1987 Fifth Avenue - Lean Burn Question

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:

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.

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

? Look at theengine 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!