-----Original Message-----
From: owner-dakota-truck@bent.twistedbits.net
[mailto:owner-dakota-truck@bent.twistedbits.net] On Behalf Of
jon@dakota-truck.net
Sent: Saturday, April 28, 2007 11:08 PM
To: dakota-truck-moderator@bent.twistedbits.net
Subject: Re: DML: Can I put an '04 gas tank in a '95 truck?
"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 --'
Thanks for the detailed reply and it wasn't to lengthy, I appreciate the
details. I'll be diagnosing the problem (using your suggestions) and see
where I end up.
Thanks again!
James
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