1987 Fifth Avenue - Lean Burn Question

Again, here I am with something regarding my 1987 Fifth Avenue with the
318/2bbl and "LeanBurn."

I removed the carb and intake the other day and cleaned out all of the
passages. There was a bit of black goo in the EGR passageways, but nothing
that stopped it completely up. Either way, I soaked the intake in parts dip
and everything looks new now and is clear as can be. Got it all reinstalled
and took the car for a test run. It still has the surging and stumbling,
whether or not I'm under a load (ie: driving the car) or just revving the
engine in neutral.

Okay, so... Just for the hell of it, I pulled off the vaccum line going to
that vaccum modulator on the aircleaner.... You know, the one attached to
the LeanBurn computer. At idle, it didn't make any noticable difference in
engine performance, but as I revved up the engine, it was SMOOOOOOOTH!
Smooth as can be and stayed smooth! No stumble, no surging! So I left the
modulator disconnected (vaccum line plugged) and took it for a short drive.
Once again, it accelerated smooth as can be and took off like a rocket
without hesitation or stumbling. So under load or in neutral it runs GREAT
with that line disconnected and plugged.

It shocked me how smooth it was.


SO, what the hell does that vacuum modulator tell the computer? I didn't
feel like tearing into the computer housing to see what it was attached to.
I'm ditching LeanBurn come this summer and converting to standard carb and
electronic ignition, but in the meantime can I leave that modulator
disconnected without hurting the engine? It's not going to cause me to run
too lean and hot or anythng else horrible? The smooth performance I gained
was really quite spectacular.

Thanks,
-Steve

 

If it is what I THINK you are referring to, it is the "vacuum advance"
for the timing. It will run well without it, but be hard on gas. You
might want to try to find a used computer at the wreckers to swap in -
sounds like you MAY have a bad computer.

 

You are probably right, but here is a photo just to be clear:

http://fifthavenue.i8.com/5thengine.jpg

 

Would a bad computer still give me decent fuel economy? I'm getting about
20-22 MPG on the highway and 17-18 around town. I've checked twice now,
figuring on an 18 gallon fuel tank and I keep coming up with good figures.

Also, it seems like the computer is "responding" properly to signals because
I can ground the O2 sensor wire and it runs like crap, smooths out as soon
as I reconnect the O2 sensor. And when the engine is cold, it runs quite
rich and will nearly stall like its loading up if I let the idle come down
too soon, then all of a sudden after it warms up there seems to be a change
in the carb (slight click, slight buzz, then silence) and the engine runs
smoother, like it's tuned properly and not running so rich. I figured it
was changing the mixture once the computer thought the engine was warm
enough. I have no idea how LeanBurn operates....

I didn't experience any pinging or th engine running hotter, so I'll give it
a few days running with that vacuum modulator disconnected and I'll see how
I do on fuel and performance...

Thanks,
-Steve

 

A bad computer can easily give you terrifically good fuel economy while
merrily burning holes in your pistons from excessively-lean mixture.

You can't generalize that the computer is working correctly because it
responds in some fashion to a gross test of ONE input.

Info on diagnosing this has been offered in great detail in previous
responses to your posts.

DS

 

Well, how does one determine if the computer is working properly? From what
I've gathered, even a LeanBurn computer in good order still produces a
surging, stumbling engine. I could easily pull two or three from the
junkyard and still not know if I have a good one.

Hell with it, I'm going to swap it all out ASAP and not have to worry about
LeanBurn toasting my engine.

 

Yes, that is the vacuum advance- or "load sensor"


Common failure point in the past.

 

Canadian or US MPG??
That car was good for about 27MPG Canadian on the highway.

Put timing light on, and apply vacuum to the diaphragm. It should
advance smoothly. If not, it is SHOT.

 

The common lean-burn surge is an EGR calibration problem, and the
Chrysler dealers should still have reference to the required vacuum
restrictor for the EGR control. There is a "vacuum amplifier" on that
thing IIRC.

