Need help with my sons car

If the exhaust is partially clogged, wouldn't the engine temp increase?

Not a flex fuel car. Regular unleaded. This problem has been ongoing for awhile now. Car never set any codes except for a occasional misfire and when the dpfe melted down

Water temp is steady at 190 and mileage never changed either

My next step would be to get a backpressure reading from the cats. Also I would get a fuel sample make sure you don’t have e85 in it.

E85 isn’t available here so that’s not a problem. What do I use for a back pressure test

You can see if you can rent a backpressure tester from advance auto, it comes with an 02 sensor adapter and a pressure gauge. You should not have more than 1-1.5 psi pressure showing on the gauge. The vacuum gauge test I mentioned earlier will show it too, if you hook it up to a vacuum source and hold the throttle to 2k and the vacuum drops you have a restriction. You can drop the exhaust and retry.

Ok quick update: I haven’t done the back pressure test yet because the car had to be inspected and running the car with no o2 would have set a code and that’s a no-no here in the great state of NY. But I do have new information if this helps. The misfire and stumble are load based. If I get on the car smoothly at a light no problem. If I give the throttle a quick stab it’s stumble city. I can get on the expressway and have the car rev up to 4K rpm no problem. Try to pass and stumble comes ( sometimes with a couple of lean pops through the intake). If the car is idling in gear and I try to powerbrake it it just coughs and sputters but in neutral I can rev it to 4K easily.

I don’t know if any of this changes anyone’s ideas or opinions but I thought I would put it out there. Now that it has been inspected I’m going to try the back pressure test and see what happens

Cracked plug wire arching on a load? Didn’t crack a plug or smash the gap closed during install?

The plugs and wires have both been changed during this issue but I’m going to recheck the wires in the dark and look for arcing

Last vechile I worked on with clogged exhaust

Once we opened the exhaust up before cat we knew instantly the problem was gone

UPDATE: After finally getting some time to look at his car again I tried to test the exhaust. Unfortunately there was no exhaust pressure gauges available anywhere near me so I improvised. I found the my compression test gauge threaded into the o2 sensor so I tried that route. After starting the car and giving it a few quick revs to about 3k rpm the needle went up to 25 psi. I went under the car and tried the downstream sensor and got 25psi. Did I do this correct? Is the gauge reading back pressure correctly like this? And if so I guess that means the rear cat is clogged

Intake melted from the EGR or melted DPFE is very common with restricted exhaust.

It’s funny you say that. I’ve now melted two dpfe sensors. I thought the first one was a fluke. But now I know better. Question now is it the rear cat or could it be the muffler?

You can use a simple vacuum gauge to test for exhaust restrictions. Hook it to an intake vacuum port and power brake. If the needle falls to zero you have a plugged cat. From there, you can remove an upstream O2 sensor , one bank at a time and give it a quick drive to narrow down which one is plugged.

I have an exhaust restriction. That I know. My question now is which is more likely- the rear cat or the muffler?

The cat for sure. The rear muffler just has baffles, and would be very difficult to plug.

Hard call on what piece.

I was thinking on that test the backside of cat would have lower pressure.

You may be stuck trying open exhaust.

Are there any breaks before and after cat that can be opened

The exhaust in this car is a little different than I’m used to. There is a upstream o2 in each manifold then a cat then the downstream o2 then a single cat where the pipes merge. Usually the o2 that measures efficiency is after the secondary cat. Not before it. I did get back pressure readings of 25psi at both the upstream and downstream o2 sensors. That eliminates the primary cats and leaves the secondary one and the muffler

toomanytoys said:

The exhaust in this car is a little different than I’m used to. There is a upstream o2 in each manifold then a cat then the downstream o2 then a single cat where the pipes merge. Usually the o2 that measures efficiency is after the secondary cat. Not before it. I did get back pressure readings of 25psi at both the upstream and downstream o2 sensors. That eliminates the primary cats and leaves the secondary one and the muffler

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Its very odd to see the secondary cat fail without one of the upstream cats fail first. Id just cut the damn thing out at this point since you have eliminated the primary cats.

I guess I could just cut it out. Since it’s after all the o2 sensors removing it shouldn’t set a cel correct?