"Jamie Calder" <jcalder3@cfl.rr.com> wrote:
> So if I have a lumpy cam with some overlap (I'd have to look up the
> exact specs) which reduced the vacuum at idle, that could cause a
> rich condition when idling?
> The reason I think it's rich is because it smells rich at idle and 2
> new high flow cats (dual exhaust) got ruined a couple months after
> the engine was built (a few years ago now, been running without
> them). If this were my case, how could I fix this (besides
> replacing the cam :)?
Yes, reduced vacuum can cause a rich condition, but I'm not sure
that its all due to the fuel pressure regulator. Granted, a lower
vacuum will indeed increase your fuel pressure because there is less
vacuum assist in the fuel pressure regulator, and this will cause more
fuel to be sprayed into the intake for each injector pulse, however, I
would expect the PCM to notice the rich condition via the O2 sensor
and decrease the injector pulse width to compensate.
A more likely reason for a decreased vacuum to cause a rich
condition may be the MAP sensor. The decreased vacuum may be being
interpreted as a higher load condition and thus the PCM itself may be
increasing the injector pulse width to richen up the mixture. Again,
I would expect this condition to be self-corrected somewhat when the
PCM checks the O2 sensor, however this is done only in closed loop
operation, so until the engine warms up and the O2 sensor starts being
used, all the PCM has to rely on are static maps and I would
definitely expect a reduced vacuum to result in a richer air/fuel
mixture, especially in open loop operation. (It may be that the
little time your truck spends in open loop was enough to destroy the
cats.) Something that might be helpful to diagnose if this is the
case (and that the PCM is compensating under closed loop) is to check
wether you notice the rich smell only while the truck is still warming
up, or even after it is fully warmed up and you have been driving for
a while.
Another possibility is simply that your O2 sensor has bit the dust.
(Just because no trouble code has been thrown doesn't mean that the
sensor is actually operating correctly.) I don't like throwing parts
at a vehicle, but in this case it might be worth your while to install
a new O2 sensor and see if it makes a difference. (I would only do
this if your rich smell is present even after the truck is fully
warmed up and operating in closed loop - the O2 sensor won't make a
lick of difference until the truck warms up. If you find the rich
smell goes away after the truck has warmed up, that is probably an
indication that your O2 sensor is functioning properly.) If you do
decide to change the O2 sensor and it is still running rich after it
has warmed up, you can always pull it out and put it on a shelf and
re-install your old one. Eventually, you are going to need a new O2
sensor and you've got a spare ready to go. :-)
If you find that the rich smell goes away when the truck is warmed
up, then it is a possibility that the reduced vacuum of the cam is
responsible for the rich condition, via the MAP sensor and the fuel
pressure regulator. If this does turn out to be the case, and you are
looking to correct this problem, there are probably a number of things
that *could* be done to lean out the mixture a bit at idle while still
allowing a safe mixture at WOT. Which one would work best I guess
depends on cost, etc. Some ideas that spring readily to mind are to
run a smaller injector (or have your existing injectors rebuilt to
flow a bit less fuel), try to fool the fuel pressure regulator into
dropping the pressure by applying extra vacuum to the regulator, or
decrease the spring pressure inside the regulator a bit, or simply
chuck it and run an aftermarket regulator that you can tune precisely.
However, I would caution against working on the assumption that it
is the cam which is causing your rich condition unless you are able to
determine beyond a reasonable doubt that this is the case. There are
many reasons why the engine might be running rich, and a solution
which relies on a false assumption may just complicate things. There
is a chance that something else is awry, so I would check everything
over to eliminate that possibility prior to applying a band-aid to the
cam situation. For example, there is a possibility is that your fuel
pressure regulator might be bad. Maybe there is a leak in the
diaphragm which is reducing the effectiveness of the vacuum assist.
Maybe the vacuum line leading to the pressure regulator has a hole or
is partially plugged.
I think you mentioned before that you don't have a fuel pressure
gauge; if that is the case, it might be worthwhile picking one up, or
maybe you can use one for free via a loan-a-tool program that some
parts stores (like Autozone) have. Ensuring that your fuel pressure
is within spec would be a good first step. Also, taking a vacuum
reading at idle would probably be helpful as well. Did you happen to
measure your vacuum prior to the cam installation? If not, maybe
somebody can tell you about what the stock vacuum should be at idle.
I'm thinking somewhere around 17-18 inches, but that's just a guess;
maybe it is listed in the FSM.
Anyhoo, sorry for the length of this message, I guess the short
version is that there are probably a number of things you should do as
far as diagnosis goes prior to moving ahead under the assumption that
the cam is to blame.
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
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