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Thread: Help with misfires, cogged catalytic converters & exhaust backpressure testing

  1. #1
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    Help with misfires, cogged catalytic converters & exhaust backpressure testing

    I've read about standard procedures for backpressure testing: plugging into the O2 sensor holes pre- then post-cat, observing pressure at idle, then at 2000-2500 rpm.

    However, that's not the condition under which I get misfires on cyls 4, 5, and 6. After a longish highway drive, if I pull off the highway and end up sitting at a light, I can feel the engine miss and the idle is unsteady. I check the codes and there they are: bank 2 misfires (almost always all 3 cyls, but if I check quickly sometimes I just see 2). And I didn't feel any misses or power loss on the hwy. If I pull off the hwy and quickly shut off the car before idling, there are no codes. City driving and brief hwy trips produce no codes. Also, once in a while I'll get a code for the bank 2 cat running below efficiency along with the misfire codes.

    I've replaced a lot: spark plugs, ignition coils, ignition wire harness, 4 O2 sensors, all vac hoses, ICV & CCV w/ associated hoses, intake manifold & TB gaskets, crankshaft and camshaft sensors, knock sensors, the thermometer in the manifold, professionally reconditioned fuel injectors, cleaned MAF. Vacuum measured at oil cap is normal.

    So I'm thinking the bank 2 cat is partly clogged: lower exhaust pressure during city driving gets through fine when the cats are just cool/warm. The cats heat up during a long hwy drive but exhaust pressure also increases and can push through. But when I come to a stop and the car idles, exhaust flow decreases and now it can't get through the cat fast enough. Exhaust backs up and causes the misfires. This seems to be a transient problem: after 10 or so minutes of city driving I don't get misses or unsteady idle at stop lights. So I'm guessing the cats cool down a little and the lower pressure exhaust can get through again.

    Let me know if you think I'm way off base. If so, what else could be the cause?

    If not, then I want to do a backpressure test but only when the cats are really hot. Can I plug in the pressure tester then, or is that dangerous?

  2. #2
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    Misfires in conjunction with a catalytic converter efficiency fault and be the result of an upset in the A/F ratio. In this case, before looking at the catalytic converters I'd want to do a smoke test and a crank case pressure check to look for intake leaks, a fuel system pressure check, and see what live data from the engine looks like. It may be necessary to do those tests after a drive that produces the symptoms.
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  3. #3
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    My mechanic said the crankcase pressure is normal and the smoke test revealed no leaks. Tomorrow he's going to do a backpressure test and probably a fuel pressure test.

  4. #4
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    Update - really need some expert help here

    Recap: after longish hwy drives I get misfires at cyls 4, 5, 6 and bank 2 cat converter below efficiency. The engine can shudder and the idle will drop then rise intermittently after exiting the hwy.
    NEW: now I also see a misfire on cyl 2 and bank 1 cat below efficiency.
    (Check post 1 for everything I've replaced. I also swapped in a brand new MAF--no difference.)

    Diagnosis: Did a fuel pressure test at the pump. Followed the procedures proscribed by the guy here, which seem adapted from Bentley.
    Jumping the fuel pump relay, the pressure hit 52psi, slightly less than the expected 55psi. Starting the engine it went to 49psi then dropped to 46, instead of the expected 52psi dropping to 49. Applying throttle, I did *not* see a pressure spike up to 60psi. And pinching the hose b/t the measurement tool & the rail did not produce a spike in pressure.
    All of that suggests that the pump was not strong enough to push the proper amount of fuel. It could push enough to drive, but the pressure was just low enough that the further injectors (bank 2) didn't get sufficient fuel.

    So I replaced the pump with a brand new one from Pelican. The car seemed to pull stronger during my 1st hour-long hwy drive. No codes, no engine shuddering, no idle bounce. I thought the problem was solved.
    Then the next day, the codes and all the symptoms returned. I even tried swapping the relay--same thing. Car feels somewhat underpowered.
    Today, my car almost stalled at a light, so it's still fuel-starved and it seems worse than before (though this typically happens in high heat like we've got on the east coast now). No fuel is leaking from the pump outlet. I don't smell leaking fuel anywhere.

    Please please help. The problem does seem to be with the fuel system but I don't know what. The prior tests pointed to the pump, not the pressure regulator. If the FPR was stuck closed and denying fuel to the injectors, that would have produced a *higher* pressure reading at the pump. Same w/ a clogged fuel filter.

  5. #5
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    what plugs did you install?
    how many miles are on the vehicle?

