I guess it is now my turn. Blown airbox

[Brtuffli]

Greetings folks,
I have a 78 SC and the other day, I turned the key, heard a loud backfire and then engine cranked with key but no joy. Opened up the read deck lid and saw, as I feared, a blown airbox. Fire inside of it went out quickly (thankfully).

Since the motor has been out and apart within the last few years, I am thinking of replacing with only a partial drop. The car now has about 46,000 miles and I believe this was the original airbox. I believe the new ones have attempted to solve the chronic problem but I understand there is no substitute for a properly tuned cis. A few questions:
I have the guages, should it do a before test?
Should I send my WUR off to tony for a double check?
What else should I investigate as I not wish to have this problem again?

I had taken it to a local well known independent shop in St. Louis to adjust the air mixture, idle and timing after I had replaced the head studs. They are an adjustment but this was probably 2 years ago.

Any and all comments and advice welcomed. By the way, there are a decent amount of threads on airbox replacements that I found but didn't see any specific to 78 or 79 sc's, which I understand are slightly different.

Thanks in advance

[Jack Stands]

I remember installing a backfire pop-off valve in my airbox back in the CIS days. It at least provides some protection in case of a backfire.

[Brtuffli]

I did have a pop off valve installed. And I have had many very small backfires in which I believe it helped. This was a BIG backfire. I'm not wanting to get into a pop off valve discussion as I believe I understand the pros and cons. In this most recent case, it didn't prevent the blown airbox.

[rwest]

the newer airboxes have a metal spider that directs the cold start valve fuel down the intake runners making it less likely that fumes will collect in the box and ignite.

I would certainly do all the tests before hand to see if there is an obvious issue.

I would also make sure my ignition components are in excellent shape.

I have a completely unscientific thought on old airboxes blowing and that is that the plastic has been heat soaked for years and becomes brittle which then takes much less force to break it then a new one that has some flexibility.

[Brtuffli]

Thanks for the reply. How do I confirm they are in good shape. I replaced the cap and rotor 10 - 15 thousand miles ago. Plug wires were replaced many years ago ( but less than 40,000 miles ago. Plugs are newish when cap and rotor were replaced and looked like they were firing appropriately. (Grayish consistent color on each plug)

[tirwin]

If you search for my CIS Troubleshooting for Dummies thread you’ll find the instructions on how to test the fuel pressure.

The older I get, the less I trust anyone to do what they should. I know there are some great shops out there that work on old aircooled cars, but I just see so many people getting short-changed because they took it to someone who they thought knew what they were doing.

Do the tests yourself. You will learn a lot and worst case you’ll have greater confidence in talking to your mechanic.

For ignition, I’d pull the coil wire and (carefully, with a helper) ground it while cranking to check the spark. It should be bright blue. If it’s orange, then you have weak spark. Next pull a plug wire an repeat.

Do the fuel pressure tests. That will tell you if the WUR is working properly. If not, then the tests will point to what needs to be calibrated. Bench test the AAR. Make sure your injectors have a good spray pattern (don’t forget the CSV!).

Check for vac leaks. Several good threads on homemade smoke machines. I replaced all my rubber braided with silicon lines.

With CIS the AFR tends to the lean side. I suspect the reason that popoff valves became popular is that CIS problems tend to slowly creep in over time. A small vac leak that gradually gets worse and worse over time is hard to notice. Then people tweak a little here, a little there to compensate until you can’t anymore. The popoff valve is a canary in a coal mine. It is telling you something is wrong (hopefully) before it becomes expensive.

[Brtuffli]

Sweet. This is very helpful. I don't think I have a vacuum leak as the rpm did drop when I removed the oil cap but i was thinking of replacing the vacuum lines anyway. Are standard sized that I can get locally?

I have read many of the cis for dummies threads when I tested my fuel pressures a few years ago. The WUR was working ok per replies on this board. I also tested the injector flow and amount at that time. I have not checked the AAR or CSV. I will need to read up on how to test these

[Brtuffli]

[boyt911sc]

Originally Posted by Brtuffli

It is nice to have these flexible fuel lines when checking the flow rates. Job well done.

Tony

[Brtuffli]

Car is finally home and in the garage. I ran the fuel pressure tests. Using HF guages with the scrhader valves removed and the relay bypass switch that I made last time. The test went well and fairly trouble free (which almost made it fun to do if it weren't for the cold temps. Anyway, here are the readings:
78 SC US
WUR 00 149 045
Temp in garage. 2 C or 36 F
WUR resistance. 24.0.
System pressure. 72psi or 5.0 bar
Cold control pressure. 16 psi or 1.1 bar
Warm control pressure. 31 psi or 2.1 bar. Took 6 min. Was 25 psi after 3 min and went up about 1 psi per 30 seconds
Residual pressure 5 min 22 psi or 1.5 bar
Residual pressure 15 min 18 psi or 1.2 bar
Residual pressure 30 min 16 psi or 1.1 bar
Residual pressure 60 min 12 psi or .8 bar

Let me know if these look ok. I will begin to disassemble the system later this week.

