CIS backfire, now won't idle

[Packy]

I had a bad backfire the other day that didn't just open the pop-up valve in the airbox but blew the thing out of the airbox. When I looked back there was smoke coming from the intake. I reglued the pop-up valve back in and the car runs fine as long as I have my foot on the gas.

Right away I noticed that the car would idle low when cold ~ 1000 rpm. I had a long drive and noticed when the car was warm (when I was on the highway going 75mph) and I put my clutch in, the motor would turn off. I essentially had to push start my motor by leaving it in gear and popping the clutch. It seems to still be pulling strong with no power loss as long as I am on the gas.

At a stop sign, speed bump, stop light or whatever the motor would turn off again. I would have to start the car and then keep my foot on the gas.

I suspect I blew a vacuum line or something but I can't seen into the back of the engine bay too well. Either that or I just need to adjust the idle speed but don't want to do that until I know it isn't a vacuum leak.

Any advice?

Thanks,

-Patrick

[tirwin]

Take the cap off the oil filler tube. If the idle doesn't change you have a vac leak. With a backfire that bad, my first guess is you cracked the airbox.

Besides the airbox, there are lots of potential leak sources. Search for posts by Tony (boyt911sc) for help on finding vac leaks.

You've got something else going on as to the cause for the backfire. Poor spark maybe. If you go to my CIS for Dummies thread you will find some pointers on things to read up on and various tests. Be sure to read Jim's Basement Workshop CIS primer.

[Reiver]

Agree with Tim, enough force to blow the valve off probably also split/cracked something else and that loss of vacuum is causing your present problem.
I had a minor backfire a week ago that was caused by too lean an AFR. Just a pop. No issues since fattening the CO.

[Howard M]

If you're lucky you just cemented the Pop-Off Valve poorly. Check the back of the cement joint (nearest the front of the car) with a dental mirror and confirm that you have a complete fillet of cement with no cracks or interruptions. Also, make sure the valve top seats well on the rubber base and alignment wasn't affected by the backfire. The valve must have "stuck" closed resulting in its launch. To avoid that in the future place a thin film of silicone on the rubber seal

[Packy]

Tim, thanks for the help. I think you're leading me in the right direction. I cold started my car, idle was still low (~1000rpm when it should be ~1800rpm cold, usually ~900 rpm warm), and opened the oil filler cap and the rpm didn't drop. SO, I definitely have a vac leak.

I'll be going over your thread here.

I think i'll drop the motor for this one so I can get a good look at everything. Not too excited to do that but I was going to do some tranny work anyways.

Howard, I didn't see any leaks with the pop-off valve epoxy. I'm going to look again to make sure.

Will follow up when I find the problem.

[timmy2]

Your airbox may be cracked. When you drop the engine do a pressure test on it.


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[Bob Kontak]

Originally Posted by Packy

I had a bad backfire the other day that didn't just open the pop-up valve in the airbox but blew the thing out of the airbox.

OUCH! Sorry about that. I can't add much to what these guys are saying regarding the current condition.

Definitely do a pressure test. If you can pull the engine, you can really check everything out (exhaust gaskets as well) with the test.

Stuff a big 'ol potato in the muffler. :-)

[Reiver]

If you know a shop with a 'smoke machine' that is an easy way to check for any leaks in the box/intake/pop off.
P.S. If your box is kaput, or one can be added, look at the later SC's that had a diffuser in place to alleviate back fire propensity.

[Packy]

Originally Posted by Reiver

If you know a shop with a 'smoke machine' that is an easy way to check for any leaks in the box/intake/pop off.
P.S. If your box is kaput, or one can be added, look at the later SC's that had a diffuser in place to alleviate back fire propensity.

Hi Reiver, can you give me more info on this diffuser? Are you talking about the AAR or the AAV?

[Reiver]

May have given you a false lead.
The Injection system was revised in '81 with a frequency valve and an improved cold running mixture control (fuel distributor) for the US market.
I suspect you have a diffuser under your pop off valve...the thing one must not drill thru when installing the PO.

[74-911]

when my air box blew, I didn't need to pressure test it to tell. the screws that hold the two halves of the air box together were blown apart on the drivers side of the air box and the 3 screws under the air cleaner just to the right of the pop-off valve were lose and would not tighten down.

[Packy]

Originally Posted by 74-911

when my air box blew, I didn't need to pressure test it to tell. the screws that hold the two halves of the air box together were blown apart on the drivers side of the air box and the 3 screws under the air cleaner just to the right of the pop-off valve were lose and would not tighten down.

I was able to see SOME of the screws and they looked fine.

Has anyone developed a retrofit for increasing the strength of these plastic boxes? It seems if we had a stronger material like aluminum and a pop-off valve, then the valve would actually do its job. If my box broke then I'm going to be pissed, maybe pissed enough to engineer ANOTHER retrofit.

I just don't have time to baby-sit my CIS intake and check the mixtures, fuel pressure and vacuum leaks to prevent the backfires. Not only that, as the valve guides wear I'm sure there are going to be more stuck valves resulting in backfires. It seems inevitable to me; and the plastic POS boxes are always going to be an issue.

-P

[vash]

so the pop off valve is okay? i scanned the thread.

if it is, then i bet you knocked the A/F mix way off. one time my car backfired, and when that the the air sensor plate, it really messed my my mixture.

it was off to begin with, hench the backfire..but it went really way off. i got it checked and adjusted and i was happy. still happy.

[tirwin]

i bet you knocked the A/F mix way off. .

OP seems to have a vac leak evidenced by no change in idle when taking off the oil cap. Adjusting the mixture would just be a futile attempt to cover up the real problem.

[vash]

Oh. I needed to read better.


Sent via Jedi mind trick.

[tirwin]

Quote de 74-911

when my air box blew, I didn't need to pressure test it to tell. the screws that hold the two halves of the air box together were blown apart on the drivers side of the air box and the 3 screws under the air cleaner just to the right of the pop-off valve were lose and would not tighten down.

I was able to see SOME of the screws and they looked fine.



Has anyone developed a retrofit for increasing the strength of these plastic boxes? It seems if we had a stronger material like aluminum and a pop-off valve, then the valve would actually do its job. If my box broke then I'm going to be pissed, maybe pissed enough to engineer ANOTHER retrofit.



I just don't have time to baby-sit my CIS intake and check the mixtures, fuel pressure and vacuum leaks to prevent the backfires. Not only that, as the valve guides wear I'm sure there are going to be more stuck valves resulting in backfires. It seems inevitable to me; and the plastic POS boxes are always going to be an issue.



-P

I don't think you need to babysit the CIS. It took me a while to get mine sorted out but that was mostly undoing years of previous ownership.

Checking the fuel pressures is so easy to me now I could do it without thinking too much.

The main thing is to get it sorted and then it is fairly straightforward to maintain.

It all boils down to good spark, eliminate vac leaks and proper adjustment of fuel pressures.

[theiceman]

i have a feeling OP will find a fairly obvious leak/ crack when he digs in ... it is posible a crack can seal up under load due to the increased vacuum the intake is pulling.

On a side note I have no problem with the airbox blowing, its like 4-500 bucks for a new updated one . to re-enforce the box or put in an aluminum one does the following
1. Not resolve the problem why it back fired
2. move the problem down the line to something a lot more expensive if the airbox doesnt give.

Just my opinion .

takes about 90 minutes to take entire CIs systrem out the first time , including jacking and dropping the engine about 3 inches. Great time to clean everything up.