Diagnosing a clogged cat or fuel starvation
 

Edited to Add: Problem is the fuel pump. It tests fine when just started but over time the presure drops to the 40s and sometimes as low as 10 psi. Free beers to logalinipoo (no lean code) and Tonik (fuel pump) and to everyone else for their help!


Dealer says my cat is fine because their diagnostic test of the "Rear to front switch ratio" on the two O2 sensors is only 0.152 and the maximum allowed is 7.999.

That seems to indicate that the cat is doing its job in cleaning up the exhaust.

Is it still possible that the cat is clogged?

When I drive it relatively hard, I can see the rear O2 sensor glowing red even in the afternoon in the garage. Is that normal? Tonight I'll drive it hard at night and see if the whole cat is glowing.

Dealer also said the fuel pump tested fine last year, but I'm also thinking that maybe it could be failing when overheating or maybe fuel filter is clogged. Dealer says that if there were a fuel pump or fuel filter issue, I would get a lean code, but I do not.

I've also read that my wiring harness could be bad, that the SSV could be sticking and that the ESS could be dirty.

I am no mechanic but I want to be able to follow along with the dealer's efforts to diagnose and fix this. I have cleaned the MAF sensor, replaced the air filter, and reset the NVRAM, pulled the room fuse, etc.

Background:
'06 MT with 81k miles, engine replaced at 40k miles, cat replaced at 50k miles, and third engine put in last week. Replaced coils, wires and plugs last year and again last week with 'new' engine. New catback put on this week. Sounds nice.

Here's the problem as I experience it:
On long Interstate drives, after a few hours of driving, summer but not real hot, engine was running a little rough at first and then progressively rougher and then would miss on hard acceleration, got progressively worse, until it would miss with mild acceleration. When I could not even maintain 30 mph, pulled over. After shutting the car off for only 5-10 minutes, and restarting, it will run OK. I didn't push it, but able to maintain highway speeds without any noticeable issues. No OBD codes.

More recently, I notice loss of power (missing or fuel starvation) above 7 or 8k even after short drives of 20-30 minutes, with full gas tank.

It takes a jack( jack stand too preferably) or some way to safely keep the car from falling). 14mm wrench 15 mm wrench and about half an hour to drop the front of the cat(3 Bolts) and look inside. It will look like a nice honey comb or piece of screen with fine square holes. It it's clogged up then it is bad.

Some wd-40 or pb blaster will help a bunch.

Dealer said it looked OK. Is a cat only clogged when the honeycomb starts falling apart? Is that what clogs it up? Probably a stupid question. Maybe that's obvious, but, like I say, I am no mechanic.

The dealer did not look at it. They checked the computer which did not show it is bad. You are asking how to diagnose it or fuel pump. the easiest way is to look at it. You've probably spent more time waiting for me to reply then it would take you to go inspect it and know if you are safe to drive.

If the honeycomb has fallen apart then it is basically gutted and not really a problem. It will become melted and clogged if it is bad. If you see a nice perfect honeycomb structure then it is not bad.


You will kill your engine driving with a bad cat.

No, they did look at it. Service tech is well known in local rotary community, very knowledgeable and trustworthy. He took if off completely. Ran the engine with cat but without catback, and without cat or catback. There was a little bit of rattle in the muffler and problem disappeared with catback off and returned with catback on. We knew this was not definitive, but this is why I replaced the catback this past week. If visible inspection does not reveal any problems, it sounds like the cat may be ruled out as the problem.

If fuel filter or fuel pump is causing a fuel starvation issue at high rpms, would I necessarily be getting a lean code? Is the service adviser right about that?

You may or may not get a lean code. I don't know what specs throw that code. I know I've run my car lean on the high end and never got a code. But that was while tuning new injectors.

It does sound like your cat is probably good, id still look for myself. Sometimes that rattle in the muffler is from a broken honeycomb that has moved back. Here is my cat that clogged then broke apart.

Thanks for your help. That was my theory last week, ie, that when the old cat failed that it might have sent sent some broken honeycomb into the old muffler. I will take a look at the cat myself at some point if the dealer cannot figure this out.

So did you run it lean enough to experience fuel starvation, loss of power, and still did not get a code?

It's the fuel pump..i don't get any cel code when mine was acting up.

