irritating idle

[rogwilson]

I have been tring to figure out a engine miss at idle speeds for months. Misbehaving 1980 911 sc. So far I have replaced the spark plugs twice, adjusted the valves, replaced the injector seals and holders, replaced the intake manifold gaskets, had the CO adjusted, replaced the oxygen sensor, and replaced the plug wires with magnecors.

Under acceleration the engine performs flawlessly. But idling around below 3000 rpms it has that iritating slight miss. I pulled the injectors today and they seem to have a good spray pattern with no leaks. I unplugged the warm up regulator and the engine would run with a erratic idle at 1000 rpms but the miss seem to be gone at idle speeds down the road!

Does this make sense? Could this mean that my warm up regulator is faulty or needs adjustment. The warm up regulator has a plugged vacumm line and an adjustment bolt. Apparently someone replaced it 10 years ago with a different model year. I am assuming that disconnecting the electrical plug would make the engine run richer? Any advice?

roger

[MAS]

Could be any number of things... I had similar probblems recently with my 77 (CIS). Finally I took it to the dealer, and they spent 2 hours tuning the engine, adjusted the mixture, and replaced some old hoses. It runs just fine now (better milage too).

-MAS

[SC-targa]

You didn't mention replacing the distributor cap and rotor.

[rogwilson]

I did replace the distributor cap and rotor when the plugs were last changed.

I think I will take it to my mechanic this week to let him take a look. He tracked down a bad lambda relay and bad connection in the wiring haress when I purchased the car a year ago.

I really enjoy doing my own mechanical work, but I suppose you have to weigh the time you are investing in investigating a problem verse letting someone else do the fix so you can back to thoroughly enjoying the ride.

[surflvr911sc]

Try unplugging the O2 sensor, drive it for a while and see if it goes away.

[singpilot]

Does the tach jump in time with the miss? Mine does, and exactly the same thing, only when warm, and only when ambient temps are above 90F.

Have done the plugs, cap and rotor, even changed the primary ign lead after opening the hood after dark and seeing it spark to another lead. Am considering changing all the wires, but they are less than a year old.

[rogwilson]

Ryan I took your advice and disconnected the oxygen sensor last night. This morning I drove to work and it ran 100 percent better!!! There still might be alittle roughness to the idle, but it could be my imagination, so I will continue to drive it this week. I did notice the idle appears to be about 100 to 150 rpms higher. (1000 to 1050). It will not adjust down any more. I have read a few of the threads on the oxygen sensor issue. There seems to be various opinions on whether to connect or disconnect. If this solves the problem, mine will be on the disconnect list. Any rate thanks!

Singpilot my symptoms of a slight engine miss at idle seemed to be independent outside temperature. I didn't notice any movement of the tack during the miss. I'm hoping that disconnecting the oxygen sensor continues to solve the problem.

[surflvr911sc]

Hmm, not so fast Rog, there is more to consider here.

By disconnecting the O2 sensor, the computer is running in open loop (rich) mode. It seems as if you were running lean on one or more cylinders (I think probably the whole system).

The fact that you can no long turn down the idle leads you to a possible vacuum leak. By unplugging the O2 sensor you have richened the mixture to compensate for the vacuum leak but there is still a problem. Check out your hoses, especially the oil tank vent hose. Squeeze it and bend it to make sure you don’t see any cracking. The injection boots could be another spot but check the easy stuff first.

Once you find what it is that’s causing the lean condition you can get it tuned up right, but that’s another thread.

[rogwilson]

Ok, looks like I am on the hunt again for a vacuum leak.

I had previously fixed a few leaks around the injectors by replacing the injector seals and the injector holders and their seals. Also, replaced the intake manifold gaskets and the rubber boots which connect the intake runners to the airbox. This helped tremendously by reducing the idle down from 1200 rpms to 900 rpm with the oxygen sensor connected.

now with the oxygen sensor disconnected the idle is back up to 1000 rpms but with the miss very much improved. Ryan, I think you are right in directing me toward finding another source of stray air. I checked the rubber hose to the oil tank, and it appears to have minute cracks like a weathered tire. Don't know if they go all the way through the hose. So, I will order a new hose to rule out this as the culprit. in the meantime, I plan to check out other sources of air leaks. I wish Bentley had a good diagram depicting all the vacuum lines and their routes.

Question? if everything is sealed tight as a drum and the engine is properly warmed up, how low should you be able to adjust the idle? Thanks.

[surflvr911sc]

I think you are heading on the right track.

W/ everything right you should be able to turn down the idle until it stops running or almost stoped where it will just stumble and cough.

While you are ordering, get a new oil cap seal too, you never know.

That made me think of something else you can try. While the engine is running, take off the oil cap, does the idle stay the same, increase, or decrease?

[rogwilson]

When I remove the oil cap the engine idle decreases.

I should get my new oil vent hoses tomorrow. I noticed that the charcoal canister has a opening on one end with no hose attached, do not know if it needs hooked up.

The engine sure seems to be running much better. It idled this morning at 900rpms to work and 1050rpms on the trip home with the idle adjustment screw all the way in.

tommorow, my day off, I plan to hunt for air leaks and check out the auxillary air regulator.

[stlrj]

How do your plugs look?

Joe

[Argeo]

I may be off, but my car had a similar problem also. My points needed to be adjusted. Easy fix. Not sure if it applies to you but it is an easy cheap fix to replace. Like $10-.

[surflvr911sc]

no points on an '80.

It's a good and bad sign that the idle decreases when you remove the oil cap. Good b/c it's normal and means that the leak isn't there. Bad b/c that means it's somewhere else, good hunting!

[rogwilson]

Well, I finally found the elusive air leak about midnight last Friday. It was late so I fired her this AM for the commute to work. The engine warmed up and then when I slowed down at the first intersection the engine quit running. A welcome sign! I pulled in the parking lot and adjusted the idle by turning out the screw to attain 850 rpms. There is plenty of adjustment and you can snuff the engine out if you turn it all the way in.

The air leak was from the intake boot. Ryan, this must be the boot you were talking about earlier. Never had it off until last night when checking to see if the throttle was fully closing. Apparently, someone had not properly aligned the boot and the opening on the throttle side was tightened at an angle. Consequently, the rubber bead that runs interior in the opening appears to have a small crease in it. I suppose I should order a new boot to be on the safe side.

The air leak appears to be fixed but now the engine idles rough with the oxygen sensor disconnected or connected. It could be related to the old vent hose to the oil tank. They sent the wrong one, so now I will have to reorder another hose.

I wonder if I need to get to CO adjusted again. It was adjusted last November with the air leak. It cost $150 to have it adjusted. Is there a instrument available for to do it in your own garage?

Thanks for the help! Oh, all the plugs look normal to me upon inspection.

[surflvr911sc]

Great Job Rog!!! Yes, now you need to reset you mixture. You need a Gunson Gastester, it's about $150 and is pretty easy to use.

If you are setting the mixture to spec and you are running a CAT, you need to disconnect the O2 sensor and test before the CAT while setting the mixture. If you don't have a test port then take out the O2 sensor and test through it's hole.

Your car is now running real rich now that you've eliminated the air leak. Get it set soon or you will clog up your CAT if alowed to run like that for a long time.

[rogwilson]

Ryan, I really appreciate your input and the board in helping me figure out the underlying problem with the air leak. Now, I think I am ready to tune her up.

Plan on ordering the Gunson Gastester Monday along with a new intake boot and oil vent hose. Probably will not be driving the P-car much this week since the seats come out Monday night for some uphostery repair. Thanks again.