diagnosing idle and low rpm acceleration issues

[StrokinIT]

Hopefully someone can confirm or deny my suspicions here. I have a 1976 280z, and I dropped my car off at a local shop, they've usually taken pretty good care of me, my dad, and my brother. However, they mostly do older muscle cars, not so many older imports pass through there. Any way, my symptoms are high idle that can't be lowered via the idle speed screw; hesitations and stumbles during acceleration under 3500 rpms; occasional backfires after these stumbles; I have no way of telling if fuel mileage is abnormally poor or if its because I just tend to keep the car at higher rpms so that it won't stumble. The car runs well above 3500 rpms though. We already replaced the spark plugs and the auxiliary air regulator, they helped a little bit but it still has most of the symptoms. I have read through the EFI Bible and come up with a few suspicions that I wanted the shop to look into for me.

 

I asked them to check:

-valve clearance

-throttle plate closing

-AFM has proper movement

-check for intake manifold leaks

-check fuel pressure

 

With the combination of issues that I have, my hunch is that there is an intake leak somewhere, or the throttle plate doesn't close all the when the throttle is released, however I could be mistaken and there could be more than 1 reason for these symptoms. I spoke with them today and asked how the tests went and they said they were trying to get it to idle properly before they would do the tests. This doesn't make a whole lot of sense to me seeing that one of the main reasons I brought it in is because it doesn't idle properly. My thoughts were that if they performed those tests, it would guide them to the problem so they could fix it. I know more about cars in theory than I do in practice, so maybe I'm missing something here, but do any of those tests require that the car is running optimally before they be performed?

 

I do have an aftermarket cam in there from the previous owner, and just sent them a scan of the cam information. I know aftermarket cams don't really get along well with the factory EFI but I would have thought that the cam would cause the idle to be lower by having less vacuum at idle and low rpms. I will attach the JPG of the cam information card for reference. In case its hard to read, the lobe separation is 110 and the intake opens at 7* BTDC.

 

Thanks in advance,

Josh

[Tennesseejed]

Josh, check the warm up air bypass system (I can't recall the proper name off the top of my head).

 

Bascially, there is a valve that shunts air across your throttle body that closes after the car warms up. It's bolted to the top of your intake manifold right behind the throttle body. The FSM says to test by pinching the hose after the valve when the car is warm to see if the idle rpm changes.

 

These things notoriously stick open allowing extra air causing both high idle and back fire from lean running condition.

[StrokinIT]

Tennesseejed, I believe you are talking about the auxiliary air regulator, but please correct me if I am wrong. I'm also almost positive we did that test and that there was no change.

[NewZed]

TJed is talking about the auxiliary air regulator. A good suggestion but sounds like you've checked it. Although the old hoses to the AAR can split leading to a vacuum leak.

 

Another device that lets air past the throttle body is the BCDD. I have read that they can get stuck but have not experienced it myself. It's the weird looking thing attached to the bottom of the throttle body, with an electrical wire attached. It bypasses air based on RPM and intake vacuum. It's described in the Emissions section of the FSM. Boost Controlled Deceleration Device.

 

The hesitation, stumble and backfire suggest a vacuum leak, leading to a lean condition. Air is getting in without moving the AFM vane. There are many places for small air leaks from the AC valve control canister and hoses to the PCV system. Most people suggest taking an unlit propane torch and blowing propane around the potential leak areas and listening for RPM change as it gets sucked in. I've never had to go that far so can't vouch for effectiveness.

[Tennesseejed]

Yes, the AAR, which from your OP I see you already replaced.

 

BCCD is good suggestion. Also check the little hose from the manifold to the fuel pressure regulator.

 

The problem with the stock EFI system is that its filled with 35 year old hoses and fittings. I was always troubleshooting FI problems on my '75 until I went Megasquirt. Then I wondered why adding Megsquirt wasn't the first thing I did when I got the car. Dead reliable.