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Also, here is a video of the jumping idle. No where near as bad as before, but a little annoying.
https://youtu.be/XH4fMsmy70c
Originally Posted by lightemup67
Hey fellas,
Thought I'd update you on the progress.
I continued checking for vacuum leaks with carb cleaner and had no luck. I moved onto grounds and everything I've checked is clean and tight. Although I've only checked about half so far.
Last night I decided to increase the idle, and I believe the previous owner had it set with the vacuum leak present. It was low (650 to 750). I increased it to 900ish by jumping g the b and c ports on the diagnostic port, and adjusting the idle hex screw on the throttle body.
I took it for a drive, and the idle bounced around slightly at stop lights. Went home and lowered the idle a little, and it seemed to cure it. It was late, so I didn't have time to test it on a cold start. I'll do that tonight.
one thing I noticed while testing things, we're these two wires with female connectors on the drivers side of the engine bay, nother connected to anything. Anyone know wherected they are supposed to go?
Just seemed odd to me.
I'll let you guys know how the cold driving goes tonight. This is the true test.
If I'm not mistaken, this is known as a "hunting idle". It is typically caused by a rich idle mixture.
My car does it when cold but settles down after it warms up.
The issue is:
1. The rich mixture raises the idle
2. O2 sensor sees the RPM rising with the closed throttle switch activated.
3. The DME takes control to lower the RPM.
All the systems are working against each other and causes the idle to hunt.
The mixture screw is located on the MAF above the idle stabilizer valve and to the left of the idle stabilizer hose.
Turning is counter-clockwise will make the mixture leaner (adds more bypass air).
I would start by turning is out by 1/2 turn until the hunting stops.
You may find the idle will need to be re-set after the mixture is correct.
If I'm not mistaken, this is known as a "hunting idle". It is typically caused by a rich idle mixture.
My car does it when cold but settles down after it warms up.
The issue is:
1. The rich mixture raises the idle
2. O2 sensor sees the RPM rising with the closed throttle switch activated.
3. The DME takes control to lower the RPM.
All the systems are working against each other and causes the idle to hunt.
The mixture screw is located on the MAF above the idle stabilizer valve and to the left of the idle stabilizer hose.
Turning is counter-clockwise will make the mixture leaner (adds more bypass air).
I would start by turning is out by 1/2 turn until the hunting stops.
You may find the idle will need to be re-set after the mixture is correct.
Thanks for that and the photo. I don't want to derail OP's inquiry, but if we're experience the same or very similar problems, maybe this is support for the cure.
My symptoms - 'hunting' idle at times, coupled with a rather severe 'on/off' fuel delivery feel during warm up - were greatly reduced when the air balance valve was cleaned. (My car was a real barn find, having sat for years).
Yet the odd fuel on/off feel seemed to happen when the car was going from open to closed loop. (Hope I have the order correct, according to my tech.) During the transition, weirdness happened, but it was a hit or miss thing.
The air balance valve cleaning definitely helped, but it persisted. He reset the idle, checked CO (was pretty off). Yesterday I unplugged the O2 sensor and ran the car.
It performed fine during what would've normally produced the on/off feel. Yet as mentioned, this is a sneaky ghost and I'm not calling victory yet. We shall see.
To be clear the screw under the (replaceable) yelllow cap is base idle. The mixture is under a cap that must be drilled to remove if not already on the airflow meter.
If you have checked for leaks then the next step is to have the idle mix adjusted with an air fuel mix meter and adjust base idle (must jump ports on dial and pull 02 plug while doing this). Get your idle mix right. Oh car must be hot. Above midway.
To be clear the screw under the (replaceable) yelllow cap is base idle. The mixture is under a cap that must be drilled to remove if not already on the airflow meter.
If you have checked for leaks then the next step is to have the idle mix adjusted with an air fuel mix meter and adjust base idle (must jump ports on dial and pull 02 plug while doing this). Get your idle mix right. Oh car must be hot. Above midway.
Thanks for the correction. Its been years since I've done this and didn't check the Bentley before responding.
Long story short, the OP's issues sounds like an idle mixture issue.
After talking to the mechanic, he suggested we do the CO test/ set the mixture, but FIRST I should do new plugs, and set the valves. (wires, cap and rotor looked good still).
So I took her home, swapped the plugs for nice NGK ones, pulled the valve covers, adjusted the valves (most felt okay, but I was super meticulous about it- one or two actually felt a little tight). Installed new gaskets and hardware as the old ones were a tad leaky.
She ran awesome after this, and the mechanic dialed her in with the machine and set the mixture for me. She seems a little rich on start up, but then levels out as she warms up and runs awesome.
I feel confident that the problem is cured. It was probably just a combination of the old plugs in there and valves slightly out of adjustment. Previous owner had mentioned he adjusted the valves in the past, but couldn't produce an invoice, so it may have been a few years.
The valve adjustment was actually kind of fun, and a great thing to know how to do if you own one of these cars. I highly recommend doing it yourself, saved me about $650 also!
09-14-2016, 02:56 PM
