Resurrecting an old thread to post what I found with this problem.
This thread was helpful to me to think through what I needed to do, so thanks to 'mud for the collective knowledge.
My car is a '95, so two in-line cats, 166,000+ miles, Supercharged for 110,000+ miles, 33" tires, I pull a small trailer frequently.
I have been experiencing noticeable power loss over the last couple months, but blamed climbing in the mountains at elevation.
Finally, on the freeway last week, the characteristic whistle noise from the engine room, and very noticeable loss of power.
Pulled over, and the rubber hose from the EGR valve to the bottom of the modulator had blown off.
Having never heard of this particular issue, and being away from home, I plugged the hose back on only to have it blow off before I got back on the freeway again. The top plastic insert on the valve cover is now melted.
Recently I had a new muffler installed and I immediately blamed it on a bad muffler, too much back-pressure.
The muffler shop assured me that this muffler would not cause that problem, but went to the trouble to do a back-pressure check before the cats.
Results: at idle about 1psi, 2500 rpm around 3-4 psi, but the engine had not been warmed up, no load just open throttle.
He offered to cut the cats off, inspect them and re-install them if they were OK visually.
I told him to go ahead and the result was very telling.
This is the front cat:
The brick in the cat is significantly melted. I can see why it still flowed, but at full temperature probably had much more resistance to flow.
The rear cat looked OK but had remnants of the front cat inside.
Conclusions:
- back pressure checks are informational, but should be done at operating temperature, under load, and should be very low, probably less than 3 psi, although right now I don't know for sure what the typical spec is.
- this is one of those surprises when running a supercharger that I didn't see coming, but it makes sense now - so much air and at a higher operating temperature and more temperature swings. The injectors can't keep up enough to keep the cats cool, they're already almost at max duty ratio at 6+ psi boost under load.
- do not clamp the rubber hose that runs the EGR valve to modulator valve, this is the 'fuse' that tells you somethings wrong - pay attention!
- it could be an old hose, but if it happens again after replacing it with the same type of hose, fix it right.
- my recommendation would be to replace both cats at the same time, don't try to save the rear cat, if the front one has gone bad, the rear one is toast as well. You'll just be in there again replacing the rear one soon after.
Remedy:
I had two new cats installed (aftermarket) fully realizing that I may be back in 5 or less years doing this again, but, life in California.
The exhaust sound is SIGNIFICANTLY better, less restricted sounding, engine has noticeably more power, feels like my old supercharged 'Cruiser again!
I will replace the EGR modulator (not the EGR valve) and the rubber hose with OE parts - 'cause I'm an OE kinda guy.
Here's to hoping it lasts!
Dan.
PS: If you're in Southern California, specifically Torrance area, I recommend Sweeney's Custom Muffler. Good guy, conscientious about his work, lots of experience.
