Perplexed - 3.2 won't idle when cold

[wjdunham]

got a problem that I've been debugging for a few weeks now with a Motronic 3.2 (now a 3.4 with 964 cams) motor, several calls with Steve Wong who has provided some debug tips but so far cannot find the issue. It's a new build with a S.W chip that fires up fine but will stall and won't hold idle for a few minutes. As soon as it warms up just a bit it's fine from then on. Here's my debug so far:

• Idle switch closes and resistance check show's it's working fine
• Disconnect the O2 sensor - doesn't seem to have any affect one way or the other
• CHT sensor is brand new
• Fuel pump, fuel pressure regulator brand new, fuel pressure checked and in spec.
• Injectors fully reconditioned, don't suspect them as the car pulls fine once warmed up
• Pulled the Idle mixture screw (the 3mm hex bolt on the bottom of the AFM), cleaned the air passages with brake cleaner. Closed all the way and backed out three turns.
• Re-tracked the AFM wiper, check with a 9v battery shows a monotonic output voltage across the range.
• same DME with a stock chip passes CA smog in my 1985 no problem

I'm out of ideas. Still think it's the idle mixture but adjust the screw on the bottom of the AFM doesn't really seem to change the behavior. I don't have a wide-band AFM to double check the mixture though.

Any thoughts as to what else I might try aside from spending the $300 for an AFM? I'm out of ideas.

Thanks in advance.
Bill

[Flat Six]

Not sure if I understand your last bullet point; are you saying this same DME w/stock chip did pass CA smog previously? Or that it does now?

Sounds an awful lot like an idle mixture issue to me. A couple of thoughts:

1) O2 sensor doesn't affect mixture until engine is warm and in closed-loop mode, you might want to connect a DVM to it though to get a general sense of cold idle mixture.

2) Just because CHT is new doesn't mean it's sending proper signal to DME; I'd measure resistance at connector to double-check its function. If OK, I'd still check at DME connector to ensure there isn't an issue w/wiring or connector(s) along the way.

3) Didn't see it mentioned, but is ICV working properly?

4) Smaller vacuum leak(s) can manifest as idle issues but less obvious when engine is warm and DME can adjust mixture. You might want to check all the usual suspects, especially for a new build check/re-tighten intake runner bolts. A vacuum gauge here can be very helpful.

5) Also might want to check your plugs for both signs of rich mixture and whether they're the proper heat range. BTW, which plugs are you running?

6) Did you verify/adjust the idle screw at the throttle body? I'm sure you know this, but as a reminder, short the B/C terminals when you do.

Hope this helps.

[911obgyn]

Did you do the IAC bypass with a jumper according to bently manual, and set idle speed while warm?
You can get afr close by using a meter on O2 sender at idle i think its supposed to dither around .06 mv

[mysocal911]

Originally Posted by Flat Six

Not sure if I understand your last bullet point; are you saying this same DME w/stock chip did pass CA smog previously? Or that it does now?

Sounds an awful lot like an idle mixture issue to me. A couple of thoughts:

1) O2 sensor doesn't affect mixture until engine is warm and in closed-loop mode, you might want to connect a DVM to it though to get a general sense of cold idle mixture.

2) Just because CHT is new doesn't mean it's sending proper signal to DME; I'd measure resistance at connector to double-check its function. If OK, I'd still check at DME connector to ensure there isn't an issue w/wiring or connector(s) along the way.

3) Didn't see it mentioned, but is ICV working properly?

4) Smaller vacuum leak(s) can manifest as idle issues but less obvious when engine is warm and DME can adjust mixture. You might want to check all the usual suspects, especially for a new build check/re-tighten intake runner bolts. A vacuum gauge here can be very helpful.

5) Also might want to check your plugs for both signs of rich mixture and whether they're the proper heat range. BTW, which plugs are you running?

6) Did you verify/adjust the idle screw at the throttle body? I'm sure you know this, but as a reminder, short the B/C terminals when you do.