Yes afr 17 and i could drive fine. But no flooring it. Id starve it about 6500 rpm.

Thanks. I've read that fuel pump issues are more likely to show up in the summer time. Did you also first notice your problems only after the car had warmed up or even been driven for a long time? For the past year, this problem was very intermittent, and especially severe during two of the only four times that I was driving for a few hours on the Interstate. That's why I was thinking the fuel pump motor might be overheating. Now it is much more common.

So that proves the service adviser wrong. No big surprise there. Thanks.

After driving on the Interstate for an hour and a half to the dealer, I was able to get a P0300 misfire code when I attempted to redline it in neutral. When it cuts out early at high rpms several times in a row, that's when I got the code. Did it again today (again in neutral) after driving it hard. When I drive it in gear, I can get the power loss at high revs but no codes. So far, only got a code twice, both times in neutral under these rare circumstances.

Another thing I tried today. If I floor it, I will get the power loss at high revs, when driving. If I slowly increase revs while driving it will make it to the normal 9.5k rpm redline on the tachometer. I was thinking that might be more of a fuel pump issue, ie, not able to supply enough fuel fast enough when flooring it, but can supply enough fuel when increasing revs more slowly. But maybe not. I suppose that air through the cat & catback might be subject to the same lack of linearity.

Am thinking about the best way to try and log AFRs vs rpms using my OBD2 sensor and smartphone and whether that might tell me anything definitive or at least suggestive.

Change coils and plugs. A missfire will cause AFRs to read high and all that unburnt fuel will cause your cat to glow red (because that's where it's burning).

Treat it soon or you will need a new CAT, and as stated above that can cause you to loose an engine.

Already changed coils & plugs & wires last year when problem first manifested and again last week when the third engine was installed.

Thanks. So a glowing cat does not necessarily mean its clogged, might just be a symptom of misfires. That's good to know.

Soon as we figure it out.

Spirited driving will cause a really good cat to glow also.

Also if you are leaning out for some reason it could cause a misfire. Or too rich but you'd have to be pouring out fuel to be that rich i think.

Hmm.... don't know what to tell you. Without more info it will be difficult to diagnose, because everything comes down to those misfires. Could be an e-shaft sensor issue, I have caused that to happen before and it did cause misfires at high rpm.

What additional information do we need? How do I go about getting it?

Cleaning the ESS is on my list. Does cleaning the ESS fix/rule out this out, or could it be more complicated than just cleaning it? Would this cause misfires intermittently, and only after the car warms up? Maybe I'll try that tomorrow.

I'd log RPM, AFR, MAF, EGT, LTFT, STFT, AFR commanded, and AFR.

If it is the e-shaft then it's not going to be obvious. The ECU does not give a e-shaft code unless it's really bad, it just causes misfires at high rpm and possibly a rough idle. Mine was from a noisy test lead I attached, and I only found the cause because when I disconnected it stopped.

I don't know what to tell you about hot vs cold missfires. Could point to it being the fuel pump, but if that was the case then full vs empty tank would cause the symptoms to change/disappear.

I doubt this will tell you much. I managed to log AFR measured briefly today during a high rpm misfire.

Jack it up and clean the e-shaft sensor. The sudden spike is probably from misfires, and from everything you've said it does not appear to be load related, so we are only left with some sort of ignition issue. If the coils/plugs and wires are good then there are very few possible parts left that could cause this kind of trouble.

At least pull the plugs and look at them. There may be some clue there.

This is random but record voltage. Maybe its doing something crazy. Have your battery checked too.

Cleaned the e-shaft sensor. No fix. Narrowing it down.

So I'm thinking of plotting AFR (commanded) vs AFR (measured). If the AFR measured is lower than commanded when it misfires, would that be an indication that the exhaust is clogged somewhere, ie, cat?

Likewise if I I see my fuel pressure or fuel flow rate cut out when it misfires, maybe that would indicated a fuel pump problem.

Sound like a plan?

Did you do the 20 brake stomp

Clogged cat would show normal AFR driving around but higher than normal EGT. Also when they cause the engine to stall they start puffing out the tailpipe. We have no fuel pressure or flow sensors so not much information you can get from there. I'll try to think of anything else.

Yep