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EGR and/or Cat System - Diagnosing

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Old 10-31-2011, 07:31 AM   #1
DeltaHotel [OP] DeltaHotel is offline
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EGR and/or Cat System - Diagnosing

I have a 2002 Tacoma 4x4, 4 cyl, 2.7l. Truck has 175,000 miles.

Recently, both O2 sensors were replaced due to engline light/codes that led to a check where both sensors were diagnosed as bad. I regret that I allowed a local shop to do that work (just because my laziness costs me quite a bit more $). But problems persists, so I'm working to resolve.

Soon after (same day) they replaced the O2 sensors, engine light came back on. Rather than return to the shop, I've decided to try and resolve on my own. So I checked code: P0420 (Cat System Efficiency Below Threshold).

Long shot as it may be, I decided I'd pull both cats and try to clean them. I soaked them overnight in a mild dish detergent/water solution. While they soaked, I renewed air cleaner and put in fresh plugs. After re-install, I cleared the codes. After about 175 miles, engine light came back on. Codes now are as follows:

Freeze Frame: P0420 (Cat System)
Pending: P0401 (EGR) and P0420 (Cat System)

I'd appreciate insights and opinions here. On the P0420, I suppose it's likely that the Cat may well be near end of life given the mileage, so I was inclined to replace the front cat to see if that would be enough, then replace the rear only if that didn't resolve the P0420.

As for the P0401, could it be related, or is this definitely a separate issue with the VSV or some such?

I have about 3 weeks to get it resolved (inspection due so need to resolve for emissions check).

Thanks so much in advanced for advice or help.
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Old 10-31-2011, 07:09 PM   #2
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Check egr system first. I'm not familiar with that system but if egr is vacuum controlled you can put a vacuum tester on valve and with engine running pull vacuum on valve. Car should idle extremely bad or die when u do that. The other way to do it if its electronically controlled is to just pull the valve off and start the car and see if it dies or runs poorly. If the intake passage to the valve is blocked you will have minimal to no change in idle with valve open or not on the car. THen you know that you need to clean that passage and good idea to clean valve too. If idle does change, as it is suppose to, then you either have a problem with the valve itself or one of its controls and will need to do further testing.
P0420 is usually solved by replacing cats. Front one should be replaced first. Though you also need to make sure there are no major vacuum leaks or exhaust leaks. The best way to determine if the cat is working properly is to use a scanner and watch the switching of the O2 sensors. Sometimes, however, o2 sensors will appear to switch properly and still produce a p0420 and in that case, replacing converter has always solved the problem for me.
I would see about getting the egr code resolved first then move onto the converter as its possible for egr problems to contribute to a cat code. Good luck.
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Old 11-07-2011, 06:21 PM   #3
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Thanks so much for the suggestions.

I removed the modulator line to the EGR, and applied vacuum at idle, and indeed the truck stalled (or ran poorly). I can hear the EGR valve operate when vacuum is applied (if truck is not running). Even so, I removed and cleaned the EGR valve, also ensured no carbon buildup or blockage at the inlet, etc. I sprayed carb cleaner on the inside, used compressed air on the filter on top of the modulator, checked the hoses for wear (they seemed okay) and put it all back together. I then replaced the front catalytic converter (the largest one) with a new unit (Walker brand, ordered from RockAuto.com). I refreshed gaskets on both ends of the new cat (as well as the gasket behind the 2nd cat (had to removed that to make room to slip out the front cat). So after all that, cleared the codes, and crossed the fingers.

Sadly, after a day or so-- just a few trips, Check Engine Light is back on. Code is P0420 (Cat System below efficiency threshold). I'm not sure what else to try but to move on to replacing the rear cat, now. Thoughts?
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Old 11-07-2011, 08:56 PM   #4
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Thats going to be your cheaper bet. Without being able to monitor oxygen sensors it can be a guessing game and that game can be expensive. Most likely is a converter issue. If code persists after replacing rear, it is also possible that aftermarket is not going to work well. I've replace converters with aftermarket units several times on different model cars and at times, I assume due to the smaller amount of converter material in aftermarket, you must go back with OEM dealer cats. They are bigger and work better. And they are much more expensive, usually. Good luck
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Old 11-08-2011, 04:23 AM   #5
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Oh my... I sincerely hope the converter I've purchased (and likely, the one I am about to) is not adequate... it's not like I can return these things, and even though cheaper than OEM, it's still a lot of money. :/ And yes, as you say, OEM cats are just ridiculously expensive.
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Old 11-09-2011, 08:22 AM   #6
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Yea that's why on cats I will often buy them from the local parts store, even if they're a few bucks more than online just because you can return them with no problem if there is a problem.
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Old 11-09-2011, 08:47 AM   #7
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That's good advice, thanks.

I went with the Walker Ultra brand (which was not the least expensive) because they boast about OE fit and construction (they apparently make some of the OE exhaust components for some mfgs). They say that they have a higer loading of the precious metals for problematic vehicles. And they also claim a 25,000 mile catalytic performance warranty (and 5 year or 50,000 mile structural integrity warranty).

Still, hoping very much I don't have to put that warranty claim to the test!

I'm going to take the truck to a local shop on Friday to put on their scope and just make SURE those new O2 sensors are doing their job, and sniff for any other issues or problems. If all looks good, but still just point to the CAT system, then I'll try replacing the rear cat and go from there.
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Old 11-09-2011, 08:37 PM   #8
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Good plan. Let us know what they say.
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