Hope this helps.

Read post #20 here; Bench testing Carrera Idle Control Valve

[wjdunham]

I neglected to mention that I did warm the car up, jumpered the ICV bypass and set the idle screw on the throttle body to 950 RPM. Rock solid once the car warms up a bit.

On the DME - I have two 911's, this '69 with the 3.4 transplant, and a bone-stock 3.2 1985. If I take the DME from the '69 and put it in the '85 with the stock chip, passes smog no problem. I mention this to rule out the DME as a potential culprit.

I checked for vacuum leaks, all the intake gaskets are new, bolt's torqued to spec. I have caps on all bit vacuum nipples for the brake booster and oil tank breather. Pretty sure I don't have a vacuum issues.

Also disconnected the O2 sensor after the car had been well warmed up and was getting about 0.26V constant reading. This was an open loop measurement, I did not try to jumper the sensor with it still connected to the DME harness.

Will double check the CHT sensor to see what it's putting out. I think it's supposed to be something like 2.7K Ohms when cold, and 270 Ohms when the engine is warmed up?
UPDATE: CHT measures 2.23K Ohms when cold.

[tholyoak]

If the AFM screw isn't doing anything, I would suggest that you may have an intake system leak or that the o-ring on the screw is missing/not sealing. Although it seems you don't have a good way to test that it really isn't doing anything. As suggested, you can just look at the narrow band O2 sensor voltage and see if the screw is really doing anything or not.

Is the hose that runs from the rubber throttle body to AFM hose to the oil tank installed? Is the restrictor (brass cone looking thing) installed in that oil line?

-Todd

[wjdunham]

Thanks Todd - I did pull the AFM screw and the o-ring is there and seems still supple enough to seal off air getting past the screw. I do not have the hose from the oil tank back to the 90 degree big rubber fitting, that is blocked off. Right now I just have a small filter installed on the oil tank fitting venting it into the engine compartment.

I do plan to take some more measurements from the O2 sensor with a few different settings on the AFM screw when I have time. I will also do some more checking for possible vacuum leaks as that could definitely explain the behavior. Wouldn't the 0.26V measurements with the engine warm indicate that it's running on the lean side at idle?

Originally Posted by tholyoak

If the AFM screw isn't doing anything, I would suggest that you may have an intake system leak or that the o-ring on the screw is missing/not sealing. Although it seems you don't have a good way to test that it really isn't doing anything. As suggested, you can just look at the narrow band O2 sensor voltage and see if the screw is really doing anything or not.

Is the hose that runs from the rubber throttle body to AFM hose to the oil tank installed? Is the restrictor (brass cone looking thing) installed in that oil line?

-Todd

[911obgyn]

Crankcase should be vented to oil tank it is a closed loop. There is your vacuum leak!

[john walker's workshop]

That was easy.

[wjdunham]

Originally Posted by john walker's workshop

That was easy.

I might be misunderstanding how it's supposed to work. I have the crankcase vented to the oil tank, and the oil tank vented to atmosphere (inside the engine compartment).

All the places where any big vent lines would connect to the intake system are blocked off, so my intake isn't pulling air out of the oil tank.

This would leave only three places for air to enter the intake south of the AFM:
1) throttle body bypass valve and air screw - set correctly when the car is warm at leas

2) AFM bypass screw, which controls how much goes past the barn door.

3) The small nipples are all either blocked off or have something connected to them. The Y connector that sends vacuum to the FPR and accumulator is one. The other is the intake that lets warm air in to prevent throttle icing in cold conditions, which is capped.


Am I missing something?

[mysocal911]

Originally Posted by wjdunham

I neglected to mention that I did warm the car up, jumpered the ICV bypass and set the idle screw on the throttle body to 950 RPM. Rock solid once the car warms up a bit.

