I'm in the market for a used car and today I looked at a very nice '92
BMW 535. However, when the owner showed me the papers from the last
safety inspection I noticed the car was very close to failing emissions.
It had a HC reading of 98 PPM, with max allowed being 100 it was only
*two* PPM below the limit. I compared this to the reading a year before
which was only 6 PPM. IOW, it had gotten 16 times worse between the
inspections. The CO went from 0.1 to 0.3 (max allowed 0.5%) during this
I'm worried that if I buy this car, it's going to fail emissions next
time. However, I managed to get the price down to $3300 from $4000, so
I'm really tempted to buy it. The question is, what is failing and how
much will it cost? High HC means the engine is running rich and has
nothing to do with the cat? The problem would more likely be a
non-working oxygen sensor, right?
Often that goes along with a bad cat: the plugged cat throws off the
which affects spark advance and other engine controls. 120K and 12 years
is certainly in the range of when a cat can/will fail.
My 318ti did that (the apply accelerator->slowdown->accelerate REALLY
SLOWLY sequence) just before the cat failed (could hear bits of it
rattling around at 3000RPM) on the A1M at the weekend. Only had 70K on
it, too >:(.
If you hear a rattling that seems to come from underneath the car when
you start it or at any point when you rev up slowly, insist on an
HC, or hydrocarbons, are the result of unburnt fuel, the catalyst will have
little affect on this measurement as its more likely attributed to an
ignition or compression problem, it could be related to an incorrect
mixture, however the CO levels would suffer as a result.
anything below 100 is a reasonable figure, the limit here in the UK is
actually 200 for HC's.
i would guess a new set of spark plugs and maybe leads will improve that
figure by a good margin.
the catalyst will only deal with carbon monoxide levels (CO). it is actually
designed to convert these gasses into carbon dioxide.
Sorry, Steve, but you are wrong, wrong, wrong. 3-way catalytic converters
"burn" three sets of combustion by-products: CO, NOx, and HC.
Googling "3-way catalyst" gives multiple explanations, here is one:
Would a broken cat affect the fuel/air ratio? Assuming it was running
rich, which I think it was, would a non-working oxygen sensor be more
likely to be the problem? Called BMW today, $1800 is the price of a new
cat. Called both a scrap yard dealing with BMWs and a couple of part
stores and non of them had anything. One person said the cat was the
same part as the headers, but there must be aftermarket cats that would
call a muffler shop
a generic CAT should be about 300 bucks installed
some cars have a pair of cats on the headers then a 3rd after the
down pipes merge... (like a Nissan maxima)
you could go with an aftermarket made for the BMW
<no i don't work for cheapexhaust, they just turned up
on a Google search to give you a starting place>
O2 sensors are relatively easy to install given the engine is cool
& you can get the car in a position where you can crawl under it
park it parallel over a dry ditch for instance
What John said.
Since a common failure mode of catalytic converters is to become
plugged, it can affect the airflow and hence manifold vacuum, which
affects fuel-air mix. Most modern engine controllers are less sensitive
to this than older vacuum-based controls, but it still affects things.
I have experienced this first-hand on my Ford E350-based motorhome
with fuel- injected engine (even though it had electronic controls).
One thing about catalytic converters you should consider before
using a generic one (of course, you can buy 6 $300 converters
for the price of an OEM one - are you sure that's not the installed
price BTW?) is that the guys writing columns in Roundel say that
non-oem ones are trash and have caused problems.
I have to agree that a $75 oxygen sensor is way cheaper to try
first if you're shot-gunning solutions. Personally, I'd take it to
an independent shop and have them put it on their analyzer
for $100 for an hour before spending any kind of money on parts.
I assume the price they gave me was if I were to buy one over the
counter. Could driving with a plugged cat damage the engine in any way,
or will it only affect fuel economy and power? I must admit, I really
like this car. Full service history, inspection II done a year ago for
$1000 at a BMW dealer, fully equipped, manual, cloth seats, original
rims, not lowered, and owned by a father of a little girl. I've sent the
seller an e-mail saying I'll buy it for $2700, so hopefully we can agree
on a price around $3000, otherwise I'll probably buy it for $3300.
Thanks for your help.
It usually doesn't *damage* the engine too much. Increases fuel
economy almost infinitely (you use *no* gas) as well as reducing engine
power by the same amount (to 'zero', to be precise). The downside is,
you have to walk home when the *engine won't run*!
(Been there; done that)
LOL. I wouldn't let it go that far... Besides, I do have two cars (well,
three until I can unload my E30 on some poor sucker...) so if one's out
of commission I'll use the other one to get parts. :-) Yeah, this means
I did buy the 535! Got the price down to $3000 just as I wanted, and
insurance is a reasonable $500/year.
Well, I don't think it's the cat anymore. The engine misfires at idle,
so I'll check the plugs, wires, and distributer next week. Also found
some more stuff to look at on this page:
Anyway, took a few pictures this weekend. Here are three of them for
those who are interested:
I must say, the 535 E34 is a lot of car for little money (so far..).
point taken, however most cars built for the UK market dont use 3 way
catalysts, and besides if the HC levels were too high i wouldnt be looking
at the catalyst....
as mentioned, and i'm sure you already know, HC is the result of unburnt
fuel.....so therefore, with this in mind why would you be looking at the
catalyst if this figure was too high.
a "3 way" catalyst may well be able to deal with these gasses and reduce
them, but this does not rectify an engine problem causing these high HC
levels to be present in the first place..
And you guys over in Europe wonder why your air stinks so bad.
3-Way cats have been required in the US since about 1990.
The fact of the matter is, at stoichiometric fuel-air mix (where
best fuel economy is) there is almost always some un-burned
fuel due to poor combustion chamber mixing.
I was under the impression that all cats no matter what final market the car
is sold in would be 3 way - saves manufacturing costs idf they only
order/fit one type of cat, let alone having different engine tuning.
Yeah, actually, since EU changed their emission standards about 10 years
ago to essentially match the US's, most cars sold in both markets have
almost-identical emission control systems. So anon's original statement
(that UK cars don't have 3-ways) was incorrect. I didn't catch that in my
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