    I had a E39 540 eat my lunch several years back. it had the same symptoms as yours. come to find out the right bank convertor was clogged. AS replacing everything under the sun and even doing a back pressure test using BMW special tools and their specs the cat passed these test. replaced the cat and the misfire faults never returned. the car had less than 80k on it. I remember this cause I was pissed it was under the 8yr/80k mile warranty and my labor time was all under warranty.

    SO if you have a lot of miles on your vehicle, don't look past them.
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  6. #6
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    FWIW: I had a code P0430 pop-up last week. I put in a post cat (bank 2) O2 and it fixed the problem.
    2001 BMW 530i e39 / Automatic / Orient on Dove

  7. #7
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    Replace bank 2 cat. Also replace your fuel filter and FPR if you want more peace of mind with the fuel system, but it's not the cause of your problems.

  8. #8
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    Update with tests and data

    Thanks for your suggestions everyone.

    My idle became unsteady so I went back and checked the ICV. Turns out the hose from the ASC+T boot to the ICV was torn right at the boot nipple, and the boot nipple itself was worn. I replaced those only 1 year ago so I'm guessing it's cheap Uro parts. This time I'll go with BMW. I cleaned the ICV and when I install the new parts I'll update with whether the misfires return. (Oh and my mechanic had recently done a smoke test so it looks like he missed this.)

    Meanwhile I've done some other testing.

    According to this guy, electrical problems can give the impression that a fuel pump is bad when it isn't. And that would explain why a brand new pump didn't fix the low pressure problem.
    So I did the voltage drop test on my new pump. Ground side was good, but the power side dropped by .6-.65V. It should be no greater than .5V. Not huge, but there's a little too much resistance in the line from the relay to the pump. The drop in voltage must be causing a mild lack of pressure. Looks like I've gotta replace that wire.

    I got AutoEnginuity's scan tool and recorded some data. The 2 figures show O2 sensor voltages recorded at the same. I plotted the banks separately. Warm colors are pre-cat, cool colors are post-cat, engine speed is in black. As you can see, the post-cat sensors are fluctuating with the pre-cat sensors. That means the cat isn't converting exhaust gasses. I don't know if it's *clogged*, so I'm not sure it's responsible for my engine issues (more likely the fuel pump voltage and/or ICV hose). But it's pretty convincing that it'll have to be replaced to pass emissions.
    AESC_328i_20130702_O2mixed2_2banks.jpg
    Anyway I'll be replacing the entire exhaust w/ a more free-flowing system. I know there are threads on this, but does anyone here have recommendations in the $800-$1200 range?


    And just to be thorough, in case people refer to this thread for help in the future, I've answered your questions below.
    Quote Originally Posted by 1sloE36 View Post
    Replace bank 2 cat. Also replace your fuel filter and FPR if you want more peace of mind with the fuel system, but it's not the cause of your problems.
    Fuel filter was replaced about 6k miles ago. FPR was replaced with no change. Also, bank 2 of the cat cannot be replaced on its own. The whole cat must be replaced, which is what I'll have to do.

    Quote Originally Posted by bcain79 View Post
    FWIW: I had a code P0430 pop-up last week. I put in a post cat (bank 2) O2 and it fixed the problem.
    Not sure what that code is for, but post-cat O2 sensors don't contribute (much?) to engine management. They just tell you if the cat is functioning. All 4 O2s were replaced in October and the post-cats fluctuate with the pre-cats, indicating a failed cat.

    Quote Originally Posted by 02325 View Post
    what plugs did you install?
    how many miles are on the vehicle?
    I had a E39 540 eat my lunch several years back. it had the same symptoms as yours. come to find out the right bank convertor was clogged. AS replacing everything under the sun and even doing a back pressure test using BMW special tools and their specs the cat passed these test. replaced the cat and the misfire faults never returned.
    Plugs are NGK BKR6EK, commonly recommended for e36s (replaced in October). Mileage is 134k (I've owned it since 95k).
    It's good to hear that replacing your cat solved the problem (especially after a backpressure test came back negative, very unexpected). As I've said I also think my cats are busted. However, cats don't fail or get clogged unless there's another underlying problem like persistent rich fuel condition. So unless I figure out why it failed in the first place I may just cook a new one. That's why I'm hesitant to install one until I've figured out the primary problem. And the one thing I've got--low fuel pressure--would create a lean condition, which I don't think would kill the cats.
    Last edited by kaivonp; 07-16-2013 at 12:14 PM.

