1973.5 911T wont run

tafttarga · 05-24-2009, 02:17 PM

I am the proud owner of a 1973.5 911T Targa with a 2.4 and c.i.s. injection. Recently after running perfect all day, i tried to start it after a quick trip to the store and it blew off the pop-off valve.The engine was warm, the hand throttle isn't connected, and it wouldn't start after reinstalling the pop-off valve or when it was cold. It has a new fuel pressure accumulator too. Now what it does is after i bleed the f.i., I'll crank it over till it starts' then it runs up to around 4500 rpm and dies. If I give it ANY throttle it backfires bad and dies. It has 89000 miles on it. any help is highly appreciated.

88-diamondblue · 05-24-2009, 02:46 PM

Can't be much help but welcome to the forum as my car is an 88. We definitely need pictures of the car before we can help,

Just kidding! Someone will chime in with help. We really would like to see your car though...

CliffBrown · 05-24-2009, 03:40 PM

Sounds like you might have developed an air leak in the system (airbox) when it backfired in spite of the pop-off valve.

Or it could be the backfire also pushed a hose loose, check to make sure all the hoses are still connected.

ossiblue · 04:08 PM

Quote:

Originally Posted by CliffBrown

Sounds like you might have developed an air leak in the system (airbox) when it backfired in spite of the pop-off valve.

Or it could be the backfire also pushed a hose loose, check to make sure all the hoses are still connected.

+1

Since it ran perfectly before the backfire, this is the most likely problem. Look closely at the airbox, especially around the screws that secure it to the throttle body. Follow Cliff's advice and check for hoses that have been loosened or cracked. Vacuum leaks cause a lean condition which is what you apparently have. Do a quick search (on the forum) for vacuum leaks and use some of the methods mentioned to find where the leak is, if you cannot see an obvious crack or loose connection.

Keep us updated on what you find.

BTW, the 73 has a micro-switch connected to the hand throttle that triggers the cold start valve when you turn the ignition to start. Since your hand throttle is not connected, you my try to depress the gas pedal slightly to engage the micro-switch on cold starts--unless you are already doing so.

ljowdy · 04:12 PM

Another thing that could happen during a backfire is that the plate that regulates the amount of air into the system could be bent or broken but, start by getting the engine running and then spray "carburetor cleaner" on the outside of the air box. If there is an air leak, the engine will stumble or quit. Broken air boxes are not uncommon.

You also mentioned that it blew off the pop off valve and thay you "reinstalled it" Did it physically come loose?? They are supposed to be glued into the air box with epoxy so if you just placed it into the hole then you have a massive air leak. Direct the carburetor cleaner to the pop off valve. If the engine stumbles, then you have a bad pop off valve or an un-sealed valved. When you squirt the carb cleaner, make sure none enter the air intake system, It won't hurt anything but it will make the engine stumble.

tafttarga · 05-27-2009, 06:58 PM

Thank You. I'll let you know what happens. Thanks again Jon

tafttarga · 05-27-2009, 07:00 PM

oh yeah fair warning, the car is rattle can flat black. It had HORRENDOUS paint

tafttarga · 05-27-2009, 07:01 PM

Thank You. I'll let you know what happens.

tafttarga · 05-27-2009, 07:03 PM

Thank You. I'll keep you posted

HarryD · 05-27-2009, 08:39 PM

Read this thread: A CIS Air Box Cautionary Tale

Find your leaks and fix them.

Get your hand throttle fixed and be sure the microswitch is operable.

Be sure your cold start injector is operable.

Follow the correct startup procedure: CIS questions on a 73.

A well setup 73.5 T is a sweet car to start and drive.

LakeCleElum · 05-27-2009, 08:43 PM

Very hard to start w/o the cold start valve working. If the hand throttle is not connected, it won't work - no matter how much you pump the pedal.....

Sunroof · 05-28-2009, 05:18 AM

As a proud owner of a 1973.5T, first congradulatons............

You must get the hand throttle working! Check to see if that plastic "grab" piece is attached to the throttle cable. Where the hand throttle is, pull back the carpet or cover to take a look at the cable, a small two inch strange looking plastic "grab" fitting should be present. Use a flashlight and view through that narrow slot in the throttle/hand brake housing. Over time these get brittle and break, thereby rendering the hand throttle useless. In all probability the piece came loose or is broke, a very common problem with the 73.5T. In the piece is a small set screw that grabs the cable for adjustment. This is where you can adjust the fitting to catch the cable for setting the amount of rpm (min 3500) for cold start. The plastic piece is not difficult to replace and the piece is not expensive. The only hangup is access. The best way for replacement is to remove the drivers seat first for the best access.

This 73.5T model with the first CIS does not use a "thermotime switch" as that was introduced in the 1974 model. Rather cold start relies on the hand throttle and its critical this works first and foremost.

The first time I experienced some backfiring, which also blew off my pop off valve it turned out I was running way too lean. I thought I blew up the airbox and had it replaced as well. The pop off valve is supposed to protect the airbox, but a major blow can easily crack the box. Think of the new owner of a 73.5T back in the day before the pop-off valve was even invented and one day on start up your airbox shattered into pieces!!!! The pop-off valve did its job for you, but for hundreds if not thousands of 73.5T owners for years it had to be a nightmare! I would venture to say that the 911 that left the most Porsche drivers stranded was the 1973.5T!! Take a good look at the airbox for any cracks, but do not get confused by the straight weld lines on the side of the box you will see and think those are cracks.

Once the airbox is checked out, expoxy the pop-off valve back with a strong two part epoxy. While your doing that check the large rubber air boot that is held on by two large clamps, one over the sensor plate, the other throttle body and remove it for inspection. It sits on top of the airbox next to the fuel distributor. This can be a real problem if cracked. Check carefully and if its cracked or worn, replace it. Clamp securely. With the air boot off, lift the large metal sensor plate by hand carefully from inside the airbox and take a rag to carefully clean around the seat under the plate where it lands. This can have oil and debris that can cause a slight leak. As mentioned, feel the plate to make sure its not warped, bent or does not seat well.

Thats a good start......................

You can take a mirror as well and look behind the airbox to check the line fittings on the cold start valve and fuel distributor.

If this is your first Porsche, welcome to the longhood community. Despite the temporary problem here, the 73.5T is an excellent car and very dependable (when working of course). Please keep us posted on any and all progress as this is how we all learn and are able to spread our collective knowledge.

Best of luck

Bob
1973.5T in Sepia

HarryD · 05-28-2009, 08:27 PM

Quote:

Originally Posted by LakeCleElum

Very hard to start w/o the cold start valve working. If the hand throttle is not connected, it won't work - no matter how much you pump the pedal.....

Mine was inadvertantly disocnnnected for 6 months and starting it was a bear until I plugged the connector back in.

Vacuum leaks can be insidous as a very small one will allow the car to start (albeit with small occasional backfires), warm up and then "seem" ok.