Bang! There Goes the Airbox

Bad news this morning I'm afraid:
I went to pick up the car from the shop after getting a little work done (alignment, adjust timing & mixture, replaced leaking distributor vacuum line) following my turbo tie-rod, ball joint, steering rack spacer install last weekend. Was greeted with the news that the car had just backfired when they started it up before I arrived, & sure enough the airbox is split. I had been meaning to install a pop-off valve, but as the car had always started fine everytime without a hint of any problems I put a couple other projects ahead on the list.

Thinking about it it does seem a little unlucky that, after lasting 190 000 miles the airbox would wait until just after this work was done (also following a recent ignition tune-up) to explode! Is it possible that the faulty distributor vacuum hose leak had been providing relief to the system, preventing backfires??

The shop said they typically do a partial engine drop for this, although the Porsche book rate I asked them to look up gives 7 hours labor (which I guess assumes a full engine drop- 7 hours seems alot!) I have to contemplate towing it home & tackling this myself- I doubt its possible without dropping the engine at least a few inches though? Has anyone who's done this have any words of advice? What would a realistic time estimate be for the shop to do this?

Obviously the moral of the story is- fit a pop-off valve if you don't have one!

Any advice appreciated. I guess no driving for me this weekend

[Leland Pate]

You must own just about the absolute LAST CIS car in existance that hasn't had a pop-off valve installed.

Sorry to hear of your luck.

If you have a garage, floor jack, and basic hand tools, you can drop the engine, remove the Injection system and replace the airbox yourself and save a wad of cash.
I agree with your mechanic that this job requires the removal of the engine.

[Jdub]

Whoa! I am not one to point fingers but if there has been a change to the CO/timing and directly after the airbox explodes I would insist on splitting the cost to replace the airbox if they have not already offered to replace it gratis.

I am very interested in hearing other's thoughts on this. My opinion is based on the idea that CO and timing must be correct to avoid this result. Am I way off here? Also, I wonder if they had FULLY warmed the car up prior to adjustment: that is key.

John

[MotoSook]

One thought. If you take it home and replace the airbox, then find out there is something else wrong with the car due to the work by the shop (or your work), then what? Would it be better to have the shop replace it so as to avoid a dispute as to who done it? You would hope that the CIS is adjusted well prior to the airbox explosion, thus after replacing the airbox, you can start her up and go. Or will there be a need to make idle adjustments, CO adjustments and such, all of which would be included in the 7 hours quoted to you by the shop.

I am the type to do this work myself, but something feels wrong (unless you trust the shop).

[MotoSook]

I agree with Jdub...something is up.

[Hladun]

Putting in a pop-off valve does protect your airbox, but take out your AAV and you'll stop startup backfires entirely. Do you really want everyone in the parking lot looking at you each time you start your car?

[john walker's workshop]

i've blown a couple over the years by just reaching in the window and cranking the key. it's fate my friend. hard to explain to the customer, but really not the fault of the shop. a carbon flake under an intake valve, that lets a flame travel up the runner and ignite the cold start spray, a WUR that runs the engine too lean, etc. fate. the 80-83 engines really have to come out. there's just too much crap in behind the intake to deal with. earlier ones can stay in, and don't even have to be dropped slightly.

Thing is, the car had never backfired at all since I bought it (admitedly only 3 months ago). My impression is that small (undamaging) backfires are relatively common with CIS?

Apparently they didn't have to tweak the mixture much. They said it was pretty close previously.

[MotoSook]

JW, I agree it is too likely for the airbox to blow, but if that is the only thing wrong with the car then good for everyone. I'm just a skeptic. If Andrew is comfortable with working on the CIS, then he should do it himself. But is there troubleshooting yet to be done post replacement of the airbox?

[Jdub]

Well JohnW is my favorite wrench, and if he says it can happen by fate then I am convinced. So take my critique above with a large helping of JohnW's comments.

John

[MotoSook]

Are JW and Jdub handles for the same J.Walker? Alter egos???

[Hladun]

This thread is interesting (and very typical), because once again the “put in a pop-off valve and let’s buy airboxes" mentality overshadows fixing the problem once and for all.

[Superman]

I agree that it's just the luck of the draw and I am not suspicious of the shop. On the other hand, I think a lean mixture (factory spec, by the way) is often the culprit and this car did just get a tuneup. Further, after replacement, the mixture will need to be reset again preferably using a gas analyzer.

I'm thinking that the ideal situation would be for the shop to cut you a special deal on replacement including labor, where the shop still makes a couple of bucks and you're happy too. That's what I would offer if I were the shop owner. But then, I'm one of those guys you can bet will never be rich.

[kristian85]

The "Pop the Magic Dragon" (worst song ever) shuffle.....

Wonder if Ferraris or Benzes with CIS ever blew their boxes!

[Jdub]

John Walker and I share only a love for Moose Drool beer. I also share my money with him ;>

John

[Langers]

Originally posted by kristian85
The "Pop the Magic Dragon" (worst song ever) shuffle.....

Wonder if Ferraris or Benzes with CIS ever blew their boxes!

Nup! 1985 W126 380SE 225 000 km and still going strong here.

[MotoSook]

I had a technician whose Mercedes with CIS sucked the idle mixture adj. screw into a cylinder...he ended up buying a Chrysler...don't ask how that screw got sucked into the motor.

Moose Drool...is it a stout? We don't have too many Mooses in the Midwest...well, there was a guy on my football team we called Moose.

[Gunter]

If you trust the shop, try to work something out. Maybe they can get a new/used airbox and you'll pay for labor (And a pop-off valve) This may be a time to consider what else can be done while the engine is out. Some hard to reach items such as oil pressure sender (for the dashboard light), oil thermostate seal, breather gasket, intake runner boots, bushings for the throttle linkage rod which is mounted under the #3 intake runner, and, of course, all the hoses behind the airbox/fuel distributor. These items are not expensive. Check for oil leaks!!!

[osidak]

I blew my airbox in my 78 and I replaced it myself with out dropping the engine at all. Taking it out was not a problem as the box was totally blown. Putting it back in took a long time and some cussing but it went back together.

I did put a pop off valve in before I installed the new box though. I have since heard that valve open twice. Sorry to hear your bad luck. I asked a shop how much he would have charged and he quoted ~$375 plus parts.

[jk911]

The exact same thing happened to me after I picked the car up from the auto electician a few months ago. see my post.

I replaced the box myself without dropping the engine at all. I did it all in one day without a problem. I purchased a used (in excellent condition) airbox for $150 with a pop of valve already installed . Don't worry. What I thought was a nightmare turned out to be the cheapest and most rewarding DIY repair i have done on the car.

Make sure the replacement airbox is the updated one which has metal tubes goind from the cold start valve towards each intake rubber boot.

good luck