  9. #9
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    Strange behavior

    New ASC+T boot & hose made no difference.

    Yesterday I got the engine up to 5k rpm and all of a sudden it started shuddering and the CEL came on. Shuddering continued even after backing off the throttle. It's not constant vibration--in the span of 30secs it'll happen a few times. When I pulled over the idle was surging up to 1500. Codes were misfires on cyls 1, 2, 4, 5, 6 and cats below efficiency (same as always).
    Cleared the codes, waited a bit, and drove on w/o incident (staying below 3k rpm).

    Today I reproduced the event at 4k rpm: shuddering, idle surging.

    What's going on here?

  10. #10
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    Cats dont only fail because of rich trims, they do fail in time. They are considered a wear item on most cars, let me ask you, do you trust BMW when they say the trans has lifetime fluid? No (you shouldnt, lol)? Then why would you trust a lifetime catalytic converter? That mileage is pretty high. Usually they get replaced around 100K.

    I would replace the catalytic converter NOW. You are ASSUMING there is an underlying problem and you are driving around a car with very bad symptoms waiting to fix the actual problem, because you think other problems caused that problem? Fix the problem you KNOW exists... then you can trace the other symptoms to see if there is something that caused the cat to fail in the first place (to me, thats unlikely, I believe the cat is just bad)

    If you are worried about fouling a new replacement cat then have the cat completely removed until you get the rest of the issue diagnosed and fixed. Have someone remove the cat and install straight pipe in it's place. Should be under $100. O2s will need to be changed too. My money is on the cat being the main problem here, and driving around with a cat that bad can and will cause other problems. I would expect a fouled O2 at the least. Expect your exhaust manifold gasket to blow out and give you a nasty exhaust leak if you keep driving with a clogged cat. If it keeps going, it can get worse from there.
    Last edited by BimmerBreaker; 07-30-2013 at 04:49 AM.

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  11. #11
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    Quote Originally Posted by BimmerBreaker View Post
    I would replace the catalytic converter NOW. You are ASSUMING there is an underlying problem and you are driving around a car with very bad symptoms waiting to fix the actual problem, because you think other problems caused that problem? Fix the problem you KNOW exists... then you can trace the other symptoms to see if there is something that caused the cat to fail in the first place (to me, thats unlikely, I believe the cat is just bad)
    Sounds reasonable. I found a Bosal cc for $373 delivered. Complete assembly so I can install it myself. Anyone know a reason not to go with that brand?

    - - - Updated - - -

    While I'm waiting for a new cat I ran a vac gauge from the intake to the cabin and drove around. I've done this before and the vac is 20" Hg when the engine isn't acting up. (all w/ the car fully warmed up btw). Noticed 2 things:

    1. When I roll to a stop and begin idling, the vac hangs up at 23" instead of smoothly falling back down to 20" like it's done previously. After 5-10secs the rpms dip to 400 and come back up to 7-800 at the same time that the vac drops to 20".

    2. If I continue to idle, I can see the vac jump quickly to 23", stay for a couple secs, then jump back to 20". This happens sporadically, a few times in a couple minutes. And there is NO concurrent change in engine speed.

    At first I thought the ICV was staying restricted (leading to high vac) then it's unsticking after 5sec. But unsticking should cause an increase in rpm, not a brief drop, right?
    The later jump from 20" to 23" and back to 20" is fairly precise, suggesting electronically controlled action like in the ICV. But I feel like a sudden restriction should lead to a drop in rpm, and I don't see that. Plus, the ICV is less than 1 year old and I just cleaned it w/ IPA a couple weeks ago.

    So while I agree that I need to replace the cats, isn't it likely that this manifold vacuum behavior is related to something else?

  12. #12
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    I paid $60 for my last catalytic converter... usually if you just take your car to an exhaust shop and say you want the cats changed they will have some on hand that will fit. Really nothing special about a BMW cat, cats only serve one purpose anyways.

    Vacuum behaviour can relate to a clogged cat. After you get the cat installed I would unplug the battery for about 30 minutes to let the car "re-learn" the adaptive settings and fuel trims. Then take it for a drive and let us know what, if any, symptoms are still present.

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  13. #13
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    I'm on to something:
    Turning off the AC eliminates the idle dip, vac rise, and engine shudder. Turning it back on immediately produces those symptoms. Running just the fan, there are no symptoms.
    Turning on the rear defogger or headlights produces the same symptoms, but to a lesser degree. (I've got HIDs so the current draw should be less than stock lamps.) But the front defroster or seat warmers produce no symptoms.

