Running lean machine (3.2L) needs help…
 

I am stumped (!) trying to understand why a 3.2L (stock in every way) would run lean only @ relatively low RPMs (1500-2200) and @ idle… (??)

Checked fuel pressure; Checked and replaced cylinder head temperature sensor; eliminate all vacuum leaks; checked ignition wires, rotor and distributor cap etc; checked all injectors pulse width and cables; checked air flow meter; checked and adjusted to spec both cams timing (stock DME and exhaust system)

Any suggestions what might be the culprit (I know that “lean is mean”)

Thanks!

Leak?
 

How about a vacuum leak or a leaky gasket?

It seems no vacuum leak in the usual places (including intake manifold)

Please keep those letters coming…

How about checking the O2 sensor? Or replacing it. Do you know the history?

You could have an AFM problem, too (air flow meter). Do some searching and you'll find a wiper issue that would read the wrong amount of air coming into the engine. Use search term, AFM, barn door, wiper and I bet you'll find it.


Doug

How did you determine it was running lean?

Joe

These are great suggestions… the AFM was replaced with a working test unit (made no discernable difference) –still wants to be lean (and mean)

O2 sensor may need a second look but was responsive (good voltage fluctuations)

Might be worth trying to run open-loop and see if false air is entering from somewhere (crankcase?)

>How did you determine it was running lean?

Joe:

Exhaust analyzer CO level below 0.5% (hesitating while coasting…)

-no hand waving with these Pelicans!

It's a long shot, but did you check the eight position rotary switch in the DME? Position #3 gives a 3.9% lean setting. Also, the Altitude sensor should only be closed loop above 1,000 feet. If your below that altitude and it's closed, it will be telling the DME to lean out.

Check out the details on both at Steve Wong's (911Chips) web site:

http://www.911chips.com/fuelsys.html

Good Luck!

Have you (long shot), done a top end recently?

I recently did one, and when I tried to set my mixture after a couple hundred miles, it was ridiculously lean. I had to turn the screw almost all the way in to get .65 % CO. Before the rings have broken in, the engine is just not pulling the vacuum it will later.

After about 2500 miles, I checked it again and it was at 3.4 % CO! Should have just left the damned thing alone!

ianc

But isn't that what it should be after the cat with the 02 sensor connected?

My 86 3.2 barely registers any CO at idle and still runs great.


Joe

That would be normal. I set mine at .8% upstream of the cat with 02 disconnected. When I plug the 02 back in, it heads down to .4% or so. Downstream of the cat will be much less, so it depends on where and how you're measuring this exactly.

If it's hesitating or bucking while coasting, there may be an issue with the idle switch which shuts off fuel flow with closed throttle above about 1100 RPM.

ianc

Doug: Did not explore that avenue (yet) since the DME was not touched but will need to consider the altitude compensator (to cover all bases).

Joe:
The measurements were made open-loop and the number I recalled was the upper limit of the very low readings (I know it was lean when the engine RPM was <2000 range). At higher (and increasing) RPMs this problem disappeared altogether.

ianc:
Normally, the air that gets to the auxiliary air-valve (idle bypass) has been measured by the air-flow meter (AFM) which should be in the equation (data map) of the air/fuel ratio settings (for the given temperature)

OR

are you referring the throttle valve (switch), mounted directly on the throttle shaft –if so, worth exploring to be sure since it plays a role in adding some compensation to the pulse-time.

Do you know if it normally signals “closed throttle” during coasting (coasting cut-off by other name) …?

Yep, that's the one.

Yes, when throttle is closed, switch is engaged and this tells the DME to activate the idle stabiliser valve. If throttle = closed, and RPM > ~1100 (can vary), then fuel flow is cut off until RPM drops again. If there's some flakiness with this switch, then that may account for the jerkiness during coasting.

ianc

<I>
<b>
If it's hesitating or bucking while coasting, there may be an issue with the idle switch which shuts off fuel flow with closed throttle above about 1100 RPM. </b> </I>

I had the same problem after pullin an engine. Throttle linkage was bent at the tranny pivot. Threw the switch helter skelter at the throttle (adjusting screw for the 4-8 mm gap is a PITA)

GO SWITCH GO SWITCH ... RAH RAH RAH

good call Ianc!

Good idea for the problem. I have the same stuff when I'm coasting along at about 2000RPM. It just wants to hunt for a rpm and misses and gurggles. I'll look into this also.

I had a lot riding (idling) on this much overlooked microswitch potential problem.

Regrettably, it does not contribute to the persistent lean mixture at idle and low RPMs. A related problem area could be the harness going to the DME. A good way to eliminate the culprit is to short (or open circiut) the harness form the switch socket cable) to the DME and see what happens. In my particular case the idle RPM went well over 1200 and returned to ~880 when the switch (jumper was put back into the circuit) which indicates that part of the A/F mixture works fine.

I still think that false air (bypassing the AFM) can be a causative factor… any more suggestions from the Pelicans…?

After the cat is not where you should be taking your readings since the job of the cat is to clean up the exhaust and make you think it's lean when it's fine.

I would concentrate on the ignition and forget the mixture.

Joe

As Ian said, fuel flow is cut off to the injectors on deceleration which activates the idle microswitch, and does not come back on until 1100 rpm. If you are still registering CO or O2 sensor voltage above 0.1 volts while decelerating above 1100 rpms, your idle microswitch may not be activating.

Another common cause of lean low rpm issues is a worn air flow meter track. Most Carreras and 964s over 70k miles have worn AFM tracks, especially in the idle and low rpm areas where the engine operates most of the time. You can see this when the black plastic cover is removed, and you see almost white wear tracks on the black resistive surface where the wiper contacts have worn through. The worn spots decrease the signal output voltage to the DME which will make it run lean and hesitate. Reposition the wiper arm up or down the shaft, and carefully bend the copper wiper arm to track on a new surface while just applying enough pressure to maintain consistent contact. Then adjust idle base CO as per spec.

To check for air leaks, use engine starting fluid (ether) and spray around all suspected points while the engine idles. Disconnect your O2 sensor and idle control valve during this test, and if you idle shoots up, you have a leak. Common causes are sucked in intake manifold gaskets between the manifold and heads.

Joe,

Just so there is no confusion, the lean condition determination is NOT based on values at the exhaust pipe (past the Cat) but at the point of the O2 Sensor, which is in front of the Cat (!)

The reason I am harping about the -false air- is that blocking (grossly) the air filter box intake does very little to change the idle (… does this hint at anything familiar such as large vacuum leak...?)

That's very helpful, Steve!

To be sure, I will chase it down and report back…