Oscillating Idle

[walt]

After 2-3 minutes of running the engine begins to oscillate wildly. Before when I removed the oil filler cap the engine speed used to drop, but not any longer. Thinking this is a sign of a vacuum leak I tried carb cleaner and propane to locate the problem without any success.

My next step would be to try a DIY smoke generator but want to make sure I am on the right path.

Thanks,

[DRACO A5OG]

Sounds like you are on the correct path.

Try blowing in cigar smoke into a vacuum line, tape off the air filter intake. Cigar will stay lit longer than a cigarette.

[gazzerr]

That works best! When mine was doing this I had a really big crack in the back side of the black CIS hat that bridges the CIS plate and throttle body. You wouldn't necessarily find that with the carb cleaner spraying around the runners. You've got a pretty big leak or a bunch of small leaks adding up. Best thing to do is just strip the whole CIS and go through everything. It's a big job but it's worth it. Over the years the CIS airbox glue on the underside seam fails too. Check out this old thread ...

Air leakage from intake manifold sleeves

[Bob Kontak]

Originally Posted by gazzerr

You wouldn't necessarily find that with the carb cleaner spraying around the runners.

I am ok with carb cleaner.

Tony is not big on it for valid reasons.

I have ignited it on a Taurus once with a cracked spark plug that was grounding to the block. Scary, but in that case harmless.

Just spray in spritzes. Does not take much.

[GothingNC]

Make sure the oil filler cap is making a tight seal.

[zelrik911]

Originally Posted by GothingNC

Make sure the oil filler cap is making a tight seal.

Yes make double sure that the cap is sealing properly - I had exactly the sale symptoms as you describe, but it turned out my oil cap was loose.

[walt]

The cigar trick worked great. I can see smoke coming from between the fuel distributor and the rubber boot in the area of the fuel/air adjustment screw. If I hold my hand there I can feel air shooting straight up, my guess is the boot is cracked. Can that boot removed without a partial engine drop and of course one of the smaller hose clamps on the rear is pointed such I can't loosen.

[timmy2]

Air coming out upstream of the air metering plate (on the air filter side of it) is not a vacuum leak.
Anything between the air metering plate and the intake runner gaskets is a vacuum leak.
Really it should be just between the top of the Throttle body to the intake runners to be considered the sealed system, as you also have the oil tank line in the rubber boot from air metering to TB top.

[vinylbro]

Originally Posted by walt

After 2-3 minutes of running the engine begins to oscillate wildly. Before when I removed the oil filler cap the engine speed used to drop, but not any longer. Thinking this is a sign of a vacuum leak I tried carb cleaner and propane to locate the problem without any success.

My next step would be to try a DIY smoke generator but want to make sure I am on the right path.

Thanks,

Oxygen Sesor

[gazzerr]

You can take the CIS boot off without a partial drop. If you have the heater blower then you will need to remove that and lay it down in the back of the engine bay. The whole thing lifts off and you can slide it over to the left. Careful you don't lose the screws to the metal bands on each side of the hat. Last time I did that I swore so hard I would have embarrassed the guys working on the NY docks. Getting the rear hoses off and on at the rear of the hat is a PITA but it's doable. Hopefully you can get a small screwdriver on that one facing to the back and loosen it enough so you can rotate it ccw and facing you.

Bob -> I know a guy that lost his eyebrows to the combo of a Ford 351 V8, Holley carb and starter spray ...

[walt]

Dennis, thanks I was considering the area beneath the air filter to be part of the vacuum side. Try again tomorrow.

[T77911S]

oscillating is too rich.

I would check fuel pressures. if you cant, check power to the WUR and the resistance of the heating element.

[walt]

Blew smoke in the vacuum line connected to the control pressure regulator and had smoke coming from the sensor plate into the air filter housing. Reinstalled the filter and closed off the air inlet with plastic and found no further leaks. I am thinking this is probably normal and should move on to checking fuel pressure. But, still concerned about no change in idle when oil filter cap was removed.

[Bob Kontak]

Originally Posted by walt

I am thinking this is probably normal and should move on to checking fuel pressure.

It's very normal but not an effective air leak test. You pushed smoke into the intake plenum and it bypassed the throttle plate.

I think you should determine that you have no air leaks then proceed to other potential culprits.

[boyt911sc]

Originally Posted by walt

Blew smoke in the vacuum line connected to the control pressure regulator and had smoke coming from the sensor plate into the air filter housing. Reinstalled the filter and closed off the air inlet with plastic and found no further leaks. I am thinking this is probably normal and should move on to checking fuel pressure. But, still concerned about no change in idle when oil filter cap was removed.

Walt,

The procedure you did was not going to help you find if you had any source of air leak/s in the system. Remove the rubber boot cover on top of the throttle body. Place a latex glove over it held by a rubber band to keep it in place. Now time to inject air pressure or your favorite cigar smoke to any of these:
a). Pop off valve if you have one installed.
b). Brake booster connection at the side of the CIS air box.
c). Injector mounting hole.
d). Others......

You don't need a lot of air pressure to do this test. A 4~5 psi. continuous supply of pressurized air will be more than sufficient plus a spray bottle with detergent in water. It is more difficult and challenging with the engine installed compared with the motor on a test stand. I will look for some pictures to post later.

Tony

[boyt911sc]

Walt,

Some old pictures looking for air leaks in a 911 CIS motor. Please take note of the very low air pressure (less than 5 psi.) in the 3rd. picture. All you need is a positive pressure in the system just to inflate the toy balloon. And spray water with dish washing detergent over the suspected areas. A home-made smoke machine would be helpful too. For this particular test, I used the brake booster connection to the air box as inlet due to convenience. You could also use the POV, throttle body vacuum port or the fuel injector port.

The trick to this procedure is to know which orifices to block off to isolate the system from atmosphere. Notice the two (2) large rubber stoppers left side of the latex glove in the last pic? There is another one used at the POV (pop off valve).

[T77911S]

that's how I did mine.
soapy water.

nice idea with the glove on the TB tony.
I also figured out best place to set the crank for minimal vavles open. I had the engine so I even adjusted the open valves so they were closed.

again, check power to the WUR and fuel pressures. rich makes it surge.

[Bob Kontak]

Originally Posted by T77911S

again, check power to the WUR and fuel pressures. rich makes it surge.

Is there a difference between surging and hunting?

Surging should smell very rich after warm up. Maybe the layman's test for this?

[walt]

Followed Tony's procedure above and found 4 of 6 fuel injector seals leaking, 1 quite bad. I also have some leakage coming from somewhere underneath the AAV. Looks like a full engine drop would be needed to diagnose this one further. Any thoughts here.

Thanks, Walt

[walt]

Tightened the hose clamps on the AAV and all seems good. Will replace injector seals next week and hopefully idle will return to normal. If not, move on to fuel pressure check. Thanks for the help.

Walt