Catalyst/Exhaust backpressure checking tool.

[clinebarger]

This subject comes up every now & then on here, So I thought I'd share..... 2006 Cadillac Escalde ESV LQ9/4L65E was brought in yesterday, Notes on the ticket stated....... *Complaint.....Low Power, Flashing CEL. *Both Catalyst inspected visually with a bore scope. *Plugs, Wires, Coils, O2 sensors, & MAF sensor replaced. *Previous shop has recommended a replacement engine. *Owner seeking second opinion. Scan revealed a P0300 (Random Misfire) & a P0101 (MAF Performance). Misfire monitor history confirmed misfires on cylinders 2,4,6,& 8. My instincts were saying clogged catalyst on Bank 2....But I took it on a test drive anyway. Low power, Heavy misfires on Bank 2, And the MAF Grams Per Second maxed out at 87.....Should be 140-160 on a V8 GMT800. Decided to 100% prove it out given the owner has spent quite a bit of money already & has nothing to show for it! Components needed..... Fuel pressure-Vacuum Combo gauge.....HERE 3/8" NPT, 90° 1/4" Barb fitting.....HERE 18mm Anti-Fouler.....HERE You will need to use sealant on the 3/8" NPT threads as it's a loose fit into a 18mm hole, I used Loctite 518 Flange sealant on mine. A longer hose than what comes with the Horrid Freight gauge would be a good idea. The Escalade had 10 psi of backpressure @ 2,000 RPM free-rev! Bank 2 catalyst DID visually look good on the Inlet side, The Outlet showed some telltale signs.......But no visible meltdown.

 

[Skippy722]

For curiosity sake, what is the normal amount of back pressure?

 

[tiger862]

I would start fresh. Compression test and if ok then smoke test. Then go vacuum test. If all is good check electronics. I never go off any other diagnostics cause most want simple code to diagnose if not let's make it so expensive they bring to someone else. (Example) I had Caravan that had 2 transmissions in less than 1000 miles (used) customer was told needed new remanufactured transmission as well as computer but since it had high mileage recommended trading it in. The customer heard about me anyhow I worked at GM from another customer and dropped it off for me then told take my time. It cost him 200 in labor as I found a wire pinched by bracket that rubbed an intermittent short to open with vibration of engine. He got money back from last shop and not only did we get a customer but he bought used car from us.

 

[tiger862]

Delete

 

[tiger862]

Originally Posted by Skippy722

For curiosity sake, what is the normal amount of back pressure?

I psi if memory serves me right

 

[KneeGrinder]

That is a pretty intelligent diagnostic process you have devised there! I wonder what the previous shop thought they were going to see with a bore scope? A Queen Bee and her worker bee's... They can break apart, crack, ect. The only way I was ever able to figure out if they are plugged up was to remove them. And they can be a pain in the rear.

 

[spasm3]

Nice work! How many miles were on it? Was the clogged cat due to mileage or was it run with a cel for a while?

 

[Trav]

Cline, could you do me a favor and shine a high intensity light through that if its still around, it looks about 50% clogged from the picture of the rear. I have no idea how they thought a boroscope would see anything at all except for an obvious meltdown. Before I got a gauge I used to just pull the front O2, if there was any improvement I would drop the cat which on most modern cars is not practical as they usually have more than one and sometimes all in one piece as you know, Before that we let the pellets out and put new ones back with a shop vac. Great idea adapting a vac gauge for this job on the cheap.

 

[das_peikko]

Originally Posted by Skippy722

For curiosity sake, what is the normal amount of back pressure?

2 PSI or less.

 

[DoubleWasp]

Hence my recommendation in the previous thread where this came up. Backs up 2 things I have said time and time again: 1. Exhaust pressure test is the best way to test for a clogged cat by a huge margin. 2. A clogged cat can absolutely be the sole cause of misfire. Good going. You spent pocket change and a pittance of time solving a problem that other mechanics took a king's ransom of time and money to get nowhere with.

 

[Linctex]

Originally Posted by Trav

Before I got a gauge I used to just pull the front O2, if there was any improvement I would drop the cat .

I'll bet that's pretty loud.... but there's enough flow from that small hole to prove your theory?

 

[Trav]

Yes there is enough additional flow to see a very noticeable improvement and yes its loud. I have a gauge and use it but lots of diy guys don't have these sorts of tools, that's what makes clinebargers tool such a good way of doing one. IIRC I paid over $100 for it, it does have a stainless braided heat resistant hose. A simple vacuum gauge will also diagnose a bad cat as a first quick diagnostic tool but low power and flashing cel is a pretty good sign. My methods without the correct tool are aimed at the diy guys without a lot of tools and don't want to buy them for one job.

 

[clinebarger]

Originally Posted by spasm3

Nice work! How many miles were on it? Was the clogged cat due to mileage or was it run with a cel for a while?

180,000 miles, Don't know the history & it already had new ignition parts & O2 sensors.

 

[clinebarger]

Originally Posted by Trav

Cline, could you do me a favor and shine a high intensity light through that if its still around, it looks about 50% clogged from the picture of the rear.

I did that when I cut it off......No light visible trough the substrate!

 

[Trav]

That's what I suspected, course removing it just for a look see is a bit much on today's cars, I did it on old cars with single converters after the diagnosis was pretty much confirmed. Great way of making that gauge.

 

[clinebarger]

Originally Posted by Trav

That's what I suspected, course removing it just for a look see is a bit much on today's cars, I did it on old cars with single converters after the diagnosis was pretty much confirmed. Great way of making that gauge.

Thanks Trav......It was a slow day & I don't do the Phone-Zombie-Facebook thing. I knew what was wrong by the super low airflow through the MAF & the misfires told me which bank. I need to learn/observe more known good MAF airflow readings on other makes! Vehicles such as this Escalade (Hydroboost) don't have anywhere to tap in for manifold vacuum readings besides the PCV line......It's easily broken so I try not to mess with it.

 

[Trav]

Yes, take off ports are getting fewer and fewer without a T but I do it often as the vacuum gauge is one of my all time favorite tools. I learned to read one properly from a guy who used it all the time and was really good with it. It is amazing what you can can learn about the engine and get further diagnosis direction from such an inexpensive tool. You and I know how to use and interpret the readings but for those who don't, this will be a big help. Sorry I don't mean to hijack your thread. http://www.secondchancegarage.com/public/186.cfm

 

[benjy]

thanks for the link Trav interesting for sure! wondering how a modern port injected ECU controlled engine could be checked similarly. great reading for later as i am retired + winter is still hangin on in Pa