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Discussion Starter #1
What the title says really. And this only happens with the AC on.

If I depress the clutch to come to a stop while the AC is on and at speeds over 30kph the idle drops the nearly zero before settling regularly. The issue doesn't happen with the AC off at all, and if I slow to under 30kph before depressing the clutch it works just fine.

Any ideas?

I've narrowed it down to either the Q20 four way pressure switch OR the speedo signal that goes into the climate control ECU (Pin B6), and my thought is to cut that wire to see if it fixes it in the hope the climate control ECU defaults to when the car's stationary.
 

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What is the A/C gas pressure?
High gas pressure can cause the compressor to work too hard which may affect engine idle.
 

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May I ask if the brakes seem to lose effectiveness at anytime during this? Like you momentarily loose brake boost?
 

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Have you tried resetting the idle actuator?

Turn on the ignition for 90 secs but do NOT touch the throttle or start the engine.

Turn off ignition for 90 secs

Turn on and start engine but do NOT touch the throttle and leave to idle for a few mins.


The other think to check is the breather pipe that runs to the intake manifold. It's not easy to see but if you take the engine cover off there are two hoses from the cam case: one goes to the intake just before the throttle and the other goes behind the throttle body to the intake. At the end is a plastic valve housing .. the mounting tabs snap and it lets in air at the wrong time which can kill the engine. I don't think you can get new ones now so don't break it!!

Several years ago I had the stalling problem but it wasn't related to the a/c. The valve at the end of that hose had been removed to do some engine work and it was missing a bit .. the valve inside! A new valve sorted it.
 

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Discussion Starter #5
Have you tried resetting the idle actuator?

Turn on the ignition for 90 secs but do NOT touch the throttle or start the engine.

Turn off ignition for 90 secs

Turn on and start engine but do NOT touch the throttle and leave to idle for a few mins.


The other think to check is the breather pipe that runs to the intake manifold. It's not easy to see but if you take the engine cover off there are two hoses from the cam case: one goes to the intake just before the throttle and the other goes behind the throttle body to the intake. At the end is a plastic valve housing .. the mounting tabs snap and it lets in air at the wrong time which can kill the engine. I don't think you can get new ones now so don't break it!!

Several years ago I had the stalling problem but it wasn't related to the a/c. The valve at the end of that hose had been removed to do some engine work and it was missing a bit .. the valve inside! A new valve sorted it.
This is really stumping me. The Idle Speed Actuator is fine, my mechanic's tested the actuator on a lift and found absolutely no issues whatsoever. I'm looking into the ABS issue that Marlon had reported on the 156 Twin Spark.
 

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Did you fix this?

Just a theory: when you go to over-run above 30 Kph the throttle probably closes almost completely, to minimise fuel consumption. If the clutch switch fails to tell the ecu when you de-clutch the ecu may hold the throttle almost closed until it sees the revs fall below idle speed, and then have to open it fast to prevent a stall. With the compressor running it will get much closer to stalling than without that extra load. With a working clutch switch the ecu would open the throttle to the normal idle position as you de-clutch. Below 30 Kph the ecu may keep the throttle in the normal idle position during over-run to prepare for the engine to idle.

(Would you get a warning for this fault? I don’t know, it might depend on the car and its age and whether you have cruise control.)
 

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Discussion Starter #7
Did you fix this?

Just a theory: when you go to over-run above 30 Kph the throttle probably closes almost completely, to minimise fuel consumption. If the clutch switch fails to tell the ecu when you de-clutch the ecu may hold the throttle almost closed until it sees the revs fall below idle speed, and then have to open it fast to prevent a stall. With the compressor running it will get much closer to stalling than without that extra load. With a working clutch switch the ecu would open the throttle to the normal idle position as you de-clutch. Below 30 Kph the ecu may keep the throttle in the normal idle position during over-run to prepare for the engine to idle.

(Would you get a warning for this fault? I don’t know, it might depend on the car and its age and whether you have cruise control.)
Now that's out of the box thinking! I haven't fixed this yet and I'll be investigating vacuum leaks because the issue has stopped being one that only happens with the AC on and now one that happens intermittently regardless of the compressor running or not. I'm putting it down on the list of things to check regardless.

Thanks for the tip!
 

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Did you fix this?

Just a theory: when you go to over-run above 30 Kph the throttle probably closes almost completely, to minimise fuel consumption. If the clutch switch fails to tell the ecu when you de-clutch the ecu may hold the throttle almost closed until it sees the revs fall below idle speed, and then have to open it fast to prevent a stall. With the compressor running it will get much closer to stalling than without that extra load. With a working clutch switch the ecu would open the throttle to the normal idle position as you de-clutch. Below 30 Kph the ecu may keep the throttle in the normal idle position during over-run to prepare for the engine to idle.

(Would you get a warning for this fault? I don’t know, it might depend on the car and its age and whether you have cruise control.)
Good thinking. I'm not sure what model the OP's car is, but all 147s, GTs & 156s from the facelift interior onwards had clutch position switches. I think early 156s (old style interior) didn't have them, but its been years since I've looked.
 

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Discussion Starter #9
Good thinking. I'm not sure what model the OP's car is, but all 147s, GTs & 156s from the facelift interior onwards had clutch position switches. I think early 156s (old style interior) didn't have them, but its been years since I've looked.
It's a Phase 2 GTV, and it seems to have a clutch switch? Hmmm.....
 

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It's a Phase 2 GTV, and it seems to have a clutch switch? Hmmm.....
In that case I would see if you can test the clutch switch in situ, if not remove it and test it. If in doubt, replace it and go from there.
 
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