New Idle Problem - Next Steps?

[ausgezeichnet]

I thought I had fixed a pulsating idle problem with my 84 3.2L Carrera by updating the cylinder head temp sensor. It helped,but did not completely eliminate the problem.

Now it seems to intermittently occur after the engine has reached normal operating temperature. I can make the problem go way for a little while by turning off the car and restarting it. It also seems that I can prevent it from occurring by letting the RPM's drop to around 1K before pushing in the clutch when coming to a stop. Do any of those symptoms make any sense?

I am considering replacing the following as my next steps. Which one should I start with given the above symptoms?

DME Relay
Speed/Reference Sensors
O2 sensor
Vacuum lines

Thanks.

[rzepko6194]

Do a search on "idle issues". There's plenty of advice out there.

It's not a bad idea to replace the DME Relay as it's cheap and you can keep the old one as a spare.

It could be that the ICV is to blame and assuming it's not worn, could be cleaned with carb cleaner. Incorrect idle mixture could also be to blame.

Check out the "free" stuff first before replacing expensive components.

Good Luck!

[scottb]

I had the idential problem in my 84. Replaced the idle control valve, head temp sensor and O2 sensor. Nothing helped. Then I had my mechanic adjust the mixture....PROBLEM SOLVED.

Sometimes the simplest solutions are the best.

[Por_sha911]

Disconnect the O2 and see if the problem goes away. That may narrow down the list of possibilities.

[Paulporsche]

are you able to lean your mixture @ the mixture control unit? Try leaning 1/16 turn. If it is better but not quite enough try another turn. If that doesn't help put it back where it was and look elsewhere.

[ausgezeichnet]

Thanks of the feed back. I adjusted the airflow bypass screw, picture 2 on page 102 of 101 projects. Is this what you mean by the mixture contol unit and adjustng the mixture?

After disconnecting the connector from the Idle Control Valve. I had to turn the air bypass screw out about 3 or 4 turns to get it up to about 750-800 RPMs. It does not see like the car has the same power it used to have. Plus it did not seem to help the problem. Yes, I did reconnect the ICV.

[ausgezeichnet]

I just answered my own question by searching a little back further in the archives. I was adjusting the airflow bypass screw no the mixture adjustment screw. Does any one know if you need to remove the AFM to remove the plug that covers the mixture adjustment screw?

[ausgezeichnet]

Unplugged the O2 Sensor and the idle surging went away. Does that mean the O2 Sensor is bad or does sit mean the richer mixture is just masking other problems?

[Paulporsche]

There is a lot of info/opinion on the O2 sensor, Mike. You should do a search.

[ausgezeichnet]

Paul, I did an extensive search and found no direct answer to my question.

I am trying to figure out if a bad O2 sensor produces the same results as if I just removed it. In other words, when they fail do they stop producing a signal (as if it was removed) or does it produce a signal that is bad. Thanks.

[Lorenfb]

You need to check the voltage output of the O2 sensor, both in closed-loop
and open-loop. The O2 voltage should be about .50 volts open-loop with
a proper CO mixture.

Check out this website ( www.systemsc.com ) on the Diagnostics page
under Poor Running.

[jtcmuga]

Check out this forum.... I posted it recently...

84 Carrera Throttle Flat Spot @2900

Check your vaccuum lines. I had a wobbly throttle and it turns out the hose from my thermo valve to my ambient air valve was disconnected at the thermo valve. Since the thermo valve isbehind the throttle housing, I couldn't see the disconnect until i took the airbox, MAF unit, and throttle housing off.

In the hunt, i checked the cyl head temp sensor, replaced cap/rotor/plugs/wires/fuel filter/o2 sensor, and adjusted valves.

Good luck

Jojo

[jtcmuga]

i would check your air/fuel ratio prior to turning any screws - the airflow bypass screw or the mixture adjustment screw...

Jojo

[ausgezeichnet]

Thanks guys for your feedback. That gives me enough info to keep tracking this problem down. This forum is great!

[ausgezeichnet]

Removed the AFM to adjust the wiper arm and remove the plug for the mixture adjustment. While I had the AFM out I found a few hoses that looked suspect. I also hooked up the line to the top of the Ambient Air Valve that had come off.
Put the AFM back in and played with the mixture adjustment. Tried turning it all the way in then out several turns it made no difference in the idle. It still pulsates after the engine gets warm. So I guess I'll still need to do a partial engine drop and replace all the vacuum lines.

Does any one know if there is a complete listing anywhere of all the vacuum lines I would need to order?


By the way: The wiper arm adjustment made a huge difference in the engine responsiveness especially in the low end.

[ausgezeichnet]

Loren,

How do you actually test an O2 sensor in both a closed-loop and open loop state. I have a multimeter but not sure what ot do with it. Do you test by removing he 02 sensor or test at the connector in the engine bay. Thanks.

-Mike

[Lorenfb]

You can test in either open or closed loop.

Open Loop (O2 disconnected) -

The voltage at the O2 sensor will give you an IDEA of the mixture value,
<.50 being lean and >.50 being rich.

Closed Loop -

You should see the voltage varying between .30 and .80 volts.

The mixture setting is probably not causing the idle problem.

Possible causes are:
1. bad contacts at the idle switch or mis-adjusted switch
2. an intake air leak bypassing the ICV
3. a bad ICV
4. a bad DME
5. a bad performance chip
6. idle air bleed screw too far out

[ausgezeichnet]

Loren,

Thanks for your quick response, but I am still a newbie with this type of stuff. When you say O2 disconnected do you mean the O2 sensor is removed from the bung on the Cat or the connector in the engine bay is disconnected? In a closed loop situation where to I actually place the black and red leads from the multimeter. In either case should the car be running for the test? I just got my Bentley manual and it does not help much with this. Thanks.

-Mike

[Lorenfb]

1. O2 sensor fully disconnected with meter leads inserted where plug would go.
2. With O2 connected, "backprobe" (insert leads where wires come into connector)
the O2 connectors.
3. The car must run and the engine must be at full temp or O2 sensor may not fully
function properly.

[ausgezeichnet]

Thanks Loren! That is exactly what I was loking for.