Another fun discussion... Vacuum impact on idle

Ok, so, as some may remember, I’ve, like others, have been chasing a surging idle issue. It’s just a slight hunting condition that doesn’t even blip the rpm gauge, but you can hear it. For me, in D or R, it goes away and after running, it’s also not always prevalent when in P, but sometimes.

In chasing the “delayed hot start” issue, I was mucking around unplugging/capping vacuum hoses, I happened to notice that when I uncapped a plug from the intake manifold (in this case the intake port that goes from the intake manifold to the FPR-B), the idle went up a touch (from like 750 to 1000), but was smooth as can be. Reconnect the plug and the slightly surge/hunting started again.

I’m not actually reconnected the hose from the intake to the FPR, it’s just a plug cap - the vacuum hose to the FPR is not connected in both tests.

The TPS was the first thing I checked a while ago and it’s operating as expected.

What insights might this give to troubleshooting? Open up the intake vacuum, idle increases and is smooth as silk versus close the intake vacuum and idle surges/hunts just enough to the ear.

Could your car be running lean? All it does to disconnect vacuum to B bank FPR is to increase fuel pressure and thus run richer.

Are you running at the correct fuel pressure? Maybe your FPR isn’t 100%.

Other reasons your car could be running lean???

And it idles better in D and R because the O2 sensors see lean and the ECU corrects.

My car has always idled a hair worse in P. What helped the most was dialing in throttle gaps to be as perfectly close to each other as can be, and at .002".

Is this a Marelli car?

Yes, it’s a Marelli.

A Marelli car does not go from open to closed loop when moving from P or N to D. (If that’s what you were referring to Greg…) It’s in closed loop regardless of gear selected.

Ahhh, OK. I had no idea the Marelli cars changed so much from the Lucas.


The one thing they could’ve changed is not running the stoopid coax wire all the way to the where the GM module used to be just to make a u-turn and go right back inside. It could just run from the boot to the RH footwell and avoid the engine compartment altogether. Nope. :angry:

I’ll double check tonight, but if my mind serves me correctly, it doesn’t matter which intake port gets unplugged, not just the FPR ones…

In that case, it should work just as well to adjust the idle speed a bit higher. Does exactly the same thing, lets a bit more air in.

My guess is that you are experiencing the normal mixture correction triggered by the oxygen sensors; oxygen sensor sees slightly rich and the ECU leans. Then the sensor sees lean and ECU goes richer. Causes about a 25 rpm undulation in the idle speed. Creating a vacuum leak causes the mixture to lean out more than the ECU can correct.
Pre- Marelli cars do not do it in park or neutral because of the system going open loop in those shift positions.
If you have a good multimeter you can back probe the oxygen sensor signal lead and watch the voltage swing from .3-.7v in time with the idle. Try both as usually one side isn’t running quite as clean. You could probably see the same at the connector in the trunk, as long as you don’t have the 47UF capacitor across the meter leads. Remember, no idle stepper motor to correct on these antiques!