Running lean machine (3.2L) needs help…

[Jascha]

I had the good fortune to locate a new Air Flow Meter from an ’94 964. The new AFM improved the low RPM response (off-throttle coasting) but my idle is still rough. I found and fixed bunches of vacuum leaks (false air)… (I decided to replace the intake gaskets -on order form the Host)

Many of you may know that it is quite possible to refurbish the AFM (redirect the rheostat sweep-arm to a new track). I recall seeing a write-up about this on the 911 List. Below is how it looks (on my now back-up unit) -note how deep are the old grooves (pitted ~ idle range -not seen) in the ceramic substrate. I repositioned the sweep to a lower (virgin) track, which makes this unit good for another 100K miles.


Tested the output voltage and was pleasantly surprised to see how (after all these years) linear (flap angle 0-110 deg) it actually is (!)

[ianc]

Yep, doing this helped mine a lot too.

ianc

[Jascha]

Below is the before picture of the sweep arc (flap angle), showing considerable pitting (first ~ 10deg) and can conceivably throw the ECU into oscillations hunting (look-up table) for the correct air-fuel mixture…

[Jascha]

I finally got around to order a set of intake manifold gaskets. When elevated the left side manifold (no need to remove the injectors or the fuel line supply) I was delighted (imagine that) to discover that indeed one of the gaskets was not seated correctly. Obviously, there must have been and air leak here for some time now (PO spent a bundle in high-profile p-shops chasing the rough idle and poor low RPM response). Another ‘tell’ was that the cylinder temperature associated with that intake leak tended to run cooler…

The use of engine starting fluid (ether is my choice) idea is a good one (Thanks Steve W!) but it tends to give lots of ‘false positives’ if you are not careful to avoid the volatile vapors reaching the air filter box. This was the lower (cylinder surface) gasket which is harder to reach with starter fluid but it worked well in my hands and gave me reason to “believe”



The take to the garage message is that persistence, aided by smart Pelicans, is at times a better solution than just taking it to the p-Pros.

After all that said…it better idle smooth

Cheers,
Jascha

[ianc]

Another ‘tell’ was that the cylinder temperature associated with that intake leak tended to run cooler…

Interesting: I would assume that due to a leaner mixture, that cyl would tend to run hotter rather than cooler.

Good find!

ianc

[strupgolf]

Yep, that's my problem too. My intake gasket was bad on # 4 and I replaced it. Now the car runs like a champ. Thanks to all for the help.

[Jascha]

Originally posted by ianc
Interesting: I would assume that due to a leaner mixture, that cyl would tend to run hotter rather than cooler.

Good find!

ianc

I agree, all things being equal, the combustion should have been hotter in that cylinder…

[strupgolf]

Maybe so, but my #4 cylinder was not firing at all. It had compression but no fire. If I would take off the spark plug wire, the engine would'nt change tune. So it was cooler.

[CliffBrown]

A lean running cylinder will be hotter than a rich cylinder but the key word is running.

Just a slight difference is A/F mixture and you have little or no combustion, just a bunch of cool air and even cooler fuel passing through an overly lean cylinder. Gasoline has a very limited flammability range.

"The flammability range of gasoline is between 1.4 and 7.6%. If the ratio of gasoline to air is less than 1.4%, then the mixture is to thin to burn. The mixture cannot burn when it contains more than 7.6% gasoline because it is too rich to burn."

[Jascha]

Cliff,

That is quite telling of what I found –an intake chamber (in question) that was still “wet”

(I ran a cold engine just long enough to put the p-car on ramps and inspect the CV boots)

[Steve W]

When installing new intake gaskets, it is imperative that the intake manifold gasket surfaces are perfectly plane with one another or the gaskets will get sucked in again in less than 10k miles! This can happen if the intake nuts were retightened repeatedly in an attempt to cure the leaks. Take the manifolds and check the gasket surface against an absolutely flat surface, such as a granite slab, or a polished piece of marble or porcelain tile. If not flat, you can take it to a machinist to have them plane them, or you can attempt to sand them flat with sandpaper taped to a granite countertop. The phenolic spacers should also not be warped, and it is also important that they still have the concentric grooves molded on both sides to grip the gasket. If they are barely there, or scraped off when trying to remove the old gasket, replace them with new ones. I use a dark brown sticky Permatex aviation gasket sealer on all gasket sides for additional insurance. Be sure to torque the manifold nuts with a torque wrench to the exact specification of 16-18 ft-lbs (IIRC) not any more or less. Come back after driving it a few thousand miles, check and retorque again if necessary, because the gaskets will have shrunk and settled in.

[ianc]

Good advice from Steve. I should probably think about doing that one of these days...

ianc

[Jascha]

Originally posted by Steve W
.... The phenolic spacers should also not be warped, and it is also important that they still have the concentric grooves molded on both sides to grip the gasket. If they are barely there, or scraped off when trying to remove the old gasket, replace them with new ones.

Steve,

I don't recall seeing such "groves" (in some circles that’s a bad sign) on any of the spacers that I removed in the past. Can you point to it (or describe it again) in the attached image...?

[Steve W]

Hard to see in your picture, but you can kind of see the concentric rings on the surface of the picture, going around the doughnut hole. They are probably milled in after molding. When you're holding them in your hand, it's very obvious. The grooves are very similar in size and spacing as the surface a strip of standard desktop staples. You can run your fingernail across the grooves making a zipping noise.