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Find a shop with an experienced technician that can do a back pressure test on the exhaust system. Theres 3 cats on that car. If you have one of them starting to clog up or something you can have a situation where without the a/c, the cooling system can handle the extra heat buildup, but once the a/c is on, it cant keep up with the extra heat from the condenser.

In some extreme cases, a completely clogged cat will not allow the vehicle to start and run. In slightly less extreme, you can get overheating issues, or even misfires. Usually accompanied with a lack of power on acceleration. In this case you may not have reached that point, and you have a partially clogged cat where its only slightly more difficult for the engine to breath out but enough to exceed the limit of the coolant system with extra load.

Also, is it just one car doing this now? or are both cars still having this same issue?
 

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Discussion Starter · #22 ·
Well, both of them just passed emissions. Still have that crap here. But both were emissions ready and passed. The SE can hold temp a lot better, but still runs hotter than in Seattle.

The GT is the one that is more problematic and what I’ve been focused on.

Would it still pass emissions fine with a clogged cat??? And have no other issues at all, and no CEL codes at all?

In my frustration, I just pulled the thermostat entirely just now as it’s a 10 min job. And ran with ODB data logging enabled.

With the AC on it still heated up to 197 just idling for about 8 minutes. Driving it, it went to 215 shortly. But it held.

A little faster to accelerate, and up a few small hills and it went to 230. With no thermostat, coasting down a hill at 50mph, it dropped to 222 in 30 sec. Then back up to 230 accelerating to 50mph. Cut off the AC and as usual, that dropped it to 220 in 90 seconds.

Its certainly related to engine RPM’s. But it’s also directly related to engine load as well. Which would explain why the AC on has an effect, along with added heat from the condenser.

Is there anything that would correlate with back pressure on the exhaust or a basic test that would at least hint of back pressure. This is a brand new thing to check. And it runs perfectly otherwise. No idle or power issues, mpg is normal. I’m thinking there should be something else that would be noticed on sensors, or performance if there was enough back pressure to make it overheat. But it’s never been brought up before, so any new ideas are good! I’m running out of possibilities!

I was starting to think there’s a blown head gasket, but it’s a hard to diagnose one that isn’t putting oil or exhaust in the coolant, or any coolant in the oil! But is somehow causing excess heat that even running with no thermostat puts it to 230F driving in normal traffic. And it’s only 92F outside. Wondering if it’s possible to have a blown head gasket that causes overheating but no exhaust is detected in the cooling system?

Many thanks!!!
 

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I'm also sorta leaning to an exhaust issue, most likely CAT. Yes, a clogged CAT can add engine heat at mid/high RPM.
Most emissions tests allow for "age/mileage" for the test, opening the numbers a bit as time goes on. It may not be enough to trip a CEL.
Now, the $100,000 question is......why both cars at the same time!!!??? Could be a change in fuel blend? Maybe more or less ethanol that previous?

Ponder with the fuel question, check different grades and blends. If no change, try test pipes in place of easy to get to CAT's.
 

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Discussion Starter · #25 ·
Hmm, ya, saw on a different post that a plugged CAT will throw a specific code.
Since there’s still emissions here, there’s lots of emissions repair shops. Perhaps they might have the best ability to test this as it’s pretty specific.
 

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Would it still pass emissions fine with a clogged cat??? And have no other issues at all, and no CEL codes at all?
Yes, can pass emissions test and not throw a light.

There is no code for a clogged cat. The computer will never throw a light for it. There are no codes even related to it.

The codes for a cat come up for the cat not doing its job cleaning the exhaust passing through it. This is measured by the secondary O2 sensor. Think about it: if the cat is clogged, less exhaust flowing through it, less for the sensor to read, computer thinks cat is doing its job.

A clogged cat also doesnt mean that the cat itself isnt functioning correctly for the exhaust passing through it in the first place.

That being said, a lack of performance issues would point away from a clogged cat. But like i said, a back pressure test is quick and easy to do, just a matter of finding a mechanic that has one. Or buy the tool and do it yourself.

 

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Discussion Starter · #27 ·
Can’t have enough tools! lol

At this point I just need to find a cause. Fixing the problem isn’t nearly as difficult!

thanks! Will report if there’s back pressure or not.
 

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Discussion Starter · #31 ·
YIKES!

I don’t have the gauge or such yet. But I might just return them as I’m not sure I even need to test it now.

Get this, my Tib isn’t rusty and the exhaust is stainless. So I ran it up to temp and did some exhaust temp checks with an IR thermometer.

Temps on the exhaust manifold cats were fairly even on inlet/outlet. About 220-240F.

But the main cat, oh man! 340F on the inlet, only 220F on the outlet!

I don’t think I need a back pressure test to say that cat is seriously plugged!
The plus, the whole bolt on assembly from manifolds, the flex pipe, and the cat is only $289 at Rock. No welding unless the bolts need to be cut. But then it bolts right up.

I’m tempted to just get that assembly and have a muffler shop just put it on for me. There’s no o2 sensors or such on the assembly and it’s a direct fit OEM.

What do ya think? Even if my readings weren’t perfect, that’s a 120 degree difference! Think that would make it overheat under load? I’m pretty sure that would do it without any CEL codes being thrown and without clear performance issues.

Very interested in your thoughts!

Thanks!
 

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Generally speaking, it should be the opposite. You could see up to 100 degrees hotter on the outlet. You need to make sure the car is at operating temp before you test though. I like to run the vehicle for 20 to 30 min before testing.
 

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Discussion Starter · #33 ·
Right, an article I read on cat testing said at least 20 degrees or more out from the cat. But it said if it’s equal, there’s a problem. But not a block, it’s just not doing what it should. And if it’s hotter on the in port, and cooler out, that’s some kind of block.

It explains why both cars overheat! Was fine in Seattle, but both were driven over passes and in heat while carrying a higher than normal payload for 1500 miles almost non stop. So at some point, the cats partially melted! So they still passed emissions, but they’re both restricted. At least it fits what’s happening. So if this fixes it, hopefully! I’ll know what my SE needs too! :)

Will see! It definitely needs a new cat. So why not. Just have to wait a week for it to get here. Can’t wait to see if this is it! I really hope so! I’m sick of not being able to jump on the throttle again!
 

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two cars though.... what are they putting in your fuel in seattle lol....
Starbucks......LOL.....

I have mentioned fuel since it's both cars.
 

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Seattle has one of the best fuels around... lol our premium 92 Octane Chevron has been dyno proven to be better than some 93 octane fuels found elsewhere. Not sure what makes our 92 so special but it makes more power than Vancouver BC's 93 Chevron and Shell.
 

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Discussion Starter · #37 ·
Lol! Most of the fuel was the $6 a gal stuff in CA! Since both would never get past 190F even on hot days in Seattle. And it was record heat when I relocated both cars, carrying a heavy load. I’m betting the cat partially melted.

Cat & pipe assembly gets here Thursday, hope to have it put on Friday. So I’ll have a look at the old cat then.
 
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