 

Steve Reinis wrote:

Well, it DEFINITELY provides a reference for timing advance (you won't
get full advance without it hooked up) and it *may* affect the fuel
mixture bias a lot like a MAP sensor on a modern car (not sure about
that- the Lean Burn may not use it for fuel mix at all). Its not going
to do anything too "horrible" although without the extra timing advance
you'll waste fuel and run the exhaust system hotter than normal.

I would be a lot more interested to see if you get rid of the surge by
removing the EGR vacuum line and re-connecting the computer's vacuum line.

 

Holy CATS thats a beautiful engine bay for an M-body! I haven't seen one
like that since those cars were still in showrooms.

 

Depends. If something fails so that the carb stays in "lean' mode, yeah
mileage can be great but driveability will suffer.

Probably not a COMPUTER problem (they almost never fail even though
they're the first thing blamed) but a sensor somewhere.

 

Have you seen the photos I have at http://fifthavenue.i8.com ? Look at the
engine bay when I had it all shined up. It's just about flawless.

I bought this car for $570 (They were asking $600) and it has four new
Cooper tires on it! Aside from the LeanBurn flakiness, it runs like a champ
and has only 103k miles. Leather, premium stereo, power everything, perfect
red vinyl roof, A/C converted to R134a and works great, exhaust is either in
damned good shape or has been replaced recently.... I keep waiting to find
out why it was so cheap... I guess maybe they thought it was in bad shape
from the way LeanBurn makes it act at times... Also, the kickdown rod was
disconnected from the carburetor and so the trans wouldn't downshift when
you floored it... It just grumbled and started shaking violently. That
would almost certainly scare anyone not too mechanically inclined into
thinking the transmission was kaput. I reconnected the kickdown cable and
put a new retaining clip on and it shifts beautifully.

It's got a few ever so minor issues, but overall is a very well preserved
car!

-Steve

 

Well, I did that as well. I pulled the vaccum line to the EGR valve and
took it for a drive... same shuddering and stumbling. It also will stumble
when being revved in neutral, so I don't think I can blame the transmission
for the stumble.

And in a previous post I already mentioned that I've pulled the intake and
dipped it to clear out all of the passages, as recommened on a couple of
M-Body forums I read. I'm pretty sure I've eliminated the EGR system as
trouble.

I replaced the fuel filter and still the same crap. I also have new plugs,
wires, cap, rotor, an a new coil because the old one was swollen. I've just
decided that I can't really go wrong with converting to a non-feeback carb
and a regular Chrysler electronic ignition and so I'm gathering the parts to
do it immediately. I have the carb already, but am waiting on the ignition
and distributor kit to arrive. The last thing I want is this silly computer
running my engine too lean and toasting everything.

 

THOSE computers DID have a history of failing. To the point there was
a thriving industry in rebuilding them. Flite systems of Mechanicsburg
PA and Mississauga Ont used to rebuild them by the skid load.

 

I'd be doing my best to keep that one stock, Steve.

 

I'll skip the snide remarks about factory part numbers on ballast
resistors and ask why your advice is to keep a common-as-dirt car stock at
the expense of driveability and utility?

DS

 

Well, it would be rather interesting 20 years down the road, maybe... But
right now I want a car that drives great instead of looks great and runs
like shit! lol

Besides, it won't look all that bad without LeanBurn in there...

 

Just my bias - if it can be made to run properly AND be original, a
car in that kind of shape is, IN MY OPINION, worth keeping original.
You, or the owner, are free to dissagree - but there are a LOT of
those lean burn engines that are still running well, with good
driveability, economy, and performance and no major modifications.

That car appears to be in much better than average condition, and
although "common as dirt" as you say, becoming hard to find in that
kind of original condition.

As I said "I" would be doing "my best" to keep that one stock.

 

No kidding, although I consider ditching the Lean Burn and putting
Chrysler EI on "leaving stock."

And I'd have bought it at TWICE that price!