popped out the pop off ... idle problem

thane · thane's Garage

The other day my 1981 SC backfired badly at start up. I took a look at the air box and my pop off valve had popped out. I pushed it back in and the car ran fine getting me home. I re-epoxied the pop off valve back in place but now I have idle hunting when the car is warm. If I blip the throttle in neutral the RPMs crash and the car stalls. When the idle is hunting if I remove the oil cap the idle smooths out and blipping the throttle does not cause the car to stall. Before the backfire incident the idle would drop 100 RPM or so when I removed the oil cap. I leaned the mixture out 1/16 turn but saw no improvement. Any ideas?

Thanks.

RFR · 09-04-2006, 07:36 PM

Free bump, I am interested in this thread

thane · thane's Garage

Anyone?

Gunter · Gunter's Garage

Removing the oil cap will change the AF-mixture and drop the RPM by about 50. That's normal.
Sounds like you have a vacuum leak somewhere and that makes it difficult to set the mixture.
Do a search on how to locate vacuum leaks. Lots of info here.

ruf-porsche · ruf-porsche's Garage

Quote:

Originally posted by thane
Anyone?

Sounds like a crack airbox.

Kelly Bell · Kelly Bell's Garage

I have to agree with ruf-porsche. Or you could have the pop-off in badly. try putting some silicone around the rim of the pop off valve and see if it gets better. Also, try the old carb spray test. You spray carb cleaner around the air box and listen for a change in idle speed. If you get a change while sparaying a certain spot, take a close look at that spot, or where the spray was going for a vacumn leak......

Kelly Bell · Kelly Bell's Garage

OMT re the carb spray test. Have a fire extinguisher handy. Carb cleaner is pretty flammable!

Kelly Bell · Kelly Bell's Garage

Also, try holding down the lid of the pop off vavle tightly with your hand while a helper starts the engine and see if the problem goes away. It could be you have damaged the O ring on the pop off vavle.....

Keithc · Keithc's Garage

I had what sounds like the same problem and it turned out to be a crack in the airbox! Cost and arm and a leg to replace. I have a pop off valve on order but it's a bit disturbing if the pop-off valve is no sure remedy!

ruf-porsche · ruf-porsche's Garage

If you have an airbox crack, leaning the mixture will only exacerbate the problem. Because you have unmetered air entering the engine you already have a leaner condition than what is normal. If you want to experiment, try richening up the mixture.

Eventually you will need to replace the airbox or at least locate the crack(s) and try to repair it/them.

Hetmann · 09-06-2006, 08:17 AM

Also agree on crack in airbox. I had a hairline crack that seem to open up when warm and close when cold. Took a long time to find it. You will need to look very closely and see if it is in a seam or crease on the airbox.

petrolblue83911 · 09-06-2006, 08:36 AM

Sounds like air box crack to me, -if you play with the pop-off valve does it open and close easily on its spring?- if it flew out during backfire it was either loose to begin with or the spring is out of place/rusty and didnt open freely. the pressure generated in the air box during a back fire should be easily freed by the pop off valve if it is working properly.

-this is a good FYI for all of us-check the pop off on occasion, maybe lube the spring as well. I did this during my last oil change but to be honest it was the first time I'd looked at it in a looooong time.

Vereeken · 09-06-2006, 09:07 AM

Try the carb spray test on your pop-off valve and the immediate surroundings. I am fairly confident that the revs will go up after 5 seconds. Hell if you do not have carb spray use WD40 less flameble.

I had the exact same problem, the epoxy simply does not seal it 100%. I had to epoxy extra under the spring as it turned out to have the biggest issues with letting air pass unmetered. Once I got it all the way epoxy-ed all was fine.

The pop-off allows so much pressure to be released that I can not believe that the remaining pressure can crack the airbox. But ten again all is possible with our cars.

Also check your Air Flow meter plate, it might have debris between the seat and the plate from teh epoxy or it might be out of alignment as a result of the backfire.

How does she run at full throttle in say 3rd gear? How is she holding up if you keep her at 4000rpm in second or third.

Good luck.

Michel

thane · thane's Garage

Thanks for all the suggestions.

Michel, the car runs great at WOT

I'll try holding the revs. Did yours exhibit problems in those scenarios?

zotman72 · 09-06-2006, 09:54 AM

In my SC, similar thing happened with symptoms. The second time around epoxy glue was not as effective in sealing, as I had used a cheaper type of hi temp epoxy from Pep Boys. Get that right Wurth epoxy stuff that comes in a white circular tab with the cute spoon that gives 2 minutes max to apply it.
HTH

Howard M · 09-06-2006, 10:08 AM

The back side of the pop-off valve (front of the car) can be difficult to see. Verify that you have a good fillet of cement on the blind side by using a dental mirror.

vash · vash's Garage

i was told that a big backfire will mess up your air/fuel mix. i would get that checked before you go investing anymore money in looking for a cracked box. a good friend chased this gremlin, and now he has two good airboxes. one in the car, and one in a box in his shelves. the same thing happened on my car. blew the valve right out. my mechanic checked the mix, and he said it was sooo lean, he was surprised it would even run. mine hunted around too.

fastfredracing · 09-06-2006, 02:17 PM

I also had a similar problem, after I re inserted the pop off valve, I had a terrible idle, but the car ran normal on the juice. I guess the backfire was violent enough to ruin the pop off valve itself. when I would hold it up to the light ,I could see that it was not making a good seal, also, make sure that the o ring is still present it the pop off valve, mine was laying in a seperate place when the valve came out.
Good luck

Vereeken · 09-06-2006, 11:37 PM

Quote:

Originally posted by thane
Thanks for all the suggestions.

Michel, the car runs great at WOT I'll try holding the revs. Did yours exhibit problems in those scenarios?

You should have no problems in those scenario's , I have found that at high revs the pop valve gest sucked in more and the % of the unmetered air versus metered air gets actually lower so the problems is less pronounced or not present. And that is something that you can not get buy with when you have a cracked Airbox.

Perhaps the engine is now running lean but do not think so; should be easy to check out if you have a A/F meter or Co meter.

I have one mechanic here in Belgium who swears by Tec 7 a silicone type glue to seal the valve, but frankly it looks like crap. Can you imagine the horror when someone opened my air box and noticed I used Tec7, hell no I'd rather spent hours getting the epoxy right ;o)

kach22i · kach22i's Garage

Quote:

Originally posted by vash
i was told that a big backfire will mess up your air/fuel mix.

Dang, that happened to me 1-1/2 years ago..................had to figure it out on my own.