Rough Idle on 73.5 CIS
 

Last december I had that 2.4 redone. All the rubber that could leak air is new, the motor was rock solid... FFWD 4 months and I had a local guy setup my suspension, but he said the pop off valve blew when he started the car - I wasn't there... He reglued it, and since then I've found that the idle is wrong. It oscillates between 500 and 900 rpm, up and down all the time and even stalls occasionally.

I use the hand throttle to make the idle a solid 1000 rpm and it works, but the car feels like it's on cruise control that way...

What could it be ? I looked inside the airbox and it's not cracked. The rubber stuff that could leak air (that I can see) is also solid... You can hear the open airbox suck air in cycles, so something's fishy.... Any advice or troubleshooting procedure welcome..

GReg.

Just sounds like it's running a little rich.

Doug

Just like that, it started running rich by itself ?
I forgot to mention, the oil tank breather hose spits out some oily smoke in the airbox, and it's kinda oily in there. Any bearing on the problem ?

Greg:

I'd say that you have two potential issues:

1) Vacuum leak from blown airbox. You can find this by using a can of carb cleaner and gently spraying it around various parts of the airbox. The idle will go up and stabilize for a moment when the chemical is pulled into the leak and the mixture changes. Patience is rewarded, here,....:)

2) Sensor plate out of adjustment

The oil smoke is likely caused by blowby from the crankcase. This can be from rings that haven't seated to other issues with the engine. Do a leakdown test and note the percentages and whether you hear air leaking out of the oil filler tube.

Breather should not be spitting anything following a rebuild.

It doesn't take much to alter the mixture. I think a big backfire could throw it off slightly. And it sounds like it's off just slightly.

Doug

Thanks guys.. One more Q...

http://members.rennlist.com/greg/911t_airbox.jpg

I circled a suspicious area here: There's a hole that looks like it should have a screw in ti or something... Should I plug it ?

The mixture screw sure is a pain to reach ;-(

PS: I didn't mean to say I had the engine rebuilt. The whole CIS was rebuilt and the mechanic said the exhaust numbers were perfect and the motor was in excellent shape (54000 original miles). So now I guess I gotta worry about the blow by too ;-) Yipee !!!

Greg:

Start the engine and spray some carb cleaner at that hole. If its leaking false air, you'll hear the idle speed change so you know that it needs attention.

I sprayed all around to no avail... I tried making the mixture a little leaner and that also made zero difference. In the end, I raised the idle to the prescribed 900+/-50, in my case more +....
The car is stable now, looks like it was dropping too low and going crazy - now how idle chages after a pop off valve detonation, I don't know... Anyway I won't think I've won the battle until I see the fully warm idle on the car...

PS: That oil vapor in the airbox from the oil return tube... Can anyone explain some more ? The oil comes from the a return tube at the top of the oil tank filler, so what would the engine have to do with this ? Is this return tube necessary BTW ?

The screw that was in that hole helps hold the two halves of the airbox together. You have at very least a split seam on the right side of the airbox. And your pop-off valve hinge should point directly backwards.

Try spraying along the underside of the airbox, on the right, right along that line of screws.

Greg,
It's possible your popoff valve was loose for some time, creating a lean condition, and someone richened the mixture to compensate. Then , when the valve was reinstalled, the mixture was too rich, causing the fluctuating idle. An alternative reason would be the guy who fixed the valve also reset the mixture too rich.

about setting your 'basic idle' with a cis car with hand throttle: the car is always intended to be started with the hand throttle fully up when cold (if you stop for a moment when warm, this is obviously not necessary). the idea is that as the car begines to warm, the idle will begin to increase..you compensate by gradually lowering the throttle will warming until it reaches operating temperature of 180 degrees. you cannot properly set the idle on a 'cold' cis car.

drive the car until you get 180 degrees, get out and adjust idle at the air-idle bypass screw to whatever your manual states (~1000 rpm)..now your basic idle is properly set. next, i recommend you take the car to a shop with a proper gas analyzer and pay what's necessary to correctly set the mixture..until you have it done on a machine, you'll never know it properly. do this after you have performed the prior mentioned check for air leaks - if you have those, they will need to be addressed first, otherwise mixture setting is a waste of money. good luck.
ryan

Thanks guys... I suspected as much (cold Idle)- in truth I did not set it 100% cold but it was not 180 either so I'll have to redo that.

The car was tuned and ran real well 100 miles ago. Really... With the CO analyzer and all... But that backfire appears to have screwed things up...


That hole, that is supposed to hold the airbox halves together ? I can poke a stick right thru it and it goes under the airbox so I don't know... I've shot carb cleaner thru and around and it does nothing, so
I don't know if it really leaks there... The hole doesn't seem to be threaded either... I'll have to investigate but I want to make sure it's not a wild goose chase. Can anybody confirm the hole circled in the picture above is indeed supposed to have a screw in it ? Steve Weiner did not pick up on it and I was hoping that meant it's normal !!

the thing is, it could still just be your 'basic idle' that isn't set properly..an incorrect basic idle setting will definitely cause the irregularity during warm-up. please try this first - it's a 'free repair' once you get it set after you'd allowed the car to reach operating temperature. this really has to be done on a car with a hand throttle..later cis models 'superceded' this method of warm-up idling speed by use of a vacuum device, the auxillary air regulator. personally, i prefer the hand throttle, it allows me to set idle speed wherever i want in the range during warmup..and lots of other folk on this board who have them agree. good luck.
ryan

Yes, that hole is supposed to have a screw in it. The screws are blown loose when the airbox splits, which is why I think you have a blown airbox. The hole goes through the top half, and into the lip or flange of the bottom half. If you spray through it you're spraying the top of the engine.