the rear O2 sensor than the muffler is clogged. It's easy, quick and effective and you don't have to remove your exhaust or bang on it... btw banging on exhaust usually lets rust inside loose and that WILL rattle and you'll risk clogs even more so.

 

Read my post. That says, " light taps". Not " bangs".
You can bang all you want if that is what you like to do.

Most of rust scale falling in the exhaust system would occur after the cat. Rust scale ahead of the cat is subjected to higher heat, velocity and vibration frequency, there is little chance of scale being that big and staying on the inside wall if you ask me.

Time advantage/concern......

My first recommendation is to remove the system and have visual inspection.

Should the "back pressure test" using the low pressure gauge reveal the cat to be clogged or faulty, the owner of the car would have to remove the exhaust system anyways. Either he does it by himself or have someone do it for him.

If he had the exhaust system removed at first, he would saved the time to rig up the testing device, never mind of time spend on finding and buying such testing device.

Cost advantage/concern.....

And how many times would average car owner would be checking his exhaust system using the device after initial use??? I wonder,,,,.For more than likely one time use test device, he would have to spend like $60 for a kit?? Or was it $170 for low pressure gauge?? Well, at any price, that is money you could have saved, in my mind.

Because of above two reasons I say, "it doesn't look like cost or time effective way to check the exhaust system. If you are in the auto repair business, it could be different story.

Reading this thread thus far, I think he (OP) is depending his mechanic for his automobile maintenance needs. Unless, OP makes a deal with his mechanic, the investment for the gauge that he is going to use for one time only is something he does not need to spend at this time. Especially, after $1200 he had spend already with not much good result.

And his problem is intermittent one. The car may have issue in some place else within the car. I just can't imagine where at this time. Anyways, that is why I think "the back pressure check" may not be effective or meaningful.

Oh, I have NEVER seen "the back pressure check" being performed on any car in my life. I don't work on cars for living. The chance for me to see such test is very slim.

Even when I was driving a 944 with AV-GAS and auto gas (50/50) in it for more than one year (60 miles each working days), I NEVER had issue with the clogged cat converter or CEL coming on with that car. You know AV-GAS contain lead,,,,.

The 93 Accord with 250000 miles I drive now has been fed with AV-GAS/Auto gas mix from time to time for the past two + years. I replaced OEM exhaust system to a header and cat-back system recently. I had a visual on OEM Cat converter. Although there was sign of lead deposit, none of core was appeared to be clogged. I did not replace it. The O2 sensor had some lead deposit but was not throwing any codes.

My point here is that if any one should have any issue with clogged cat or malfunctioning O2 sensor, I would be a good candidate. But, knock on wood, I have been lucky after all these years. Anyways, I have stopped using AV-GAS now. I hope I don't have to face for the need of conducting "the back pressure check" for years to come.