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305 E4ME Qjet Fast Idle when warm: what to check

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Old 11-14-2017, 05:27 PM   #1
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305 E4ME Qjet Fast Idle when warm: what to check

So I have been through many threads, the haynes manual, and many internet resources on the E4ME and am still not sure where to go from here. Here is what happened...

The subject: 1983 305 with E4ME with electric choke, ISC solenoid, 160k miles. I know the history of this car (was parent's car). Most likely, very minimal has been done to the tune. Just enough to keep it passing CA emissions testing.

BEFORE:

Engine ran fine. Choke worked as expected. Idle was ok. I removed some emissions components and advanced the timing a bit which saw some increase power. But the choke and idle behavior pretty much stayed the same.

THEN:

It was discovered, after replacing the non-functioning temperature gauge that the engine has run hot nearly %100 of the time (in the red). Who knows how long this has been the case.

I proceeded to replace the thermostat, which evolved into replacing the intake manifold with Edelbrock Performer (2101 I think). That is another story.

NOW:

Engine starts, choke works as expected. But the engine runs at a fast idle (1500-2000 rpm) when the engine is fully warmed up. (thermostat fixed the hot running problem and the gauge reads 220-240ish).

THE PROBLEM: What caused, and how to adjust the fast idle?

MORE INFO:

Everything is reconnected back the way things were (except the EGR was deleted, but solenoid still connected). There are no vacuum leaks. In fact less leaks than before probably.

No codes are being thrown.

The throttle linkage is FULLY retracted. ILC solenoid is fully retracted. The the left hand linkage set screw is backed off and not touching the linkage.

I can "push" the throttle linkage forward (towards the front) a bit, with a stronger-than-return-spring force, which lifts the TPS plunger a bit, and the engine nearly drops back to normal idle. But returns to fast idle when the force is released.

I see no obvious issues with the throttle linkage at the carb (i.e. broken/bent bits).

SO...

What is causing the fast idle?
Forcing the throttle forward a bit improves idle by affecting two things I can Identify:
- TPS sensor, could this tell the ECM to back off enrichment causing idle to drop?
- Butterfly valves are closing just a little bit more, restricting air and causing idle to drop?
- both?

The thing is, the linkage on the carb is 'at rest' in its most 'closed' position. Nothing is forcing the linkage 'open'. It cant get anymore 'closed'!

Or is there some other setting I have missed?

Or, could this be the result of better flow performance of the new intake? If so, how can I dial back the carb idle?

I will probably overhaul the E4ME, with new float, new electric choke, sometime in the future. But, before I do that, knowing how the idle is being affected would be very helpful.

Thanks all for any input.
Robert
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Old 11-14-2017, 10:01 PM   #2
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Car: 1987 Z28 Camaro
Engine: 305 LG4 4bbl
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Re: 305 E4ME Qjet Fast Idle when warm: what to check

vacuum leak would be my first port of call.
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Old 11-17-2017, 01:52 PM   #3
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Car: 1985 Trans Am
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Re: 305 E4ME Qjet Fast Idle when warm: what to check

I had this problem on my '85 LG4, and was going through all the same problems you are. It turned out that the carburetor mounting gasket was hanging up on the butterfly and not allowing it to completely close. I guess it was evident when the shop pulled the carb off. Yes, I finally threw in the towel and brought it to a shop that deals with carburetors.
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Old 11-17-2017, 05:22 PM   #4
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Re: 305 E4ME Qjet Fast Idle when warm: what to check

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Originally Posted by gearhead141 View Post
I had this problem on my '85 LG4, and was going through all the same problems you are. It turned out that the carburetor mounting gasket was hanging up on the butterfly and not allowing it to completely close. I guess it was evident when the shop pulled the carb off. Yes, I finally threw in the towel and brought it to a shop that deals with carburetors.
Thanks for that. Yeah, I have a new gasket coming in the mail. The current one is a bit ratty. who knows,maybe there is a vacuum leak in there.

After fussing with things a bit I have determined the idle mix is a bit on the rich side.

But, more specifically, the fast idle on the choke side is getting stuck and/or the fast idle set screw is cranked too high.

But why this would not cause a problem on the old manifold is unclear.

Some of this diagnosis is blind as without a dwell meter, I cannot verify if the ECM is in open or closed loop. I know this nneds to be be considered when choosing what to adjust.

Got a $20 dwell meter coming soon (off of eBay).
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Old 11-18-2017, 01:12 PM   #5
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Car: 1985 Trans Am
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Re: 305 E4ME Qjet Fast Idle when warm: what to check

The idle circuit does not involve open loop or closed loop operation. It is simply the idle mixture screws, the choke, and the curb idle screw. Yes, the choke linkage could very well be at fault. Once the car starts (cold), the idle should be anywhere from 1500-1800rpm. A quick stab of the throttle should unlatch the fast idle cam on the carb. Then the idle should drop down to 1100-1200rpm. The choke is keeping the idle high at this time. As the choke heats up, the idle will drop down to the curb idle setting.
Closed loop doesn't come into play until the O2 sensor heats up enough to send a signal to the ECM.
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Old 11-19-2017, 12:14 AM   #6
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Re: 305 E4ME Qjet Fast Idle when warm: what to check

So far all indications are a stuck fast idle linkage, but even with it manually dropped idle is still a high (and the curb idle screw is fully retracted).

I am hearing what sounds like a vacuum leak when I shut off the engine, just forward of the driver side firewall. It does not appear to be the power brake assist. I think the MAP sensor is there? It seems there is a small black rigid vacuum line to the manifold from there.
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Old 11-19-2017, 12:28 PM   #7
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Re: 305 E4ME Qjet Fast Idle when warm: what to check

Quote:
Originally Posted by zed-028 View Post
vacuum leak would be my first port of call.
Yes! This was it!

Despite having labelled all hoses and connections before the manifold swap, upon reconnecting, I managed to connect the left wiper washer hose to manifold vacuum!

I rechecked and rechecked everything and somehow missed this.

The high hissing sound near the windshield gave it away.

Gawd I am an idiot.

Everything fine now. And I know a lot more about this carburetor now!
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