On the DME - I have two 911's, this '69 with the 3.4 transplant, and a bone-stock 3.2 1985. If I take the DME from the '69 and put it in the '85 with the stock chip, passes smog no problem. I mention this to rule out the DME as a potential culprit.
I checked for vacuum leaks, all the intake gaskets are new, bolt's torqued to spec. I have caps on all bit vacuum nipples for the brake booster and oil tank breather. Pretty sure I don't have a vacuum issues.

Also disconnected the O2 sensor after the car had been well warmed up and was getting about 0.26V constant reading. This was an open loop measurement, I did not try to jumper the sensor with it still connected to the DME harness.

Will double check the CHT sensor to see what it's putting out. I think it's supposed to be something like 2.7K Ohms when cold, and 270 Ohms when the engine is warmed up?
UPDATE: CHT measures 2.23K Ohms when cold.

So what! Just because it passes smog, doesn't imply that all functions within the DME ECM are fully functional.
Once the temp sensor is below about 200-300 ohms, the sensor has NO effect on the mixture.
Did you perform the test in post #4?

You indicate; "won't hold idle for a few minutes when cold", then fully disconnect the temp sensor when cold to check for a mixture problem.
If the startup idle improves, then the overall mixture is too lean. Your measurement of the O2 sensor when warm of .27 volts
indicates a LEAN mixture which can cause a low idle when cold. You can push on the AFM flap to richen the mixture when cold to check for an idle improvement.

Your problem isn't that difficult to solve! A 911 3.2 DME engine running problem is extremely simple to troubleshoot.

[wjdunham]

Originally Posted by mysocal911

So what! Just because it passes smog, doesn't imply that all functions within the DME ECM are fully functional.
Once the temp sensor is below about 200-300 ohms, the sensor has NO effect on the mixture.
Did you perform the test in post #4?

You indicate; "won't hold idle for a few minutes when cold", then fully disconnect the temp sensor when cold to check for a mixture problem.
If the startup idle improves, then the overall mixture is too lean. Your measurement of the O2 sensor when warm of .27 volts
indicates a LEAN mixture which can cause a low idle when cold. You can push on the AFM flap to richen the mixture when cold to check for an idle improvement.

Your problem isn't that difficult to solve! A 911 3.2 DME engine running problem is extremely simple to troubleshoot.

Since it is easy enough to debug the temp sensor, I did that first. It does seem to be a potential culprit. I disconnect it and leave it dangling, won't hold idle. Jumper with a 2.7K resistor, engine will hold an idle although not super smooth and a bit low RPM, but at least it stays running. When I pull the jumper it dies.

Tried a 270 Ohm resistor and a wire to short it completely, neither will get it to hold an idle.

Ugh, it's a bit of a pain to replace that sensor so I'd really like to make sure it's the problem. Again, when I measure the resistance across it when it's cold, I get something less than 2K Ohms. I'll take a few measurements when the engine warms up and see if it is showing at least some response to the engine temp. If the mixture is off due to some other cause is it possible I see the above behavior?

[mysocal911]

Originally Posted by wjdunham

Since it is easy enough to debug the temp sensor, I did that first. It does seem to be a potential culprit.

Not necessarily. It basically indicates you have a mixture problem when 1st starting.

Originally Posted by wjdunham

I disconnect it and leave it dangling, won't hold idle.

The mixture is now too rich.

Originally Posted by wjdunham

Jumper with a 2.7K resistor, engine will hold an idle although not super smooth and a bit low RPM, but at least it stays running. When I pull the jumper it dies.

The mixture now is close to being correct.

Originally Posted by wjdunham

Tried a 270 Ohm resistor and a wire to short it completely, neither will get it to hold an idle.

It's again too lean a mixture.

Originally Posted by wjdunham

Ugh, it's a bit of a pain to replace that sensor so I'd really like to make sure it's the problem. Again, when I measure the resistance across it when it's cold, I get something less than 2K Ohms.

You need to try a 3-4K resistor. Remember, higher resistance causes richer starting mixture.

