Motronic start up - need to hear from people in the know

[redcoupe86]

ok, Im still dealing with a start up issue I never got resolved.

All the sensors have been checked and all seem to be working in order.

I have the new CHT sensor (by PO)

I have a new ICV

I've checked all large vacuume lines and replaced all small vacuume lines. While not fullproof, if I remove oil cap idle gets noticably rougher.

the Mixture has been set at idle with a CO analyzer by myself and another Pelican. However, Im not sure this is correct b/c I've read somewhere it seems that it needs to be checked at 2000 rpm.

Here's the deal:

When I start the car it revs up and idles fine, but if I just barely blip the throttle, say 20 seconds after startup, the car drops low, real low to like 200 rpm (almost dies - sometimes does) then rockets back up to the correct idle speed. This goes on until the car is warmed up then there are zero problems.

Can idle mixture have anything to do with this? I cant tell if its because it could be a too rich condition or too lean. I'd raise the idle speed up but that doesn't seem the right way to go about fixing the problem. Plus when the idle speed is raised up the CO readings get lean and you have to richen it up the get to the correct CO reading again.

Another thought I have b/c i have not checked to see if it is working is the thermo switch no back of the throttle body. Im not sure what this does.

Help.

[ZOA NOM]

I would replace the O2 sensor, then re-do the mixture with a gas analyzer.

[redcoupe86]

I should have mentioned - new O2 sensor.

As I understand it, the O2 sensor doesn't come into play until the car is warm anyway, and once mine is warm I don't have a problem.

[ZOA NOM]

My bad, you're correct.

Another possibility is the resistive wiper inside the AFM. This can develop a worn spot which can cause erratic idling. There is a procedure to adjust the wiper to ride along a different, unworn path , which provides a cleaner voltage to the DME.

[redcoupe86]

dont want to discourage you frame more input, but I have checked the wiper, and have set it to a new track. I've also used a friends known good AFM and had no luck with that either.

Keep the ideas going though, Im up for any insightful info. No matter how small the detail may seem. Im at the bottom of the barrel on ideas so I can only go up -

[jbrinkley]

I still think its the old(er) chip. if mine fit's in your socket you can have it till steve updates yours

[redcoupe86]

Hey Jerry,

must be getting close for you to be heading home. might have to take you up on that offer! Let me know when you'll be coming in and you when you will be having some free time.

[stlrj]

What's the CO like after the engine is warm?

[redcoupe86]

Well, we set the CO with the engine warm so it is fine. Should we have set it cold?

[ChrisBennet]

Originally posted by redcoupe86

I've checked all large vacuume lines and replaced all small vacuume lines. While not fullproof, if I remove oil cap idle gets noticably rougher.
...
When I start the car it revs up and idles fine, but if I just barely blip the throttle, say 20 seconds after startup, the car drops low, real low to like 200 rpm (almost dies - sometimes does) then rockets back up to the correct idle speed. This goes on until the car is warmed up then there are zero problems.

If it was my car this is what I would be thinking (right or wrong):
Some data points:
1. Idle dropping to almost nothing and then jumping up sounds like the engine is either trying correct for air/fuel ratio that is outside of the window it can correct for (engine wants to idle too high and ISV is trying correct it) or the idle stapilizer isn't working correctly.
2. If the oil cap is removed it should experience a hiccup and then a stable idle - not a rough idle. (From my poor memory.)
3. With the car warm and the ISV disabled (by putting a jumper in the test socket) the idle should be around 8-900rpm. (Judging from your thoroughness on the items you mentioned you probably already did this.)

Proper idle depends on:
- Correct cold mixture
The DME has to know that it the engine is cold so it enriches the mixture. The CHT does this. Probably not the problem if you've got a new (2 wire) CHT.

- The "Off idle switch" indicating DME that throttle is closed.
A too tight throttle cable, sticky throttle linkage (look at the bellcrank on the transmission) or a wayward cruise control cable can interfere with the throttle closing correctly.

- A working ISV
It is not unhead of for the DME's ISV driving transistors to fail. I suspect a bad ISV toasts them but I'm not sure. If you can, try your DME in someone else's 911. That's the safest way.

-Chris

[redcoupe86]

Thanks for the reply Chris:

Let me reply to some of your coments by number.

1 - I think this might be the problem, but im not sure where to go with the mixture since it was set at idle, with a CO analyzer, jumpers on, no cat.

2 - I may be over emphasizing this point, it the cap is removed the idle speed remains the same just not the same silky smoothness.

3 - you are correct this is how the idle speed was set.

- I have checked the "off idle" switch and everything seem to be in order.

- My ISV is new and when I installed it the "jump" back up in rpm whent the idle dips was much quicker, but it didn't solve the problem.


So I belive its #1 from you comments, but again, I don't know how to go about achieving it since i thought i did everything right the first time.

Originally posted by ChrisBennet
If it was my car this is what I would be thinking (right or wrong):
Some data points:
1. Idle dropping to almost nothing and then jumping up sounds like the engine is either trying correct for air/fuel ratio that is outside of the window it can correct for (engine wants to idle too high and ISV is trying correct it) or the idle stapilizer isn't working correctly.
2. If the oil cap is removed it should experience a hiccup and then a stable idle - not a rough idle. (From my poor memory.)
3. With the car warm and the ISV disabled (by putting a jumper in the test socket) the idle should be around 8-900rpm. (Judging from your thoroughness on the items you mentioned you probably already did this.)

Proper idle depends on:
- Correct cold mixture
The DME has to know that it the engine is cold so it enriches the mixture. The CHT does this. Probably not the problem if you've got a new (2 wire) CHT.

- The "Off idle switch" indicating DME that throttle is closed.
A too tight throttle cable, sticky throttle linkage (look at the bellcrank on the transmission) or a wayward cruise control cable can interfere with the throttle closing correctly.

- A working ISV
It is not unhead of for the DME's ISV driving transistors to fail. I suspect a bad ISV toasts them but I'm not sure. If you can, try your DME in someone else's 911. That's the safest way.

-Chris