    So clearly there's an electrical problem. Not sure if the 3 troublemakers are all connected to a common ground, but that's what I'm checking. Unless anyone has better suggestions.

  14. #14
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    You need to check all the major grounds and look at your alternator and battery too. Normally with just bad or irregular idle and shuddering dependent on A/C on or off it can be the A/C compressor on it's way out. These symptoms occuring with the headlights or defogger on tell us that its most likely an electrical issue and the only way an electrical issue could show those symptoms is if a ground is very poor, the alternator is not functioning properly or the battery is bad.

    When was the last time you replaced the battery?
    How about the alternator?

    Looks like you have two bad cats and a possible diagnosis as to how it happened: the bad battery/charging system gave poor spark, the poor spark prevented complete combustion thus allowing rich conditions, these rich conditions clogged the cat. This could have been going on for awhile, you wouldnt even notice it was running on bad spark as long as it was getting spark... that is, you wouldnt notice until the cat became clogged and it started throwing codes.

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  15. #15
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    I'm having a shop replace the cats tomorrow. Decided not to tackle the job myself, since the header studs tend to break and I don't want to deal with that.

    Your theory about how the cats clogged sounds valid. However, I've kept a close eye on the spark plugs and they've never looked fouled or burned from running rich.
    The chassis ground near the passenger shock tower had a lot of rust. Cleaned up all the metal but that didn't make a difference. Battery is about 18 months old. Alternator is probably original--I'll look up some vids on testing it.

  16. #16
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    Aftermarket cats are crap, your old ones are worth more in scrap than what muffler shop new ones cost. (They'll fight to keep your old ones, watch.)There is a reason for that. You'll be better served finding a good used OE part. The muffler shop cats will be toast in a short time ESPECIALLY if you have fueling issues.

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  17. #17
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    Chasing the electrical issue

    The hood ground strap is a little frayed. Not sure if that would make a huge difference but it'll be replaced.

    More suspect:
    At some point a cell phone system was installed in my car including an antenna, a mic, a speaker, & a phone mount. All are connected to a box behind the passenger footwell carpet. To power that box, someone ran one wire (with in-line fuse) from the battery positive post in the engine bay. The ground wire is the one that's screwed against the chassis just right of the strut tower. Those wires (and a 3rd wire that ran to the fuse box, also with in-line fuse) were spliced into the black adapter shown in the photo below.
    customPhoneInstall2.jpg
    The brown arrow points to the ground, the red points to the power line going to the Motorola box, the purple points to the spliced-together input. The words on that adapter are, from top to bottom: Ignition, Power, and Ground (on the left), & 2A and 10A (on the right).

    I'm very suspicious that this setup is partly to blame for the electrical issues. Anyway I've pulled it all out but now my problem is where to attach the other end of the ground cable. What was it originally grounding? Or did the installers create a new ground spot just for this system?

  18. #18
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    The behavior of the engine when the A/C is switched on doesn't sound like an electrical problem to me. The biggest effect of turning on the A/C is the additional load on the engine caused by the compressor. In a case where the engine isn't running quite right that additional load will make the faults much more evident.

    If a back pressure test of the converters doesn't indicate a problem, it is hard to imagine case for the converters to become more obstructed as the get hotter given the internal construction.

    As to the fuel pressure, connect a gauge to the rail zip tie it to a wiper and see what the pressure does while the car is beingg driven.
    The car makes it possible, but the driver makes it happen.
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  19. #19
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    People have been asking for an update after replacing the cats:

    The data definitely showed a difference: the post-cat O2 sensors, which tracked the pre-cat sensors prior to replacing the cats, are now steady. That means the catalyst is converting the exhaust.

    Unfortunately, I still get codes for misfires in cylinders 4, 5, and 6. Occasionally "cat below efficiency" for bank 2 and, rarely, bank 1. So after everything that's been replaced, the problem is exactly the same as it was nearly 2 years ago.

  20. #20
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    Which means that there is a problem with engine management that hasn't been diagnosed. That problem almost certainly led to the damage to the catalytic converters. You need a really good BMW indie shop of a dealer to diagnose the car.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  21. #21
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    Sorry to hear it's still giving you trouble. My money was on the cats fixing it. I had similar symptoms on an M54, except the backpressure test was obvious, and new cats did the trick.

    Compression and leakdown tests should be in your near future. If they check out I would have it recoded at a dealer. Sometimes computers get effed up.

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