Originally Posted by wjdunham

I'll take a few measurements when the engine warms up and see if it is showing at least some response to the engine temp. If the mixture is off due to some other cause is it possible I see the above behavior?

Yes!

You have two DME ECUs, one with a stock chip and the other with a modified chip
and they both cause the engine to run the exactly same, right? If that's true, then it's not an ECU problem.

Again, try and richen the mixture by pushing on the AFM flap and/or remove the cover and mark the flap position and then loosen the flap spring, when first starting the cold engine.

[john walker's workshop]

So hook up the hose from the filler neck to the back of the intake air boot. Gotta have a closed system, not vented to atmosphere.

[r_towle]

Use a potentiometer (volume dial) to tune the CHT circuit until you get it perfect.
Measure resistance across the volume switch
Remove it and install the resistor that you need to replace the volume switch.

[mysocal911]

Originally Posted by r_towle

Use a potentiometer (volume dial) to tune the CHT circuit until you get it perfect.
Measure resistance across the volume switch
Remove it and install the resistor that you need to replace the volume switch.

No need to do that step. If engine runs smoothly at startup, then temp sensor is bad or
something else is causes the lean condition. You can't leave too large a resistor in place of the temp sensor,
the engine will run too rich when warm.

[wjdunham]

Taking all of the above, it would seem "something else is causing the lean condition" and per John W.'s guidance it sounds like my lack of a breather hose from the oil tank to the intake boot. I think I have a hose lying around that might fit, will try that tomorrow when I have some time.

I'm still not understanding the physics though. I would have thought that the crankcase ventilation is a separate system from the intake. When I hook up the hose, if there are no air leaks in the hoses or the tank, and the oil cap is on tight, how would this cure a lean condition at startup. If anything, I would think it would cause a more lean condition...

Sounds like my analysis is incorrect here, but I'm not understanding what I am missing.

[mysocal911]

Originally Posted by wjdunham

Taking all of the above, it would seem "something else is causing the lean condition" and per John W.'s guidance it sounds like my lack of a breather hose from the oil tank to the intake boot. I think I have a hose lying around that might fit, will try that tomorrow when I have some time.

I'm still not understanding the physics though. I would have thought that the crankcase ventilation is a separate system from the intake. When I hook up the hose, if there are no air leaks in the hoses or the tank, and the oil cap is on tight, how would this cure a lean condition at startup. If anything, I would think it would cause a more lean condition...

Sounds like my analysis is incorrect here, but I'm not understanding what I am missing.

It's used to prevent the engine blow-by, causing air pollution, by sucking it into he intake. Remember, the crankcase is sealed from the atmosphere and will always have a vacuum,
i.e. no external air leaks. You can test for this by removing the oil cap, causing the idle to drop, i.e. a lean condition. Remove the oil cap and see what happens, no change
and you have an intake air leak causing a lean condition at idle. If that intake blow-by hose is blocked, then crankcase fumes won't enter the intake. Then that's NOT causing
your problem, if there's really an air leak in the crankcase venting system.

[john walker's workshop]

There is a restricter in that hose also. It keeps the engine from stalling when the oil cap is removed.

[tholyoak]

Based upon the description of how the oil tank is plumbed, I don't see how it is providing a source of false air. My understanding is you have the breather hose from the case connected to the oil tank, the other connection at the tank is vented to the atmosphere to allow the case not to create a vacuum. The port on the throttle body boot that would normally connect to the tank is plugged. Thus no air leak on the intake side.

Have you checked or eliminated the IAC valve as the culprit. I can imagine if it is stuck in the open state it will continuously leak air past the throttle plate and result in a lean mixture.

Based upon your resistance values I don't think there is anything wrong with your CHT sensor. As stated above, mucking about with resistors to simulate different engine temps is just allowing for compensation of the lean mixture you have when the DME knows the correct engine temperature.
Again, this is all based upon the idea that you have a lean mixture that cannot be compensated for within the AFM adjustment screw.

-Todd