[boyt911sc]

Originally Posted by Brtuffli

Car is finally home and in the garage. I ran the fuel pressure tests. Using HF guages with the scrhader valves removed and the relay bypass switch that I made last time. The test went well and fairly trouble free (which almost made it fun to do if it weren't for the cold temps. Anyway, here are the readings:
78 SC US
WUR 00 149 045
Temp in garage. 2 C or 36 F
WUR resistance. 24.0.
System pressure. 72psi or 5.0 bar
Cold control pressure. 16 psi or 1.1 bar
Warm control pressure. 31 psi or 2.1 bar. Took 6 min. Was 25 psi after 3 min and went up about 1 psi per 30 seconds
Residual pressure 5 min 22 psi or 1.5 bar
Residual pressure 15 min 18 psi or 1.2 bar
Residual pressure 30 min 16 psi or 1.1 bar
Residual pressure 60 min 12 psi or .8 bar

Let me know if these look ok. I will begin to disassemble the system later this week.

The WCP is too low. Next time you do this test, measure the WCP with and without vacuum applied. And the CCP is also out of spec.

Tony

[Brtuffli]

Thanks for the direction. I will get on with replacing the airbox. Meanwhile reading how to adjust/impact CCP and WCP

[stlrj]

Originally Posted by Brtuffli

I will get on with replacing the airbox.

Hopefully the new one will have the internal metal plumbing that should prevent future explosions without the need of the pop-off valve which doesn't work anyway.

Cheers,

Joe

[Brtuffli]

Ok. My Bentley manual arrived today. Very helpful. Still need to learn all the different names for these things.

1). I didn't find any directions on removing the airbox in the Bentley manual. I assume they expect you to remove engine and then remove the entire CIS as a unit. Correct?
2). Should I test the CSV prior to removing the CIS? Does it matter if I test it with the pressure guage still attached?
3). Thermo-time switch checking. Same questions?
4). Looks like the way to check the AAR is remove it and check to see if the valve is open. Correct? I can't check it with the engine running as the airbox has not been replaced yet. ( or do I replace the airbox first, get the engine running and then perform the tests?
5). For retesting the WCP, is the vacuum line I am removing attached to the WUR? Do I need to apply vacuum if connected?
6). I have not found probable causes for a low CCP. Suggestions?

Thanks in advance for your counsel and time

[mike sampsel]

I have removed my CIS with engine in the car, runners and all to fix leaks from the trinity. Then my wife decides we need to paint the car! So the engine comes out. Then Bruce convinced me to get the valve guides done, from there the dykes failed.

Anyway with engine out CIS came off. It’s much easier to install the CIS system as a unit with the engine out. You can get at it from all angles. It’s a real challenge to install the runners on the intake studs in situ. It’s easy to get over anxious and crack a new airbox by forcing your runners on. So if you are a first timer, I’d drop the engine, but this takes a couple of hundred extra bucks probably, which is a pittance for these cars.

[mike sampsel]

I found a crows foot wrench which fit my oil line. Then used a wrench (30 mm, I think) to hold the crossover pipe and had a floor jack under the pipe to stabilize. With breaker bar oil nut came off pretty easy. I did this once before when I changed my old lines (they started to leak). I tried the 32 mm wrench which I bought here years ago with no luck. Breaker bar and crows foot are easy.

I pulled both engine and transmission too. Lots of various opinions on which method. If just the engine getting the angle correct between engine and transmission require a Jack which can accurately get the angle right. Not so easy for the home mechanic

[Brtuffli]

Thanks for the replies. I have removed the engine and trans 3 times in the 40,000 miles I have owned the car. Can get it done in under 2 hours. I would only considering removing both the engine and trans as a unit. No downside as I think the most difficult part is mating the trans to the engine. My thinking is that the 2 hours I spend removing and the 2 hours I spend installing can go to the airbox removal and install.

I Hadnt considered that the airbox could be broken during the install if I become over zealous.

Any thoughts to my CIS questions?

[boyt911sc]

Originally Posted by Brtuffli

Thanks for the replies. I have removed the engine and trans 3 times in the 40,000 miles I have owned the car. Can get it done in under 2 hours. I would only considering removing both the engine and trans as a unit. No downside as I think the most difficult part is mating the trans to the engine. My thinking is that the 2 hours I spend removing and the 2 hours I spend installing can go to the airbox removal and install.

I Hadnt considered that the airbox could be broken during the install if I become over zealous.

Any thoughts to my CIS questions?

Perform all the necessary tests before you install the individual CIS components. Don’t wait afterwards because you will be disappointed. And after the engine has been completely assembled, do a smoke or pressure test while the complete motor is sitting on your engine stand. If you are not familiar with the different tests simply inquire.

Tony

[Brtuffli]

Why do I question the folks here? After removing the air box, I realize that I should have removed the engine and trans. I am planning to remove them for the CIS install. Time to clean and check stuff

[Brtuffli]